Emerging Photovoltaic Technologies: Photophysics and Devices [1 ed.] 9814800694, 9789814800693

Table of contents :
Cover
Half Title
Title Page
Copyright Page
Table of Contents
Preface
1: Fullerene-Based Organic Solar Cells
1.1 Introduction
1.2 Working Principle of Organic Solar Cells
1.2.1 Structure of Bulk-Heterojunction OPV
1.2.2 The Power Conversion Process
1.2.3 Device Performance
1.3 Materials Used in Fullerene-Based OPV
1.3.1 Fullerene-Based Acceptor
1.3.2 Donor Material
1.4 The Morphology of Active Layer and the Processing Method
1.5 Stability Problem
1.6 Conclusion
2: Non-Fullerene-Based Polymer Solar Cells
2.1 Introduction: Principle of Operation of All-Polymer Organic Solar Cell
2.1.1 Basic Operation Principles
2.2 Calculation of Power Conversion Efficiency
2.3 Polymer/Polymer Blends
2.3.1 Cyanated Phenylenevinylene (CN-PPV)-Based Polymer Acceptors
2.3.2 Benzothiadiazole-Based Polymer Acceptors
2.3.3 Rylene Imide Dyes
2.3.3.1 Perilene diimide-based polymer acceptor
2.3.3.2 Naphthalene diimide
2.4 Morphology
2.4.1 Solvent
2.4.2 Thermal Annealing
2.4.3 Molecular Weight
2.4.4 Donor/Acceptor Blend Ratio
2.5 Conclusion
3: Ternary Sensitization of Organic Solar Cells: A Multifunctional Concept to Boost Power Conversion Efficiency
3.1 Introduction and Motivation for Organic and Ternary Solar Cells
3.2 Fundamental Principles
3.2.1 Charge Transport/Transfer Mechanisms
3.2.1.1 Cascade charge transfer
3.2.1.2 Energy transfer
3.2.1.3 Alloy model
3.2.1.4 Parallel-like model
3.2.2 Beyond the Sensitization Concept: Controlling Recombination
3.3 Nature of the Third Component: A Review on the Experimental Results
3.3.1 Polymeric Ternary Components
3.3.2 Small Molecule-Based Ternary Components
3.3.3 Dye-Based Ternary Components
3.3.4 Non-Fullerene Acceptors
3.3.4.1 D:FA:NFA ternary solar cells
3.3.4.2 D:NFA:NFA ternary solar cells
3.3.4.3 D:D:NFA ternary solar cells
3.4 Conclusion and Outlook
4: Dye-Sensitized Solar Cells: Photophysics of Coordination Complex
4.1 Introduction
4.1.1 Fabrication of Dye-Sensitized Solar Cells
4.1.2 Photovoltaic Efficiency Measurement
4.1.3 Time-Resolved Techniques
4.1.4 TiO2-Based DSCs
4.1.5 NiO-Based DSCs
4.1.6 DSCs Tandem Architectures
4.2 TiO2-Based DSCs
4.2.1 Molecular Design of the Photosensitizer
4.2.2 Influence of Photosensitizer Structure on Electron Transfer
4.2.3 Photosensitizer Injection and Regeneration Kinetics
4.3 Summary and Outlook
5: Fe Complexes as Photosensitizers for Dye-Sensitized Solar Cells
5.1 Introduction
5.2 FeIIL6 Complexes
5.2.1 FeII Polypyridyl Complexes
5.2.2 Cyano-Bearing Complexes
5.2.3 Photosensitization Studies on FeIIL6 Complexes
5.3 Modified FeIIL6 Complexes
5.3.1 FeIIL6 Complexes of Higher Octahedricity
5.3.2 Equilibrium with Another State
5.3.3 FeII NHC Complexes
5.3.3.1 FeII NHC complexes based on normal NHCs
5.3.3.2 FeII NHC complexes based on mesoionic NHCs
5.3.4 Cyclometalated FeIIL6 Complexes
5.4 Sensitization Beyond the 1/3MLCT Transition of FeIIL6 Complexes
5.4.1 Exploring the 5/7MLCT States of FeIIL6 Complexes
5.4.2 The LMCT States of FeIIIL6 Complexes
5.5 Conclusion
6: Quantum Dot-Sensitized Solar Cells
6.1 Introduction
6.2 Structure and Performance Parameters
6.2.1 Structure of a QDSSC
6.2.2 Performance Parameters
6.3 QDSSCs Materials and Preparation
6.3.1 Photoanode
6.3.1.1 Metal oxide semiconductor layer
6.3.1.2 QDs photosensitizer layer
6.3.2 Electrolyte
6.3.3 Counter Electrode
6.3.3.1 Noble metals
6.3.3.2 Carbon materials
6.3.3.3 Metal chalcogenides
6.4 Charge Transport in QD-Sensitized Solar Cells
6.4.1 Charge Carrier Dynamics in QDSSC
6.4.1.1 Electron injection in n-type QDSSC
6.4.1.2 Hole injection in p-type QDSSC
6.4.1.3 Charge transfer states
6.4.2 Charge Transport in QD Heterojunction Assembly
6.4.2.1 Exciton migration in CdSe based QD assembly
6.4.2.2 Charge carrier transport in PbS based QDs assembly
6.5 Multiple Exciton Generation in QDs Solar Cells
6.5.1 MEG Dynamics in QDs-Basic Concept
6.5.2 MEG in QDs Solar Cells
6.5.3 Bottleneck for MEG in the QD-Sensitized Solar Cells
6.6 Summary and Outlook
7: Time-Resolved Spectroscopic Studies of Perovskites
7.1 Introduction
7.2 Femtosecond Pump-Probe Transient Absorption Spectroscopy
7.3 Ultra-Fast Time-Resolved Terahertz Spectroscopy
7.4 Electronic Structure of Lead Halide Perovskites
7.5 Trap States in Perovskites
7.6 Mono, Bimolecular, and Auger Recombination in Perovskites
7.7 Efficient Transport of Electrons and Holes in Perovskites
7.8 Charge Carrier Transfer to Holes and Electrons Extracting Layers
7.9 Conclusions and Remarks
8: Using First-Principles Simulations to Understand Perovskite Solar Cells and the Underlying Opto-Electronic Mechanisms
8.1 Introduction
8.2 Basic Features of Hybrid Perovskites Semiconductors
8.2.1 Band Structure of Hybrid Perovskites and the Band Gap Problem
8.2.2 Ferroelectricity in Hybrid Perovskites: Spontaneous Polarization of Periodic Systems and Rashba–Dresselhaus Splitting
8.2.3 Prediction of Spectroscopic Observables: Vibrational Response of Hybrid Perovskites within the Harmonic Approximation
8.3 “E pur si muove” (Still, It Moves): Ionic Motion at Operating Temperature
8.3.1 Prediction of Spectroscopic Observables: Vibrational Properties of Hybrid Perovskites Beyond the Harmonic Approximation
8.3.2 Band Gap Oscillations at Room Temperature in CH3NH3PbI3 and Possible Contribution of the Electrostatic Disorder to Charge Separation and Hindered Recombination
8.4 Beyond Ideal Crystals: Surfaces and Point Defects
8.4.1 Surfaces
8.4.2 Point Defects: Energetics and Their Possible Role on the Material Photo-Response
8.5 Advanced Methods: Excited State Properties
8.5.1 Role of the Electron–Hole Interaction in Excited State Properties of Hybrid Perovskites
8.5.2 Beyond Born–Oppenheimer Approximation: Non-Adiabatic Quantum Dynamics and Electronic De-Excitation
8.6 Synopsis
Index

Citation preview

edited by Carlito S. Ponseca, Jr.

EMERGING PHOTOVOLTAIC TECHNOLOGIES PHOTOPHYSICS AND DEVICES

EMERGING PHOTOVOLTAIC TECHNOLOGIES

EMERGING PHOTOVOLTAIC TECHNOLOGIES PHOTOPHYSICS AND DEVICES

edited by

Carlito S. Ponseca, Jr.

Published by Jenny Stanford Publishing Pte. Ltd. Level 34, Centennial Tower 3 Temasek Avenue Singapore 039190

Email: [email protected] Web: www.jennystanford.com British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

Emerging Photovoltaic Technologies: Photophysics and Devices Copyright © 2020 by Jenny Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher.

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher.

ISBN  978-981-4800-69-3 (Hardcover) ISBN  978-0-429-29525-6 (eBook)

Contents

Preface 1.

Fullerene-Based Organic Solar Cells Wanzhu Cai, Zesen Lin, and Lintao Hou 1.1 Introduction 1.2 Working Principle of Organic Solar Cells 1.2.1 Structure of Bulk-Heterojunction OPV 1.2.2 The Power Conversion Process 1.2.3 Device Performance 1.3 Materials Used in Fullerene-Based OPV 1.3.1 Fullerene-Based Acceptor 1.3.2 Donor Material 1.4 The Morphology of Active Layer and the Processing Method 1.5 Stability Problem 1.6 Conclusion

xi 1

1 3 3 5 6 10 10 12 14 16 17

2. Non-Fullerene-Based Polymer Solar Cells 31 Alice Corani 2.1 Introduction: Principle of Operation of 32 All-Polymer Organic Solar Cell 2.1.1 Basic Operation Principles 34 2.2 Calculation of Power Conversion Efficiency 36 2.3 Polymer/Polymer Blends 38 2.3.1 Cyanated Phenylenevinylene 38 (CN-PPV)-Based Polymer Acceptors 2.3.2 Benzothiadiazole-Based Polymer Acceptors 42 2.3.3 Rylene Imide Dyes 44 2.3.3.1 Perilene diimide-based 44 polymer acceptor 2.3.3.2 Naphthalene diimide 46 2.4 Morphology 47 2.4.1 Solvent 48

vi

Contents

2.4.2 Thermal Annealing 2.4.3 Molecular Weight 2.4.4 Donor/Acceptor Blend Ratio 2.5 Conclusion

48 50 50 51

3. Ternary Sensitization of Organic Solar Cells: A Multifunctional Concept to boost Power Conversion Efficiency 57 Negar Kazerouni, Marcella Guenther, Barry C. Thompson, and Tayebeh Ameri 3.1 Introduction and Motivation for Organic and Ternary Solar Cells 57 3.2 Fundamental Principles 59 3.2.1 Charge Transport/Transfer Mechanisms 59 3.2.1.1 Cascade charge transfer 60 3.2.1.2 Energy transfer 66 3.2.1.3 Alloy model 68 3.2.1.4 Parallel-like model 70 3.2.2 Beyond the Sensitization Concept: Controlling Recombination 73 3.3 Nature of the Third Component: A Review on the Experimental Results 78 3.3.1 Polymeric Ternary Components 79 3.3.2 Small Molecule-Based Ternary Components 86 3.3.3 Dye-Based Ternary Components 90 3.3.4 Non-Fullerene Acceptors 98 3.3.4.1 D:FA:NFA ternary solar cells 99 3.3.4.2 D:NFA:NFA ternary solar cells 104 3.3.4.3 D:D:NFA ternary solar cells 106 3.4 Conclusion and Outlook 112

4. Dye-Sensitized Solar Cells: Photophysics of 121 Coordination Complex Vanira Trifiletti and Norberto Manfredi 4.1 Introduction 122 4.1.1 Fabrication of Dye-Sensitized Solar Cells 129 4.1.2 Photovoltaic Efficiency Measurement 131

Contents





4.1.3 Time-Resolved Techniques 134 4.1.4 TiO2-Based DSCs 138 4.1.5 NiO-Based DSCs 139 4.1.6 DSCs Tandem Architectures 141 4.2 TiO2-Based DSCs 142 4.2.1 Molecular Design of the Photosensitizer 143 4.2.2 Influence of Photosensitizer Structure on Electron Transfer 148 4.2.3 Photosensitizer Injection and Regeneration Kinetics 151 4.3 Summary and Outlook 157

5. Fe Complexes as Photosensitizers for Dye-Sensitized Solar Cells Yizhu Liu and Kenneth Wärnmark

167

5.1 Introduction 167 5.2 FeIIL6 Complexes 171 5.2.1 FeII Polypyridyl Complexes 171 5.2.2 Cyano-Bearing Complexes 177 II 5.2.3 Photosensitization Studies on Fe L6 Complexes 179 5.3 Modified FeIIL6 Complexes 181 5.3.1 FeIIL6 Complexes of Higher Octahedricity 182 5.3.2 Equilibrium with Another State 183 5.3.3 FeII NHC Complexes 184 5.3.3.1 FeII NHC complexes based on normal NHCs 184 5.3.3.2 FeII NHC complexes based on mesoionic NHCs 190 5.3.4 Cyclometalated FeIIL6 Complexes 193 5.4 Sensitization Beyond the 1/3MLCT Transition of FeIIL6 Complexes 194 5.4.1 Exploring the 5/7MLCT States of FeIIL6 Complexes 194 5.4.2 The LMCT States of FeIIIL6 Complexes 196 5.5 Conclusion 197

vii

viii

Contents

6. Quantum Dot–Sensitized Solar Cells 209 Huifang Geng and Kaibo Zheng 6.1 Introduction 209 6.2 Structure and Performance Parameters 211 6.2.1 Structure of a QDSSC 211 6.2.2 Performance Parameters 212 6.3 QDSSCs Materials and Preparation 213 6.3.1 Photoanode 213 6.3.1.1 Metal oxide semiconductor layer 214 6.3.1.2 QDs photosensitizer layer 217 6.3.2 Electrolyte 221 6.3.3 Counter Electrode 222 6.3.3.1 Noble metals 223 6.3.3.2 Carbon materials 223 6.3.3.3 Metal chalcogenides 224 6.4 Charge Transport in QD-Sensitized Solar Cells 226 6.4.1 Charge Carrier Dynamics in QDSSC 226 6.4.1.1 Electron injection in n-type QDSSC 227 6.4.1.2 Hole injection in p-type QDSSC 230 6.4.1.3 Charge transfer states 231 6.4.2 Charge Transport in QD 233 Heterojunction Assembly 6.4.2.1 Exciton migration in CdSe based QD assembly 234 6.4.2.2 Charge carrier transport 236 in PbS based QDs assembly 6.5 Multiple Exciton Generation in QDs Solar Cells 238 6.5.1 MEG Dynamics in QDs-Basic Concept 238 6.5.2 MEG in QDs Solar Cells 239 6.5.3 Bottleneck for MEG in the QD-Sensitized Solar Cells 240 6.6 Summary and Outlook 242

7. Time-Resolved Spectroscopic Studies of Perovskites 251 Piotr Piątkowski 7.1 Introduction 251

Contents



7.2



7.4



7.3 7.5 7.6 7.7 7.8 7.9

Femtosecond Pump-Probe Transient Absorption Spectroscopy 253 Ultra-Fast Time-Resolved Terahertz Spectroscopy 256 Electronic Structure of Lead Halide Perovskites 260 Trap States in Perovskites 269 Mono, Bimolecular, and Auger Recombination in Perovskites 276 Efficient Transport of Electrons and Holes in Perovskites 282 Charge Carrier Transfer to Holes and Electrons Extracting Layers 288 Conclusions and Remarks 290

8. Using First-Principles Simulations to Understand Perovskite Solar Cells and the Underlying Opto-Electronic Mechanisms

303

Claudio Quarti 8.1 Introduction 303 8.2 Basic Features of Hybrid Perovskites Semiconductors 306 8.2.1 Band Structure of Hybrid Perovskites and the Band Gap Problem 307 8.2.2 Ferroelectricity in Hybrid Perovskites: Spontaneous Polarization of Periodic Systems and Rashba–Dresselhaus Splitting 313 8.2.3 Prediction of Spectroscopic Observables: Vibrational Response of Hybrid Perovskites within the Harmonic Approximation 318 8.3 “E pur si muove” (Still, It Moves): Ionic Motion at Operating Temperature 322 8.3.1 Prediction of Spectroscopic Observables: Vibrational Properties of Hybrid Perovskites Beyond the Harmonic Approximation 325

ix

x

Contents



8.3.2 Band Gap Oscillations at Room Temperature in CH3NH3PbI3 and Possible Contribution of the Electrostatic Disorder to Charge Separation and Hindered Recombination 329 8.4 Beyond Ideal Crystals: Surfaces and Point Defects 333 8.4.1 Surfaces 333 8.4.2 Point Defects: Energetics and Their Possible Role on the Material Photo-Response 338 8.5 Advanced Methods: Excited State Properties 341 8.5.1 Role of the Electron–Hole Interaction in Excited State Properties of Hybrid Perovskites 342 8.5.2 Beyond Born–Oppenheimer Approximation: Non-Adiabatic Quantum Dynamics and Electronic De-Excitation 346 8.6 Synopsis 353

Index

367

Preface

Preface

This review volume came to fruition after a decade of dedicated research to the investigation of various emerging photovoltaic material systems. The field of solar cell science and technology is vast and spans from theoretical chemistry, photophysics/photochemistry, ultrafast spectroscopy, imaging, and material sciences to interface engineering, to name a few. The chapters included here present a few of these sub-fields and how they could contribute to the optimization of photovoltaic devices. Three chapters are dedicated to polymer solar cells, i.e., fullerene, non-fullerene, and ternary organic photovoltaic devices. Three chapters examine hybrid solar cells, that is, dyesensitized, Fe-based sensitized, and quantum-dot sensitized solar cells. The last two chapters report on the first-principles calculation and time-resolved studies of recently emerging organo-metal halide perovskite solar cells. The editor and the chapter contributors present the fundamental photophysical properties of the abovementioned emerging photovoltaic technologies and discuss the role of these properties in enhancing the power conversion efficiency. This book would not have been possible without the guidance of Prof. Villy Sundström, my mentor in ultrafast spectroscopy. Villy, with his effortless enthusiasm, expertise, and wisdom, has inspired me to pursue this very challenging and exciting field. He has given me the freedom to navigate the field and re-orient me whenever I seemed lost. I am also grateful to Prof. Arkady Yartsev, who unselfishly gave his time whenever I needed, especially on the technical details of the experiments. Thank you so much, Villy, Arkady, and the other researchers in the Division of Chemical Physics of Lund University, Sweden. Special thanks to all the chapter contributors, who have dedicated their time, knowledge, and expertise for writing their chapters. Finally, my unending thanks and love to Courtney Huang, my dedicated wife, and Kenshin Teodor and Mary Kerstin, my kids, who are the source of my inspiration. Ad maiorem Dei gloriam! Carlito S. Ponseca Jr.

xi

Chapter 1

Fullerene-Based Organic Solar Cells

Wanzhu Cai, Zesen Lin, and Lintao Hou Department of Physics, Jinan University, Huangpu Avenue West 601, Guangzhou, Guangdong 510632, China [email protected]

1.1 Introduction The history of organic photovoltaic (OPV) research dates back to more than a half-century ago, since the first observation of photovoltaic effect on anthracene crystals in 1959 [1]. Despite this relatively long history, its potential for commercial application has long been underestimated. The situation changed when conducting polymer was discovered in the late 1970s [2–5], which opened a new chapter in OPV research. It brought opportunity to synthesize materials with desired properties, and led to the acceleration in its power conversion efficiency (PCE). Currently, PCE of OPV devices with single junction reaches 14% [6], and PCE of multi-junction geometry reaches 17% [7]. High-throughput production of OPV devices and modules has already been demonstrated [8–10] and launched in many countries for commercial use [11]. Emerging Photovoltaic Technologies: Photophysics and Devices Edited by Carlito S. Ponseca Jr. Copyright © 2020 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4800-69-3 (Hardcover), 978-0-429-29525-6 (eBook) www.jennystanford.com

2

Fullerene-Based Organic Solar Cells

The fundamental component in OPV is the organic conjugated material, which is considered as “the fourth generation of polymeric materials” [12, 13]. Similar to most polymeric materials, Van der Waals force dominates the intermolecular interaction, inducing some typical “plastic” properties, such as soft and lightweight. The backbone of organic conjugated molecular typically contains alternating single bonds and double bonds with many repeat times known as conjugation. This conjugated chemical bonding leads to one unpaired electron per carbon atom and a continued overlapping of Pz orbital, which was theoretically demonstrated using the Su– Schrieffer–Heeger (SSH) model [3, 14]. This unpaired electron is called π electron. The induced split energy bands are called π band and π* band, which is analogous to the conduction band and the valence band, respectively, in the Energy Band Theory [15]. The organic conjugated materials used as a light absorber in OPV are usually intrinsic, meaning the molecule has its π band filled and π* band empty. The gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is corresponding to the bandgap. This kind of material inherits the convenience of chemical structure tailing, the easily processing properties and mechanical properties from the old generation of polymeric materials. In OPV, when the light absorber materials absorb light, strongly bonded electron–hole pair is generated. This quasi-particle called exciton has a binding energy (an electrostatic attractive force) measured to be around 0.5 eV or more [16, 17], which is much higher than the thermal energy of the particle at room temperature, ~25 meV. Because organic conjugated materials have a relatively low dielectric constant, consequently resulting in a large electrostatic attractive force [13]. Besides, the exciton in OPV has a short diffusion length. Typically, the diffusion length is around 20 nm long for polymer [18, 19], although long-range diffusion happens in crystallized molecule domain [20] or with triplet exciton [21] were demonstrated. For high PCE, high exciton splitting rate is a prerequisite; therefore, overcoming the high binding energy and limited diffusion length of the exciton in OPV device is the most important at the very beginning of its operation. For quite a long time, devices with a single component as light absorber were used but presented extremely low photocurrent

Working Principle of Organic Solar Cells

(1950s–1980s) [22]. The breakthrough came when fullerene-based materials were introduced. In 1984, Tang’s bilayer OPV device first showed a dramatically enhanced photocurrent compared to the single component device [23]. Later, researchers found that the interface between two materials with different electronegativity and electron affinity is a splitting area for hot exciton. These two materials are named as electron-donor (D) and electron-acceptor (A), respectively, indicating the former is the hole transport dominated material, and the latter is electron transport dominated material. In 1992, timeresolved photoluminescence (PL) measurement revealed that the photo-induced electron transfer from the conjugated polymer (MEH-PPV) donor to fullerene (C60) acceptor is in the time scale of 50–100 femtoseconds [24], which is 1000 times faster than any exciton decay processes, such as photoluminescence (nanosecond scale) and charge recombination (microsecond scale). In 1995, the boom of PCE arrived through the use of a donor: fullerene derivate blend in the active layer, which is called bulk-heterojunction active layer [25, 26]. In this chapter, we will review the investigations on fullerenebased OPV, to elucidate the understanding of fullerene-based OPV regarding working principle, materials, morphology and operation stability.

1.2  Working Principle of Organic Solar Cells 1.2.1  Structure of Bulk-Heterojunction OPV

OPV device is a typical two-terminal diode using a layer-by-layer stacked sandwich geometry. The total thickness of the device is commonly less than half a micrometer, although micrometer thick device has also been reported [27]. In the laboratory, the OPV device is fabricated with a rigid substrate, whose active area is defined by the electrode pattern as shown in Fig. 1.1. The device structure contains three types of functional layers. They are the active layer with light absorber material, metallic electrodes used for the electricity connection (anode and cathode), and carrier transport layers for the charge carriers transport and collection (the hole transport layer (HTL) and the electron transport

3

4

Fullerene-Based Organic Solar Cells

layer (ETL)). Figure 1.1 shows a conventional geometry of the device, in which light is injected through the anode electrode (high potential electrode). Another device geometry is an inverted structure with the light injected through the cathode, which is not shown here. Due to the performance issue, bulk-heterojunction is the most popular structure for the active layer till now [25]. The electronic structure of devices is mainly decided by the nanoscale morphology in the active layer [28]. Also shown in Fig. 1.1 (top, right) is a typical morphology of donor and acceptor molecules separated in randomly distributed phases, some domain form continuous interpenetrating networks. More details on morphology will be discussed in Section 1.4. The ETL and HTL are used to improve the transport and collection efficiency of electron and hole, respectively, by aligning the energy level between the active layers and the electrodes. In general, a waterfall-like potential alignment is desired, which reflects as the contact resistance approaching zero [29–32].

Figure 1.1  Top left: photograph of OPV device in the laboratory; top right: schematic of the morphology of the active layer, area with different color represents different material domain. Exciton splits at the two-material interface. Bottom: conventional device geometry.

Working Principle of Organic Solar Cells

1.2.2  The Power Conversion Process In OPV, the quasiparticle species involved in the energy conversion process, with different energy states are identified as photon, exciton, interfacial hole–electron pair and free charge carriers (hole or electron). When the incident light hit the light absorbers, either electron donor or electron acceptor, a photon with energy larger than the optical bandgap (Eg) is converted into a neutral exciton. Excitons are generated inside the molecule with a size of few nm [33–35], and they split at the D/A interface. The induced exciton concentration gradient provides the driving force for the exciton to diffuse toward D/A interface during its lifetime. Else, excitons decay via radiative recombination or non-radiative recombination pathways [36].

Figure 1.2  Energy conversion process schematic in a state energy diagram, 1D* is the singlet excitonic level, D+A– is the CT state, and A– is the transport state in acceptor.

At the D/A interface, the exciton converts into a bound hole– electron pair at the heterojunction. This hole–electron pair is settled at a D/A complex energy state called charge transfer (CT) state [37, 38]. Charge recombination could happen and is identified as geminate

5

6

Fullerene-Based Organic Solar Cells

recombination at this step [39, 40]. The geminate recombination here commonly involves both radiative recombination and nonradiative recombination. Free hole and electron are born at the next stage when they are fully separated into different components. Electrons and holes are transported in an interpenetrated pathway toward the electrode. During this period, the recombination also can happen due to the Coulombic forces or trap states [39, 41, 42]. Due to the energetic disorder of transport states caused by the structural disorder of conjugated polymerized materials, charge carriers are transported by hopping among the localized states in a broad distribution of states, not excluding multiple trapping and release [43]. The whole energy conversion process is depicted in Fig. 1.2; the solid line arrow shows the photocurrent generation direction.

1.2.3  Device Performance

The performance of OPV device is generally evaluated using four parameters: Open-circuit voltage (VOC), Short-circuit current density (JSC), Fill Factor (FF) and PCE. They are extracted from the current density-voltage (J–V) response curve of the device as indicated in Fig. 1.3, measured under a standard simulated light called AM1.5G (ASTM standard). FF is defined by FF = Pmax/ (VOC × JSC), where Pmax is the maximum output power and equal to VMPP × JMPP. In the 2D plot of J–V curve, FF is the ratio of size from square filled with dots and the square filled with solid line. Therefore, J–V curve is more approaching a perfect square, the higher FF is obtained. The PCE is defined as

PCE =

VOC ¥ JSC ¥ FF (1.1) Pin

Pin is the incident light power, it is standardized as 100 mW/cm2 for AM1.5G. To gain insight into the improvement strategy of performance, we discuss the relationship between each parameter and the device working mechanism, as well as the correlation between material engineering method and corresponding material properties in the following.

Working Principle of Organic Solar Cells

Figure 1.3 Standard J–V curve of a OPV device.

Solar radiation presents a continuum optical spectrum covered from ultraviolet (UV) light to infrared light. Receiving the photon with such a broad range of radiation energy, for OPV materials with a specific bandgap, the spectral mismatch is inevitable. Therefore, two types of possible spectrum losses could happen. As mentioned above, photovoltaic materials can only convert the photons with energy larger than Eg into exciton. The photons with less energy than the Eg would be turned into heat or radiation. Photons with energy larger than bandgap would transfer the excessive energy to exciton and release this excess energy in a process called thermalization. In 1961, Shockley and Queisser considered a solar cell is an ideal blackbody, and the spectrum loss is back to the environment by photon emitting only. They predicted the ultimate efficiency of photovoltaic devices in a detailed balance approach (an emissionabsorption equilibrium), known as Shockley–Queisser (SQ) limit [44]. They predicted the maximum power conversion efficiency is 30% when the energy gap is 1.1 eV. This inevitable loss also reflects on the limitation of VOC; it causes a VOC loss DEsq typically between 0.25 and 0.3 eV. For easier evaluation of other loss of VOC, we separate the other VOC loss into two types, non-radiative recombination

7

8

Fullerene-Based Organic Solar Cells

(induced) loss DEnon–rad and radiative recombination (induced) loss DErad. In fullerene-based OPV, there is a strong non-radiative recombination loss compared to its counterpart like inorganic PV. In the well‐known P3HT: PCBM system, it presents a large DEnon– rad as high as 0.67 eV [45]. It is identified as the most critical VOC loss for bulk-heterojunction-based OPV device [46, 47]. This loss is suggested as an intrinsic loss in organic materials system due to the energy coupling between electron and C- C bond vibration [48]. This loss manifests on the device as extremely low electroluminescence emission. To decrease DEnon–rad, increasing the high radiative quantum efficiency of the low-bandgap component is found to be an efficient method [46, 49], which could be realized by reducing the vibrionic coupling between the CT state and the ground state claimed by some recent studies [50, 51]. DErad for fullerene-based OPV is also largely due to the existence of CT state and commonly manifest as PL emission. Considering Eg is the maximum limit of VOC, VOC can be written as the following expression.

VOC = Eg - DEsq - DE rad - DE non-rad (1.2)

For most of the fullerene-based OPV device, the gap between the theoretical Eg and the actual VOC is relatively large. For the state-ofthe-art devices, this loss is still up to 0.8 V. Furthermore, there is possible VOC loss at the electrode, which can be readily remedied by making sure that there is Ohmic contact between the electrode and active layer [31]. JSC can be obtained by

JSC =

q hC

lmax

Ú EQE ◊ P (l )dl , (1.3) in

lmin

where q is the elementary charge, h is Planck constant, C is the speed of light and EQE is the external quantum efficiency, referring to the ratio of output electrons to the incident photons. EQE is equal to the product of all efficiencies in the energy transfer processes as written in the following equation:



EQE = habs ◊ hdiff ◊ htc ◊ htr ◊ hcc , (1.4)

where habs is the photon absorption efficiency. The last four parameters, namely ηdiff, ηtc, ηtr, and ηcc, are the components of internal quantum efficiency (IQE), which represents the efficiencies

Working Principle of Organic Solar Cells

of the exciton diffusion process, the hole–electron separation process, the carrier transport process and the charge collection process, respectively. ηabs depends on the overlapping of the absorption spectrum with the solar radiation spectrum, as well as the absorption coefficient of the material. Optical engineering is an efficient way to enhance ηabs [52]. ηdiff represents the efficiency of the exciton diffusion from bound to the separated state. As discussed above, the typical exciton diffusion length in semiconducting polymers is less than 20 nm. Therefore, the distance between the spatial location where exciton is generated, and the interface should be short enough. The recombination of exciton would result from the geminate recombination, which induces the loss of VOC and JSC. Once the exciton reaches the interface, the energy conversion from exciton to free carriers would happen. ηtc is used to describe this charge transfer efficiency. In fullerene-based OPV, large energy offset between donor and acceptor provides the sufficient driving force for the charge separation [53]. For electron, the driving force is the LUMO/LUMO offset [54]. Apparently, it leads to an energy loss. The ηtr depends on the carrier’s transport, which is usually evaluated by the carrier mobility and the lifetime [55, 56]. In practically, the intrinsic characteristic of organic conjugated materials, such as high traps density [57, 58], delocalized transport state and imperfect carrier transport network [59], is harmful for the charge transport. Molecular design and morphology engineering are needed for improving htr [60–63]. Note that for higher htr, ohmic and selective contacts are required [29, 64]. It has been shown that that OPV device can possibly reach 100% IQE [65] if and when all the efficiency factors are optimized. FF is a more complex factor, it reflects the recombination happens in devices in a general view [39, 60]. In most cases, sufficiently high and balanced electron and hole mobility will lead to positive change of FF [66, 67], unbalanced carrier transport would induce the carrier accumulation, then bimolecular recombination. Injection/ extraction barriers at the electrode would greatly reduce the FF due to the same carrier accumulation [68]. More detail discussion on the device physics can be found in [36, 56, 69].

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1.3  Materials Used in Fullerene-Based OPV 1.3.1  Fullerene-Based Acceptor Fullerene acceptor is a big family, containing pristine C60, C70, C78, C80, C84, etc., as well as plenty of their derivatives (Fig. 1.4 ). They all featured a spherical cage, with carbon atom placed at each vertex and covalently conjugated over the surface. In 1985, H. W. Kroto discovered and named the first fullerene, showing it is an all-carbon molecule buckminsterfullerene (C60) with 12 pentagonal and 20 hexagonal faces [70]. Soon after, its high electron affinity gained much attention. For a single C60 molecule, it can accept up to six electrons due to its triply-degenerated low-lying LUMOs [71]. In 1992, the photoinduced transfer from MEH-PPV to C60 was identified by Sariciftci [24]. This work starts the study on fullerene-based OPV. Various fullerene derivatives were further developed, and they commonly show similar electronic structure as C60, such as the high electron affinity [72]. In 1995, Wudl group reported the synthesis and use of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) [73], which can be dissolved in common organic solvents. Later, the idea of bulk heterojunction was conceptualized by using this soluble fullerene, and the breakthrough in the PCE is initiated. The simple physical blending of polymer and PCBM lead to the way on building interesting electronic structures for OPV. Since then, PCBM became the dominating acceptor in OPV application. One of the significant advantages is its unique solution processing ability. The solubility of PC61BM in toluene is 35 g/L [72, 74, 75]. The good solubility ensures well mixing of donor and acceptor in the solution, consequently giving a fine phase separation. Besides the high electron affinity and excellent solubility, fullerene and its derivative present high electron mobility as well. The fullerene C60 has a high electron mobility of 1 cm2 V–1s–1. For PC61BM, it is 1 ¥ 10–2 cm2 V–1s–1 from FET measurement, and 2 ¥ 10–3cm2 V–1 s–1 measured by space-charge-limited-current (SCLC) method [76]. One of the possible reasons is the spherical conjugated carbon network supports electron transport in three dimensions (3D). Since fullerene has a strong tendency to form clusters of different sizes in the film, the close-packed molecule can make the

Materials Used in Fullerene-Based OPV

inter-molecular transport easier. This strong interaction is due to the π-π interactions between aromatic rings. Generally, the modification of fullerene would decrease mobility [72, 77]. The electron mobility is 7 ¥ 10–4 cm2 V–1 s–1 for bisadduct analog of PC61BM [78, 79], and an electron trapping effect is observed in the trisadduct analog of PC61BM [80]. Some groups also observe there is some trade-off when the fullerene structure is modified with other functional side chains.

Figure 1.4  Molecular structure of some popular fullerene accepters: C60, PC61BM, PC71BM, bisPC61BM, bisPC71BM, and IC60BA.

The absorption spectrum of the fullerene C60 is broad from ultraviolet light to the visible light range [81]. However, the intensity is quite weak [45]. Since singlet transition is forbidden in the symmetrical structures like C60 [83], the contribution from C60 to photocurrent is limited. A similar property is found on its derivative such as PC61BM. Lowering down the symmetry of structure can be an efficient way to improve the absorption intensity. A successful case for enhancing the absorption intensity is PC71BM, which has a strong absorption from 300 to 600 nm. The performance of OPV based on PC71BM is usually better than that with PC61BM [84]. In 2016, Zhan’s group used PTB7-th as the donor and PC71BM as the acceptor to achieve a PCE of 10.42%[85].

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In fullerene-based OPV, VOC is found very early that it positively correlates to the difference between the donor HOMO and acceptor LUMO [86]. Upshifting the LUMO of fullerene or downshifting the HOMO of donor can both result in a large VOC. Note that the energy gap between the donor LUMO and acceptor LUMO is one of the energy loss pathways. For less energy loss, acceptor with higher energy positioned LUMO is desirable. Based on this consideration, a successful fullerene acceptor, indene-C60 bisadduct (bisPC60BM) was designed and synthesized. It has about 0.1 eV LUMO up-shift compared to PC61BM and electron mobility of 7×10–4 cm2 V–1 s–1, which resulted in less energy loss. Similarly, PC71BM-bisadduct (bisPC70BM) is introduced, and in 2016, Jianhui Hou reported solar cell based on IT-M and bisPC70BM as double-acceptor which achieved a PCE of 12.2% [87]. Yongfang Li’s group synthesized a new kind of bisadduct of fullerene, indene-C60 bisadduct (IC60BA) and indene-C70 bisadduct (IC70BA), which has a 0.17 eV [88] and 0.19 eV [89] higher positioned LUMO comparing with the PC61BM and PC61BM, respectively. The OPV devices based on P3HT /IC60BA achieved a PCE of 6.48% after pre-thermal annealing at 150°C for 10 min in 2009 [90]. The OPVs based on P3HT /IC70BA achieved a PCE of 7.40% after pre-thermal annealing at 150°C for 10 min in 2012 [91]. In 2015, devices based on PTB7/PC71BM/IC60BA achieved a PCE of 8.43% [92]. Finally, in 2018, Zhan’s group used PTB7-th as donor and PC71BM and F8IC as double acceptors realized a record PCE of 12.3% [93].

1.3.2  Donor Material

In practice, the structural modification and purification are more difficult for fullerene compared to the polymer. Tailoring donor structure is more flexible. From literature, the LUMO of most fullerene derivatives is usually located at –4.3 eV, while the HOMO of the donor is reported from –4.5 to –5.8 eV [94, 95]. If one wants to enlarge the VOC, which is dependent on the difference between the donor HOMO and acceptor LUMO, upshifting HOMO position of the donor is a more common strategy. Most material development has been focused on the efficient donor with an appropriate energy level optimized specifically for the fullerenes.

Materials Used in Fullerene-Based OPV

Reducing the optical bandgap of the donor to extend its absorption into the near infrared light range for higher photocurrent is another important approach to increase PCE. Take for example the early reported donors, poly (p-phenylene vinylenes) (PPVs) and polythiophenes, and their bandgaps are 1.85 and 2.2 eV. The bandgap of 1.85 eV (absorption wavelength edge was at 670 nm) only allowed 46% of photons to be collected. If the bandgap can be pushed down to 1.1 eV (like Si), it will allow more than 90% of photons to be harvested [69]. In 2006, Brabec’s group reported design rules on donor’s energy level. They conclude that the donor with Eg of 1.7 eV and HOMO of 5.6 eV can obtain PCE over 10% [86]. To lower Eg and obtain appropriate HOMO, chemists devised plenty of strategies for the synthesis of narrow bandgap donor [96–100]. A successful structure design is called “push-pull” structure with electron rich and electron deficient units together in the same polymer/molecule backbone. More informative and detailed reports can be found in several review papers [95, 101]. Some of these donors are shown in Fig. 1.5.

Figure 1.5  Molecular structure of some popular donors: P3HT, PCDTBT, PTB7.

Besides the energy level difference, molecular structure has a dramatic influence in donor–acceptor interaction and the resulting bulk heterojunction morphology. Some effective donors have electron-rich moieties that can sterically accessible for the interactions with the fullerene acceptors [102, 103]. McGehee’s group pointed out donor should have more sterically accessible electron-deficient moiety and more sterically hindered electronrich donating moiety [104]. In-depth discussion on the influence of molecular structure parameters can be found in the review papers [105, 106].

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1.4  The Morphology of Active Layer and the Processing Method Solution processing is one of the advantages of polymer solar cells, which enables easy fabrication and brings the cost low. In 2002, Sariciftci et al. reported a PCE booms from 0.9% to 2.5% based on MDMO-PPV: PCBM active layer. Their data strongly indicated that the degree of intermixing of donor and acceptor is of critical importance to the device’s performance [107]. Since then, the relationship between morphology and PCE was intensively investigated for every material system, as well as ways to manipulate morphology and the morphology characterization methods [28, 105]. For achieving an ideal morphology, materials are preferred to be well dissolved and blended in an appropriate solvent. Therefore, sufficiently high and comparable solubility of the donor and acceptor are desired. It was reported that when the solubility of fullerene derivatives increased from 0 to 20 mg/ml [108], the corresponding device performance likewise increased. It should be noted that the solubility of PC71BM in chlorobenzene can reach 80 mg/mL, while the solubility of PC61BM in chlorobenzene is 50 mg/mL [108, 109]. Therefore, the solubility of donor is more of the concern in most material systems. Replacing the solvent from a relative bad solvent to a better one for the donor is a favorable strategy for better performance [110]. However, in some cases, poor solvent can be used for promoting the crystallization of polymer, which is favorable for the carrier transport in devices [61]. Solution recipes are then optimized specifically for each active layer materials system, tailoring the blend composition, concentration, solvent temperature, host solvent, blend solvent or using liquid or solid additives. Annealing methods are also commonly applied before or/and after the film is done, along with or without solvent vapor [111]. Even some very rarely used methods, such as light activation, magnetic field, and electrical field treatment were explored. For OPV active layer, spin coating is the most widely used film deposition method. It can easily produce a smooth and uniform film with thickness down to 10 nm. The donor and acceptor blend can spontaneously form a morphological microstructure with bicontinuous interpenetrating three-dimensional network during

The Morphology of Active Layer and the Processing Method

the film drying. Large donor/acceptor interface can also be created in the film [25, 26]. Large phase separation should be prevented to create sufficient interface area. The desirable domain size is considered appropriate if comparable to 20 nm, which is close to the exciton diffusion length. Too fine mixing would cause discontinuous network, consequently hindering the carrier transport. As to the phase separation in the vertical direction, donor materials should be more enrich approaching the anode, while the acceptor concentrates at the cathode side. This will minimize the unfavorable charge carrier distribution, resulting in a higher collection efficiency. The impact of many other factors on phase separation and domain size, e.g., molecular miscibility [112], surface tension [113], and drying dynamic process control [114] are briefly discussed below. Molecular miscibility is one of the most important properties that fundamentally influences the thermodynamic processes of morphology evolution [115, 116]. Good miscibility provides a large driving force for fullerene diffusing in the donor phase, consequently, high intermixing degree can be formed. As we know, good intermixing would result in a higher exciton slip efficiency, which is usually found in amorphous donor domains. In the regionrandom grade of P3HT, the miscibility of PCBM is 16–22% [117]. The miscibility of PCBM in the semi-crystallized PTB7 domain is 27% [118]. However, in another aspect, the diffusion of fullerene in donor domain decreases the purity and would hinder parking of the donor molecules. Domain purity has been demonstrated to be critical for the performance both qualitatively and quantitatively [105]. Increasing volume faction of fullerene rich domain in the polymer-rich domain lead to a more efficient charge collection pathway as evidenced by higher charge density and carrier lifetime [112]. In some films, the PCBM-rich domain can easily be coarsened by the agglomeration of some smaller PCBM domain [119]. It must say morphology control is a game for a delicate manipulation to trade-off the miscibility and purity of material domains. For the nanoscale morphology of either polymer or fullerene, high crystallinity is desired, because it was shown to be the origin of better charge transport [120–122] and separation [123]. To probe the different crystallographic orientation of different materials, specialized experiments based on either the high-energy neutron or synchrotron X-ray sources are used [124]. For example, grazing

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incidence scattering technique enables the understanding of statistical orientation of specific molecules. Based on the orientation of conjugation plane of the polymer with respect to the substrate plane, polymer crystals can be defined in three orientation schemes [120]. (1) Edge-on orientation, in which the polymer side-chains are in close contact with substrate surface; (2) Face-on orientation, in which the polymer conjugation plane is in close contact with substrate surface; and (3) Side-on orientation, in which the polymer chain is normal to the substrate. In some high-performance donor materials, face-on structure can usually be found [118]. One popular assumption is that the face-on structure with the π-conjugation plane being parallel to the substrate is favorable for the charge transport [28], because the π-conjugation length is extended in the transport direction. For the charge separation, the relative orientation of donor and acceptor is suggested more important. It has been demonstrated that the photocurrent generation is more efficient when the donor and acceptor both prefer face-on orientation [125]. Correlation between orientation and performance needs to be clarified in the next step [126, 127]. Spin coating constraints the size of fabricated devices. To upscale its area, more solution processing techniques need to be developed. Studies on different processing optimization techniques for better morphology are intensively studied, some of which can be found in the following reviews [8, 128–130].

1.5  Stability Problem

In order to commercialize OPV, its stability problems must be addressed. Organic materials are known to be oxygen and water sensitive [131, 132]. The early study shows the device based on P3HT/PCBM under ambient conditions has 20% loss of its original conversion efficiency after 40 days [133]. Therefore, more and more studies aiming to address the problem of stability should be carried out. During the device operation, not only the chemical stability of the organic molecule is the utmost concern; others include structural stability, device geometry, and its nanoscale morphology. Photochemical degradation happens during the operation of the device. Some of the oxidizable chemical units were identified

Conclusion

containing exocyclic double bonds, quaternary carbon atoms and easily removable bonds [132, 134–136]. This oxidation can induce the change in the electronic structure as reflected in the changes of photon absorption and charge motilities. Another problem is the oxidizable low work function material, especially metals. To prevent the oxidation, first is to avoid the application of oxidable unite in the molecule design. Researchers also found that crystallinity of the polymer can reduce the oxidation, which can be enhanced by increasing the rigidity and the polymerization [131, 137]. The morphology stability is a more complex stability problem. The donor and fullerene acceptor are mixed in a nanoscale, but it is a metastable morphology. Although pristine PCBM has good thermal stability, its crystallization temperature and melting points are 195 and 290°C [138]. As discussed above, due to its high diffusion constant, and at elevated temperature, PCBM diffuses and aggregates into the crystals with a few micron length scales, leading to large-scale phase separation in the film [139, 140]. For a donor, inconjugated side chain like alkyl side chain is used for a good solubility of the polymer in the solvent, but it also promotes separation from fullerene resulting phase separation in the operation condition. Brabec’s group highlights the molecular incompatibilities between the donor and acceptor as a dominant mechanism for the major short-time device degradation [141]. During the operation, the heat from the irradiation or other factors as moisture can induce the phase separation. Besides, note that this is a layer-by-layer device structure, where the films are not as condensed as the inorganic films. The intermolecular interaction is the Van der Waals force. Therefore, diffusion of electrodes and buffer layer material into the active layer is inevitable. Furthermore, the mechanical deformation change can induce material incoherence and fracture [142].

1.6 Conclusion

This chapter provides an overview of OPV devices based on fullerene acceptor with the development history, the basic materials and device knowledge, the optimization of the device structure, morphology engineering and stability issue. Although nowadays the new star acceptor materials system is non-fullerene, which presents

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some advantages such as broad absorption spectrum, low energy loss, and good stability. Fullerene as a successful acceptor has been widely used for a very long time, and is compatible for various donor materials, providing important model system for the OPV study. Learning from OPV-based fullerene helps us to understand the problems in OPV, and to seek the breakthrough for OPV material and devices in the future.

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Chapter 2

Non-Fullerene-Based Polymer Solar Cells

Alice Corani Division of Chemical Physics, Kemicentrum, Lund University, SE-22100 Lund, Sweden [email protected]

Fullerene-based polymer solar cells have reached more than 10% power conversion efficiency (PCE) in the recent past proving its improvement as one of the most important among emerging photovoltaic technologies. They are known to be an efficient electron acceptor but have low absorption in the solar cell spectrum, especially in the red part, a tight chemical geometry resulting in low tunable energy level, and are known to be chemically unstable. These limitations led to the replacement of the fullerene acceptors by polymer molecules. The recent rise in the PCE using polymer/ polymer blends, currently at ~13%, is quite promising considering that this field started just a few years ago. This chapter presents the principle of the operation of a polymer/polymer solar cell and the design rules to achieve higher efficiency. The photochemistry of the polymer materials used as well as some examples of the blends will also be discussed. Emerging Photovoltaic Technologies: Photophysics and Devices Edited by Carlito S. Ponseca Jr. Copyright © 2020 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4800-69-3 (Hardcover), 978-0-429-29525-6 (eBook) www.jennystanford.com

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2.1  Introduction: Principle of Operation of AllPolymer Organic Solar Cell The limitless energy provided by the sunlight and the constant increasing need for renewable energy with preferably low environmental impact has oriented researchers to investigate solar cell materials. Despite the great conversion efficiency reached by crystalline silicon solar cells 29.8%, above the theoretical limits of 29.4% [1] strong interests have been focused on organic solar cells (OSCs). First, OSCs have good mechanical properties. They are flexible, light, and easy to produce in large quantities. The production cost is therefore low. Moreover, chemically, both donor and acceptor energy levels can be tuned by organic synthesis [2]. As the previous chapter presented, large interest has been shown to fullerene acceptors which resulted in quite efficient solar cells. However, fullerene efficiency is mainly limited by its morphology. The LUMO (lower unoccupied molecular orbital) level is barely tunable and, as will be explained later, it can limit electron transfer. In addition, fullerene extinction coefficient in the solar spectrum is low, while conjugated polymers have a very wide absorption spectrum fitting sunlight’s emission spectrum. Consequently, in the polymer/fullerene blend the absorbed energy needed to produce excited electron–hole pairs, is mainly induced by the polymer donor. A better absorber should supply more photocurrent which then will increase the efficiency of the device. Finally, fullerene has strong tendency to crystallize when produced in large scale that results in an unstable device [3]. There are two types of organic photovoltaic cell; the photoactive layer can be either a bilayer of the donor/acceptor (Fig. 2.1A) or a bulk heterojunction (BHJ) (Fig 2.1B). The polymer used for OSC are conjugated polymers which are known to have interesting optical and electronic properties due to their delocalized π-electrons, typical for structure having alternating single and double bonds. All polymers cell uses two polymers one as an electron donor and the other as an electron acceptor.

Introduction

Figure 2.1  OSC solar cell disposition: (A) bilayer structure, (B) bulk heterojunction solar cell.

The BHJ seems so far to give better results than the bilayer [4]. The BHJ increases the exciton creation and diffusion especially because the interface contact between the donor and acceptor is larger resulting in a better charge separation. Moreover, it has been demonstrated that an ideal BHJ morphology displays nanostructural layers of donor and acceptor alternatively separated by 2 ¥ 10 nm. The 10 nm separation being the typical exciton, i.e., excited hole– electron pair, diffusion length (LD) (Fig. 2.2).

Figure 2.2  Ideal BHJ morphology for an OCS.

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As presented in Fig. 2.1, the device is generally made of a photoactive layer added between a transparent anode, usually made of a thin layer of indium tin oxide (ITO) and a metal cathode Al or Ca. To improve the conductivity of the device, interfacial layers can be added between the anode and/or the cathode and the photoactive layer. The interfacial layer allows a stabilization of device performance [5].

2.1.1  Basic Operation Principles

To adjust the solar cell efficiency, it is essential to understand how an organic solar cell works. The power efficiency conversion can then be improved by adjusting specific parameters. Typically, the processes are listed below and presented in Fig. 2.3.

(1) The photoactive material, that is herein the BHJ polymer/ polymer blend, absorbs light. (2) Light absorption induces the formation of hole–electron pairs, called excitons. (3) These excitons then split into free charges, holes, and electrons. The excitons only split in the donor/acceptor (D/A) interface—heterojunction phase—this means that in order to have a successful exciton splitting, it is important that excitons have high mobility in the photoactive medium. Excitons’ mobility is dependent on their diffusion length which is known to be about tens of nanometers in organic material [6]. This means that excitons produced at a greater distance than 10 nm from the D/A interface will not contribute to the extraction of photocurrent. For the splitting to occur, the energy difference between the LUMO of the donor and the LUMO of the acceptor must be close to the exciton binding energy, generally a few tens of an electronvolt. Consequently, the LUMO of the donor and the acceptors energy level must be different. (4) The free charges, holes, and electrons migrate toward the anode and the cathode, respectively. It is the donor/acceptor interface that facilitates the mobility of the free charges toward the electrodes leading to a potential difference and current extraction.

Introduction

Figure 2.3  OSCs’ energy production principle under illumination.

Exciton binding energy in Si cell (inorganic PV cell) is quite low ~0.1 eV compare to typical organic semiconductor (~0.5 eV). As previously discussed, in an OSC exciton splitting occurs only in the photoactive material. This means that the energy difference of the donor and acceptor LUMO must be in this order of magnitude [7]. From the operation principle of an OSC, obviously the factors to optimize to get an efficient solar cell are

∑ extinction coefficient of the photoactive layer over the solar spectrum to absorb a maximum of photons; ∑ exciton mobility in the BHJ interface to collect and split the exciton; ∑ efficient exciton splitting (binding energy); ∑ free charges mobility toward the electrode.

The exciton quenching, charge recombination and charge leakage will interfere with the efficiency of a solar cell, these phenomena need to be controlled and limited as much as possible. Below will be discussed the different types of polymer/polymer blends for OSC device and the important role of the photoactive layer morphology. First, some mathematical notions will be introduced.

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2.2  Calculation of Power Conversion Efficiency Regarding a solar cell, it is important to be able to compare different devices efficiency. Therefore, it is relevant to define several parameters:





∑ Open-circuit voltage (Voc), is the voltage record in a cell exposed to sunlight where no current is running (see Fig. 2.4). It can be tuned by changing HOMO and LUMO energy level of both donor and acceptor ∑ The short-circuit current, Isc is the current passing through a cell exposed to sunlight without external resistance. ∑ The short-circuit photocurrent density Jsc (see Fig. 2.4).

Figure 2.4  Typical current density vs. voltage plot with no illumination and under illumination (left panel) and external quantum efficiency as a function of wavelength for an OSC (right panel) [8].

∑ The Fill factor, FF is the ratio between the actual and theoretical maximal output power when Isc and Voc are maximum. A high FF value can be an indication for high charge separation and a good charge mobility [8]









FF =

Vmpp Impp Voc Isc



(2.1)

∑ The power conversion efficiency, PCE is the ratio of the output power Pout to the input power Pin PCE =

Pout FF(Voc Isc ) = , Pin Pin

(2.2)

Calculation of Power Conversion Efficiency

where Pout is the maximum electrical output power under illumination and Pin is the intensity of the incident illumination to the device. ∑ The external quantum efficiency, EQE, represents the device efficiency as function of energy, or wavelength absorption. The EQE allows the construction of a photocurrent spectrum. The spectrum indicates the ability of an OSC to transform photons of a certain energy into electron. Clearly the higher the PCE the more efficient the solar cell. It is thus important to be able to tune

∑ the HOMO and LUMO energy levels of both polymer donors and acceptors; ∑ the absorption of the photoactive layer, to fit the solar emission spectrum preferably; ∑ the morphology of the photoactive layer, which has a direct influence on the excitons and the free charges production and mobility.

An optimization of all these parameters will lead to a higher extraction of electrons from absorbed photons [9, 10]. The photostability and the electrochemical properties should not be overlooked. However, one of the most critical parameters to control is the HOMO and LUMO energy levels of both the donor and the acceptor. The energy level difference between the LUMO of the acceptor and the HOMO of the donor must present a large difference. Nevertheless, to have an efficient electron transfer, the donor LUMO energy must be higher than the acceptor LUMO energy. Several polymers show good properties to be efficient acceptor. A good polymer acceptor need to have low-lying energy level (low HOMO) and to be able withdraw electrons, and with high mobility. Moreover, the interface between the two polymers should have a good crystalline structure to allow the charge splitting. Below are some of the first polymers used for OSC devices as well as those with high PCE.

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2.3  Polymer/Polymer Blends 2.3.1  Cyanated Phenylenevinylene (CN-PPV)-Based Polymer Acceptors In 1995, Halls et al. [11] were one of the first groups to make an all-polymer OSC device. They used two poly(p phenylenevinylene) (PPV) derivatives as the photoactive layer, poly[2-methoxy-5-(20ethyl)-hexyloxy-p-phenylenevinylene] (MEH-PPV) and cyano-PPV (CN-PPV). The chemical structures are shown in Fig. 2.5. Using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and parallel electron-energy-loss spectroscopy (PEELS) to investigate the morphology of the blend. They detected a small phase separated structure of 10 to 100 nm, which was confirmed by the photoluminescence (PL) quenching experimentally. They concluded that the polymer absorption matches the polymer photoresponse. Similar observations were reported by Yu’s group in the same year [12] using MEH-PPV:MEHCN-PPV blend. No ground-state charge transfers were detected from the absorption and the emission spectra of single and blend polymer films. The charge transfer observed was thus only photoinduced. The energy conversion efficiency was increased by a factor of 20 and 100 compared to the energy conversion of photodiodes made of pure polymers MEH-PPV and MEH-CN-PPV, respectively. However, the PCE obtained for the polymer blend was very low 0.9%. This result clearly shows that OSCs made from these polymers generate a lot of charges but the charge mobility was very low. Despite this low efficiency, these first observations were important to prove the necessity to have a large donor/acceptor interface but also the requirement to have interconnection to transport the free charges toward the electrodes. This important discovery is the beginning of the research on all-polymer OSC devices. However, the PCE obtained using polymer blend photoactive phase had difficulty to overcome the 1% PCE. It took over a decade to get a PCE above 1%. In 2005 a PCE of 1.7% was obtained by Kiezke et al. using PPV derivatives and spin-coated deposition technique [13]. Poly[2,5-

Polymer/Polymer Blends

dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2ethylhexyloxy)-(1,4-phenylene-1,2-ethenylene)] (M3EH-PPV) and poly[oxa-1,4-phenylene-1,2-(1-cyano)ethylene-2,5-dioctyloxy-1,4phenylene-1,2-(2-cyano)ethylene-1,4-phenylene] (CN-ether-PPV) were used to investigate the difference using a polymer blend BHJ and a two-layer device. Before this study, the highest PCE for all polymers device was obtained with a bilayer structure. The M3EH-PPV:CNether-PPV blend was spin-coated using chlorobenzene as a solvent and evaporated by Ca/Al electrodes. The structural morphology of the blend polymers explains the larger PCE obtained. The different solubility of the two polymers in chlorobenzene allows them to crystallize in different orientations that provide vertical structures. The obtained structure was close to the ideal BHJ morphology (Fig. 2.2), which led to a larger interface between the two polymers and thus resulted in efficient exciton separation and charge transport. For comparison, the bilayer device using the same polymers gave a PCE of 1.3%.

Figure 2.5  Chemical structure of the CN-PPV acceptor used in all-polymer cells mentioned in this chapter.

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Polymer/Polymer Blends

Figure 2.6  Chemical structure of the donor used in all-polymer cells mentioned in this chapter.

Koetse et al. [14] also reported that the addition of an acceptor thin layer, about 5 nm, between the photoactive blend and the collecting electrode increased the PCE. The highest PCE obtained using PPV derivatives was achieved by Fréchet et al. [15]. They obtained a PCE of 2% using poly[3-(4-n-octyl)-phenylthiophene] (POPT) and MEHCN-PPV. POPT, whose chemical structure can be found in Fig. 2.6, was synthesized applying Grignard metathesis (GRIM) technique. The synthesis led to a large average molecular weight of POPT and extensive regioregularity allowing a deposition of the MEH-CN-PPV acceptor polymer by spin coating. The obtained device is a bilayer OSC with a large region of donor/acceptor interconnection.

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2.3.2  Benzothiadiazole-Based Polymer Acceptors Benzothiadiazole (BT) heterocycles are good candidates as polymer acceptor because of their ability to withdraw electrons [8], while their low-lying energy is necessary for acceptor polymers [16]. The combination of the PFB, a triarylamine-based polymer, and PF8BT was first investigated by Arias et al. [17]. PF8BT is known to have high luminescence and a high electron affinity. In addition, the hole mobility in PFB is high. The films were prepared by either spin-coating or drop-casting using chloroform or xylene as solvents. Atomic force microscopy (AFM) reveals that the use of a solvent with a low boiling point as chloroform leads to a fine phase separation of the two polymers on the order of the exciton diffusion length. Consequently, the PL of the film prepared by spin-coated were highly quenched demonstrating efficient exciton separation. The best EQE was obtained from the film prepared by spin-coating in chloroform where an EQE of 4% at 3.2 eV was measured, while the corresponding EQE for the film spin-coated in xylene was 1.8%. The drop-casted films present a much larger phase separation, despite the large D/A interface. PL quenching was also observed and the maximum EQE for the dropcasted samples were only two times less than the film fabricated by spin-coating process. This raises the interesting aspect of the balance between the charges separation and the charges mobility in the device. Bradley et al. [18] investigated the P3HT/PF8BT blend and tried to optimize several parameters to obtain an ideal morphology. They varied the D/A ratio, the film thickness, and the solvent. They could improve the PCE by more than a factor of 6 but still got a low PCE of 0.13%. TA spectroscopy reveals that electrons have an extremely low mobility in PF8BT. As an alternative, PF8TBT6 was used as the polymer acceptor and for an optimized device using LiF/Al electrodes a PCE of 1.8% was measured [19]. PF8TBT6 presents a better electron mobility, μe–. Using the same blend P3HT/PF8TBT6, Friend’s group achieved a PCE of 1.85% and a maximum EQE 50% higher using nanoimprint lithography (NIL) fabrication method. They obtained a pattern size of 25 nm close to the LD, favoring charges separation and limiting geminate recombination [20].

FIgure 2.7  Chemical structure of the BT acceptor used in all-polymer cells mentioned in this chapter.

Polymer/Polymer Blends 43

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Non-Fullerene-Based Polymer Solar Cells

The highest PCE obtained for BT polymers derivatives was obtained by Ito et al. [21] with a PCE of 2.7% using a polymer blend of P3HT/PF12TBT, with the acceptor having a high molecular weight. The chemical structure of PF12TBT can be found in Fig. 2.7. The high molecular weight of the polymer acceptor as well as the annealing (see Section 2.4, Morphology) of the film led to an adequate morphology, and large avenue for improving charge separation and mobility. poly(3-hexylthiophene) (P3HT) and poly[2,7-(9,9-didodecylfuorene)alt-5,5-[40,70-bis(2-thienyl)-20,10,30-benzothiadiazole]] (PF12TBT).

2.3.3  Rylene Imide Dyes Rylene diimide-based acceptors are the most efficient molecules used for all-polymer OSCs.

2.3.3.1  Perilene diimide-based polymer acceptor

Perilene diimide (PDI) are interesting acceptors due to their ease to attract electrons leading to a better exciton splitting. PDI also have high electron mobility while its energy levels are easily tuned by chemical synthesis [22, 23]. Zhan et al. in 2007 [24] introduced the first all-polymer OSC using PDI acceptors. The device made from a polythiophene derivative as the donor and P(PDI2DD-DTT) as acceptor have a good absorption over the visible down to nearIR spectral region. See the chemical structure of P(PDI2DD-TT) in Fig. 2.8. The first results were just above 1% PCE. Low PCEs obtained for PDI derivatives are due to strong self-aggregation, limiting the D/A interface and consequently charge separation. In order to overcome this difficulty chemists have introduced functional groups to avoid aggregation and increase crystallinity [25, 26]. A PCE of 4.4% was obtained by Bao et al. [27] by adding side groups on the main polymer PDI chain. The blend polymer used was insoindigobased polymer donor with polystyrene side chains (PiI-2T-PS5) and a PDI-thiophene copolymer [P(TP)] as acceptor. The highest PCE obtained using PDI is 9.5%. It was obtained by Yan et al. [28] using P3TEA/SF PDI2. The OSC shows large charge separation and very low voltage loss.

Figure 2.8  Chemical structure of the PDI acceptor used in all-polymer cells mentioned in this chapter.

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Non-Fullerene-Based Polymer Solar Cells

2.3.3.2  Naphthalene diimide The introduction of the naphthalene diimide (NDI) polymer as an acceptor in an OSC was done by Fiacchetti in 2009 [29]. They investigated the P(NDI2OD-T2) acceptor, whose chemical structure can be found in Fig. 2.9. High electron mobility of 0.85 cm2V–1s–1 was observed using top-gate organic field effect transistor (OFET). The high electron mobility is a result of the high crystalline structure of the film [30], which also affects the exciton mobility. NDI has been deeply reviewed; thus its electronic properties are well known [2, 31–34]. NDI can be easily modified chemically to match the energy level properties for an efficient OSC. They present a large absorption band over the solar emission spectrum and they are known to have a deep LUMO energy and a great stability in normal condition. High FF of 70% was obtained using P3HT/P(NDI2O-T2), comparable to that obtained from polymer/fullerene blend indicating efficient exciton splitting and high charge mobility. However, the blend presents a low Jsc, which is directly correlated to the low PCE obtained with these copolymer, less than 0.2% [4, 15, 35]. Optical measurements where the incoming light intensity is varied showed that losses were due to monomolecular recombination, i.e., of exciton or geminate pairs [8]. These observations were confirmed by TA measurements performed on P3HT/P(NDI2OD-T2) were a sub-nanosecond decay was assigned to charge-pair recombination. The addition of functional groups to limit the aggregation of the polymer induced great improvement of Jsc of the photoactive phase as well as the PCE. A PCE of 1.4% was achieved using cyano-naphthalene [36, 37]. To achieve a better PCE, several solutions have been investigated. McNeill et al. [38] used a low-band gap polymer PTB7 with P(NDI2OD T2). The absorption spectrum of PTB7 has a larger extinction coefficient over the solar emission spectrum than the P3HT usually used. A PCE of 1.1% is observed. Marks et al. [39] enhanced the PCE to 2.66% using spin coating deposition in xylene solvent. A PCE of 4.8% was obtained for the PSEHTT and PNDIS-HD [40]. It was optimized by selecting appropriate solvent for the spin coating, a mixture of chlorobenzene and dichlorobenzene. The efficiency was explained by balanced transport of the holes and the electrons.

Morphology

Figure 2.9  Chemical structure of the NDI acceptor used in all-polymer cells mentioned in this chapter.

The morphology is a critical factor to obtain an efficient all-polymer OSC. Even polymer with properties such as a high electron mobility like P(NDI2OD-T2) is not a guarantee of a better performance. Therefore, one should have an utmost control of the blend morphology to find a good balance between charge separation and mobility. The last part of this chapter focuses on morphology and how to obtain an ideal blend structure.

2.4 Morphology

A homogeneous photoactive phase, ordered polymer, and a low phase separation in the order of LD are the key to a good device efficiency. Many studies, in particular the groups of Ito [4], have investigated the blend morphology and tried to adjust several parameters to obtain a structure close to the ideal morphology. One of the key parameters is the D/A contact interface phase. Morphological studied are reviewed here. Several parameters were examined; solvent, thermal annealing, molecular weight, and D/A blend ratio [17, 41–44].

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Non-Fullerene-Based Polymer Solar Cells

2.4.1 Solvent A blend of two polymers has the tendency to create two separated phases due to their long chain. The phase separation decreases the heterojunction phase and results in a poor exciton splitting and high ratio of geminate recombination. Ito et al. studied the effect of spin coating using solvent with various boiling point [21]. When the kinetic of the film preparation is long, the phase separation between the two polymers is large. The use of a solvent with a low boiling point like Chloroform (CF), could freeze the polymer blend in the early time of film formation. As such, the solvent with lower boiling point allows the formation of a well-mixed blend, limiting phase separation and leading to a favorable morphology for excitons and charges transport. Furthermore, it was noticed that a solvent containing a small proportion of additive can considerably improve the PCE of the device. For example, the addition of 1.25% (volume) of 1,8 diiodooctone in the solvent, CF during the spin coating shows considerable improvement in the PCE from 3.47% to 4.6% in PTB7 Th/ P(NDI2OD-T2) [45]. The 1,8 diiodooctone additive increases the crystallinity of the film and thus enhanced the electron mobility µe. Drying of the film under argon atmosphere was also explored and compared to thermal annealing treatment. PTB7-Th/ P(NDI2ODHD) devices obtained a 7.7% PCE for the film dried under argon and an EQEmax of 85%. The slow drying of the spin coated film in a glove box under argon atmosphere allows a smaller average crystallization phase of the polymers and a larger amorphous phase of the blend resulting in a better µe [46].

2.4.2  Thermal Annealing

In the work of Jenekhe [46], thermal annealed films were compared with argon atmosphere drying. As observed with the sample dried under argon atmosphere, thermal annealing also has an impact on the crystallization of the polymer blend. Ito et al. [21, 47] show the correlation between annealing temperature and the efficiency of a solar cell using a P3HT/PF12TBT. The polymers films are blended by spin coating in CF solution. Without annealing, the PCE is measured to be 0.27%; annealing at 140°C for 10 min gives the best PCE of 2.0%, while at higher temperatures PCE decreases again (see Fig. 2.10).

Morphology

The increase of the annealing temperature on the polymer blend has two effects: Maximum PCE can be reached and it contributes to a better D/A phase organization.

Figure 2.10  PCE of P3HT/PF12TBT as function of the annealing temperature according to Ito et al.’s [48] data.

Donor polymer P3HT are small chains included in the acceptor polymer favoring exciton—generated in the acceptor—separation at the donor interface. The interface area is smaller or in the range of LD. However, the charges generated in the donor polymer, the holes, have poor mobility because of the small network built by small donor polymer chains. Above the temperature leading to the maximum PCE, the phase separation between the polymers donor and acceptor is larger. Thus, limiting the exciton splitting and geminate recombination occurs before charges separate. The interface area is larger than the LD resulting in quenching of the excitons, consequently Φq, increasing the photoluminescence PL quenching efficiency and Jsc decrease. The quenching of the photoluminescence Φq gives information about the blend morphology. Φq is directly correlated to the size and the purity of the polymer blend. Indeed, the PL quenches when generated excitons reach the polymer interface giving information on the distance between the polymers interface. A strong correlation between Φq and Jsc was also observed [4]. Furthermore, it was also reported that [48] FF increases with annealing temperature

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Non-Fullerene-Based Polymer Solar Cells

increase. All parameters impacting the morphology also have strong correlation with charge mobility. Consequently, thermal annealing has a direct influence on the charge carrier mobility as it impacts the morphology. Both electron and hole mobility µe - and µH, respectively, can be improved. The first indicates that an elevation of temperature purify the acceptor polymer and isolate it from the donor as probed by transient absorption (TA) [21] on P3HT/PF12TBT. In addition, annealing also leads to a better donor crystalline structure producing a well-connected interface between both donor and acceptor polymer. This results to an efficient charge separation and mobility toward the electrodes. The equilibrium between exciton splitting and charge transport is an important feature of the solar cell material [48].

2.4.3  Molecular Weight

The impact of the molecular weight has also been investigated by Ito’s group [21] as well as Kim et al. [49]. Both groups observed an increase of the PCE with the increase of the acceptor polymer molecular weight. The FF, Jsc and Voc improve as well with a longer acceptor polymer chain. A high molecular weight allows a preferential crystalline orientation resulting in a better and larger mixed interface between the donor and the acceptor polymer. Charge generation and migration are therefore enhanced.

2.4.4  Donor/Acceptor Blend Ratio

Efficiency of the photovoltaic device is strongly dependent on the donor/acceptor proportion. Ito et al. showed that [50] the larger the donor proportion, the larger the PCE. The proportion of the polymer directly influences the Jsc and FF parameters and both show improvement when donor proportion is large. For the PTQ1/ P(NDI2OD-T2) blend, they find the maximum PCE of 4.1% for a blend 70/30. They observed that increasing the donor proportion increased the µH; µe stays nearly constant as the donor proportion increases and is always larger than µH. Increasing µH by increasing the ratio of the donor polymer results in a better-balanced charge mobility and therefore a higher FF [51, 52].

Conclusion

2.5 Conclusion The fast improvement of the PCE of the all-polymer cells shows that they are good candidates for overcoming the mentioned limitation of fullerene-based OSC. However, despite the efforts exerted to find good polymer blends, until now, the electron mobility µe is lower than fullerene derivatives and leads to large losses due to the charge recombination. It is though important to focus on the morphology and sharpen the structure morphology in the interface of the D/A using new fabrication techniques [2]. In addition, most of the donors used for all-polymer cells have been developed to increase the performance of the fullerene acceptor; however, a new donor adapted to the polymer blend would efficiently enhance cell properties. As previously shown, a good donor for a fullerene cell is necessarily adapted to a polymer cell [8]. The combination of good chemical and electronics properties of the donor and the acceptor as well as the development of managing structure morphology would lead to an efficient solar cell product, more stable and with less production cost than a fullerene solar cell.

List of Abbreviations:

PCE: power conversion efficiency OSC: organic solar cells LUMO: lower unoccupied molecular orbital HOMO: higher occupied molecular orbital BHJ: bulk heterojunction ITO: indium tin oxide D/A: donor/acceptor Voc: open-circuit Voltage FF: Fill factor Isc: short-circuit current Jsc: short-circuit photocurrent density Pout: output power Pin: input power TA: transient absorption

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39. Zhou, N., et al., Morphology‐performance relationships in high‐ efficiency all‐polymer solar cells. Adv. Energy Mater. 4, 1300785 (2014). 40. Earmme, T., Hwang, Y.-J., Subramaniyan, S., and Jenekhe, S. A. Allpolymer bulk heterojuction solar cells with 4.8% efficiency achieved by solution processing from a co-solvent. Adv. Mater. 26, 6080–6085 (2014).

41. Yan, H., et al. Correlating the efficiency and nanomorphology of polymer blend solar cells utilizing resonant soft X-ray scattering. ACS Nano 6, 677–688 (2012).

References

42. Snaith, H. J., Arias, A. C., Morteani, A. C., Silva, C., and Friend, R. H. Charge generation kinetics and transport mechanisms in blended polyfluorene photovoltaic devices. Nano Lett. 2, 1353–1357 (2002).

43. Shikler, R., Chiesa, M., and Friend, R. H. Photovoltaic performance and morphology of polyfluorene blends: The influence of phase separation evolution. Macromolecules 39, 5393–5399 (2006).

44. Swaraj, S., et al. Nanomorphology of bulk heterojunction photovoltaic thin films probed with resonant soft X-ray scattering. Nano Lett. 10, 2863–2869 (2010).

45. Kang, H., Kim, K.-H., Choi, J., Lee, C., and Kim, B. J. High-performance all-polymer solar cells based on face-on stacked polymer blends with low interfacial tension. ACS Macro Lett. 3, 1009–1014 (2014).

46. Hwang, Y.-J., Courtright, B. A. E., Ferreira, A. S., Tolbert, S. H., and Jenekhe, S. A. 7.7% efficient all-polymer solar cells. Adv. Mater. 27, 4578–4584 (2015). 47. Ito, S., et al. Development of polymer blend solar cells composed of conjugated donor and acceptor polymers. J. Photopolym. Sci. Technol. 26, 175–180 (2013).

48. Mori, D., Benten, H., Ohkita, H., and Ito, S. Morphology-limited free carrier generation in donor/acceptor polymer blend solar cells composed of poly (3-hexylthiophene) and fluorene-based copolymer. Adv. Energy Mater. 5, 1500304 (2015).

49. Kang, H., et al. Determining the role of polymer molecular weight for high-performance all-polymer solar cells: Its effect on polymer aggregation and phase separation. J. Am. Chem. Soc. 137, 2359–2365 (2015). 50. Yu, W., et al. Control of nanomorphology in all-polymer solar cells via assembling nanoaggregation in a mixed solution. ACS Appl. Mater. Interfaces 6, 2350–2355 (2014).

51. Scully, S. R., Armstrong, P. B., Edder, C., Fréchet, J. M. J., and McGehee, M. D. Long-range resonant energy transfer for enhanced exciton harvesting for organic solar cells. Adv. Mater. 19, 2961–2966 (2007). 52. Wang, Y., Ohkita, H., Benten, H., and Ito, S. Efficient exciton harvesting through long-range energy transfer. Chemphyschem 16, 1263–1267 (2015).

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Chapter 3

Ternary Sensitization of Organic Solar Cells: A Multifunctional Concept to boost Power Conversion Efficiency

Negar Kazerouni,a,* Marcella Guenther,b,* Barry C. Thompson,a and Tayebeh Amerib aDepartment of Chemistry and Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089-1661, USA bChair of Functional Nanosystems, Research Area of Physical Chemistry, Department of Chemistry, University of Munich (LMU), Butenandtstr. 11 (Haus E, Gerhard-Ertl-Building), 81377 Munich, Germany [email protected], [email protected]

3.1  Introduction and Motivation for Organic and Ternary Solar Cells Organic solar cell (OSC) technology has seen a rapid evolution over the last decade, based on comprehensive understanding of the physics and chemistry of fundamental mechanisms and working principles of the device, development of characterization, engineering and *Both authors contributed equally.

Emerging Photovoltaic Technologies: Photophysics and Devices Edited by Carlito S. Ponseca Jr. Copyright © 2020 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4800-69-3 (Hardcover), 978-0-429-29525-6 (eBook) www.jennystanford.com

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processing techniques, implementation of high efficiency materials, introduction of novels contexts and concepts for efficient design and light management, and progress of machinery and printing methods [1]. The unique selling points of organic photovoltaics (OPVs), such as excellent light harvesting capability, freedom of form, color and transparency, environmental friendliness, easy scalability and lower manufacturing costs based on roll-to-roll printing methods, position this technology for the mobile power market, and this most properly reflects the state of the art in commercialization. An important milestone towards OPV commercialization has been surpassed by reaching a power conversion efficiency (PCE) of over 13% [2–4] for OSCs. By highlighting the underlying principles and important challenges that have limited the device efficiency to date, an outlook to guide OSCs toward 15% efficiency regime could be provided [5–12]. However, despite the significant improvements achieved, the PCE of OPV has to be increased to compete with commercial inorganic solar cells. The main limitation is due to the intrinsic narrow absorption window (~100–200 nm) of polymers compared to inorganic semiconductors such as Si, which makes it challenging to fully cover the solar spectrum with a single junction device. Furthermore, due to the low mobility of the photoactive materials, organic solar cells present a limited thickness, typically around 100 nm that dramatically limits light harvesting. To overcome the absorption limitation, ternary blend organic solar cells represent one of the dominant strategies that have been explored in the past decade. The outstanding advantage of ternary blends consists of maintaining the simplicity of the processing conditions used for single active layer devices [13, 14]. Moreover, all the optimization strategies developed for binary cells can be also effectively applied for ternary solar cells. In this elegant configuration, all three photovoltaic parameters (VOC, JSC, FF) can be tackled simultaneously or individually by optimizing the ratio between the three materials used in the photoactive layer. The fundamental complexity and unpredictable composition dependence of all performance parameters make it non-trivial to precisely calculate the ultimate potential of ternary solar cells. Ternary blends are an auspicious strategy to fabricate high performance solar cells surpassing ~14% PCE [15]. Recently, thick layered novel ternary organic solar cells (~300 nm) could overstep

Fundamental Principles

the efficiency boundary and deliver 11% PCE by controlling charge carrier recombination [16–19]. The reviews by our group [14], as well as several other groups [13, 20–23], have summarized the progress in ternary solar cells. According to the growing interest of the community and the special relevance of this topic, we were encouraged to present a follow-up contribution on organic ternary solar cells. We will discuss the core concept of ternary solar cell operation, the proposed charge transfer/transport models, the developing understanding of morphology, the explanation for the origin of the tunable open circuit voltage, and considerable advances towards controlling recombination. Furthermore, we will review briefly the performance of high efficiency ternary solar cells based on the nature of their ternary sensitizer.

3.2  Fundamental Principles

3.2.1  Charge Transport/Transfer Mechanisms In general, ternary organic solar cells are composed of three components in the photoactive layer. The two main ternary configurations are the donor:donor:acceptor (D1:D2:A) and the donor:acceptor:acceptor (D:A1:A2), where the third component (sensitizer) can be a polymer, a small molecule, a dye, or an inorganic/hybrid quantum dot/nanoparticle. Nevertheless, the fundamental principles that control the photovoltaic behavior in a ternary solar cell are much more complicated than a binary solar cell. The governing mechanisms are affected by many parameters such as the nature and the amount of the sensitizer, structure and properties of the host matrix and photo-physical properties of the incorporated compounds [13, 14, 20]. Depending on the chemical and electronic nature of the sensitizer and its relationship with the host, the optoelectronic mechanisms in ternary solar cells are not a simple superposition of the photovoltaic processes of each individual solar cell. In fact, the third component can act as antenna and/or a charge relay material, favor a preferred bulk heterojunction (BHJ) morphology acting as a template, improve the charge dissociation and transport properties of the “host” material or form an “alloy” with the host. Importantly, all these effects are strictly dependent of

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the position of the third agent in the bulk. Kemerink et al. [24] suggest that even if a fortunate morphology would lead to the presence of two (hole-) percolating networks that are electrically isolated from each other, the presence of metal contacts does unavoidably enforce equal quasi-Fermi levels in the sub-cells, seemingly ruling out a “parallel” type model (vide infra). They have found that by applying a simple state filling model with the disordered Gaussian density of states for the two donor polymers and the acceptor and by assuming a constant FF, only narrow but strong absorbers give rise to systems where ternary compositions offer improved performance over binary ones; broadening the absorption or weakening the absorption length shifts the balance to binary-dominated ones (Fig. 3.1a,b). However, they illustrated the critical role of the composition dependent fill factor in achieving ternary systems that outperform their binary counterparts, where the PCEs over 14% with more than 22% improvement compared to the binary reference performance are reachable (Fig. 3.1c,d). To date, various mechanisms have been proposed to elucidate the charge transfer/transport mechanism in ternary blends. In our comprehensive review in 2013, we analyzed different physical models discussed within the literature and reduced the complexity of previous models by presenting the well-defined physical models of (i) cascade charge transfer model, (ii) energy transfer model, (iii) alloy model, and (iv) parallel like model. Either an individual mechanism or a combination of them governs dominantly the charge transfer/transport dynamics in these relatively complex systems and the mechanism(s) of operating are likely system dependent. In the following, each mechanism is briefly updated and discussed.

3.2.1.1  Cascade charge transfer

As mentioned above, the charge transfer and transport in a ternary blend is more than a simple superposition of individual phases. In these intricate blends, the content and location of the added component alongside its energy properties direct the charge transfer and transport properties of the final film and could properly circumvent the traps [25]. In a cascade charge transfer setting, the most effective location of the ternary sensitizer is at the interface between the (semi-)crystalline domains of donor and acceptor materials, where its molecules can act as an energy funnel for the

Fundamental Principles

host excitons at D1:A interface. This situation was found by Ohkita et al. [26] and Ke et al. [27] for phthalocyanine sensitizer in poly(3hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) matrix.

Figure 3.1  (a) Optimal fraction of donor1 and (b) associated PCE for all different highest occupied molecular orbital (HOMO) level combinations for an absorption width (full width at half maximum (FWHM) = 0.8 eV), the lowest unoccupied molecular orbital (LUMO) level was fixed at –3.8 eV, and a constant fill factor of 0.65 was used. The diagonal dashed line indicates equivalent D1 and D2 and small ternary regions are indicated by dashed ellipses. A highest achievable PCE of 13% is found for binary devices, shown as a yellow cross; (c) Optimal fraction of donor1 and (d) associated PCE for all different HOMO level combinations. The calculation parameters are same as above but with a composition-dependent fill factor (FF1 = 0.65, FF2 = 0.5) leading to a maximum FF of 0.7 at D1:D2 ratio of 0.66:0.44. The dashed ellipse indicates the best overall PCE of 14.1% at a D1 fraction of 0.66. Reproduced from [24] with permission from The Royal Society of Chemistry.

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A very powerful and easy method to track the charge transfer mechanism is photoluminescence (PL) spectroscopy (Fig. 3.2a). However, the charge transfer between the donor and acceptor in ternary blends does not dramatically alter the PL spectrum compared to the binary cells. Therefore, the charge transfer between the sensitizer with either the donor or the acceptor needs to be scrutinized. General speaking, if energy transfer between the two polymer donors occurs, a decreased photoluminescence from one polymer with a concomitant increase in photoluminescence emission from the other polymer is expected based on the assumption that the two polymers have similar quantum yields [28, 29]. On the contrary, if charge transfer occurs between the donor and the sensitizer, the emission intensity of the donor will be quenched, without increased emission of the sensitizer. This method is widely used in the ternary community (for example Yu et al. [29]) in order to discriminate charge vs. energy transfer. Recently, relevant reports shine light onto the charge transfer mechanism in ternary composites by employing current-voltage measurements. Firstly, Zhang et al. [28] investigated charge transfer between two donors (without the acceptor) with J–V characteristics under light conditions (Fig. 3.2b). If an effective charge transfer between two polymers based on their cascade energy alignments prevails, one would expect that the active layer of two admixed polymers would display the largest short circuit current (JSC) compared to single donor active layer. Meanwhile, the VOC of the ternary blend is determined by the smallest difference between the HOMO and LUMO energy levels in the system, which would pin the voltage [29]. Furthermore, our group also developed an elegant method for tracking the charge transfer between the donor and the sensitizer materials, consisting of the fabrication of a bilayer single carrier device (Fig. 3.2c). For hole transfer the donor and the sensitizer are stacked on top of each other and depending on the symmetry of the J–V curve, we are able to ascertain whether hole transfer between the HOMOs of the two donor materials is energetically favored. Notably, proper ink formulation based on for example orthogonal solvents is essential

Fundamental Principles

Figure 3.2  Examples of ternary solar cells characterized by the charge transfer mechanism. (a) PL spectra with different two-dimensional conjugated small molecule (SMPV1) doping ratios of films P3HT:SMPV1 under 490 nm wavelength light excitation. Pure SMPV1 film has a strong emission with a maximum at 720 nm, while P3HT exhibits a photoluminescence emission signal with a peak at 660 nm and the photoluminescence of P3HT was quenched by increasing SMPV1 fraction in the blend, reprinted with permission from [28], Copyright 2015 American Chemical Society; (b) J−V curves for PSCs with polymer only active layer (no PC70BM) of P3HT, SMPV1, and P3HT:SMPV1 (1:1) under AM 1.5 G illumination at 100 mW cm–2, adapted with permission from [28], Copyright 2015 American Chemical Society; (c) Bilayer hole-only devices and proposed charge transfer/transport mechanism. The fabrication of the hole-only bilayer devices is made by first coating Si-PCPDTBT from a 20 mg/ml solution in chlorobenzene and then depositing PTB7 from a 10 mg/ml solution in toluene, reprinted by permission from Springer Nature: Nature Energy [25], Copyright 2016.

for the fabrication of bilayer single carrier device. Finally, Koppe et al. [30] studied the charge dynamics processes in ternary solar cells via transient absorption spectroscopy (TAS) for the first time,

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by selective photo-excitation of the sensitizer and the consequent polaron formation of the donor, where these species were negligible without incorporation of the sensitizer. The detected P3HT polarons induced upon selective excitation of poly[2,6-(4,4-bis-(2ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b¢]dithiophene)-alt-4,7(2,1,3benzothiadiazole)] (PCPDTBT or C-PCPDTBT) in the ternary film of P3HT:PCPDTBT:PCBM (0.8:0.2:1) indicated an efficient hole transfer from PCPDTBT to P3HT upon electron transfer from PCPDTBT to PCBM. We employed ultrafast time-resolved pump-probe spectroscopy, as a detailed exciton/charge transfer study for different ternary systems. Differential absorption spectra revealed that, in ternary P3HT:PCPDTBT:PCBM 0.9:0.1:1 films, upon photo-exciting PCPDTBT, charges are created on P3HT. Importantly, the charges are the result of a hole transfer from PCPDTBT to P3HT (Fig. 3.3a). On the one hand, hole transfer starts at times faster than 1 ps, which is consistent with the mechanism of direct hole transfer, shown in Fig. 3.3b. Nevertheless, hole transfer extended, throughout the investigated timescale of 7500 ps containing a dominant component of 140 ps, indicating the mechanism of hole transfer upon diffusion with a calculated diffusion length of around 1.4 nm (Fig. 3.3c) [31]. Very similar results were achieved for the poly[2,1,3-benzothiadiazole4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta2,1-b:3,4-b¢] dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) based ternary systems, investigated by Koppe et al. [32]. Using picosecond time resolved pump probe spectroscopy, they showed that photogenerated positive polarons are transferred from Si-PCPDTBT to P3HT within a few hundreds of picoseconds. This charge transfer from a low-bandgap polymer to the host polymer can contend with non-geminate charge carrier recombination of the holes in the low-bandgap polymer phase with electrons in the PCBM phase. Furthermore, our study on two different ternary systems based on aza-4,4-difluoro-4- bora3a,4a-diaza-s-indacene (Aza-BODIPY) dye sensitizers revealed the importance of a cascade charge transfer mechanism in those ternary systems in which the sensitizer does not possess a significant charge transport property and, therefore, charge transport in the ternary system relies on the host matrix [33].

Fundamental Principles

65

Figure 3.3   (a) Absorption spectrum (solid line, upper part), differential absorption spectra upon excitation at 775 nm (energy laser pulses of 100 nanojoule) recorded with different time delays, and time absorption profiles at 565 nm (red, lower part) and 650 nm (black, lower part) of a P3HT:PCPDTBT:PCBM 0.9:0.1:1 film; (b) Proposed mechanism of charge separation in ternary films without diffusion in C-PCPDTBT domains—including, firstly, generation of an exciton in C-PCPDTBT, secondly, electron transfer from PCPDTBT to PCBM, and, thirdly, hole transfer from PCPDTBT-positive polaron to P3HT; (c) Proposed mechanism of charge separation in ternary films with diffusion in C-PCPDTBT domains—including, firstly, generation of an exciton in C-PCPDTBT, secondly, electron transfer from PCPDTBT to PCBM, thirdly, hole diffusion to an interface between C-PCPDTBT and P3HT, and, fourthly, hole transfer from C-PCPDTBT-positive polaron to P3HT. Reproduced from [31] with permission from John Wiley and Sons.

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3.2.1.2  Energy transfer For an energy transfer to occur, one of the compounds needs to have an absorption spectrum that overlaps with the emission spectrum of the other one. This mechanism can become a relevant relaxation pathway for the primary photo-excited states in the ternary blends, depending on the domain sizes of the individual components [34– 40]. If the sensitizer is surrounded with host donor or acceptor pure domains, energy transfer from the sensitizer to either of them is the only functional way to suppress the recombination of the photo-induced excitons of the sensitizer. As previously described, PL measurements can distinguish between charge and energy transfer [14, 20]. Moreover, time-resolved techniques, in particular time-resolved transient photoluminescence (TRTPL) and TAS are useful tools for energy transfer detection. Förster resonance energy transfer (FRET) introduces non-radiative decay energy states into the system. Therefore, Förster theory anticipates that excited state lifetime of the FRET donor decreases relative to increasing FRET acceptor concentration [41]. The fluorescence decay of P3HT as a function of 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIB-SQ) concentration was probed by Taylor et al. [35]. Using a femtosecond fluorescence upconversion technique, the samples were pumped at 500 nm and examined at 690 nm. The average lifetime of a neat P3HT film was reported at 223 ps and decreased to 52.4 ps in the presence of 1 wt.% SQ, and further to 9.9 ps with 5 wt.% SQ. Similarly, An et al. [42] measured the carrier lifetime in neat films and in blended donor:sensitizer composites and were able to detect an energy transfer mechanism from SMPV1 (synthesized by introducing bithienyl‐benzo[1,2‐b:4,5‐b¢]dithiophene (BDT-T) as the core unit and 3-octylrodanine as the electron-withdrawing endgroup) to DIB-SQ. The TRTPL spectra of blend films were analyzed by applying 460 nm light excitation and 705 nm emission. The lifetime of neat SMPV1 films decreased from 0.99 to 0.44 ns in SMPV1:DIBSQ blend films with a weight ratio of 9:1 and further to 0.33 ns for a weight ratio of 1:1. Cnops et al. [43] presented a simple three-layer configuration of a donor and two non-fullerene acceptors, where the cascade energy arrangement of the components facilitates an efficient two-

Fundamental Principles

step exciton dissociation. By an accurate thickness control of the ternary component, i.e., working as a spacer layer, the donor and the acceptor material were separated. The three-layer stack devices were structured in a way that boron subnaphthalocyanine chloride (SubNc) was positioned in the middle of its homologue, boron subphthalocyanine chloride (SubPc), and a-sexithiophene (a-6T) as the donor. In doing so, the exciton quenching caused by direct charge transfer at the interface is suppressed by the spacer, while the longrange energy transfer across the spacer is still possible. Excitons that are generated in the wider-bandgap acceptor are relocated to the smaller-bandgap acceptor through long-range FRET and thereafter dissociate at the donor:acceptor interface. Recently, we have demonstrated that by incorporating 50 wt.% poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b¢] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b] thiophene-)-2-carboxylate-2-6-diyl)] (PCE10, known also as PTB7-Th) into the highly ordered wide-bandgap polymer of poly[5,5¢-bis(2-butyloctyl)-(2,2¢-bithiophene)-4,4¢-dicarboxylatealt-5,5¢-2,2¢-bithiophene] (PDCBT) blended in a [6,6]-phenyl-C71butyric acid methyl ester (PC70BM) it is possible to achieve PCEs approaching 10%, mainly due to energy transfer from the host to the guest material. The enhanced face-on orientation and improved π–π stacking of poorly ordered PCE10, induced by blending in PDCBT:PC70BM matrix, caused a significant reduction of trapassisted recombination in the ternary device, leading to a high fill factor of 73%. A comprehensive study based on steady state as well as time-resolved photoluminescence spectroscopy (TRPL) and Fourier-transform infrared photo-induced absorption spectroscopy (FTIR-PIA) revealed a very efficient FRET from PDCBT to PCE10 (depicted in Fig. 3.4a,b,c). These results were in agreement with the transport analysis based on electron paramagnetic resonance measurements (EPR), suggesting that the transport properties of the ternary blend are localized on the sensitizer of PCE10 (Fig. 3.4d). Employing transmission electron microscopy (TEM) and its analytical methods, we visualized a very unique microstructure formed in PDCBT:PCE10:PC70BM ternary blend, whereas the needlelike structures of PDCBT were still preserved. It proposes that PDCBT needle-like fibers are surrounded by the amorphous matrix of PCE10, allowing an efficient FRET from PDCBT to PCE10 [44].

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Figure 3.4 (a) Steady state PL spectra excitation at 400 nm; and (b) corresponding transient PL spectra at 800 nm, considering only polymer emission, for blends and pristine materials used in this study; (c) FTIR-PIA spectra in middle IR region for ternary and the reference binary blends; and (d) corresponding EPR spectra, measured under white light emitting diode (LED) illumination at 70 K. These results revealed that charge transport in optimized ternary system is happening through the PCE10 and PCBM phase channels [44].

3.2.1.3  Alloy model The alloy model was introduced for the first time by Thompson et al. [45–49] as a mechanism describing morphology and charge transport in ternary systems, either for host donor:sensitizer or host acceptor:sensitizer. In this case, the definition of host and sensitizer is often fluid as device operation is often found to be effective across all ratios of the synergistic components. Formation of an organic alloy elucidates the composition dependence of the VOC through a proposed hybridization of frontier orbitals between synergistic

Fundamental Principles

components. When this model operates in a ternary system, the transport levels and charge transfer (CT) states at alloy:acceptor or donor:alloy interface, which define VOC, are weighted averages of the components. This is in contrast with the cascade model where VOC has a limiting value. However, in the case of two polymers and an acceptor, an intimate mix of donors is necessary in order to form an alloy. To address this, the possibility of alloy formation was probed in both alloying and non-alloying systems by Thompson et al. [47–50]. In this comprehensive study, they applied photocurrent spectral response (PSR) into ternary systems and measured the optical absorption of the heterojunction interfaces which relates to the CT state (Fig. 3.5a) [51–54]. They observed a steady shift of CT state as the ratio of indene-C60 bisadduct (ICBA) was increased in the P3HT:PCBM system. Khlyabich et al. proposed a monotonic, but not necessarily linear relationship for alloy formation between ionization potential (IP) and composition (Fig. 3.5b) [46]. In ternary blends of poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTTDPP-10%), poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25) and PCBM, they showed a decreasing IP trend with increasing P3HT75-co-EHT25 fraction which originates from the energy difference between the LUMO of the acceptor and the composition dependent HOMO of donors. In the following section, we will discuss how this phenomenon leads to a non-limiting VOC but permits the additive absorption of components. Moreover, they presented the role of co-crystallization of two polymers in ternary blends as the first direct structural study through Grazing Incidence X-ray Diffraction (GIXRD) and its relation to a tunable VOC [46]. They observed a single diffraction peak (100) in a blend of P3HTTDPP-10% and P3HT75-co-EHT25 (Fig. 3.5c,d) shifting toward smaller qz with increasing P3HT75-co-EHT25. The monotonic shift of d-spacing according to the addition of P3HT75-co-EHT25, proved the existence of co-crystallization in this ternary blend and correspondingly an intimate interaction between two polymers, which leads to an alloy formation (Fig. 3.5e). This model with or without a direct evidence of alloy formation is invoked by other groups as well [55–59].

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Figure 3.5  (a) PSR data as a function of PCBM fraction of the total acceptor for the ternary blend of P3HT:PCBM:ICBA. The CT state shifts continuously with composition, supporting the hypothesis that the LUMO of acceptor blends and the HOMO of donor blends are composition dependent. Reprinted with permission from [47], Copyright 2013 American Chemical Society. (b) Voc (squares) of ternary blend solar cells and ionization potential (stars) of blends of the two polymer donors as a function of the fraction of P3HT75-co-EHT25, reproduced from [46] with permission from John Wiley and Sons. (c) (100) diffraction peaks in GIXRD representing that the reflection in ternary blends is gradually shifting. (d) The primary reflections executed from GIXRD as a function of composition. (e) Images of GIXRD reflections, reproduced from [46] with permission from John Wiley and Sons.

3.2.1.4  Parallel-like model Alternative to the cascade charge transfer mechanism, a parallellike charge transport mechanism of two or more materials (mostly

Fundamental Principles

donor polymers) with different bandgaps but similar polarity can be employed for the design of ternary systems. In this mechanism, reported first time in 2012 by You et al. [60], it is proposed that excitons generated in each individual donor polymer would migrate to the respective polymer-acceptor interface and then dissociate into free electrons and holes. Electrons are transported via the acceptor domains towards the cathode as in normal binary solar cells. In this case, charge transfer between the two polymers is suggested to be absent and holes will be transported towards the anode via the two parallel percolation pathways formed by the two polymers [60– 62]. Although it is very likely that the sensitizer, particularly those with decent transport properties, at a specific concentration starts forming its own charge transfer channel in parallel to the host charge transfer channel, there is not a trivial test method to include or exclude the existence or coexistence of these mechanisms in ternary blend devices. As with the alloy model, the parallel-like model also purports to explain the often-observed tunable VOC in ternary systems. However, as noted previously, Kemerink et al. have possibly ruled this out [24]. So often, the parallel-like model has been invoked in many ternary systems with some morphological evidence [63]. Kim et al. [64] invoked the model and showed a gradual monotonic shift in 100 peak in Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) data by adding the sensitizer. This shift is known as clear evidence of co-crystallization in many studies and would seem to be more consistent with the alloy model and distinct from a picture of two independent donor pathways [65, 66]. On the contrary, Sun et al. [62] investigated a ternary blend of a wide-bandgap polymer donor poly(dithieno[2,3-d:2¢,3¢-d¢]benzo[1,2-b:4,5-b¢]dithiophene-co-1,3-bis(thiophen-2-yl)-benzo-[1,2-c:4,5-c¢]dithiophene4,8-dione (PDBT-T1), PC70BM acceptor, and a deep absorbing nonfullerene acceptor, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2¢,3¢d¢]-s-indaceno[1,2-b:5,6-b¢]dithiophene (ITIC-Th). A new out of plane reflection in grazing-incidence X-ray diffraction (GIXD) results showed that ITIC-Th crystallizes readily and forms its own domains when it is added to the blend. Therefore, the system has PDBT-T1 polymer domains, ITIC-Th domains, and PC70BM-rich domains when the ITIC-Th content is >30% in the ternary blends. This conclusion correlates well with phase separation results using Resonant

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Soft X-ray Scattering (RSoXS) where 0% ITIC-Th has a shoulder at ~0.014 Å–1 which corresponds to a length scale of 45 nm for the phase separated domains. By adding ITIC-Th up to 30%, the phase separation length scale is slightly shifted to lower q meaning that its size is increased. Although the VOC was linearly tuned between 0.92 and 0.95 V, they concluded that parallel like model is the controlling mode in their system. However, many other factors can characterize this small deviation. We employed the calculated internal quantum efficiency (IQE) spectra as a simple method to individualize the parallel-like charge transport presence in our ternary devices. In this regard, a novel silicon naphthalocyanine dye (SiNC) was introduced to the P3HT:PCBM host system. The SiNC showed strong photosensitivity in the near IR region (650–950 nm) in ternary devices, as revealed by external quantum efficiency (EQE) measurements (Fig. 3.6a), yielding an improvement of up to 10% for JSC and 24% for PCE compared to P3HT:PCBM. Interestingly, the calculated IQE spectra of the SiNC ternary blend devices showed a significant improvement in near IR region from 5 wt.% to 15 wt.% and levels out beyond 20 wt.% (Fig. 3.6b). It suggests that by forming a mature percolation pathway of SiNC domains adjacent to the P3HT crystallites, a parallel-like hole transfer is possibly produced. The decent hole transport property of the SiNC compound was furthermore verified by testing the SiNC:PCBM binary device, where an acceptable performance of around 1% with a VOC = 0.59 V, JSC = 4.83 mA cm–2, and FF = 31.89% was achieved. However, by further in-depth study based on the complementary PL and PIA measurements, complex charge transfer and transport kinetics, including co-existence of cascade charge transfer and energy transfer mechanisms in addition to the parallel-like charge transfer was revealed in these ternary devices [67]. It is important to mention that a combination of the aforementioned mechanisms may occur depending on the final formed microstructure, thus making it difficult to predict the mechanisms that govern the photovoltaic processes. Significantly, the governing transport mechanism affects the charge carriers’ recombination dynamics in the ternary systems, which is in turn an alternative approach to additionally improving the VOC and FF of the device if the ternary composite is strategically designed.

Fundamental Principles

Figure 3.6  (a) EQE spectra of the P3HT:SiNC:PCBM ternary devices with various composition ratios. (b) Calculated IQE spectra with SiNC content from 5 wt.% to 30 wt.%. In part from [67] with permission of The Royal Society of Chemistry.

3.2.2 Beyond the Sensitization Concept: Controlling Recombination In an ideal organic solar cell, every incident photon generates a hole and an electron which move to the corresponding electrode and are extracted there. However, in fact, holes and electrons can recombine at different stages of charge extraction, following various recombination mechanisms [68–71]. When an exciton is generated, it has to move to a donor:acceptor interface, where it forms a coulombically bound CT state, and finally is separated into free charge carriers. The recombination of the exciton before this separation is referred to as geminate recombination since the recombining charges originate from the same incident photon. In contrast, recombination of already separated holes and electrons, which were generated by different photons, is called nongeminate recombination. Geminate recombination is a monomolecular process and scales linearly with the number of absorbed photons, but it is worth noting that the recombination probability for each geminate pair remains unchanged. However, nongeminate recombination has been identified as the dominating mechanism for organic solar cells [72]. This type of recombination can be further distinguished into trap-

73

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Ternary Sensitization of Organic Solar Cells

assisted/Shockley–Read–Hall (SRH) recombination, bimolecular/ Langevin recombination and Auger recombination. For trap-assisted or SRH recombination, firstly, a charge carrier is captured in a trap and then an oppositely charged carrier finds the trapped one and recombines. Since those two steps happen consecutively, the process is considered as first-order recombination. In contrast to this, for Langevin recombination a free hole and a free electron recombine with each other, which corresponds to a second order process. Auger recombination, finally, is a trimolecular mechanism, where an electron in the LUMO recombines with a hole in the HOMO and the released energy excites another electron to an energetically higher state. However, for Auger recombination a very high charge carrier density is necessary, which normally is not the case for organic solar cells [71]. To estimate bimolecular and trap-assisted recombination, the current-voltage characteristics can be studied as a function of the light intensity Pin. Koster et al. showed in 2005 that according to Eq. (3.1) this dependence of the VOC reveals information about the recombination type [73].

VOC =

E gap q

-

kT È (1 - P )g Nc2 ˘ ln Í ˙ , q ÍÎ PG ˙˚

(3.1)

Here, Egap is the energy difference between the HOMO of the donor and the LUMO of the acceptor, q is the elementary charge, k is the Boltzmann constant, T is the temperature, P is the dissociation probability of electron-hole pairs, γ is the recombination constant, Nc is the density of states in the conduction band and G is the generation rate of electron-hole pairs. Since P, γ and Nc are independent of the light intensity, while G linearly depends on it, the logarithmic plot of VOC versus light intensity shows a slope of kT/q for bimolecular recombination and a steeper slope for additional trap-assisted recombination [44]. Moreover, according to Eq. (3.2), the dependence of the JSC on the light intensity Pin can be used as a measure for bimolecular recombination, since the exponent α is close to one for weak or no second-order recombination and smaller than one for bimolecular recombination occurring [74].

JSC ~ Pina

(3.2)

Fundamental Principles

Finally, the recombination order can be determined with the help of the relation between charge carrier lifetimes τ and corresponding charge densities n, according to Eq. (3.3), as Shuttle et al. showed [75]. Addition of one to the recombination exponent λ results in the recombination order R (R = λ + 1), which indicates perfectly bimolecular recombination behavior for values close to 2. For R > 2, higher recombination orders are associated due to the influence of traps. l



Ên ˆ t Dn = t Dn0 Á 0 ˜ Ë n¯

(3.3)

For binary organic solar cells various possibilities exist to reduce or suppress recombination, for instance by improving phase separation, minimizing chemical impurities or adjusting the active layer thickness [71]. The addition of a third component generally was thought to deteriorate recombination behavior, since mismatches in morphology can serve as trap sites and recombination centers [20]. However, recently, it has been found that the additional component in ternary solar cells, too, can offer new and unexpected possibilities to influence the recombination behavior positively. In 2015, we investigated ternary solar cells with indacenodithieno[3,2-b]thiophene,2,3-bis(3-(octyloxy)phenyl) quinoxaline (PIDTTQ): PC70BM as the host system and Si-PCPDTBT as a sensitizer [76]. We observed that the addition of the sensitizer indeed increases the JSC from 10.14 mA cm–2 to 10.91 mA cm–2 due to an enhanced absorption in the near infrared region, but at the same time the VOC and FF decrease from 0.84 to 0.64 V and from 60% to 52%, respectively, leading to a downgraded efficiency of 3.6% under 1 sun conditions, compared to 5.1% of the binary device. The reduced VOC was attributed to the higher lying HOMO energy level of Si-PCPDTBT, but the attempt to understand the reduction of the fill factor lead to a surprising result: at 0.01 sun conditions the ternary system delivered a PCE of 6.1%, while binary cells only exhibited 2.4% efficiency, which was mainly caused by the ternary blend’s three times higher JSC of 0.16 mA cm–2. The further evaluation of the open-circuit voltage’s light intensity behavior resulted in a slope of 1.0 kT/q for PIDTTQ:Si‑PCPDTBT:PC70CM devices and 1.47 kT/q for binary devices (see Eq. 3.1). Consequently, there should

75

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Ternary Sensitization of Organic Solar Cells

be no trap-assisted recombination in the ternary blend. Instead, the worse efficiency at 1 sun can be explained with an unfavorable morphology of the ternary blend with much larger domains. Under low light levels, the bimolecular recombination is negligible and the ternary system free from the trap-assisted recombination benefits a significant performance improvement over its binary reference. This represents a very important finding for indoor applications. In our subsequent work, we showed that the suppression of trap-assisted recombination with a third component can also lead to increased efficiencies under 1 sun conditions for a well-designed ternary system [25]. We sensitized the host system of PTB7:PC70BM with 15% Si-PCPDTBT and thus improved the efficiency from 7.52% in the binary blend to 8.6%, primarily due to the fill factor, that increased significantly from 68.5% to 77.1%. The EQE of the ternary blend showed increased values in the 600–750 nm region, where mainly the host materials absorb. Therefore, it was concluded that the sensitizer improves charge separation and/or transport. With photo charge carrier extraction in linearly increasing voltage (photo-CELIV) measurements (Fig. 3.7a) the charge carrier mobility of ternary devices was found to be 28% higher than those of the binary ones. Furthermore, the charge carrier lifetime τ versus the charge carrier density n, was measured by transient photo voltage (TPV) and charge extraction (CE) (Fig. 3.7b), and the determined recombination order R showed, that in contrast to the binary blend, the ternary blend with 15% sensitizer demonstrated an enhanced lifetime and yielded R = 2.09, a nearly ideal bimolecular recombination behavior. By fabricating bilayer hole-only devices of PTB7 and Si-PCPDTBT, it was found that a hole transfer occurs from the rather amorphous PTB7 to the semi-crystalline Si-PCPDTBT, where charge transport is facilitated due to a lower degree of energetic disorder (Fig. 3.7c,d). The generality of this method was proven by designing a ternary solar cell based on an amorphous donor, PTB7-Th. In comparison to the binary cell of PTB7‑Th:PC70BM with a PCE of 8.32%, the ternary device with Si-PCPDTBT showed an enhanced efficiency of 9.08% with an increased fill factor, thus suggesting that in this way recombination can be reduced generally in organic photovoltaics.

Fundamental Principles

Figure 3.7  (a) Photo-CELIV curves of PTB7:Si-PCPDTBT:PC70BM blends with different compositions. (b) Charge carrier lifetime τ as a function of charge density n. (c, d) Schematics of the suggested three-phase morphology with the more ordered polymer Si-PCPDTBT, that provides efficient charge transport in the system with a narrow density of states. The density of states (DOS) were calculated by charge extraction measurements. Reprinted by permission from Springer Nature: Nature Energy [25],Copyright 2016.

Furthermore, our latest work shows that adding a third component can improve the VOC by influencing the recombination behavior [77]. A dithienylthienothiadiazole-based small-molecule sensitizer (SM1) was incorporated into a binary blend of P3HT:PCBM, which increased the PCE by 30% to 3.7% due to a significant enhancement of the VOC up to 0.75 V. The HOMO and LUMO energy levels of SM1 are located between those of the P3HT and PCBM, therefore the tunable VOC partially originates from a proportional contribution of P3HT and SM1. Besides, with Fourier transformation photocurrent spectroscopy a decreased non-geminate recombination was found in the presence of SM1, leading to VOC losses lowered by 130 mV compared to the binary reference. Time-of-Flight secondary ion

77

78

Ternary Sensitization of Organic Solar Cells

mass spectrometry (ToF-SIMS) measurements suggested that SM1 mainly aggregates at the interface of the active layer and the hole transport layer PEDOT:PSS instead of being distributed in the host matrix. Therefore, we concluded that the interfacial presence of SM1 suppresses recombination and facilitates hole extraction of the active layer. We observed the same effects for ternary blends of other aggregating small molecules and another host matrix, thus confirming the applicability of this approach. Adjustment of the recombination behavior upon adding a third component is also observed and reported by some other groups. Kumari et al. [78] designed a ternary OSC based on PTB7-Th and PC70BM with (5Z,5¢Z)-5,5¢-(((4,8-bis((2-ethylhexyl)thio)benzo[1,2b:4,5-b¢]dithiophene-2,6-diyl)bis(3,3¢¢-dioctyl-[2,2¢:5¢,2¢¢terthiophene]-5¢¢,5-diyl))bis(methaneylylidene))bis(3-ethyl-2thioxothiazolidin-4-one) (DR3TSBDT) as second donor and achieved an average PCE of 11.78%, which corresponds to an improvement of 20% compared to the binary device. This enhancement was attributed not only to an enlarged absorption range but also to improved charge transport with reduced recombination. From the light intensity dependence of the J–V curves, the authors suggested a reduced bimolecular recombination in the ternary blend while trapassisted recombination remained unchanged.

3.3  Nature of the Third Component: A Review on the Experimental Results

Based on the type of the sensitizer and its properties and concentration, the fabrication method, processing conditions, post treatments and the active layer thickness may vary. Following our review from 2013 [14], we review the most efficient ternary solar cells reported in recent years in this section. In this regard, the ternary organic devices are divided into five categories depending on the nature of the third component, as polymer-based, small molecule-based, dye-based, hybrid solar cells and novel nonfullerene-acceptor-based systems.

Nature of the Third Component: A Review on the Experimental Results

3.3.1  Polymeric Ternary Components Before 2012, the most studied organic solar cell in literature was the binary system of P3HT:PCBM with a limited absorption window in the visible region (up to ~650 nm). For this reason, the early studies of ternary OSCs and the proof of concept were started with sensitization of P3HT-based system. Many sensitizers with photon absorption not only in the visible region but also in the near IR region were integrated into P3HT-based solar cells to improve light harvesting [79]. In 2010 Koppe et al. incorporated a low-bandgap polymer PCPDTBT into the P3HT:PCBM blend resulting in an extended absorption at 800 nm and a consequentially improved PCE from 2.5% to 2.8% (the polymer structures are shown in Fig. 3.9 and photovoltaic performances are presented in Table 3.1) [30]. Later, we introduced a Silicon substituted PCPDTBT (Si-PCPDTBT) into P3HT:PCBM with higher sensitizer loading (up to 40% by weight), resulting in PCE exceeding 4% [80]. However, the moderate PCE of P3HT-based system was not giving enough breath for this technology. In addition to the limited absorption profile and the high VOC losses in P3HT-based binary devices further limited improvements in their performances even in ternary blends. The situation dramatically changed after 2013 with the commercialization of high performing middle and low-bandgap polymers, such as PTB7, PTB7-Th, 5,6-difluoro-4,7-bis[4-(2octyldodecyl)thiophene-2-yl]benzo[c][1,2,5] thiadiazole (DTFFBT), named PBDT-DTFFBT [81], etc., In particular, Yu et al. reported two different ternary blends based on binary hosts of PTB7:PC70BM [29] and PTB7-Th:PC70BM [40] and a ternary agent as PID2, resulting in PCE of 8.22% and 9.20%, respectively. The improved power conversion efficiency was not only due to the extended light harvesting, but also due to optimized morphology and reduced bimolecular recombination. The weak absorption of PTB7-Th was compensated by a highly efficient wide-bandgap polymer PDBT-T1 by Sun et al. [82]. As 10% sensitizer was added to the PTB7-Th:PC70BM system, the synergistic effects of enhanced light absorption and charge transport, efficient energy transfer from sensitizer to host polymer, improved charge generation and BHJ morphology resulted in improved current and FF. Adopting a similar approach, Yang et al. added a DPP-based polymer (PBDTT-SeDPP)

79

80

Ternary Sensitization of Organic Solar Cells

to PTB7:PC70BM delivered a ternary solar cell with up to 8.7% efficiency [66]. In the same system, sensitization with poly[2,7-(5,5bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:20,30-d]pyran)-alt-4,7(5,6-difluoro-2,1,3-benzothiadiazole)] (PDTP-DFBT) was represent by Zuo et al. [83] to increase the efficiency by 7% due to the improvement in crystallinity and molecular orientation. Our group could successfully engineer the ternary devices through reducing carrier recombination and boosting FF to 77% with no significant impact on the host system morphology [25]. In this system, our highly crystalline guest polymer (Si-PCPDTBT) was selected strategically so that its energetic levels would allow charge transfer from the trap-limited PTB7 into sensitizer. Adding 15% Si-PCPDTBT resulted in a rather ideal bimolecular recombination order (R = 2.09) compared to the recombination order of binary reference (R = 2.22). To prove the broad applicability of the achieved results, this system was compared to PTB7‑Th:Si-PCPDTBT:PC70BM. The results clearly showed that this sensitization mechanism indeed has the potential to suppress recombination losses in other similarly affected binary systems. Importantly, this design rule creates an opportunity to reconsider low-FF materials providing efficient charge generation as host materials in the design of advanced composites for highefficiency applications. In another study in our group, a novel high performance ternary,system based on PBTZTx-stat-BDTTy-8 [84] (figure ? with x being the BDT-T block (donor, D) and y the BTZ-T block (acceptor, A)), PTB7-Th and, PC70BM thick film was shown to be able to extend the absorption window of solar cells without sacrificing the charge transport [17]. The ternary devices provided optimum efficiency beyond 11% (Fig. 3.8a). This strategy showed over 90% internal quantum efficiencies with the active layer thickness above 300 nm (Fig. 3.8b), which has a great potential for up-scaling ternary devices. Significant research effort has been directed toward understanding the opportunities and limitations of ternary organic solar cells with respect to charge generation and charge transport as well as establishing morphology and performance relations. Wang et al. [34] developed a high efficiency ternary system by incorporating a highly crystalline sensitizer to the PTB7-Th:PC70BM system. With 15% PffBT4T-2OD, the blend possesses hierarchical phase separation within 25 nm and phase purity was improved

Nature of the Third Component: A Review on the Experimental Results

remarkably, which not only provided sufficient interfaces for exciton separation, but also assisted excellent charge transport. This system was then contrasted with PTB7:th: poly[2,5-(2-octyldodecyl)-3,6diketo-pyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b] thiophene)] PDPP2TBT:PC70BM [85].

Figure 3.8  (a) Efficiency chart for binary and ternary devices as a function of active layer thickness. (b) Fill factor (top), charge carrier mobility (middle) and integrated IQE (bottom) as a function of the active layer thickness for PBTZT-STAT-BDTT-8:PC70BM, PBTZT-STAT-BDTT-8:PTB7-Th: PC70BM and PTB7‑Th:PC70BM devices. Reproduced from [17] with permission of The Royal Society of Chemistry. (c) PCE and FF for devices with PTB7-Th:BTR:PC70BM ratios of 1:0:1.1 and 0.75:0.25:1.1 as a function of active layer thickness. (d) EQE of ternary devices with PTB7-Th:BTR:PC70BM ratios of 1:0:1.1 and 0.75:0.25:1.1 with thin and thick active layers. Independent from the nature of the sensitizer, ternary sensitization successfully overcame the thickness limitation of the host system for high efficient ternary devices. Reprinted with permission from [16], Copyright 2017 American Chemical Society.

Although the PCE was enhanced in the PffBT4T-2OD-based system, it decreased dramatically in the PDPP2TBT-based system despite the fact that the hole mobility and light absorption were

81

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Ternary Sensitization of Organic Solar Cells

improved. It was concluded that contrary to the first system, energy transfer from PTB7-Th to PDPP2TBT worsens the efficiency of the ternary device due to inefficient charge dissociation between PDPP2TBT and PC70BM. Wei et al. approached the two-polymer-donor ternary systems in a different strategy. They showed that by adopting the same donor chemical units in the two conjugated polymers of benzodithiophenethiophene-benzooxadiazole (PBDTTBO) and PTB7-Th, the carrier transport is improved and hence the PCE is optimized [86]. In this system, two polymers featured the same benzo-dithiophene (BDTT) donor, with complementary light absorption and the side chains were engineered in order to tune the packing of these semi-planar polymers. Polymer sensitization has been used successfully even in all polymer solar cells (all-PSCs). Su et al. [87] showed that in their system if a crystalline guest polymer, poly[[5,7-bis(2-ethylhexyl)-4,8dioxo‑4H,8H-benzo[1,2-c:4,5-c¢]dithiophene-1,3-diyl][3,3¢¢-bis(2ethylhexyl)-3¢¢,4¢-difluoro[2,2¢:5¢,2¢¢:5¢¢,2¢¢-quaterthiophene]5,5¢¢-diyl]], PBDD-ff4T could fully embed into an amorphous host polymer (PTB7-Th), they could form a donor alloy which not only provides smoother energy level gradients, but also optimized morphology in all-PSCs. Ito et al. [88] demonstrated that the use of a wide-bandgap polymer (PCPDTBT) in an efficient low-bandgap donor/acceptor polymer blend of PTB7-Th and poly{[N,N0-bis(2octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt5,50-(2,20-bithiophene)} [P(NDI2OD-T2); Polyera ActivInkt N2200] could efficiently contribute in photocurrent generation and enhance the performance of the ternary devices. However, the highest efficiency for all-PSCs was reported up to 9% by Xu et al. as they benefited from the synergistic effects of extended absorption, more photocurrent generation and optimal morphology simultaneously in ternary devices by incorporating high-bandgap poly[[4,8-bis[5((2-octyl)thio)thiophen-2-yl]benzo-[1,2-b:4,5-b0]dithiophene2,6-diyl]]-alt-[bis(5-thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)2H-benzo[d]-[1,2,3]triazole-4,7-diyl] (PBDTTS-FTAZ) as the second donor into PTB7-Th and poly{{[N,N¢-bis(2-octyldodecyl) naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5¢-(2,2¢-

Nature of the Third Component: A Review on the Experimental Results

bithiophene)}-ran-{[N,N¢-bis(2-octyldodecyl)naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-2,5-thiophene}} (PNDI-T10) blends [89].

Figure 3.9  Chemical structures of polymeric sensitizers discussed in Section 3.3.1.

83

D:D:A

D:D:A

D:D:A

D:D:A

D:D:A

D:D:A

D:D:A

2014

2015

2015

2015

2016

2016

D:D:A

2010

2012

Type

Year

Table 3.1

0.8:1.5:0.2 0.8:1.1:0.2 1:2:1 0.8:1.5:0.2

PTB7-th:PC70BM:PDBT-T1

PTB7:PC70BM:PBDTT-SeDPP

PTB7:PC70BM:PDTP-DFBT

PTB7-th:N2200:PBDD-ff4T

1.35:1:0.15

0.9:1.5:0.1

PTB7-th:PC70BM:PID2

PTB7:PC70BM:PID2

0.6:1:0.4

0.8:1:0.2

Ratio (D:A:A2/D2)

P3HT:PCBM:Si-PCPDTBT

P3HT:PCBM:PCPDTBT

Ternary blend (Donor 1: Acceptor 1: Acceptor 2 / Donor 2)

15.7

16.3

18.7

17.8

16.7

16.8

11.0

8.0

Isc [mA/cm2]

Ternary performance

PCE [%]

8.2

0.82 56

7.2

0.74 70.7 8.6

0.69 67.4 8.7

0.81 69.5 10.2

0.78 70.8 9.2

0.72 68

0.59 62.1 4.0

FF [%]

0.62 55.4 2.8

Voc [V]

22

7

21

15

17

13

29

12

PTB7-th:N2200 (1.5:1)

PTB7:PC70BM (1:1.5)

PTB7:PC70BM (1:2)

PTB7-th:PC70BM (1:1.1)

PTB7-th:PC70BM (1:1.5)

PTB7:PC70BM (1:1.5)

P3HT:PCBM (1:1)

P3HT:PCBM (1:1)

Binary reference Increase (Donor: PCE [%] Acceptor)

Photovoltaic parameters of reviewed ternary solar cells based on polymeric sensitizers

13.9

15.1

15.1

16.1

14.9

15.0

8.6

7.1

Isc [mA/ cm2]

[80]

[30]

Ref.

[66]

[82]

0.80 53

5.9

[87]

0.74 72.2 8.08 [83]

0.72 66.3 7.2

0.80 67.9 8.9

0.75 70.3 7.88 [40]

0.72 67.1 7.25 [29]

2.5

PCE [%]

0.57 63.6 3.1

FF [%]

0.57 63

Voc [V]

Binary performance

84 Ternary Sensitization of Organic Solar Cells

D:D:A

D:D:A

D:D:A

D:D:A

D:D:A

2016

2017

2017

2017

2018

0.85:1.2:0.15

1:1:0.15

0.9:1.5:0.1 PTB7th:PC70BM:PBDTTBO(OC2C6)2

PTB7-th:PC70BM:PffBT4T2OD

PTB7-Th:PNDI-T10:PBDTTSFTAZ

19.8

18.9

19.0

14.6

19.4

18.7

0.5:1.5:0.5

PBTZT-STAT-BDTT8:PC70BM:PTB7-th

15.9

0.85:1.5:0.15

PTB7:PC70BM:Si-PCPDTBT

14.4

0.9:1:0.1

Isc [mA/cm2]

PBDTTT-EF-T:N2200:PCDTBT

Ratio (D:A:A2/D2)

FF [%]

PCE [%]

0.8

0.8

71.8 11.4

27e

17.5

17.8 PTB7-th:PC70BM (1:1.5)

PTB7-th:PC70BM (1:1.2) 14d

68.4 10.3

15

PTB7-Th:PNDI-T10 12.8 (1:1)

11.03b

25

15.0

2

12.4

9.0c

PTB7:PC70BM (1:1.5)

PCDTBT:N2200 (1:1)

PBDTTT-EFT:N2200 (1:1)

Isc [mA/ cm2]

PBTZT-STAT-BDTT- 16.7 8:PC70BM (1:1.5)

14

17

Binary reference Increase (Donor: PCE [%] Acceptor)

10.21a 28

8.6

0.78 72.6 10.7

0.84 73

0.77 74

0.77 71

0.70 77

0.79 58.3 6.7

Voc [V]

FF [%]

0.8

5.7

PCE [%]

7.2c

0.78 66

9.0d

0.79 66.6 9.3

0.82 69

0.78 67.1 8.6a

0.73 68.5 7.5

0.97 40

0.81 57

Voc [V]

Binary performance

area of 10.4 mm2. bActive area of 2 mm2. cInverted structure. dElectron transport layer (ETL) of ZnO. eETL of ZnO:7%PEI.

D:D:A

2016

aActive

Type

Year

Ternary blend (Donor 1: Acceptor 1: Acceptor 2 / Donor 2)

Ternary performance

[86]

[34]

[89]

[17]

[25]

[88]

Ref.

Nature of the Third Component: A Review on the Experimental Results 85

86

Ternary Sensitization of Organic Solar Cells

3.3.2  Small Molecule-Based Ternary Components The research community started to consider small molecules as ternary agents mainly due to their superior transport properties and a more ordered microstructure compared to polymers. They profit from reduced batch-to-batch variation, well-defined molecular structure and simple purification processes [90]. In a recent study by Stingelin et al. [91] SMs were used as an additive to create cascade energy bands in ternary blends. The first small-molecule donor (SMD1) based on diketopyrololpyrrole (DPP) (Fig. 3.10) was introduced in 2008 by Nguyen et al. into the P3HT:PCBM system [92]. Compared to polymer sensitization where the guest polymer may disrupt the crystallization of the host, due to the unique crystallization behavior of the small molecules, conjugated SMs mostly enhance phase separation and induce higher crystallinity of the host. Moreover, due to their high mobility, they may facilitate charge transport by providing another channel and hence reducing bimolecular recombination [93]. Table 3.2 reports the state-of-the-art high performing ternary systems based on SMs reported to date. We first discuss small-molecule donor ternary sensitizers. Among those, Wei et al. combined a high crystalline small molecule (BDT-3T-CNCOO), a D-A polymer with a high open circuit voltage containing benzo[1,2-b:4,5-b¢] dithiophene and thieno[3,4-c] pyrrole-4,6-dione groups (PBDTTPD-HT) and PC70BM to achieve an open circuit voltage ~1 V [94]. The small molecule could increase the order of the host polymer and form a favorable nanostructure for charge generation and collection. The ternary shows a broad absorption covering from 300 to 700 nm resulting in PCE of 8.40%. This small molecule was also successfully blended with another BDT-based polymer, 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b :4,5-b¢ ]dithiophene (PBDTTT-C-T) to optimize morphology and efficiency from 7.6% to 8.6% [95]. The important 10% milestone was achieved by Wei et al. in a PTB7‑Th:PC70BM binary system. They introduced an ordered small molecule p-DTS-(FBTTH2)2, achieving average PCE of 10.5% [56], obtained by improved JSC and FF. As discussed in Section 3.2.1.3, the formation of an alloy may facilitate charge separation and reduced charge recombination [56]. In this system, two donors

Nature of the Third Component: A Review on the Experimental Results

are miscible, and an alloy is formed. As a result, a highly ordered face-on orientation of the host polymer was favored due to alloy structure of polymer/small molecule, in other words, both the crystallinity and the face-on preferential orientation with respect to the substrate are enhanced. Hence charge separation and transport were enhanced and recombination was reduced. Later, Peng et al. [96] sensitized PTB7:PC70BM system with the low-bandgap small molecule of 5,15-bis(2,5-bis-(2-ethyl-hexyl)-3,6-di-thienyl2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione-50-yl-ethynyl)10,20-bis(4-octyloxy-phenyl)-porphyrin zinc (DPPEZnP-O) which had a complementary absorption to the host polymer. They could effectively broaden light absorption in the near IR and not only enable bridging effect between the host and sensitizer to provide more charge transfer routes at the D/A interfaces but also suppress recombination centers. In 2015, the very low-bandgap DPPEZnP-TEH molecule with porphyrin ring linked to two diketopyrololpyrrole units by ethynylene bridges, which had a broad light absorption up to above 900 nm, was designed and synthesized by the same group [97]. Later, Russell et al. [98] incorporated this molecule as a near IR sensitizer into PTB7:PC70BM system. The enhanced charge extraction and reduced recombination led to increased JSC and FF and achieved over 11% efficiency. Moreover, the morphology investigation by TEM and atomic force microscopy (AFM) showed higher phase separation for ternary blends containing 10% and 30% DPPEZnP-TEH. Therefore, the charge dissociation probability and charge transport were facilitated. However, in the presence of 50% sensitizer, larger phase separation was induced which resulted in decreasing hole mobility and efficiency. Recently, by adopting the PTB7‑Th:PC70BM host system, Yang et al. [78] reported an impressive 12.1% PCE by incorporating DR3TSBDT (a benzo[1,2-b;4,5-b0] dithiophene (BDT)-based small molecule). Again, not only improved photon absorption range but also enhanced charge transport and suppressed recombination through a combination of cascade energy levels and optimized morphology were the reasons for the high efficiency reported. A highly efficient thick-film ternary system with introducing a liquid crystalline small-molecule benzodithiophene terthiophene rhodanine (BTR) [99] into PTB7-Th:PC70BM was reported by Huang et al. [16].

87

8.40

aCathode

0.9:0.8:0.1

12.82 0.93 71.1

8.48e

12

14.97 0.90 76.5 10.26d 15

0.75 70

61

0.75:1.1:0.25 21.4

11.4c

21

20

17.24 0.76 69.6

0.75:1.5:0.25 23.31 0.77 70.4 12.10

17.53 0.81 65.3 9.20

11.79 0.99 58.1 6.85

FF PCE [%] [%]

DR3TBDTT: PC70BM (1:0.8)

PTB7Th:PC70BM (1:1.1)

PTB7th:PC70BM (1:1.5)

[98]

[96]

[56]

[94]

Ref.

12.73 0.91 65.8 7.59e

0.77 50.6 7.07c

[16] 13.52 0.88 74.9 8.90d [57]

17.8

19.70 0.79 65.3 10.10 [78]

PTB7:PC70BM 16.96 0.75 70.0 9.09a (1:1.5) 15.47 0.74 66.6 7.85b

PTB7:PC70BM 14.99 0.74 67.4 7.47 (1:1.5)

PTB7th:PC70BM (1:1.1)

PBDTTPDHT:PC70BM (1:1)

Isc [mA/ Voc cm2] [V]

Binary performance

interlayer of ZnO:PBI-H. bCathode interlayer of ZnO. cThickness of 250 nm. dWith post treatment. eWithout post treatment.

2018 D:D:A DR3TBDTT: PC70BM:DR3TBDTT-E

2017 D:D:A PTB7-Th:PC70BM:BTR

2017 D:D:A PTB7th:PC70BM:DR3TSBDT

9.52b

18.68 0.77 74.9 11.03a 24

0.7:1.5:0.3

2016 D:D:A PTB7:PC70BM:DPPEZnPTEH

12

14

23

17.22 0.72 67.7 8.39

0.85:1.1:0.15 18.44 0.76 75.3 10.5

12.17 0.97 71

0.6:1:0.4

Binary reference Increase (Donor: PCE [%] Acceptor)

2016 D:D:A PTB7:PC70BM:DPPEZnP-O 0.8:1.5:0.2

2015 D:D:A PTB7-th:PC70BM:p-DTS(FBTTH2)2

2015 D:D:A PBDTTPDHT:PC70BM:BDT-3TCNCOO

Year

FF PCE [%] [%]

Isc [mA/ Voc cm2] [V]

Ratio (D:A:A2/ D2)

Ternary performance

Photovoltaic parameters of reviewed small molecule-based ternary solar cells.

Ternary blend (Donor 1: Acceptor 1: Type Acceptor 2 / Donor 2)

Table 3.2

88 Ternary Sensitization of Organic Solar Cells

Nature of the Third Component: A Review on the Experimental Results

Figure 3.10  Chemical structures of small-molecule sensitizers discussed in Section 3.3.2.

This highly crystalline small-molecule BTR was chosen in order to decrease the π−π stacking distance, enlarge the coherence length, and enhance the domain purity of the blend film, thus significantly increasing the hole mobility of the ternary active layer. Although PTB7-Th gives a high PCE, its hole mobility is relatively low in binary systems (1.22 × 10−3 cm2 V−1 s−1). Hence the optimal active layer thickness is limited to approximately 100 nm. This ternary system allowed the increase of active layer thickness to 250 nm (Fig. 3.8c). Moreover, BTR has a complementary absorption with host polymer

89

90

Ternary Sensitization of Organic Solar Cells

which results in a broad absorption range from 300 to 800 nm (Fig. 3.8b). This strategy could increase the efficiency from 9.03% (~100 nm thick) to 11.40% (~250 nm thick) (Fig. 3.8d). Significantly, this work and our work on PBTZT-stat-BDTT-8:PTB7-Th:PC70BM [17] system demonstrated that ternary sensitization concept for both polymeric and SM-based sensitizers overcomes successfully the thickness limitation of the host systems. High efficiency performance accompanied with several hundred nm thick active layer are crucial characteristics for large area roll-to-roll production. Very recently, Wei et al. [57] reported all-small-molecule ternary solar cells consisting of two compatible small-molecules donors: DR3TBDTT (M1) and DR3TBDTT-E (M2) and PC70BM as an acceptor. Both M1 and M2 contain benzo[1,2-b:4,5-b¢]dithiophene (BDT) unit. A high PCE of 10.26% was achieved after optimized thermal and solvent vapor annealing (TSA). Based on the recombination mechanism analysis, they concluded that TSA treatment decreases the bimolecular recombination which results in higher current and subsequently more efficient device. Morphological investigation is supporting this information, where crystallinity is enhanced by TSA treatment and the lengths of (010) peaks increases in GIWAXS patterns. They realized that TSA shifts the DR3TBDTT-E to the D/A interfaces, which ameliorates the charge transfer and separation and transforms the system from alloy-like model to cascade model.

3.3.3  Dye-Based Ternary Components

Dyes are an extremely interesting class of organic materials commonly used across industry and research. They are characterized by a very high absorption coefficient and narrow absorption profile making them very appealing for the sensitization effect. Moreover, dye sensitized ternary solar cells have already attracted significant attention because they are easy to adapt in ternaries in respect to microstructure compatibility. Among this class of materials, impressing results were obtained by Ito et al. in 2009 in which multi-colored dye sensitization of P3HT:PCBM, based on SiPC and SiNC were explored. Devices, with PCE of 2.7% [26] and 4.1% [93] were achieved with SiPC and SiNC sensitizer-based ternary,

Nature of the Third Component: A Review on the Experimental Results

respectively. Then, they studied the difference in absorption and device performance between SiPC and a planar molecule, zinc 2,3,9,10,16,17,23,24-octak-is(octyloxy)-29H,31H-phthalocyanine (ZnPc) [100]. They concluded that due to ZnPc’s structure, it stakes likely in the direction normal to the phthalocyanine plane and aggregates, which is in contrast with SiPC with two bulky groups in axial direction perpendicular to the phthalocyanine plane. However, Kymakis et al. [101] for the first time could successfully incorporate ZnPc up to the ratio of 20 wt.% of the host polymer into the high efficient PTB7:PC70BM system. Since this ternary blend formed a cascade energy structure, based on the results, they claimed that the charge transfer barrier was minimized, and the recombination was decreased. The efficiency of 8.52% was reported for 10% loading of the sensitizer which was 15% higher than the corresponding binary reference efficiency. Ito et al. further reported a multicolored dye sensitization by employing both SiPC and SiNC which meets the energetic requirement of the cascade energy level of the host system [100]. By reason of wider light harvesting (up to 800 nm) compared to individual ternary systems and hence improved photocurrent, the quaternary blend improved the efficiency to 4.3%. Therefore, it was concluded that no unfavorable interaction between the two dye molecules was present. In 2015, Ohkita et al. successfully fabricated heterostructure SiPC derivatives with asymmetric axial ligands in order to control the interaction of SiPC sensitizer with host P3HT and PCBM components, whereas one of the ligands is compatible with the host polymer with a lower surface energy and the other one is compatible with the acceptor with a higher surface energy [102]. It let them to incorporate high concentration of SiPC dye molecules into the amorphous interfaces of P3HT/PCBM (15 wt.%), preserving desired nanomorphology of the host system [102]. Among SiPcBz6, SiPc6 and SiPcBz ternary devices, P3HT:PCBM:SiPcBz6 cells exhibit the best PCE of 4.8% at 15 wt.% with an EQE of more than 60% at the dye absorption. These results showed that controlling the D/A interfaces in crystalline host systems by ligand engineering of sensitizer components is a smart strategy to achieve highly efficient multi-composite solar cells.

91

92

Ternary Sensitization of Organic Solar Cells

We investigated a series of novel silicon phthalocyanines (SiPcPy-1, SiPc-Py-2, SiPc-Py-3, SiPc-Py-4) and SiNC functionalized with tert-butyl groups on the periphery position and pyrene acid groups in the axial position into P3HT:PCBM as near IR sensitizers for ternary solar cells [27]. Optoelectronic properties revealed that the length of the alkyl chains between the core and pyrene ligands controls the intermolecular and intramolecular interaction in the solid state. SiPc-Py-1, which had no alkyl chains as linker group, showed a relatively broadened absorption in the film and higher JSC, compared with the other three SiPC sensitizers. However, SiPcPy-4, which had the longest of alkyl chain as linker group, revealed the overall best fill factor and highest charge carrier mobility among all the sensitizers. The photosensitization mechanisms were investigated to yield an increase of up to 21.9% of JSC, 16.1% of VOC and 7.2% of FF, leading to an improvement of up to 51.6% of PCE for SiPc-Py‑4 based ternary solar cells compared to the reference P3HT:PCBM binary device. Notably, the EQE spectra showed a strong photosensitivity in the near IR region, up to 800 nm for SiPc-Py-1 and 750 nm for the other three sensitizers [27]. On the other hand, the PCE of P3HT:PCBM was improved around 30% by implementing 10 wt.% SiNC, owing to the significant near IR light harvesting effect up to 950 nm. The complementary absorption profiles and morphological compatibilities of the SiPC and SiNC compounds (Fig. 3.11a) allowed us to successfully fabricate functional panchromatic dye sensitized quaternary solar cells. Adding 7.5 wt.% SiPc-Py-2 plus 5 wt.% SiNC, the PCE was improved ~20% compared to the binary P3HT:PCBM reference cell, ~15% compared to the 7.5 wt.% SiPc-Py-2 based ternary device and ~13% compared to the 5 wt.% SiNc based ternary device. Contribution of each dye sensitizer into the photocurrent was obviously observed in the EQE spectra (Fig. 3.11b). Interestingly, the quaternary device showed the same sensitizer loading limitation as the SiNC based ternary device (10– 15 wt.% in overall) which implies on the P3HT/PCBM interface limitation, where the dye sensitizers are supposed to locate to avoid the host system’s morphology and transport disruption [67].

Nature of the Third Component: A Review on the Experimental Results

Figure 3.11  (a) Chemical structures of SiPc-Py-1, SiPc-Py-2, SiPc-Py-3, SiPcPy-4, and SiNC. Reproduced with permission from [27]; (b) EQE spectra of the P3HT:SiNC:SiPc-Py-2:PCBM quaternary devices under different sensitizer contents. Reproduced from [67] with permission from The Royal Society of Chemistry.

Taylor et al. [35] incorporated 1 wt.% SQ-based dye molecule (DIB-SQ) into P3HT:PCBM and had above 38% performance improvement from PCE 3.27%. According to the results from femtosecond fluorescence and transient absorption spectroscopic, they reported a highly efficient energy transfer which contributes to JSC. SQ sensitization also promoted phase separation since SQ molecules prefer to be located at the D/A interfaces, hence highly ordered crystalline P3HT domains formed. SQ molecules were modified through side chain engineering and besides DIB-SQ [103], other types such bis[4-(2,6-di-tert-butyl)vinyl-pyry- lium]

93

94

Ternary Sensitization of Organic Solar Cells

squaraine TBU-SQ [104], and squaraine-triarylamines with parafluoro and meta,meta-bis-trifluoromethyl (SQ-TAACF3) [105] were synthesized as additional sensitizers to improve P3HT:PC70BM device performances. The high intrinsic carrier mobility, high absorption coefficient and intense near IR absorption of DIB-SQ up to 800 nm, had the potential to improve the efficiency of P3HT:PC70BM from 3.05% to 3.72%. Moreover, the energy levels of P3HT:DIBSQ:PC70BM formed a cascade structure which enhanced the exciton dissociation and facilitated charge transport by doping 1.2 wt.% of the sensitizer. Similarly, enhanced near IR harvesting capability of 2.5 wt.% TBU-SQ could improve the efficiency even further up to 5.15%. The improved performance observed for devices with 10 wt.% of SQ-TAACF3 likewise was attributed to an energy level cascade in the active layer components and improved light harvesting in red and near IR regions. Later, they incorporated asymmetrical 2-[4-(N,Ndiisobutylamino)-2,6-dihydroxyphenyl]4-diphenylamino] squaraine (ASSQ) and 2,4-bis[4-(N,N-diphenylamino)-2,6dihydroxyphenyl] squaraine (DPSQ) in three highly efficient systems of PTB7:PC70BM, PTB7‑Th:PC70BM, and PT8:PCBM as quaternary systems and achieved an increases in PCE of 33% (PCE = 9.46%), 19% (PCE = 10.45%), and 30% (PCE = 6.15%), respectively, mostly due to the improvement in JSC and FF [106]. From GIWAXS results, they concluded that ASSQ and DPSQ forms cocrystalline phase which results in better device performances. According to these results, the ASSQ:DPSQ mixed sample showed a strong scattering signal compared to the neat ASSQ and DPSQ films without any post-heat treatment. Furthermore, smaller domains in devices with ASSQ and DPSQ facilitate the formation of donor/acceptor microfibril interfaces [107], which is the favorable microstructure for charge collection [108]. Lastly, Lin et al. [109] improved the JSC and morphology of PTB7-Th:PC70BM binary by introducing a small molecule with four bulky pyrene rings, named as DFNPy (structures are shown in Fig. 3.12). The efficiency was recorded as 10.27% by 15 wt.% of sensitization and it was attributed to the complementary absorption of DFNPy and PTB7-Th in short wavelength region (350–

Nature of the Third Component: A Review on the Experimental Results

450 nm) which significantly enhanced light harvesting and hence the improvement in JSC. It was also ascribed to DFNPy aggregation pattern via π–π stacking which facilitate charge transport. Moreover, they claimed that this pattern promoted the morphology of the ternary blends and improved electron mobility, leading to higher FF. The performances of all mentioned ternary solar cells based on dye sensitizers and the corresponding binary references are presented in Table 3.3.

Figure 3.12  Chemical structure of dye sensitizers discussed in Section 3.3.3.

95

2:1:0.18

2:1:0.15

P3HT:PCBM:SiPcBz6

P3HT:PCBM:SiPc-Py-1

P3HT:PCBM:SiPc-Py-3

2015 D:D:A

2016 D:D:A

2013 D:D:A

P3HT:PCBM:SQ

P3HT:PCBM:SiPc-Py-4

P3HT:PCBM:SiPc-Py-2

0.99:1:0.01

2:1:0.2

2:1:0.15

0.85:1:0.15

1:1.5:0.1

0.57 69

0.55 0.68

0.58 59

FF [%]

0.62 61.46 3.8

0.62 61.58 3.79

4.8

0.62 61.57 3.78

0.53 59

8.43

4.3

3.7

2.7

PCE [%]

11.6

0.60 64.8

4.51

10.02 0.65 63.54 4.14

9.99

9.91

9.88

13

17.88 0.77 61.4

9.94

7.9

Isc [mA/ Voc cm2] [V]

1:1:0.1:0.03 10.9

PTB7:PC70BM:ZnPc

D:D:D:A P3HT:PCBM:SiPC:SiNC

2017 D:D:A

1:1:0.03

P3HT:PCBM:SiNC

2010 D:D:A

NR*

P3HT:PCBM:SiPC

Type

Ternary blend Ratio (Donor 1: Acceptor 1: (D:A:A2/ Acceptor 2 / Donor 2) D2)

Ternary performance

Photovoltaic parameters of reviewed dye-based ternary solar cells

2009 D:D:A

Year

Table 3.3

38

52

39

39

38

37

14

23

6

23

P3HT:PCBM (1:1)

P3HT:PCBM (2:1)

P3HT:PCBM (1:1)

10.3

8.22

10

8.96

6.5

FF [%]

3.5

7.38

3.5

2.2

PCE [%]

0.59 53

3.27

0.56 59.27 2.73

0.55 63

0.55 0.71

0.58 0.59

Isc [mA/ Voc cm2] [V]

Binary performance

PTB7:PC70BM 16.32 0.76 59.6 (1:1.5)

P3HT:PCBM (1:1)

P3HT:PCBM (1:1)

Binary reference Increase (Donor: PCE [%] Acceptor)

[35]

[27]

[102]

[101]

[100]

[26]

Ref.

96 Ternary Sensitization of Organic Solar Cells

P3HT:PC70BM:TBU-SQ

P3HT:PCBM:SQTAACF3

2014 D:D:A

2014 D:D:A

PTB7th:PC70BM:DFNPy

1:1.5:0.15

0.8:1:0.01: 0.01

1:1.5:0.01: 0.01

1:1.5:0.01: 0.01

9.7

1:1.1:0.1

0.73 65

0.66 62

0.70 58

0.61 56

0.58 66

0.59 62 27

16

221

48

6.15

30

10.45 13

9.46

4.62

5.15e

4.55d 31

2.66c

30

0.46 55

PTB7th:PC70BM (1:1.5)

PT8:PCBM (0.8:1)

PTB7th:PC70BM (1:1.5)

0.95 53.2 18.51 0.77 65.8

9.33

17.05 0.79 68.6

5.6

0.66 56

0.61 62

0.57 56

0.57 57

aWith

FF [%]

PTB7:PC70BM 16.74 0.73 66.3 (1:1.5)

P3HT:PCBM (1:1.1)

5.6

8.4

8.7

Isc [mA/ Voc cm2] [V]

[103]

Ref.

9.44

4.72

9.22

8.18

1.44

[109]

[106]

[105]

3.47d [104]

2.10c

2.86b

3.05a

PCE [%]

Binary performance

P3HT:PC70BM 9.4 (1:1)

P3HT:PCBM (1:0.8)

11

3.39a 3.72b

PCE [%]

19.72 0.78 68.15 10.59 12

10.73 0.97 59.1

17.82 0.79 71.7

17.88 0.72 71.1

12.6

11.2

7.8

9.7

9.3

1:1:0.025

1:0.8:0.012

FF [%]

Binary reference Increase (Donor: PCE [%] Acceptor)

NR*, not reported. pre-annealing treatment. bWith post-annealing treatment. cNo treatment. dCast from THF solution. eThermally annealed.

2018 D:D:A

PT8:PCBM:ASSQ:DPSQ

PTB7th:PC70BM:ASSQ:DPSQ

2016 D:D:D:A PTB7:PC70BM:ASSQ: DPSQ

P3HT:PCBM:DIB-SQ

Type

2014 D:D:A

Year

Isc [mA/ Voc cm2] [V]

Ternary performance

Ternary blend Ratio (Donor 1: Acceptor 1: (D:A:A2/ Acceptor 2 / Donor 2) D2)

Nature of the Third Component: A Review on the Experimental Results 97

98

Ternary Sensitization of Organic Solar Cells

3.3.4  Non-Fullerene Acceptors In recent years, non-fullerene acceptors (NFAs) have attracted significant attention because of their favorable properties. In contrast to fullerene based acceptors (FAs), the NFAs show strong absorption in the visible region, tunable energy levels and high stability [110, 111]. To date, the binary organic solar cells based on non-fullerene acceptors have reached efficiencies over 13% [4, 112–115].

Figure 3.13  (a) Architecture, energy levels and molecule structures of the ternary device with the highest efficiency to date. (b) J–V curve for the record device with and without annealing. (c) J–V curve and (d) absorbance of the active layer for different annealing steps. Reprinted from [15], Copyright 2018, with permission from Elsevier.

Nature of the Third Component: A Review on the Experimental Results

With this rapid and significant progress, application of NFAs in ternary solar cells became a widely pursued strategy [22, 23, 116]. As a third component, NFAs are well suited to complement light absorption of the binary host since their synthetic flexibility enables broad absorption in the visible or near-infrared range. Moreover, NFAs as a third component were observed to improve the morphology of the bulk heterojunction. Especially in binary blends with FAs, the addition of a NFA causes smoother films, appropriate domain sizes and purer phases [117, 118]. Additionally, the presence of NFAs can have a positive impact on charge transport, fill factor and the open circuit voltage. Since the energy levels of HOMO and LUMO in NFAs can be precisely adjusted by varying the structure, they are often used to create a cascade-like energy level alignment [117, 119, 120]. This facilitates exciton dissociation, reduces bimolecular recombination and, thus, increases the open circuit voltage [121]. To exploit these advantages, a wide variety of materials exist. The most promising NFA classes to date are based on the perylene diimides (PDIs) or naphthalene diimides (NDIs), and the components based on fused aromatic cores with electron-withdrawing and electron-donating moieties [122, 123] (fused-ring electron acceptors = FREAs). The structures of the most commonly used NFAs and those mentioned in this section can be found in Fig. 3.14. In the following section, recent developments of ternary solar cells containing NFAs are discussed. For this purpose, they are classified in three groups depending on the composition: (i) one donor with a fullerene and a non-fullerene acceptor (D:FA:NFA), (ii) one donor with two nonfullerene acceptors (D:NFA:NFA), and (iii) two donors with one nonfullerene acceptor (D:D:NFA).

3.3.4.1  D:FA:NFA ternary solar cells

Despite the aforementioned advantages of NFAs, they also have some drawbacks like low electron mobility and morphology issues due to of aggregation. Hence, one frequently used approach is to combine a non-fullerene acceptor with a fullerene acceptor to benefit from the merits of both. Such systems can be further distinguished according to the ratios of NFA and fullerene: either the NFA is used as an additive or as an equal host.

99

100

Ternary Sensitization of Organic Solar Cells

It has been demonstrated that the introduction of a small amount of a NFA can improve the light harvesting of a polymer:fullerene solar cell. For example, Zhang et al. added 20% 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4hexylphenyl)-dithieno[2,3-d:2¢,3¢-d¢]-s-indaceno[1,2-b:5,6-b¢] dithiophene (ITIC) to a binary blend of 4,8-bis(4,5-dioctylthiophen2-yl)benzo[1,2-b:4,5-b¢]dithiophene-2,6-diyl-alt-N-(2-hexyldecyl)5,5¢-bis(thiophen-2-yl)-2,2¢-bithiophene-3,3¢-dicarboximide (PDOT) and PC70BM and achieved a PCE of 10.74% [124]. This improvement was mainly reached due to an extended EQE response in the range from 650–800 nm and the corresponding increase of JSC from 11.75 mA cm–2 to 17.49 mA cm–2. Moreover, a NFA as an additive can have a positive effect on the active layer’s morphology. Huang and coworkers designed a device based on PTB7-Th and PC70BM with 3% of an acceptor consisting of a tetraphenylethylene core connected with four perylene diimide (TPE-4PDI), which led to a simultaneous increase of JSC, VOC, and FF and resulted in a PCE of 10.09% [125]. The higher VOC was attributed to the higher LUMO level of TPE-4PDI, while the enhanced JSC and FF were caused by a higher EQE across the spectrum due to better charge splitting and transporting. Similar results were achieved with a macromolecular additive, P(NDI2OD‑T2) [126]. With 0.8% P(NDI2OD-T2) added to a binary blend of PTB7-Th:PC70BM, charge mobilities were increased and a finer phase-separation of the hosts was achieved, which resulted in a maximum PCE of 11.6%. Lu et al. presented an organic solar cell with a NFA as equal host, consisting of the wide-bandgap polymer donor poly{4,8bis(4-(2-ethylhexylthio)phen-1-yl)benzo[1,2-b:4,5-b¢]dithiophen2,6-yl}-alt-{5,5¢-(5-(2-decyl-tetradecyloxy)-6-fluorobenzo[c][1,2,5] thiadiazole-4,7-diyl)di(thiophen-2-yl)} (PPBDTBT) in combination with the acceptors (PC70BM) and ITIC in similar amounts and obtained a maximum efficiency of 10.41% [127]. Compared to the binary systems PPBDTBT:ITIC and PPBDTBT:PC70BM, the efficiency in the ternary cell was improved by 35% and 43%, respectively. The authors explained their good results with the exploitation of both acceptors’ advantages. While the non-fullerene ITIC extends the light absorption and thus increases JSC, PC70BM disrupts the aggregation of ITIC and causes a favorable morphology and a high electron

Nature of the Third Component: A Review on the Experimental Results

mobility. This synergistic effect is dependent on the weight ratio of the two acceptors: VOC increases with the amount of ITIC while the fill factor increases with the amount of PC70BM. In this way, the efficiency shows an inflection point at a ratio of 1.2:0.8 ITIC:PC70BM. In addition to the beneficial properties of each material, the energy level alignment of the donor and the two acceptors allows a cascade charge transfer, which leads to a high exciton dissociation efficiency. The synergistic behavior of a fullerene and a non-fullerene acceptor was utilized in many other ternary material systems. For example, it was observed that the incorporation of PC70BM into a binary blend of (5Z,5¢E)-5,5¢-((4,4¢,4¢¢,8,8¢,8¢¢-hexakis(5(2-ethylhexyl)thiophen-2-yl)-[2,2¢:6¢,2¢¢-terbenzo[1,2-b:4,5-b¢] dithiophene]-6,6¢¢-diyl)bis(methanylylidene))bis(3-ethyl-2thioxothiazolidin-4-one) (DRTB-T) and the non-fullerene acceptor 2,2¢-((2Z,2¢Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2b:5,6-b¢]dithiophene-2,7-diyl)bis(methaneylylidene))bis(3-oxo-2,3dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) causes purer domains with decreased sizes and thus increases JSC and the fill factor, leading to an efficiency of 10.48% [128]. The positive effect of a mixed NFA and FA even can be observed with a minute amount of fullerene sensitization, as Hou and coworkers showed [129]. Adding 5% Bis-PC70BM to a blend of PTB7‑Th and 2,2¢-((2Z,2¢Z)((5,5¢-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno [1,2-b:5,6-b¢]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene)) bis(3-oxo-2,3-dihydro-1Hindene-2,1-diylidene))dimalononitrile (IEICO) caused an improved PCE of 10.21% due to the fullerene inducing a more suitable domain size of the polymer and thus facilitating charge transfer. Importantly, with the approach of combining FAs and NFAs, Ding and coworkers demonstrated the first organic solar cells exceeding 14% efficiency, thus reaching the highest singlejunction PCE to date [15]. They introduced a new non-fullerene acceptor with a carbon-oxygen-bridged unit, COi8, and difluorosubstituted 1,1-dicyanomethylene-3-indanone (IC) end groups. The corresponding molecule structures and device architecture are shown in Fig. 3.14. The binary blend of COi8DFIC and PTB7-Th has already reached a PCE of 12.16% with an impressing JSC of 26.12 mA cm–2, a VOC of 0.68 V and a fill factor of 68.2% [130]. Addition

101

102

Ternary Sensitization of Organic Solar Cells

of the PC70BM improved electron transport due to the stepwise alignment of LUMOs, reduced charge recombination, and optimized the morphology towards nanoscale phase separation. In this way, JSC and FF were enhanced to 28.2 mA cm–2 and 71.0%, respectively, and a PCE of 14.08% was achieved [131]. A post-annealing process at 80°C, eventually, led to a record efficiency of 14.62% due to an increase of the VOC to 0.73 V and the FF to 73.4% (Fig. 3.13b,c,d), attributed to a further optimized morphology. Moreover, the authors observed a PCE over 13.5% for annealing temperatures in the range of 70–160°C, therefore concluding a good thermal stability of the devices.

Nature of the Third Component: A Review on the Experimental Results

Figure 3.14  Structures of the non-fullerene acceptors reported in Section 3.3.4.

103

104

Ternary Sensitization of Organic Solar Cells

All these systems benefit from the cascade charge transfer, but there are also systems that show interesting results with other mechanisms. Liu et al. built up a ternary system with the donor PDBT-T1 and the acceptors PC70BM and ITIC-Th that demonstrated a champion PCE of 10.5% with a VOC of 0.95 V, a JSC of 15.60 mA cm–2 and a FF of 71.1% [62]. In this system, the energy levels are not suitable for a cascade charge transfer since the LUMOs of the two acceptors with 3.91 and 3.93 eV are too similar. Correspondingly, Photoluminescence measurements suggested that no energy or charge transfer occurs between the two acceptors. Instead, grazingincidence X-ray diffraction and resonant soft X-ray scattering results indicated a parallel-like morphology in which each acceptor forms its own transport network. Furthermore, Fan et al. observed that in a blend of benzodithiophene-alt-difluorobenzo[1,2,3]triazole (PBTA-BO) and PCBM the addition of (5Z,5¢Z)-5,5¢-((7,7¢-(4,4,9,9-tetrakis(4hexylphenyl)-4,9-dihydro-s‑indaceno[1,2-b:5,6-b¢]dithiophene2,7-diyl)bis(6-fluorobenzo[c][1,2,5]thiadiazole-7,4-diyl)) bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (IFBR) enhances the VOC because IFBR, which has a higher LUMO level than PCBM, forms an alloy with the fullerene and thus the VOC can be tuned with the composition [117].

3.3.4.2  D:NFA:NFA ternary solar cells

Since the synthetic flexibility of NFAs is one of their greatest advantages, using two NFAs and one donor provides various possibilities to improve the device performance. Baran and coworkers first demonstrated this kind of ternary solar cell by combining the two non-fullerene acceptors IDTBR and IDFBR with the low-cost donor P3HT and achieved a highly stable, affordable device with an efficiency of 7.7% [132]. The improvement of 22% compared to the binary device of P3HT:IDTBR is assigned to the enhanced absorption range and to the fact that both acceptors build a disordered solid solution while the crystalline order of the donor remains unchanged which reduces charge recombination and increases the photovoltage. Using this strategy with the low-bandgap donor PTB7‑Th even results in a VOC of 1.03 V and an efficiency of 11%.

Nature of the Third Component: A Review on the Experimental Results

The approach of enhancing the absorption range by using two NFAs with complementary bandgaps also can be realized with the combination of a 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene) malononitrile (IC) based NFA with low bandgap and a PDI or NDI based NFA with a medium bandgap [59, 133, 134]. For example, Sun and coworkers combined ITIC-Th and the selenophene-containing perylene bisimide acceptor SdiPBI-Se with the conjugated copolymer PDBT-T1 and observed absorption across the entire visible spectrum [133]. Thereby, a PCE of 10.3% with a VOC of 0.94 V, an JSC of 15.4 mA cm–2 and a fill factor of 71.3% was achieved, which corresponds to an improvement of 26% and 60%, respectively, compared to the PDBT‑T1:SdiPBI-Se and PDBT-T1:ITIC-Th binary cells. These promising results were also partially attributed to the improving interactions of the two acceptors, which lead to an intimate molecular mixing and thus to a decreased crystallinity and optimized morphology. Yu et al. used two non-fullerene acceptors 3,9-bis(2-methylene((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11tetrakis(4-hexylphenyl)-dithieno[2,3-d:2¢,3¢-d¢]-s-indaceno[1,2-b:5,6-b¢]dithiophene (IT-M) and IEICO with very similar chemical structures but different bandgaps in combination with the widebandgap donorbithienyl-benzodithiophene-alt-fluorobenzotriazole copolymer (J52) and achieved an efficiency of 11.1% [135]. In comparison to the binary cells, the JSC and FF was increased due to the wider absorption range and especially due the efficient energy transfer from IT-M to IEICO, providing an additional pathway for charge generation. Using two different NFAs even can be beneficial when they do not change the blends absorption range. Cheng et al. added 1% of the perylene diimide-based acceptor PDI-2DTT as a third component to a system consisting of PTB7-Th and (5Z,5¢Z)-5,5¢-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b¢]dithiophene-2,7-diyl)bis(benzo[c][1,2,5] thiadiazole-7,4-diyl))bis(methaneylylidene))bis(3-ethyl-2thioxothiazolidin-4-one) (IDT-2BR) and improved the efficiency from 8.2% to 9.7% [136]. This small amount of PDI-2DTT did not affect the morphology or the light harvesting, but it served as an

105

106

Ternary Sensitization of Organic Solar Cells

energy driver. The LUMO and HOMO offset between PTB7-Th and IDT-2BR with 0.1 and 0.32 V, respectively, is found to be insufficient for efficient charge transfer. Therefore, the lower LUMO and HOMO energy levels of PDI-2DTT enhance the driving force. Su et al. demonstrated a ternary solar cell with 11.1% efficiency consisting of ITIC, IDIC and the polymer donor PSTZ [58]. ITIC and IDIC have nearly identical absorption spectra but the energy levels are slightly different, thus enabling a smooth energy gradient and increasing the VOC due to the higher LUMO of ITIC compared to IDIC. Moreover, the weak crystallinity of ITIC in combination with the highly crystalline IDIC improves the phase separation in the ternary blend leading to an enhanced JSC and FF.

3.3.4.3  D:D:NFA ternary solar cells

In 2016 Jenekhe et al. developed the new non-fullerene acceptor 2,5-bis(8-(17-phenyl)-7,9,16,18-tetraazabenzodifluoranthene3,4,12,13-tetracarboxylic acid diimide)-3,4-ethylenedioxythiophene (DBFI-EDOT) and demonstrated for the first time that a NFA is compatible with two different donor polymers (thiazolothiazoledithienosilole copolymer PSEHTT and PTB7-Th) [137]. The ternary solar cells reached the maximum efficiency of 8.5%, which corresponds to an improvement of 7% and 31%, respectively, in comparison to the PSEHTT:DBFI-EDOT and PBDTT-FTTE:DBFIEDOT binary blends. This positive effect was mainly caused by the enhanced absorption range and the resulting higher JSC. Hou et al. refined this concept further by using the ultra-narrow-bandgap acceptor 2,2¢-((2Z,2¢Z)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9dihydro-sindaceno[1,2-b:5,6-b¢]dithiophene-2,7-diyl)bis(4((2-ethylhexyl) oxy)thiophene-5,2-diyl))bis(methanylylidene)) bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile (IEICO-4F) in a ternary OSC with the polymer donors PTB7-Th and J52 [138]. The complementary bandgaps yielded a wide absorption from 300 to 900 nm and thus a high JSC of 25.3 mA cm–2, whereas the VOC remained unchanged due to the donor’s similar HOMO levels. Overall, a PCE of 10.9% was achieved. In a lot of ternary systems, the third component does not enlarge the absorption range by directly generating free charges

Nature of the Third Component: A Review on the Experimental Results

but by transferring the excitation via Förster resonance energy transfer. Benten et al. used the n-type conjugated polymer N2200 as a non-fullerene acceptor in combination with the donor PTB7Th and introduced PCDTBT as a second donor [88]. Since the emission spectrum of PCDTBT overlaps with the absorption spectra of the other two components, excitons generated in PCDTBT are transported to the host polymers through FRET. In this way, the EQE is increased by over 60% in comparison to PTB7-Th:N2200, while the good charge transport characteristics of the binary blend are preserved. Besides, Zhang and coworkers showed that PCDTBT also has a positive influence on the morphology of PTB7-Th:N2200 [139]. They claimed that PCDTBT causes smaller domains and increases the interface of host donor and acceptor. The suitability of PCDTBT as a guest donor was also shown in combination with PTB7-Th and the small-molecule acceptor ITIC [140]. In this ternary blend, it was observed that PCDTBT is embedded in both PTB7‑Th and ITIC and, accordingly, the PL-measurements indicated an energy transfer from PCDTBT to both host components. Additionally, GIWAXS patterns showed clear diffraction peaks in the out-of-plane direction, which were used to determine the π–π stacking distances. With increased doping concentration of PCDTBT, the host’s d-spacings became smaller which might facilitate electron hopping. The addition of a second donor can even be beneficial without increasing the absorption range. Liu et al. constructed a ternary blend with two donors that possess similar absorption ranges but different morphological behavior [141]. The used binary blend of PTB7-Th and ITIC is thermodynamically miscible and thus the resulting film lacks distinct domains that are necessary for efficient exciton dissociation. To solve this problem, PffBT4T−2OD was added as second, highly crystalline donor and as a result an interpenetrating network with appropriate domain sizes was achieved. The performances of all mentioned ternary solar cells and the corresponding binary references are presented in Table 3.4. The structures of the related non-fullerene acceptors are shown in Fig. 3.14.

107

16.66

2:1:0.024 PTB7Th:PC70BM:P(NDI2OD-T2)

PPBDTBT:PC70BM:ITIC

PBTA-BO:PCBM:IFBR

DRTB-T:PC70BM:IDIC

2016 D:FA:NFA

2016 D:FA:NFA

2017 D:FA:NFA

2017 D:FA:NFA

1:0.5:0.5

1:1:0.8

1:0.8:1.2

23.00

1:0.05:1.45

PTB7-Th:PC70BM:TPE4PDI

2017 D:FA:NFA

15.36

13.45

17.44

17.49

1:1.6:0.4

PDOT:PC70BM:ITIC

Type

2018 D:FA:NFA

Year

Jsc [mA/ cm2]

Ratio (D:A:A2/ D2) FF [%]

0.99 67.7

0.93 65.1

0.89 68.0

0.81 61.0

0.78 73.9

0.96 66.8

Voc [V]

Ternary performance

17.3

44.8

8.11a 10.48

35.1

16.9

9.4

51.9

—

PTB7Th:P(NDI2OD-T2)

DRTB-T:IDIC (1:1)

DRTB-T:PC70BM (1:1)

PBTA-BO:IFBR (1:1.6)

PBTA-BO:PCBM (1:1.6)

PPBDTBT:ITIC (1:2)

PPBDTBT:PC70BM (1:2)

—

—

0.99 65.4 6.04

1.06 43.5 3.65a

0.87 73.0 5.53a

13.87 0.99 63.5 8.79

9.36

8.34

8.93

13.00 0.93 63.0 7.66

12.25 0.76 75.0 7.09

—

19.20 0.78 61.0 9.62

0.89 50.3 1.88

16.47 0.77 71.2 9.22 4.26

PTB7-Th:PC70BM (1:2)

PCE [%]

13.39 1.03 49.8 5.90

FF [%]

11.75 0.93 69.7 7.07

PTB7-Th:TPE-4PDI (1:1.5)

PTB7-Th:PC70BM (1:1.5)

PDOT:ITIC (1:2)

PDOT:PC70BM (1:2)

Increase Binary reference PCE [%] (Donor : Acceptor)

10.41

11.60

10.40

11.21

PCE [%]

Jsc [mA/ Voc cm2] [V]

Binary performance

Performances of reported NFA based ternary solar cells and their corresponding binary blends.

Ternary blend (Donor 1 : Acceptor 1 : Acceptor 2 / Donor 2)

Table 3.4

[128]

[117]

[127]

[126]

[125]

[124]

Ref.

108 Ternary Sensitization of Organic Solar Cells

28.20 27.39

1:0.45:1.05 1:0.45:1.05 1:0.5:0.5

PTB7Th:PC70BM:COi8DFIC

PTB7Th:PC70BM:COi8DFIC

PDBT-T1:PC70BM:ITIC-Th

PBDB-T:ITIC-Th:TPE-4PDI

2017 D:FA:NFA

2018

2017 D:FA:NFA

2017 D:NFA:NFA

D:FA:NFA

1:0.1:0.9

18.92

1:0.06:1.14

PTB7-Th:BisPC70BM:IEICO

2017 D:FA:NFA

17.20

15.54

15.82

1:1.2:0.3

PDTP4TFBT:PC70BM:ITIC

Type

2017 D:FA:NFA

Year

Jsc [mA/ cm2]

Ratio (D:A:A2/ D2) FF [%]

0.87 72.6

0.93 70.5

0.73 73.4

0.70 71.0

0.83 65.0

0.84 69.3

Voc [V]

Ternary performance

Ternary blend (Donor 1 : Acceptor 1 : Acceptor 2 / Donor 2) 6.4

11.0

PBDB-T:TPE-4PDI (1:1)

PBDB-T:ITIC-Th (1:1)

12.58 0.95 59.0 7.05a PDBT-T1:ITIC-Th (1:1)

PDBT-T1:PC70BM (1:1)

10.50a 10.7

23.84 0.69 63.8 10.17

PTB7-Th:COi8DFIC (1:1.5)

8.92

0.95 55.8 4.73

16.45 0.85 69.7 9.75

13.24 0.92 76.2 9.23a

16.21 0.75 60.2 6.99

23.84 0.69 63.8 10.17

PTB7-Th:PC70BM (1:1.5)

—a

PTB7-Th:COi8DFIC (1:1.5)

14.62a 42.3

—

16.21 0.75 60.2 6.99

—

17.31 0.83 58.0 8.33a

—

PTB7-Th:IEICO (1:1.2)

PTB7-Th:BisPC70BM

14.18 0.90 59.3 7.40

PTB7-Th:PC70BM (1:1.5)

11.04

PCE [%]

PDTP4TFBT:ITIC (1:1.6)

34.8

14.08

FF [%]

PDTP4TFBT:PC70BM 15.35 0.82 69.6 8.54 (1:1.4)

Increase Binary reference PCE [%] (Donor : Acceptor)

10.21a 22.6

9.20

PCE [%]

Binary performance Jsc [mA/ Voc cm2] [V]

Ref.

[125]

[62]

[15]

[131]

[129]

[120]

Nature of the Third Component: A Review on the Experimental Results 109

PTP8:P(NDI2HD-T):ITIC

Type

2017 D:NFA:NFA

Jsc [mA/ cm2]

1:0.5:0.5

PTB7-Th:IDTBR:IDFBR

PDBT-T1:SdiPBI-Se:ITICTh

J52:IT-M:IEICO

PTB7-Th:IDT-2BR:PDI2DTT

PSTZ:ITIC:IDIC

2016 D:NFA:NFA

2016 D:NFA:NFA

2017 D:NFA:NFA

2017 D:NFA:NFA

2017 D:NFA:NFA

1:0.1:0.9

1:1:0.02

1:0.8:0.2

1:0.5:0.5

1:0.7:0.3

16.90

14.50

19.70

15.37

17.2

14.4

1.5:0.85:0.15 12.60

Ratio (D:A:A2/ D2) FF [%]

0.95

1.03

0.85

0.93

1.03

0.82

64.1

65.0

66.8

70.2

60.0

64.0

0.98 57.0

Voc [V]

Ternary performance

2016 D:NFA:NFA P3HT:IDTBR:IDFBR

Year

Ternary blend (Donor 1 : Acceptor 1 : Acceptor 2 / Donor 2)

24.4

10.30a

11.10

10.10

36.1

18.3

11.10a 19.8

-

22.2

7.70a

11.00

17.8

7.4 —

P3HT:IDFBR (1:1)

PTB7-Th:IDFBR

—

—

—

—

15.10 0.85 51.0

14.30 0.92 56.7 7.79

-

14.90 1.01 49.3 7.75

PSTZ:IDIC (1:1)

PSTZ:ITIC (1:1)

-

-

12.80 1.05 61.1 8.20

6.50a

17.10 0.84 65.1 9.10a

11.70 0.92 59.4 6.39a

12.64 0.94 68.2 8.12a

—

—

0.89 68.0 4.50a

0.72 60.0 6.30a

PTB7-Th:PDI-2DTT

PTB7-Th:IDT-2BR (1:1)

J52:IEICO (1:1)

J52:IT-M (1:1)

PDBT-T1:ITIC-Th (1:1)

PDBT-T1:SdiPBI-Se (1:1)

—

13.9

P3HT:IDTBR (1:1)

PTB7-Th:IDTBR

PCE [%]

1.00 58.0 5.01

FF [%]

10.71 0.98 57.0 5.79

8.76

PTP8:ITIC (1.5:1)

PTP8:P(NDI2HD-T) (1.5:1)

Increase Binary reference PCE [%] (Donor : Acceptor)

7.01

PCE [%]

Binary performance Jsc [mA/ Voc cm2] [V]

Ref.

[58]

[136]

[135]

[133]

[132]

[59]

110 Ternary Sensitization of Organic Solar Cells

0.9:2:0.1

PBDTTT-EFT:IEICO-4F:J52 0.7:1.5:0.3 1:1:0.2 1:1:0.3 0.8:1.3:0.2 0.8:1.5:0.2

PSEHTT:DBFIEDOT:PBDTT-FTTE

PBDTTT-EFT:N2200:PCDTBT

PTB7-Th:N2200:PCDTBT

PTB7-Th:ITIC:PCDTBT

PTB7-Th:ITIC:PffBT4T2OD

2017 D:NFA:D2

2016 D:NFA:D2

2016 D:NFA:D2

2017 D:NFA:D2

2017 D:NFA:D2

aAnnealed.

Type

2016 D:NFA:D2

Year

Ratio (D:A:A2/ D2)

15.36

16.71

11.44

14.30

24.10

15.48

Jsc [mA/ cm2]

0.84

0.80

0.78

0.79

0.73

0.91

Voc [V]

Ternary performance

Ternary blend (Donor 1 : Acceptor 1 : Acceptor 2 / Donor 2)

62.6

55.9

56.0

57.0

58.9

59.5

FF [%]

8.22

7.51

5.11

26.8

15.4

19.9

17.5

PBDTTT-EFT:IEICO4F (1:1.5)

10.90a 8.2 6.65

PSEHTT:DBFI-EDOT (1:2)

7.0

8.52a

1.76

PCDTBT:N2200 (1:1)

2.56

9.95

0.95 40.0 0.94

0.79 54.0 4.17

0.88 56.3 4.42

PffBT4T-2OD:ITIC (1:1.5)

8.91

PTB7-Th:ITIC (1:1.5) 15.06 0.82 51.2 6.35

0.97 33.1 0.69

PCDTBT:ITIC (1:1.3) 2.15

PTB7-Th:ITIC (1:1.3) 13.89 0.83 56.7 6.51

PCDTBT:N2200 (1:1)

PTB7-Th:N2200 (1:1)

0.96 37.0 0.63

PBDTTT-EF-T:N2200 12.70 0.80 54.2 5.49 (1:1)

21.40 0.73 58.5 9.30a

22.40 0.74 59.4 9.70a

13.50 0.95 50.0 6.42a

PCE [%]

13.51 0.92 63.1 7.85a

FF [%]

J52:IEICO-4F (1:1.5)

PBDTT-FTTE:DBFIEDOT (1:2.5)

Increase Binary reference PCE [%] (Donor : Acceptor)

PCE [%]

Binary performance Jsc [mA/ Voc cm2] [V]

[141]

[140]

[139]

[88]

[138]

[137]

Ref.

Nature of the Third Component: A Review on the Experimental Results 111

112

Ternary Sensitization of Organic Solar Cells

3.4  Conclusion and Outlook Ternary blends have been a successful platform to overcome the efficiency limitation for OPVs. The investigations on this method over the last decade, starting with limited efficiency P3HT:PCBM binary systems, have led to significant improvement in device performances (Fig. 3.15). The breakthrough efficiency of 14.62% recorded for OSCs has been achieved by sensitization of a highly efficient binary system. Most of the work in this field has been focusing on employing a long wavelength absorber sensitizer and enhance the JSC. Hence, the transport mechanisms have been broadly explored. Along with the advantages of ternary method, there are still several challenging unanswered questions regarding the fundamental physical and electronic interactions, for example the origin of the compositiontunability of VOC and the optimal electronic correlations between the components. Moreover, the complex morphology of ternaries, and even more complicated dependence of morphology on electronic behavior and the ability to tune the VOC with composition need to be understood and addressed in order to fully utilize the potential of this method.

Figure 3.15  Research ternary organic solar cells efficiency for different sensitizers reviewed in this paper and also collected from references [14] and [142].

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To date, binary organic systems have been sensitized through a broad range of different organic components, such as middle- and/ or low-bandgap polymers, small molecules and dyes. Furthermore, innovation of hybrid functional systems not only opened a novel processing window in this field, but also showed a prospective future in other electronic applications. This cost-effective strategy is targeted to close the gap between the efficiency of OPVs and perovskite solar cells. We believe that it would be of significant interest to establish clear guidelines for the design of an optimal ternary system which can thus be further implemented in large scaled manufacturing.

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138. H. Yao, Y. Cui, R. Yu, B. Gao, H. Zhang, J. Hou, Angew. Chem. Int. Ed. 2017, 56, 3045. 139. R. Zhang, H. Yang, K. Zhou, J. Zhang, J. Liu, X. Yu, R. Xing, Y. Han, J. Polym. Sci. Part B Polym. Phys. 2016, 54, 1811. 140. P. Bi, F. Zheng, X. Yang, M. Niu, L. Feng, W. Qin, X. Hao, J. Mater. Chem. A 2017, 5, 12120. 141. X. Liu, J. Wang, J. Peng, Z. Liang, Macromolecules 2017, 50, 6954. 142. S. R, K. Aa, A. T, J. Mater. Sci. Eng. 2017, 6, 1.

Chapter 4

Dye-Sensitized Solar Cells: Photophysics of Coordination Complex

Vanira Trifiletti and Norberto Manfredi University of Milano—Bicocca, Department of Materials Science and Milano—Bicocca Solar Energy Research Center (MIB-SOLAR), Via Cozzi 55, 20125 Milano, Italy [email protected], [email protected]

In the past decades, dye-sensitized solar conversion technologies have gained more and more interest in the scientific community. Dye-sensitized solar cells (DSCs) are a very promising technology that can combine the high-stability features common to the inorganic semiconductors and the wide flexibility of the organic compounds. Indeed, these novel solar cells are multi-component devices that can be prepared using different metal oxides as charge transport materials in combination with organometallic or organic sensitizers as absorber. Because of their great flexibility, they immediately became a source of great interest from different points of view, especially with regard to the studies of the phenomena of charge generation and transport. These phenomena are strictly related to the structure of the cell itself and the nature of the sensitizers. Emerging Photovoltaic Technologies: Photophysics and Devices Edited by Carlito S. Ponseca Jr. Copyright © 2020 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4800-69-3 (Hardcover), 978-0-429-29525-6 (eBook) www.jennystanford.com

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These cells can be made as n-type or p-type with a passive counter electrode, depending on the metal oxide semiconductor, as well as in combination in a tandem configuration with both active anode and cathode. However, the most important part of the cell is the sensitizer. Indeed, since their report, the rush to find novel more efficient sensitizers started. From the first ruthenium complex, the rush to more efficient and more stable dyes led to the production of a plethora of different molecular sensitizers, either organometallic and organic, capable of astonishing properties setting higher and higher the attainable efficiency and improving, the stability of the overall device. In this chapter, after a presentation of the main type of DSCs, attention will be paid to the photophysics of devices sensitized with metallo-porphyrins based dyes and the effect of the different functionalzation of their tetra-aryl porphyrinic core. Different techniques have been used to evaluate the main photophysics parameters to point out the critical effect of proper functionalization over the final efficiency of the devices.

4.1 Introduction

Among all renewable energy resources, solar energy is the one present in greater amount and more uniformly distributed over the entire planet. The Earth is invested in every moment by an electromagnetic radiation with a power of about 1.7 ¥ 105 TW and, just outside the Earth’s atmosphere, with a power density 1367 W m–2 (AM0). Considering reflection and absorption by the atmosphere, on the Earth’s surface, the solar spectrum has an average power density of about 1000 W m–2 (AM1.5) [1]. This variation can be explained considering what happens along the way that photons move through the atmosphere. Solar radiation has a spectrum unperturbed that does not differ much from that of a black body at T ≈ 6000 K, the passage through the atmosphere, where there are mainly O2, H2O, CO2, O3 and particulate, reduces the radiation because of the aforementioned considerations and removals and it is clear that the greater the portion of atmosphere will be crossed, the greater the attenuation. This optical path through the atmosphere (L) is defined by the air mass coefficient AM.

Introduction



AM =

L 1 , = L0 cos(J z )

(4.1)

where L0 is the normal path to the Earth’s surface and Jz it is the zenith angle in degrees. Every square meter of the Earth’s surface receives an average variable annual amount of energy, depending on the latitude, between 1 and 2 MWh that, even considering the current efficiency of photovoltaic devices, potentially could supply coverage for much of the global energy needs. The photovoltaic effect was seen for the first time in 1839 by French physicist Edmond Becquerel while devoting himself to studies of electrolytic cells. The first solar cell was built in 1883 by the American inventor Charles Fritts and consisted of a junction formed from selenium and gold with an efficiency of only 1%. The first devices with significant efficiencies are reported in the second half of the last century, based on the development of semiconductor studies. In particular, scientists D. Chapin, Fuller and C. G. Person of Bell Laboratories in 1954 put in place the first silicon photovoltaic cell with an efficiency of about 6% [2]. From that first device, studies have focused on the development of ever more efficient cells. This development has made it possible to achieve high conversion efficiencies dramatically reducing the cost of production. Current technologies are divided into three generations that consider of the different degrees of technological advancement and their availability in terms of costs and materials. The first two generations include semiconductor devices: the first one includes the monocrystalline or polycrystalline silicon cells which are those studied for longer and that are the most commercially widespread. The second generation regards the thin-film devices that have lower efficiency than silicon but a lower cost. To date they have been unable to find much space in the photovoltaic market but, at the research level, they are still a topic of great interest. Finally, shall be deemed third-generation, all the emerging technologies still in the basic research stage such as organic cells (OPV), the dye-sensitized cells (DSCs), perovskite solar cells (PSC), and quantum dot and some types of multi-junction cells. The third generation promises to take the best from the past to reach a cost/efficiency ratio, which makes photovoltaic a major player among the different energy sources used for meeting the global need.

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DSCs (Fig. 4.1), broke onto the photovoltaics research field in 1991 [3]: Michael Grätzel and Brian O’Regan described a photoelectrochemical solar cell based on a nanocrystalline thin film of titania (TiO2), sensitized by a dye, immersed in an electrolyte and provided with electrical contacts. Some components of the DSCs are inorganic and others organic, and, for this reason, they are defined as “thin-film hybrid devices”.

Figure 4.1  Examples of dye-sensitized large area solar cells and module with different pattern (University of Milano-Bicocca, 2013).

Unlike silicon solar cells, in which light absorption and transport of photo-generated charges take place in the same semiconductor, in DSCs these roles are separated. The DSCs are easily processable: Conventional roll-printing techniques are involved, and most of the

Introduction

employed materials are low-cost. While its conversion efficiency is inferior to that of the best thin-film cells, the low production cost, and the ability to work in low-light conditions make DSCs an attractive technology. The first DSCs sensitized with dyes in the form of organometallic ruthenium complexes bearing bi-pyridines bi-carboxylated to activate the titanium oxide came from the early 1980s [4], and their development continued in the second half of the 1980s using ruthenium complexes with three bi-pyridines bicarbossilate [5]. Since the famous publication in Nature magazine of 1991 [3], various complexes of ruthenium have been tested, and in 1993 Graetzel reported as sensitizers a series of mononuclear ruthenium complexes, of which the best performing was N3, where ruthenium shows two thiocyanate ligands and two bi-pyridines substituted in position 4, 4¢ with carboxylic acid units [6]. The carboxyl groups allow the anchoring of the titanium dioxide and are conjugate to the pyridine groups. In the ground state electron density, it is concentrated on the metal centre, but in the excited state resulting in the absorption of electromagnetic radiation, such density is concentrated on carboxylate groups linked to TiO2. The thiocyanate groups are electron donors which increase the absorption coefficient of the complex. Nazeeruddin and coworkers have tested different forms of protonated N3: N712 has four carboxylate groups with tetra-butyl ammonium salt, N719 has two carboxylate groups and two carboxylic acids [7, 8]. The carboxylic acids allow a better anchorage of titanium oxide and higher current, but carboxylates increase the voltage of the cells. The cells up to now carried more performing with ruthenium complexes make use of N719 dye and the redox couple I–/I3– as the electrolyte (12% efficiency). It is now normal practice for each new tested dye, to use N719 as reference. Changes on the molecular structure allowed synthesizing many other photosensitizers. For example, Z907 was synthesized, in which a bi- pyridine bi-carboxylate substituent is replaced by a nonyl-4,4¢bipyridine [9]. The presence of alkyl hydrophobic chains makes the molecular structure most stable. The cell constructed making use of hexa-decyl-phosphonic acid as co-absorbent, showed efficiency around 7%, which is also kept when subjected to thermal stress. The structures of the two reference dyes, N719 and Z907, are shown in Fig. 4.2.

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Figure 4.2  Structures of the two reference dyes, N719 and Z907, presented by Nazeeruddin in 1999 and Wang in 2003.

With the aim of improving the optical properties of such complexes, in the following years, many new ruthenium-based complexes have been synthesized, in which one of the two bipyridine ligands has been replaced with more conjugated systems. All these derivatives have showed improved optical properties, but lower efficiencies than those obtained with the reference dyes [10–15]. Also metal free organic dyes have been deeply investigated. These new dyes with a dipolar donor-acceptor molecular structures such as D-π-A, in which the acceptor is also linked to the anchoring group that is a unit vinyl-cyano-acetic. As donor groups, different electro-rich donor moieties have been investigated and different π-spacers have been bonded on each. As for the latter, the most efficient are those thienyl or poly-thienyl based. Among the different investigated donor groups, the most performing were found to be coumarin, indolines, tetraidrochinoline, carbazoles, dialchilaniline, and especially trifenilammine. Further structural modifications, followed by an optimization of the devices have allowed obtaining results comparable with those of the organometallic reference dyes [16–21]. The literature on these compounds is very extensive (Fig. 4.3), since it is simple, from the synthetic point of view, to obtain a great variety of molecules. In particular, using as spacers 3-hexyl-thienyl unit, 8.5% efficiency were obtained [22]. The best efficiencies with organic dyes using triphenylamine as donor group have been obtained with alkoxy-triphenylamine linked to spacers thieno[3,2-b]thiophene. In particular, the C217 in which a tri-

Introduction

phenylamine is conjugated to a 3,4-ethylendioxithiophene (EDOT) and a thieno[3,2-b]thiophene, has allowed a record efficiency of 9.8% [23].

Figure 4.3  Examples of linear organic dyes.

More recently, Abbotto and co-workers have published a major work on a new class of multibranched triphenylamine based derivatives [24]. These derivatives present a structure of the type D-(π-A)2, where a single triphenylamine donor is connected to two conjugated π-spacers and, as a result, two anchoring groups. This configuration allows to obtain a longer time stability compared to the corresponding linear system. Another important development of this class of compounds was subsequently proposed by synthesizing a system in which the two branches exhibited different structures connected to a more complex donor centre with the objective of improving the optical properties [25, 26]. The validity of this approach (Fig. 4.4) has been demonstrated by many other works published in the following years [27]. After such a strong development of sensitizers, the focus has shifted to the development of the device to make it more commercially attractive. Indeed, a further weak point of traditional DSC technology, to which it is important to pay attention, is the presence of a liquid electrolyte containing iodine. Up to now, such an electrolyte is the best redox mediator to obtain high efficiencies. There have been some attempts to replace iodine with more efficient redox systems such as the redox couple Co2+/Co3+ with which, using

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Figure 4.4  Examples of multibranched organic dyes.

a zinc porphyrin YD2-o-C8, Yella and coworkers have reached and exceeded the previous record efficiency bringing the value 12.7% [28]. This outstanding result, however, was not enough to push the marketing of this type of devices, because, once again made with sensitizers with a complex synthesis and in the presence of a liquid electrolyte particularly toxic due to the presence of cobalt salts. In order to make these devices attractive to the market, it was necessary to develop solid-state systems, thus eliminating the risk related to the presence of electrolytes containing solvents and toxic or corrosive components [29, 30]. In this scenario, the best devices are those made using the molecular organic hole transport material (HTM) called 2,2¢,7,7¢-tetrakis (N,N-dimetoxyphenylamino)-9,9¢spirobifluorene (spiro-MeOTAD) (Fig. 4.5), which allowed Graetzel and co-workers in 2011 to reach a certified efficiency of 6.08% [31]. Despite the several efforts made in research, the most widely used HTM is still the spiro-MeOTAD which, however, has the problem of being easily oxidized to air and is expensive; a complicated synthesis and very few commercial sources make the product difficult to find. The growing interest in solid-state devices has prompted researchers

Introduction

to further investigate on more efficient systems. In 2012 were presented for the first time ssDSC cells (solid-state dye-sensitized solar cell) that used the traditional configuration with the spiroMeOTAD as a hole conductor and a hybrid perovskite with structure CH3NH3PbI3 sensitizer with an efficiency of 10.9%, the highest ever recorded for systems of this type [32].

Figure 4.5  Benchmark hole transport material, Spiro-MeOTAD.

4.1.1  Fabrication of Dye-Sensitized Solar Cells A DSC is an electrochemical solar cell consisting of a working electrode, that absorb the light, and a counter electrode. The working electrode is usually prepared on fluorine-doped tin oxide (FTO) coated glasses, cleaned in a detergent solution, and then rinsed with pure water and ethanol. After treatment in a UV-O3 system, the FTO plates were treated with an aqueous solution of TiCl4 and then rinsed with water and ethanol. A first transparent TiO2 nanocrystalline layer (about 20 nm particles) is screen-printed and dried. Then, a thin scattering layer, containing optically dispersing anatase particles, is screen-printed over the transparent layer. Figure 4.6 shows pictures of the deposition of a titania precursor paste on the screen-printer mask and the printing operation. The coated films are then thermally treated at 500°C for 15 min and treated with an aqueous TiCl4, rinsed with ethanol, and heated again at 500°C. After cooling down, the TiO2-coated plate is immersed into a solution of the dyes (Fig. 4.7).

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Figure 4.6  Deposition of a titania precursor paste on the screen-printer mask and the printing operation (University of Milano-Bicocca, 2013).

Figure 4.7  FTO glass coated by a titania thin film, after the immersion into the sensitizing solution (University of Milano-Bicocca, 2013).

Introduction

The counter electrode is prepared making a 1 mm hole in an FTO plate by using diamond drill bits, which is cleaned with a detergent and with a HCl diluted solution, and finally acetone. After the UV-O3 treatment, a drop of a solution of H2PtCl6 in ethanol is added and treated at 400°C. The dye-sensitized TiO2 working electrode and Pt counter electrode are assembled into a sealed sandwich-type cell by heating with a hotmelt ionomer-class resin as a gasket between the electrodes. A drop of the electrolyte solution is introduced inside the cell by vacuum backfilling. Finally, the hole is sealed with a sheet of ethylene acrylate resin and a cover glass. A reflective foil at the back side of the counter electrode is taped to reflect unabsorbed light back to the photoanode [26].

4.1.2  Photovoltaic Efficiency Measurement

A DSC is a multi-component device comprising (a) a dye-sensitizer S; (b) a n-type semiconductor metal oxide (typically TiO2); (c) a p-type semiconductor or a redox electrolyte (typically a redox couple); (d) a transparent working anode and a counter electrode (based on fluorine-doped tin oxide, FTO). Under light irradiation the sensitizer S passes to its excited state S* from which an electron is injected into the conduction band (CB) of TiO2, leaving the dye in its oxidized state S+. The collected electrons at the photoanode are then transferred through the external load to the counter electrode where, via Pt catalysis, sensitizer regeneration takes place (S+ Æ S). If a p-type semiconductor is used in place of the electrolyte (solid-state devices), dye-regeneration occurs via hole transfer from S+ to the HOMO of the hole transporter. A DSC is a very efficient device where, formally, one photon is converted to one electron without permanent modification of any part. In addition to the main processes, several undesired pathways and losses are present including recombination of injected electrons from TiO2 to either S+ or the oxidized form of the electrolyte, incomplete light harvesting, and inefficient electron transfer from S*. The main source of loss-in potential of a DSC is the high overpotential needed to regenerate the dye, which strongly limits the maximum attainable photovoltage [33]. The performance of a DSC, or in general of a PV cell, is determined by measuring the overall power conversion efficiency (sometimes referred to as PCE) from the ratio of maximum output power density

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(Pout, in W m–2) and the input light irradiance (Pin, in the same units) as shown below. Under standard reporting conditions the light intensity Pin is 1000 W m–2, the sun spectrum is AM 1.5 G [1], and the sample temperature is 25°C [34, 35].

Pout Pin

(4.2)

Pout = Vmp J mp

(4.3)

h = PCE =

According to the Shockley–Queisser model, the maximum theoretical efficiency for a single junction device under non-concentrated sunlight is ~30% [36]. The maximum power point Pout of a cell is given by the following equation, where Jmp and Vmp represent the current density and voltage at the maximum power point.

By defining the fill factor ff as the ratio (values between 0 and 1) of Pout and the product of the maximum attainable voltage (open circuit conditions) Voc (in V) and current density (short circuit conditions) Jsc (in mA cm–2), the efficiency relationship can be rewritten as follows, which is used to determine the cell performance.



ff =

h=

Pout Voc Jsc

Voc Jsc ff Pin

(4.4)

(4.5)

The Jsc, Voc, and ff values are measured by plotting the current density as the bias voltage is varied while irradiating the PV cell by a calibrated solar simulator (Fig. 4.8). A typical diode current/voltage characteristic is shown in Fig. 4.9. DSC researchers usually report J and V as positive values, but other J/V curve notations are used as well. Another PV parameter, which is routinely employed to determine the quality of a PV device, is the external quantum efficiency (EQE), usually referred to as the incident photon-to-current conversion efficiency (IPCE) by the DSCs community. IPCE(λ) is defined as the number of collected electrons under short circuit conditions per number of incident photons at a given excitation wavelength λ and gives the ability of a cell to generate current as a function of the wavelength of the incident monochromatic light. IPCE is calculated by measuring the short-circuit photocurrent as a function of the monochromatic photon flux.

Introduction

Jsc ( l )

IPCE( l ) =

Pin ( l )

e = hc Jsc ( l ) = 1240 Jsc ( l ) le Pin ( l ) l Pin ( l ) hu

(4.6)

Figure 4.8  Solar simulator under measurement condition (University of Milano-Bicocca, 2013).

IPCE is determined by the sensitizer light harvesting efficiency at λ (LHE), the quantum yield for electron injection from S* to the semiconductor oxide (Φinj), and the charge collection efficiency (ηcoll), the product of the latter two parameters giving the absorbed photon-to-current efficiency (APCE) or internal quantum efficiency.

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IPCE ( l ) = APCE ( l ) LHE ( l )

(4.7)

The integral of IPCE with the AM 1.5 G spectrum gives the photocurrent, which should match that measured under the solar simulator. Therefore, higher IPCE and broader spectra correspond to higher Jsc.

Figure 4.9  Current–voltage characteristics of a DSCs device under light irradiation.

4.1.3  Time-Resolved Techniques The electron transport and mobility, the electron lifetime and concentration in a DSCs under operating conditions can be characterized through the time‐resolved techniques. The charge carrier mobility can be studied by the transient photocurrent (TPC) technique: the device is illuminated, charges are generated, and the transient current is measured in short circuit condition or with an applied voltage. The TPC rise and decay are correlated to the charge carrier mobilities. This technique can also be used as a small-signal technique when, at a given working point, a small light pulse is added. On the other hand, to find the electron lifetime and have information about the recombination mechanisms, the transient photovoltage/open circuit voltage decay (TPV/ OCVD) method is used: a large perturbation is applied, and the system response is recorded in the time domain. The solar cell is illuminated when it is at open circuit to reach a steady-state voltage and then, when the light is switched off, the slope of the decay lead to calculate the electron lifetime and, if the open-circuit voltage decays only by charge recombination, information about the recombination

Introduction

mechanisms. Photocurrent and photovoltage transients are usually taken using a flash lamp, a potentiostat and bias light. Two modulation techniques, using light with modulated intensity, can be also used to evaluate the electron transport and recombination in DSCs: the intensity-modulated photocurrent spectroscopy (IMPS) and the intensity modulated photovoltage spectroscopy (IMVS). In IMPS, the voltage is kept constant and the photogenerated current is measured: a time constant is determined, which depends on electron transport. In IMVS, the DSC is maintained at open circuit conditions and the characteristic time constant for photovoltage rise is measured, containing information about recombination. The electron transport time is calculated based on the capacity of the TiO2 and the substrate/electrolyte interface. The charged density in the TiO2 film under working conditions can be studied by the charge extraction (CE) methods: a reverse extraction voltage is applied after the light is switched off, and the charge in the film is extracted. Under short-circuit conditions, after the light off-switching the integration of the transient photocurrent gives the photogenerated charge density at open-circuit. When the light is off and, at the same time, the connection switches from short-circuit to open-circuit condition, the measurement gives information on the charge-potential decay. In these measurements a potentiostat, an oscilloscope or a fully integrated photoelectrochemical measurement system, often designed specifically for the characterization of DSCs, are employed (Fig. 4.10). These techniques analyse phenomena in second and microsecond time scale.

Figure 4.10  Fully integrated photoelectrochemical measurement equipment for steady-state and transient characterization of solar cells [37].

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Other techniques are applied to reach the nanoseconds range, such as the transient absorption spectroscopy (cartoon of a typical set up in Fig. 4.11a). This method is used to register the absorption spectra, saw at a single delay time, and the kinetic behaviour, recorded at a single wavelength as a function of time, of all the chemical species with a short lifetime. A pulse perturbs the equilibrium of the system, promoting the molecules to an excited state, and the transient species and their evolution are seen spectroscopically.

Figure 4.11  (a) Schematic of a transient absorption experiment and (b) ΔA spectrum contributions: excited state absorption (blue), ground state bleach (red), stimulated emission (green).

The time resolution of the measure is given by the duration of the excitation pulse, which must be intense enough to generate a detectable amount of transient species. The excitation pulse hits the sample with a delay time, τ, with respect to the analysis pulse (Fig. 4.11a), then the difference between the absorption in the excited and ground states is calculated (ΔA). Therefore, τ is changed and ΔA calculated for each delay time, to collect ΔA as a function of τ and of the wavelength λ. Therefore, the transient absorption experiment reveals the changes in the sample absorbance at a given wavelength and time ΔA(λ, τ), after the absorption of the excitation light. ΔA spectrum can contain various contributions, given by (i) the groundstate bleach, recognizable as a negative signal in the wavelength range of ground state absorption (Fig. 4.11b, red); (ii) stimulated emission, which occurs only for the allowed transitions and it is Stokes shifted with respect to the ground-state bleach, resulting in a negative ΔA signal (Fig. 4.11b, green); (iii) excited-state absorption,

Introduction

when optically allowed transitions from the excited states to a higher ones occur, and a positive signal is registered (Fig. 4.11b, blue) [38, 39]. To reach the femtosecond time resolution, pump and probe methods were developed, using for the analysis light a short pulse in the pico- and femtosecond range. A general stretch for pumpprobe experiments is depicted in Fig. 4.12: the laser pulse is split by a semi-transparent mirror in two beams that hit the sample: one, the pump, is earlier reflected and the other, the probe, previously passes through a delay line. The right-angle reflector in the delay line is equipped by a stepping motor, therefore, the travelling distance can be tuned and so the delay of the probe related to the pump pulse [40, 41].

Figure 4.12  Schematic of a pump and probe experiment.

Photoinduced absorption (PIA) spectroscopy, quasi steady-state and time-resolved PIA, is a compelling investigation method to check the dye electrons injection into TiO2 and the dye regeneration. An optical chopper or pulsed light beam of a specific wavelength illuminates the sample, that can be placed in a cryostat for temperature dependent measurements (ranging from 5 to 320 K). In quasi steady-state technique, a continuous wave laser or a LED, modulated by a mechanical chopper, are employed together with a white light. The absorption is registered from reflectivity and transmittance; the photoluminescence can be neglected thanks to the use of a monochromator behind the sample, that avoids the occurrence of false signals. The lock-in amplifier uses the chopper frequency as the reference, leading to an accurate quantification of the absorption change induced by light: these variations can be

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related to long-lived excited species, such as charges or triplets [16, 39].

4.1.4 TiO2-Based DSCs

In the conventional device architecture, the dye acts as photonabsorber and generates electrons that are injected into the TiO2 conduction band; the dye, therefore, makes the titania sensitive to the visible radiation. This peculiarity can be easily and economically tuned. The typical DSC architecture is shown in Fig. 4.13a: The photo-electrode is a nanoporous 10 μm film composed by TiO2 particles of about 20 nm in diameter, stained with a dye. The film is deposited on a transparent conductive oxide (TCO), which covers a glass plate or a plastic substrate. The counter electrode is also a glass coated with a conductive layer, on which typically platinum particles are deposited, acting as a current collector. The electrodes are held together with a thermoplastic polymer gasket, which acts both as a sealant and as a spacer, thus forming a stratified thin film device in which an electrolyte fills the space between the two electrodes.

Figure 4.13  (a) structure and (b) principle operations of a typical DSC.



The principle operations are as follows (Fig. 4.13b):

1. The dye absorbs a photon, and an electron is excited from the ground state to the excited state of the molecule. 2. The generated electrons are injected into the conduction band of the titania. 3. The oxidized dye is regenerated by capturing an electron from the reduced electrolyte.

Introduction



4. The electrons, injected, travel by diffusion in the TiO2 layer until they reach the electrical contacts and, then, enter the external circuit. 5. The electrons return to the cell, through the counter electrode, and reduce the electrolyte. 6. The electrical circuit is completed by transporting the redox couple through the electrolyte.

The main reverse reactions that limit the solar cell performances are (a) non-radiative recombination of the dye excited state; (b) electronic recombination with the oxidized dye; (c) electronic recombination with tri-iodide.

Other detrimental phenomena such as thermalization, trapping and interfacial transfer can be checked only on the femtosecond to picosecond timescales. Therefore, the understanding of the ultrafast carrier dynamics in the DSCs is essential for improving the photovoltaic performance.

4.1.5 NiO-Based DSCs

Lately, the interest in the inverted device architecture is increasing. The operation principles are reversed compared to those for TiO2based DSCs: the photoexcited sensitizer is reduced by hole injection into the NiO valence band (VB), generating a cathodic photocurrent (Fig. 4.14). A photocathode, a passive anode (a Pt-coated conductive glass) and a solution employed as a redox mediator, usually based on iodide ions, compose the p-type DSCs. The p-type semiconductor employed is NiO, which has a band gap of 3.5 eV [42]. The photovoltaic performances are remarkably lower (about 0.4%) than those for n-type TiO2-based DSCs, mainly because of the low Voc (0.1 V) which stems from the small potential difference between the NiO VB level and the redox potential of the electrolyte. Mesoporous NiO films have been prepared to enhance the dye loading, giving some improvement in the light harvesting efficiency. Using a cobalt complex-based electrolyte, a Voc value of 0.35 V has been reported [42]. The PIA spectra of a NiO film sensitized by the coumarine dye C343 [43], were recorded using a white probe light generated by a 250 W

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tungsten-halogen lamp, which was superimposed to a square-wave modulated laser (50 mW nominal power, λ = 488 nm) as excitation; pump intensity of 12 mW cm–2 and modulation frequency of 170 Hz is used. Time-resolved and steady-state photoluminescence measurements are performed using pulsed excitation sources at 515 and 690 nm. The photovoltaic performances of the NiO-based DSC reach a photovoltaic conversion efficiency of about 0.1%, comparable value to those reported in the literature [43–45].

Figure 4.14  Schematic representation of the operational principles of a NiObased DSC.

Figure 4.15  PIA measurements on p-type liquid dye-sensitized solar cell obtained with 488 nm excitation and 230 Hz frequency. (A) In-phase and out of phase PIA signals obtained for coumarine DSC working cell; (B) in phase and out of phase PIA signals obtained for a dummy pDSC cell with no dye. Reprinted and adapted with permission from Vanira Trifiletti, Vittoria Roiati, Silvia Colella, et al. Applied Materials. Copyright (2015) American Chemical Society [45].

Introduction

Figure 4.15A shows a broad absorption of the I2− redox intermediate (750 nm; in-phase channel) and the absorption of NiO+ species after the hole injection that is efficiently happening in the oxide semiconductor (out of phase channel). The electrolyte signal is isolated in a dummy cell (fabricated following the same procedure but avoiding the dye loading) and displayed in Fig. 4.15B [45].

4.1.6  DSCs Tandem Architectures

The DSC technology employing a single light absorber has a theoretical largest efficiency of 31%. To overcome this limit, one workable way is to fabricate tandem solar cells, with a theoretical limit of 43%, by making both electrodes of DSC photoactive. Figure 4.16 shows a schematic energy level diagram of a tandem DSC: illumination comes from the n-type semiconductor side, which works as a conventional DSC, absorbing the high-energy photons.

Figure 4.16  Schematic energy level diagram of a Tandem DSC.

Low-energy photons can pass and are absorbed by the p-type DSC, where the photoexcited dye reduces the redox couple and regenerated by an electron in the valence band of the p-type semiconductor. Therefore, in n-type DSCs an electron is injected from the excited state of the photosensitizer to the CB of an n-type semiconductor, generating an anodic photocurrent, meanwhile in p-type DSCs the photosensitizer is reduced by hole injection into the

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Dye-Sensitized Solar Cells

VB of a p-type semiconductor, generating a cathodic photocurrent. The photoconversion efficiency of a tandem DSCs is expected to be higher than that of the single type DSCs, due to the broadening of the efficient light-harvesting capabilities to a wider spectral range of sunlight. The Voc results to be the difference between the quasi-Fermi level above the valence band of the p-type semiconductor and that one below the conduction band of the n-type semiconductor, being indeed the sum of the Voc of the two devices. The redox potential of the electrolyte is not involved in the tandem Voc, but it must be matched with the energy levels of both photoelectrodes. Since this configuration is comparable to two generators connected in series, the current is limited by the lower performing electrode, therefore the currents must be matched [42].

4.2 TiO2-Based DSCs

The most investigated and highest performing architecture is the n-type configuration. Although several metal oxides have been tested as n-type semiconductors, so far TiO2 has given the best results because of its large surface area and high photoelectrical response as a porous photoelectrode. Grätzel and co-workers have optimized the device fabrication using a colloidal paste of TiO2 nanoparticles which is screen-printed via a multilayer deposition process until the desired film thickness has been reached and sintering at ca. 500°C follows [46]. The active layer of the photoanode is composed, typically, by a 12 µm-thick film of transparent 20 nm TiO2 nanoparticles covered by a 4 µm-thick film of larger (350 nm) particles which scatter photons back into the transparent film; following light absorption, the excited dye injects electrons into the TiO2 [16]. The charge transport through the TiO2 layer happens by diffusion since the electric field through the film is attenuated by the ionic strength of the electrolyte. The use of high ionic electrolytes ensures that electroneutrality is kept through all the lattice, and that the ionic diffusion remains strongly correlated with the electronic transport. The main consequence of electroneutrality is that the electron spread is ambipolar; in this case, the transport is known as “chemical diffusion” or “mutual-diffusion”. The electrons’ motion creates a charge imbalance and the resulting electric field drags

TiO2-Based DSCs

cations, while the negatively charged ions are repelled by electrons. The clear effect is that the drift of the electrons is slowed down, and the diffusion of the ions is increased; it must also be considered the inverse effect for which the motion of electrons is favoured and the ions’ one is delayed. Therefore, the simultaneous and inseparable motion of electrons and ions supplies a single diffusion coefficient [47–50].

4.2.1  Molecular Design of the Photosensitizer

The photosensitizer plays a strategic role in DSCs, absorbing light and promoting the formation of an electron-hole pair, which is then separated, transported, and collected. The HOMO and LUMO energies of the sensitizer must fit the semiconductor CB of the electron semiconductor and level of the redox couple, Nernst potential, to give efficient charge transfer mechanisms [16, 51]. Therefore, the ultimate sensitizer should absorb the whole solar spectrum with maximum efficiency, convert all the collected photons into electrons, inject them quickly in the conduction band of the semiconductor, regenerate immediately and avoid any undesirable recombination process; it should also be stable for a long period and numerous cycles [52]. The 2,2¢-Bipyridyl Ru(II) complexes, such as the benchmark dyes N719 and Z907 are amongst the best performing photosensitizers (Fig. 4.2), the main drawback of such complexes is the presence of NCS (thiocyanate) ligands. Thiocyanate is an ambidentate ligand, i.e., it can attach to the central atom in two places, and a monodentate ligand, which can be easily replaced by other competing ligands yielding fewer active species [53, 54]. Inspired by the process of solar energy collection by photosynthetic cores of bacteria and plants, involving a porphyrinic centre as light harvesting chromophore, porphyrinic structures have been considered as attractive dyes such that some of them have been synthesized and investigated for applications in DSCs. With ruthenium complexes, high performances of up to 8–12% have been achieved [55, 56]. However, the choice of the sensitizer heavily influences not only the device efficiency but also the environmental impact and costs related to the technical production [57]. The excessive cost of ruthenium and its relatively low molar absorption coefficients pose difficulties for

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Dye-Sensitized Solar Cells

straightforward scalability of the technology; it follows that interest in research on a large variety of alternative organic dyes is steadily increasing [16, 58]. Porphyrins are a promising class of sensitizers because of their large absorption coefficients in the visible region of sunlight. Moreover, the possibility of fine-tuning their optical properties by chemical functionalization allowed the design of innovative dyes specifically tailored for DSCs [52, 58–60]. In particular, push-pull porphyrinic dyes characterized by an ethynyl-linked donor group and an ethynyl-carboxyphenyl moiety as acceptor/anchoring group, at the opposite meso positions, present a strong charge-transfer from the push moieties to the pull ones, through the ZnII di-aryl porphyrinate ring acting as a π bridge [61]. This highly directional electron flow from the donor to the acceptor moieties, where the HOMO and the LUMO of the molecule are, respectively, localized, as indicated by theoretical calculations [61], produce a better electron injection into the semiconductor oxide leading to exciting performances of around 9% [60]. Nevertheless, these porphyrinic systems are generally characterized by a strong tendency to form π-π aggregates, which significantly reduce the electron injection efficiency in the TiO2, limiting the photovoltaic performances of the device [16, 52, 58]. Moreover, the Zinc(II) ion coordinated inside of the porphyrinic core can interact with the I3– species in the electrolyte, promoting a faster recombination reaction with injected electron and leading to a detrimental effect on the final device performance [62, 63]. Since the presence of bulky t-butyl groups in meso push-pull ZnII di-aryl porphyrinates is not enough to overcome these inconveniences, the introduction of long alkoxy chains, at the ortho-positions of the two opposite phenyl rings, can fit these problems by an enveloping effect on the porphyrinic ring, thus reducing π–π aggregation and hindering a facile interaction between the ZnII centre and I3– [60, 64, 65]. This approach on a meso push-pull gave an appreciable performance enhancement reaching a value over 10%, with dodecyloxy chains [66]. The same strategy applied to meso di-substituted push-pull ZnII di-aryl porphyrinates with an amino-linked donor group led to exceed, for the first time, the efficiencies of ruthenium dyes in combination with a second organic co-sensitizer and a Co-based redox electrolyte [59]. Nowadays, this latter kind of porphyrins produce performances in

TiO2-Based DSCs

DSC with efficiencies of over 13% in the absence of a second cosensitizer, but still working coupled with a Co-based electrolyte [67, 68]. However, long multistep syntheses are required to obtain such a kind of meso di-substituted push-pull ZnII di-aryl porphyrinates resulting in meager overall yields [59–61, 65, 67, 68]; on the other hand, β-substituted tetra-aryl porphyrins present a straightforward synthesis, which can be scaled up to multi-grams quantities [69–71]. The β-substituted tetra-aryl ZnII-porphyrinates, which involve a tetra-aryl porphyrinic core, are synthetically more attractive since they can be easily obtained by means of a one-pot reaction between pyrrole and the proper aryl aldehyde. Moreover, the attachment to this core of the functionalization carrying a carboxylic group can be obtained by a one-step reaction. Further advantages can be taken into consideration: first, the more sterically hindered architecture of a tetra-aryl porphyrinic core should guarantee a decrease of dye aggregation when adsorbed on TiO2. Furthermore the enhancement of the solubility in the most common organic solvents should facilitate the purification processes to produce very pure porphyrinic dyes [72]. The effects on the electrochemical properties and absorption spectra produced by the presence of diverse groups on the β-ethynylphenyl moiety (Fig. 4.17) was experimentally investigated, and their electronic nature was studied using density functional theory (DFT) and Time-dependent density functional theory (TDDFT) calculations. The HOMO-LUMO energy gap is lower in the case of the meso di-substituted push-pull porphyrinic dyes, not only when compared to β mono-substituted but also to β di-substituted push-pull porphyrinic dyes. This shows that the role of the porphyrinic ring as linker between the push and the pull ethynyl-phenyl substituents is more effective when the push-pull system involves the 5,15 meso positions (dyes 6, 7 and 8) instead of the 2,12 β-pyrrolic positions (dyes 2, 5) of the porphyrinic ring. A more efficient electron transfer by excitation along the push-pull system towards the anchoring carboxylic group for dyes 6, 7 and 8 was confirmed by the higher intensity and the lower energy of their single Q band, originated largely by the HOMO-LUMO transition, when compared to the rather weak Q bands at lower energy of dyes 2 and 5, still mainly originated by the HOMO-LUMO transition (Fig. 4.18). The performance of DSCs

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Dye-Sensitized Solar Cells

based on β di-substituted dyes 2 and 5 was found to be higher than those of DSCs based on β mono-substituted 1 and 4, and of those based on meso di-substituted dyes 6 and 8.

Figure 4.17  β-substituted tetra-aryl ZnII-porphyrinates. Reprinted and adapted with permission from John Wiley and Sons [61].

Figure 4.18  Electronic absorption spectra in THF solution of 1–8 ZnIIporphyrinates.

TiO2-Based DSCs

The higher efficiency of DSCs based on dyes with a push-pull system along the 2,12 β positions of the porphyrinic ring is an unexpected result, since the electron transfer by excitation along the push-pull system is more efficient when it occurs along the 5,15 meso positions, thus suggesting a relevant role of the localization of the anchoring group. Moreover, as a general feature, the presence of a cyano-acrylic group in the pull ethynyl phenyl substituent of all the porphyrinic dyes investigated was found to produce higher DSCs performances, probably due to a more efficient electron injection into the TiO2, since the cyano-acrylic group favours the localization of excited electrons on the pull ethynyl-phenyl substituent carrying the anchoring group. Interestingly, the incident monochromatic photonto-current conversion efficiency (IPCE) spectra of the β mono or disubstituted porphyrinic dyes are more intense, particularly in the presence of a cyano-acrylic group over a broad range of wavelengths (350–650 nm), while those of the meso di-substituted push-pull porphyrinic dyes are less intense and show the limitation of two well separated and less intense peaks, corresponding to the B and the single Q bands, with weak IPCE values in the mid-range (530–610 nm) of only 20–30%. In fact, DSCs based on both dyes 4 and 5 show an IPCE spectrum with a broad and intense coverage of the wavelengths range between 350 and 650 nm, like that of dyes based on ruthenium complexes such as N719. To obtain such a broad and intense coverage with porphyrinic dyes substituted in the meso position, the addition of a co-sensitizer or the introduction of two thienylene-vinylene units to bridge the porphyrinic ring and the anchoring cyano-acrylic group are suggested. In summary, the increased light harvesting properties characterizing the β substitution of the porphyrinic ring cannot be ascribed to more intense B and particularly Q bands, but to a more facile charge injection into the TiO2 surface, consistent with the DFT electron distribution of the β mono-substituted porphyrinic dye 1 interacting with the TiO2 surface. The electrochemical and TDDFT investigations have shown that the positive effect produced by the cyano-acrylic group is due to an increased conjugation of the pull ethynyl-phenyl substituent with the porphyrinic ring. TDDFT calculations show also that such an electron transfer involves, by excitation, not only the Q absorption bands at lower energy but also the B band [52, 72].

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4.2.2  Influence of Photosensitizer Structure on Electron Transfer Time-resolved photophysical studies, photo-electrochemical investigations, and electrochemical impedance spectroscopy (EIS) can be combined with the aim of characterizing the fundamentals of the interfacial charge separation and collection processes on the porphyrin-sensitized TiO2 surface. A qualitative way to evaluate the dye charge injection is to check the emission quenching, comparing the value obtained for the sensitizer absorbed on TiO2 with that one for a semiconductor in which this process is forbidden. ZrO2 is a good counterpart, because it has a lower conduction band edge (ca. −2 V vs. SCE) and a film with properties (transparency and dye loading) comparable to TiO2 can be done. When the charge injection is forbidden, the only deactivation way for the excited state is through both radiative and non-radiative recombination. In Fig. 4.19 the spectral properties of two porphyrinic dyes, one β mono-substituted treat-aryl porphyrinates, ZnB (n.4, Fig. 4.3), and one meso di-substituted push-pull treat-aryl porphyrinates, ZnM (n.8, Fig. 4.3) are compared, showing the typical two bands: the Soret B-bands at around 450 nm and the Q bands in the nearinfrared range. The ZnM push−pull architecture gives an intense Q-band at 670 nm, due to the strong dipole moment, meanwhile the more symmetric ZnB shows the Q-bands in the 530−630 nm range.

Figure 4.19  Absorption spectra of porphyrinic dyes (A) ZnM and (B) ZnB in EtOH/THF solution (solid line), loaded onto TiO2 (dash) and ZrO2 (dot) thin films. Normalized emission spectra in EtOH/THF solution (dash-dot). Reprinted and adapted with permission from Gabriele Di Carlo, Stefano Caramori, Vanira Trifiletti, et al. Applied Materials. Copyright (2014) American Chemical Society [73].

TiO2-Based DSCs

A strong emission quenching happens for both ZnM and ZnB anchored on TiO2, that is related to an effective photoinduced electron transfer (Fig. 4.20).

Figure 4.20  Integrated emission spectra of ZnM and ZnB adsorbed on ZrO2 (squares) and TiO2 (triangles) in the presence of a 0.1 M Li+ in CH3CN solution. Integrals are averaged over a batch of four electrodes per substrate type. Reprinted and adapted with permission from Gabriele Di Carlo, Stefano Caramori, Vanira Trifiletti, et al. Applied Materials. Copyright (2014) American Chemical Society [73].

A dye adsorbed on a semiconductor surface has an emission lifetime given by

t=

1

kr + knr + kinj

,

(4.8)

where kr is the radiative rate, knr is the nonradiative rate, and kinj is the charge injection rate constant. The nonradiative rate constant, knr, includes the Singlet to Triplet intersystem crossing (ISC) and the Internal conversion (IC) vibrational relaxation to the ground state. If TiO2 and ZrO2 have the same surface chemistry and polarity, it can be postulated that knr and kr are identical in the two cases. Therefore, by finding the excited state lifetime on TiO2 and ZrO2, kinj can be calculated through the difference between the emission lifetimes of dyes adsorbed on TiO2, τTiO , and ZrO2, τZrO :

2

kinj =

1

t TiO2

+

1

t ZrO2



2

(4.9)

Emission lifetimes determined by time-correlated single photon counting (TCSPC) are reported in Table 4.1 and the relative emission decays are represented in Fig. 4.19 [73].

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Dye-Sensitized Solar Cells

Table 4.1

dye

τ

Emission lifetime determined by TCSPC upon 460 nm excitationa

(solution)b

ZnM 1.202 ± 0.001

ZnB

1.792 ± 0.006

(ns)

τ (ZrO2)b τ (ZrO2)c τ (TiO2)b τ (TiO2)c (ns) (ns) (ns) (ns) 0.922 ± 0.012

0.991 ± 0.018

1.116 ± 0.022

1.173 ± 0.026

0.965 ± 0.011

1.040 ± 0.011 0.954 ± 0.030

Source: Reprinted and adapted with permission from Gabriele Di Carlo, Stefano Caramori, Vanira Trifiletti, et al. Applied Materials. Copyright (2014) American Chemical Society [73]. aLifetimes are reported with their confidence intervals extracted from the fitting. bDeconvolution (monoexponential function). cTailfit (monoexponential function).

The lifetimes in solution are similar and consistent with singlet emissions. Meanwhile the lifetimes on TiO2 are slightly lower than those on ZrO2. The long-lived emission tail originates from the coupled molecules that are unable to perform electron transfer. The excited state lifetime of the two porphyrinic dyes can be rate combining the stationary quenching with the lifetimes on ZrO2. The integrated emission spectra (Fig. 4.6) corrected for the light harvest efficiency (LHE) at the excitation wavelength (460 nm) are proportional to the emission quantum yields Φem, which are related to the radiative constant and to the lifetime: F em = kr t



(4.10)

Considering that kr has the same value on TiO2 and ZrO2, it follows that



ITiO2

IZrO2

=

F TiO2

F ZrO2

t TiO2 =

ITiO2 IZrO2

=

t TiO2

t ZrO2

t ZrO2 .



(4.11)

(4.12)

Therefore, the excited state lifetimes on TiO2 surface is 95 ps for ZnM and 220 ps for ZnB, the respective injection rate, according to Eq. 4.2, are 9.4 × 109 s−1 (ZnM) and 3.7 × 109 s−1 (ZnB) and the injection quantum yields (Φinj = kinj τTiO2) are 89% for ZnM and of 81% for ZnB.

TiO2-Based DSCs

These high quantum yields, more than 80%, are a consequence of the population of the oxidized dyes generated within the nanosecond laser pulse. When the injection rate constant is calculated by quenching experiments, it is an average value because the dye populations can interact in slightly diverse ways with the surface, conditioning the electron transfer kinetics. Indeed, the electron injection in porphyrin-sensitized photoanodes has an inhomogeneous behaviour because some molecules inject on ultrafast time scales, causing a deactivation of a fraction of dye population also in the sub-nanosecond timescale [74]. On the other hand, the non-anchored molecules give a contribution visible in the residual emission seen on TiO2 in the TCSPC results. These molecules follow the same deactivation pathways to those one registered on the inert ZrO2. Therefore, the average injection rate constant, kinj, on TiO2 is underestimated and the excited state lifetime τTiO2 is overestimated. Thus, the injection quantum yield Φinj (kinj ¥ τTiO2) is a quantitative parameter useful for appreciate the average performance of the sensitizers, in chemical conditions that reproduce those that happen in the working devices.

4.2.3  Photosensitizer Injection and Regeneration Kinetics

The kinetics processes are usually investigated by optical techniques able to detect ultrafast phenomena, where a pulse excites the sample, and after a tunable delay time, another pulse measures its transmission or reflection. Within a laser pulse at 532 nm, the lowest triplet state (T1) is populated and its lifetimes, obtained by monoexponential fitting, are 270 ns for ZnM and 560 ns for ZnB. The intense bleaching of the B band, related to the ground state S0 Æ S2 absorption, is prevalent in the differential transient absorption (TA) spectra (Fig. 4.21), therefore the ΔA results to be negative. TA spectra of ZnM show in the 500−610 nm region the T1 Æ Tn absorption and the bleaching of the Q bands in the 630−710 nm interval. After the bleaching of the B band, ZnB displays a remarkable triplet−triplet absorption in the 490−580 nm range, which is followed by a relative weak absorption up to 850 nm, interrupted by the bleaching of the Q-band.

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Dye-Sensitized Solar Cells

Figure 4.21  TA spectra of (A) ZnM and (B) ZnB in EtOH/THF solution. Spectra recorded at various time delays after the 532 nm laser excitation are represented. Single wavelength (450 and 500 nm) decays fitted with a monoexponential function are depicted as insets. Reprinted and adapted with permission from Gabriele Di Carlo, Stefano Caramori, Vanira Trifiletti, et al. Applied Materials. Copyright (2014) American Chemical Society [73].

The transient difference spectra (laser excitation 532 nm) of ZnB and ZnM adsorbed on ZrO2 are confirm the formation of the triplet state with a lifetime of 350 ns: Figure 4.11A shows an intense absorption band with maximum in the 500−520 nm range, that is correlated to the triplet state of ZnB, followed by the Q bands bleaching. The Soret band bleaching is recognizable after the 440 nm. ZnM (Fig. 4.22C) is, on the other hand, pointed out by the intense Soret bleaching at 450 nm and by an absorption peak at 550 nm, followed by a weak Q-band bleaching, in good agreement with the TA recorded in solution. The transient emission spectra of ZnB and ZnM (Figs. 4.22B,D, respectively) checked at early delays after the laser pulse are in excellent agreement with those registered in solution. TA spectra of porphyrins adsorbed on TiO2 (Fig. 4.20) do not show significant contributions from the triplet state: the charge separated state, i.e., the injected dye cation and electron (dye+/ e−(CB), is formed in prevailing proportions within the laser pulse. Therefore, the predominant deactivation pathway of the excited state is the charge injection, as expected considering the charge injection quantum yields given in Section 4.2.2. Moreover, Fig. 4.23 shows the bleaching of the B band at 450 nm and the deep bleaching of the Q bands at 630−700 nm (ZnM) and at 530−630 nm (ZnB). Without the electrons donated by the electrolyte,

TiO2-Based DSCs

the dye cation takes the required electrons from the semiconductor conduction band ((e−)CB + dye+ Æ dye), and the kinetics is welldescribed by a biexponential function, with an average lifetime [75]:

·t Ò = S i

Ait i2 , Ait i

(4.13)

Figure 4.22  Transient absorption and emission spectra on semi-transparent ZrO2 thin films following 532 nm laser excitation. (A) ZnB. In (A) t0 corresponds to a delay of ca. 20 ns, to eliminate the contribution of the stimulated emission of the chromophore. The Soret bleaching could be only partly explored due to the strong absorption in the Soret region, behaving as an optical wall below 440 nm. (B) Transient emission spectrum of ZnB monitored at early delays after the laser pulse. In (B) t0 corresponds to the peak of the laser pulse. (C) ZnM. Here t0 corresponds to a delay of ca. 20 ns, to eliminate the contribution of the stimulated emission of the chromophore. (D) Transient emission spectrum of ZnM monitored at early delays after the laser pulse. In (D) t0 corresponds to the peak of the laser pulse. Thin films immersed in CH3CN/0.1 M LiClO4. Reprinted and adapted with permission from Gabriele Di Carlo, Stefano Caramori, Vanira Trifiletti, et al. Applied Materials. Copyright (2014) American Chemical Society [73].

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Dye-Sensitized Solar Cells

where A1, A2 are the amplitudes and τ1, τ2 the lifetimes related to the fitting function

y = A1e

-

t t1

+ A2e

-

t t2

+ y0 .

(4.14)

·τÒ is equal to 800 ns for ZnM and 680 ns for ZnB cations, but the dye cations did not regenerate entirely.

Figure 4.23  Time evolution of TA spectra of (A) ZnM and (B) ZnB loaded onto TiO2 thin films in contact with 0.1 M LiClO4 in CH3CN. Reprinted and adapted with permission from Gabriele Di Carlo, Stefano Caramori, Vanira Trifiletti, et al. Applied Materials. Copyright (2014) American Chemical Society [73].

Figure 4.24 represents the regeneration of ZnM (A) and ZnB (C) cations, probed at 680 nm, varying the laser excitation energies in the presence of an electrolyte (0.6 M AMII + 0.1 M LiI): the electron transfer by I− accelerate the dye regeneration, and in 1 μs the 90% is almost complete. The regeneration efficiency, η, can be calculated by the reciprocal of both the average lifetime ·τÒ and by the half-life τ1/2 (at 50% of the initial decay), according to the Eq. 4.8 and 4.9:

1 ·t reg Ò

h=

1 ·t reg Ò

+



1

(4.15)

·t rec Ò

1 ·t 1 Ò

h=

reg

,

2

1 ·t 1 2

reg

Ò

+

1 ·t 1 2

rec

Ò

(4.16)

TiO2-Based DSCs

where ·τregÒ is the average regeneration lifetime calculated by Eq. 4.6, considering the electron donating species (I−) and ·τrecÒ is the average recombination lifetime in an inert solution of LiClO4.

Figure 4.24  Dye cation recovery in the presence of I−. (A) ZnM at 680 nm; (B) ZnM at 800 nm in the presence (red) and in the absence (black) of I−. Excitation energy 7.9 mJ/pulse/cm2. (C) ZnB at 680 nm. In A−C, various laser excitation energies (mJ/pulse/cm2) were employed in order to investigate the dependence of the dye recovery from the excitation energy in the presence of electrolyte containing electron donating I−. Reprinted and adapted with permission from Gabriele Di Carlo, Stefano Caramori, Vanira Trifiletti, et al. Applied Materials. Copyright (2014) American Chemical Society [73].

Under excitation energy of 7.9 cm−2 pulse−1 at 680 nm, corresponding to a laser pumping at 1.3 kV, the regeneration efficiency η calculated for ZnB was 75 ± 1% by using both ·τÒ and half-life τ1/2. ZnM regeneration efficiency, instead, resulted in being 78% using ·τÒ and 85% by τ1/2. Moreover, under excitation energy of 7.9 cm−2 pulse−1 at 800 nm, the ZnM regeneration efficiency is 73% using ·τÒ and 77% by τ1/2, that are lower than those calculated at 680 nm. The incomplete TA recovery at 800 nm is due to the contribution of photoinjected electrons trapped in long-lived surface states (Fig. 4.21B, red line). An increase in excitation pulse energy generates an acceleration in the recombination process, which is balanced by an acceleration of the dye recovery. Nevertheless, in Fig. 4.21A,C it is shown how a triplication in pulse energy has a minor impact on the acceleration of the dye recovery: regeneration process dominates over recombination. Therefore, ZnM and ZnB are very well regenerated by the mediator I−, with regeneration efficiencies of about 80% for ZnM and 74% for ZnB. The performance of devices based on ZnB and ZnM dyes were 6.1%, and 3.9%, respectively. The greater efficiency of ZnB is due to

155

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Dye-Sensitized Solar Cells

a general improvement of all photovoltaic parameters with respect to those of ZnM: Jsc is increased from 8.6 mA cm–2 (ZnM) to 11.7 mA cm–2 (ZnB); Voc is increased by ca. 10% (612 mV for ZnM and 675 mV and for ZnB); the fill factor undergoes a 2.6% enhancement (0.75 for ZnM and 0.77 and for ZnB). Photophysical investigations have given information about the quantum yield of charge injection and the regeneration efficiency, but it is also the kinetics of the electronic recapture by the oxidized electrolyte has a preeminent role in the photosensitizer regeneration. On the other hand, EIS investigations can discriminate differences in kinetics of the electronic recapture by the oxidized electrolyte, and indeed, ZnB-based DSCs show better performances thanks to a better screening of the semiconductor substrate effect [73]. One way to increase the charge injection is to suppress the aggregation by adding a disaggregating agent, like chenodeoxycholic acid (CDCA), to the up-taking solution. Figure 4.25 displays the TCSPC analysis for ZnB absorbed on Al2O3, varying the CDCA amount. Al2O3 is another well-known inert substrate, with an overall surface area comparable to that one provided by TiO2. The beneficial effect of CDCA in preventing dye self-quenching is clearly expressed by the long decay time of the excited state for the dye solution containing 1 mM of CDCA [70].

Figure 4.25  TCSPC analysis on Al2O3 films sensitized with dye 4 (ed. ZnB) [70].

Summary and Outlook

4.3  Summary and Outlook The investigation on more stable, high performing sensitizers started more than 20 years ago and is still going on with interesting results. Starting from the most credited ruthenium complexes that have ruled the top efficiencies in DSCs devices for more than a decade, the researchers devoted their efforts to the investigation of more complex structures. Indeed, the development of different class of organic sensitizers pushed cost/efficiency ratio lower and lower due to the removal of costly rare metal atoms and pushing the efficiency and stability up to a comparable level. Moreover, the multi-components structure of this very peculiar solar cells, allowed the optimization of every single components in the cell making these devices appealing for many different application, especially indoor and disposable photovoltaic, as well as building integration. Also, the investigation of their properties, either chemical or photovoltaic has been improved over the ages leading to a deeper knowledge in the field. Nonetheless, the work to do is still not complete. The DSCs are close to commercialization: the production cost estimates are close to those of other photovoltaic technologies. To make them more competitive, efficiency must be improved, and production costs of traditional structures must be further reduced. On the other hand, developing DSCs in tandem architecture or in solid-state configuration can be the easiest way for DSCs to break into the market. The porphyrinic structures have been synthesized and studied for applications in DSCs, inspired by the process of collecting solar energy by photosynthetic nuclei of bacteria and plants, which involve a porphyrin centre as a chromophore for the collection of light. In the last decade, many efforts have been dedicated to the design of sensitizers with porphyrinic structure, so much that high performance has been achieved, approx. 13%, comparable to conventional ruthenium sensitizers. This optimization was possible thanks to the simplicity with which the values of the HOMO-LUMO levels can be modulated and by the high coefficient of absorption of the porphyrins. Therefore, porphyrin-based sensitizers are the best candidates for high-efficiency DSCs. To break the record of 13% there should be (i) improved light collection capacity in the near IR; (ii) suppressed aggregation on the surface of the working electrode;

157

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Dye-Sensitized Solar Cells

(iii) perfected electronic injection; (iv) designed new redox pairs to increase the VOC; and (v) increased the long-term stability under operating conditions.

References

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4. J. Desilvestro, M. Gratzel, L. Kavan, J. Moser, J. Augustynski, Highly efficient sensitization of titanium-dioxide, J. Am. Chem. Soc. 107(10) (1985) 2988–2990. 5. N. Vlachopoulos, P. Liska, J. Augustynski, M. Gratzel, Very efficient visible-light energy harvesting and conversion by spectral sensitization of high surface-area polycrystalline titanium-dioxide films, J. Am. Chem. Soc. 110(4) (1988) 1216–1220.

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53. A. Abbotto, C. Coluccini, E. Dell’Orto, N. Manfredi, V. Trifiletti, M. M. Salamone, R. Ruffo, M. Acciarri, A. Colombo, C. Dragonetti, S. Ordanini, D. Roberto, A. Valore, Thiocyanate-free cyclometalated ruthenium sensitizers for solar cells based on heteroaromatic-substituted 2-arylpyridines, Dalton Trans. 41(38) (2012) 11731–11738. 54. C. Dragonetti, A. Valore, A. Colombo, D. Roberto, V. Trifiletti, N. Manfredi, M. M. Salamone, R. Ruffo, A. Abbotto, A new thiocyanate-

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57. A. Fakharuddin, R. Jose, T. M. Brown, F. Fabregat-Santiago, J. Bisquert, A perspective on the production of dye-sensitized solar modules, Energy Environ. Sci. 7(12) (2014) 3952–3981. 58. L.-L. Li, E. W.-G. Diau, Porphyrin-sensitized solar cells, Chem. Soc. Rev. 42(1) (2013) 291–304.

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60. Y.-C. Chang, C.-L. Wang, T.-Y. Pan, S.-H. Hong, C.-M. Lan, H.-H. Kuo, C.-F. Lo, H.-Y. Hsu, C.-Y. Lin, E. W.-G. Diau, A strategy to design highly efficient porphyrin sensitizers for dye-sensitized solar cells, Chem. Commun. 47(31) (2011) 8910–8912. 61. A. Orbelli Biroli, F. Tessore, V. Vece, G. Di Carlo, P. R. Mussini, V. Trifiletti, L. De Marco, R. Giannuzzi, M. Manca, M. Pizzotti, Highly improved performance of ZnII tetraarylporphyrinates in DSSCs by the presence of octyloxy chains in the aryl rings, J. Mater. Chem. A 3(6) (2015) 2954– 2959.

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Chapter 5

Fe Complexes as Photosensitizers for Dye-Sensitized Solar Cells

Yizhu Liua and Kenneth Wärnmarkb aInstitute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne Lausanne, CH-1015, Switzerland bCentre for Analysis and Synthesis, Lund University Lund, SE-221 00, Sweden [email protected]

5.1 Introduction Efficient utilization of solar energy is a strategic topic for the longterm well-being of mankind. In this regard, dye-sensitized solar cells (DSSCs) present a low-cost solution thanks to their inexpensive components and simple fabrication [1–3]. Compared to conventional p–n heterojunction solar cells based on semiconductors with energydemanding manufacturing procedures, DSSCs comprise only easily accessible materials such as titanium dioxide (TiO2), molecular chromophores or redox pairs, and so on. After smart design and careful thermodynamic and kinetic engineering, a combination of Emerging Photovoltaic Technologies: Photophysics and Devices Edited by Carlito S. Ponseca Jr. Copyright © 2020 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4800-69-3 (Hardcover), 978-0-429-29525-6 (eBook) www.jennystanford.com

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these components can deliver light-to-electricity energy conversion efficiency of up to more than 20% [4], making DSSCs extremely promising for large-scale application. The functioning of a typical n-type DSSC starts with the light absorption by a photosensitizer (PS), which is covalently anchored on a thin layer of semiconductor such as TiO2. The PS then injects an electron from its excited state (PS*) into the conduction band of TiO2, which transports these photoelectrons to the external circuit. The thus oxidized photosensitizer (PS+) is regenerated by the redox shuttle (RS), the latter exchanging electrons between the PS+/PS pair and the counter electrode where electrons are collected from the external circuit. Figure 5.1 provides a schematic illustration of such a cycle from the energetic point of view. Apparently, the generation of the photocurrent as the first step of this cycle plays a key role in determining the eventual performance of the DSSC device; the effectiveness of photocurrent generation at a specific wavelength can be quantitatively evaluated by the incident photon-to-current conversion efficiency (IPCE). IPCE is the product of four components: [3, 5]

IPCE = fA ¥ finj ¥ freg ¥ fC,

(5.1)

where fA, finj, freg, and fC are the efficiencies of the light absorption, electron injection, PS regeneration, and charge collection, respectively. A typical IPCE spectrum is shown in Fig. 5.2 [6]. Since the short-circuit current (Jsc) of a DSSC is directly proportional to the integration of the IPCE spectrum against the wavelength axis [3], an IPCE curve covering a broad area, for example the Ru-based N3 PS (1) in Fig. 5.2, is thus preferable. This means that an ideal PS should possess high light absorptivity (fA) and efficient excited-electron injection efficiency (finj). Molecular materials have traditionally dominated the field of PSs [7–9], and transition metal complexes (TMCs) have been widely explored in this context [10], thanks to their versatile electronic properties and structural motifs [11]. Prominent examples are the d6 ruthenium(II) complexes [12–17], which are a renowned prototype in photochemical investigations [18] and were employed as the sensitizer in the seminal report of dye-sensitized solar cell (DSSC) [19]. Indeed, such complexes represent the most extensively

Introduction

investigated classes of molecular PSs thanks to (1) their unique metal-to-ligand charge transfer (MLCT) transition which is strongly absorbing in the visible light wavelength range, (2) with the corresponding MLCT excited state (ES) finely tunable in redox potentials in order to match the energetics of the semiconductors, and (3) sufficiently long-lived so that the ES electron injection is kinetically competitive relative to the intrinsic ES decay processes [11]. Indeed, as illustrated in Fig. 5.1, a d6 TMC-based PS can inject its ES electron from the 1/3MLCT manifold to the conduction band of TiO2 semiconductor (kinj). The 1/3MLCT manifold can also decay to the metal-centered (3/5MC) states (kdec) to dissipate instead of making use of the ES energy.

Figure 5.1  Schematic presentation of a DSSC using a d6 transition metal complex photosensitizer (d6 TMC PS). The PS is excited into the 1/3MLCT manifold, which by itself decays through the 3/5MC states to the ground state at a rate of kdec, but also injects the excited-state electrons into the conduction band (C.B.) of the semiconductor at a rate of kinj. The dye cation generated after the electron injection can be recovered through the mediation of the redox shuttle (RS).

From a structural point of view, a MLCT state is geometrically little deviated from the ground state (GS). Therefore, it is conceivable that electron injection from the MLCT state of the sensitizer to the TiO2 conduction band requires minimal reorganization energy,

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Fe Complexes as Photosensitizers for Dye-Sensitized Solar Cells

which is consistent with the usually observed ultrafast injection kinetics given matched energetics [5]. Moreover, the MLCT state is featured with substantial localization of the electron density on the ligand moiety, through which the sensitizer is usually anchored onto the surface of the semiconductor. Therefore, there is favorable electronic coupling between the MLCT state and the conduction band of the semiconductor. The MC state is however contrasting the MLCT state on both aspects. Due to the population of the eg* antibonding orbitals pointing towards the ligand, resulting in a weakening of the metal–ligand bonding, the MC state is usually significantly distorted from the GS. This necessitates much larger reorganization energy of electron injection thereupon. As the name suggests, the electron density in a MC state is mainly localized on the metal center, which leads to little overlap with the conduction band of the semiconductor.

Figure 5.2  IPCE spectra of the prototypical Ru-based N3 PS (1) with its structure shown together. Adapted with permission from J. Am. Chem. Soc. 1993, 115, 6382–6390 [6]. Copyright (1993) American Chemical Society.

Due to the above reasons, the 1/3MLCT manifold rather than the 3/5MC states is almost the elusive injecting state of d6 TMC PSs, and the relative kinetic ratio between kinj and kdec thus determines the efficiency of the ES electron injection from the 1/3MLCT manifold as the very initial photoelectric conversion step:

FeIIL6 Complexes



finj =

kinj kinj + kdec

=

kinj -1 kinj + t MLCT

,

(5.2)

where tMLCT = (kdec)−1 is the intrinsic lifetime of the MLCT manifolds. For RuII complexes, this step is usually highly efficient thanks to its sub-ms MLCT ES lifetime (kdec = (tMLCT)−1 < 106 s−1) versus the subps injection kinetics (kinj > 1012 s−1).5 This is key to the successful application of Ru-based PSs in DSSCs [18]. While the RuII complexes had dominated the sensitizers for DSSCs for a long time, research on inexpensive and abundant alternatives started almost as early [20]. As a lighter congener to Ru, yet more than 7 orders of magnitude more abundant in the Earth’s crust [21], Fe presents the natural alternative. The low-spin (LS) octahedral iron(II) complexes (FeIIL6) share similarly intense MLCT transitions. However, their MLCT manifold is traditionally very short-lived due to the energetically low-lying MC states, and more often a spincrossover behavior is observed for this class of complexes [22, 23]. This has precluded both their practical applications in solar energy conversions and a thorough understanding of their fundamental light-induced photophysics. In this chapter, FeIIL6 complexes relevant to the application as PSs in DSSCs will be reviewed. Section 5.2 will provide an overview of the advances in the understanding of the fundamental photophysics as well as in-device performances of FeIIL6 complexes. Section 5.3 will summarize the recent efforts on improving the photophysical properties of FeIIL6 complexes for photosensitization purposes. A miscellaneous of other Fe complexes in line with solar energy conversion beyond FeIIL6 complexes will be covered Section 5.4. The readers are also referred to recent excellent review articles on the relevant topic [24–29].

5.2 FeIIL6 Complexes

5.2.1 FeII Polypyridyl Complexes The so-called Ru-based N3 PS 1 (Fig. 5.2 and Scheme 5.1) set the energy conversion efficiency of DSSCs to above 10% for the first time [6], making it a benchmark PS that has been extensively studied by the community [5, 17]. However, the most relevant FeII analogue

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Fe Complexes as Photosensitizers for Dye-Sensitized Solar Cells

(2, Scheme 5.1) was reported to only inefficiently sensitize a TiO2 semiconductor [30]. While 1 roots its excellent photophysical properties in RuIIL6 complexes [18], with [Ru(bpy)3]2+ (bpy = 2,2¢-bipyridine, 3, Scheme 5.1) as the representative that has been widely employed in all kinds of photochemical applications, it is naturally necessary to understand that of the closely relevant [Fe(bpy)3]2+ (4, Scheme 5.1). Complex 4 had been reported as early as 1898 by Blau [31]. Some of the early studies on the chemistry of 4 as well as the related FeII complexes of 1,10-phenanthroline (5, Scheme 5.1) and 2,2¢:6¢,2¢¢-terpyridine (6, Scheme 5.1) were summarized in the review by Brandt et al. [32]. The intense absorption of these complexes had soon become the subject of extensive research and further investigations established their electronic structures from spectroscopic [33, 34] and electrochemical measurements [35, 36]. COOH

HOOC

COOH

2+

HOOC N

N

Ru N

NCS

Fe

NCS

N

HOOC

N

Fe N

N

4

N

N

N N

N

3

2

2+

N

Ru

CN

COOH

1

N

N

N

CN

N

HOOC

COOH

N

N

N

2+

N

Fe N

2+

N

N

5

N N

N

N Fe

N

N N

N

6

Scheme 5.1  The chemical structures of the Ru-based N3 PS 1, the seminal FeII PS 2 used in DSSC, the benchmark RuIIL6 complex 3, and the relevant prototypical FeIIL6 complexes 4–6.

As the first study on the ES behavior of FeIIL6 complexes, in 1969, Fink and Ohnesorge reported the absence of photoluminescence of

FeIIL6 Complexes

complexes 4–6 even in liquid nitrogen (80 K) [37]. This was in great contrast to the equally ligated RuII complexes, and was attributed to the smaller ligand field strength of the analogous Fe complexes resulting in their low-lying high-spin (HS) MC ESs, leading to ready deactivation of the charge-transfer ESs. This suggests that the photochemically interesting MLCT state of FeIIL6 complexes, as discussed in Section 5.1, is unusually short-lived, which may pose stringent time limit for efficient electron injection. In 1976, Kirk et al. examined the ES behavior of 4 using picosecond transient absorption spectroscopy [38]. It was found that the groundstate bleach (GSB) appeared instantaneously upon excitation into the lowest-energy MLCT band, and the corresponding ES with a lifetime of 830 ps was essentially not absorbing in the whole visible region, in addition to the absence of photoluminescence as reported by Fink and Ohnesorge for this class of complexes. A similar result was obtained later by Street et al. for complex 5 with an ES lifetime of 710 ps, but discussions on the multiplicity of the HS MC states had still been limited to triplet [39]. Soon afterwards, Creutz et al. confirmed the experimental results reported earlier by Kirk and Street for complexes 4–6 [40]. They systematically studied a series of polypyridyl TMCs down the group 8 triads in the periodic table, and found that an opposite trend of ligand dependence of the ES lifetimes was observed for FeII than for RuII and OsII, with the usually considered strongerfield ligands (1,10-phenanthroline and its derivatives) resulting in shorter lifetimes for FeII complexes. Moreover, they reported the excited-state absorption (ESA) spectrum of 4, which significantly differs from that of both its RuII and OsII analogues as well as the radical anion of the 2,2¢-bipyridine ligand itself (Fig. 5.3), leading to the exclusion of a MLCT nature of the sub-ns lowest-energy ES. Combining the spectroscopic measurements and electron-transfer quenching experiments, the ES was postulated to be a MC state lying no higher than 0.9 eV above the GS, and the MC state was proposed for the first time to be of electronic quintet multiplicity. McCusker and co-workers studied the ES evolution of tethered FeIIL6 complexes 7–9 (Scheme 5.2) as unambiguous benchmarks of lowest-energy 5T2 MC states [41]. By comparing the qualitatively similar behaviors between 4–6 and 7–9, it was concluded that the 5T state is the lowest energy states for 4–6. Furthermore, using a 2

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Fe Complexes as Photosensitizers for Dye-Sensitized Solar Cells

sub-ps laser pulse, they were able to set the upper limit of the lifetime of the MLCT manifolds (or the population of the 5T2 MC state) to be