Organic Photovoltaics E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 1151

Veröffentlichungsjahr: 2014

Beschreibung

The versatility of organic photovoltaics is already well known and this completely revised, updated, and enlarged edition of a classic provides an up-to-date overview of this hot topic.

The proven structure of the successful first edition, divided into the three key aspects of successful device design: materials, device physics, and manufacturing technologies, has been retained. Important aspects such as printing technologies, substrates, and electrode systems are covered. The result is a balanced, comprehensive text on the fundamentals as well as the latest results in the area that will set R&D trends for years to come.

With its combination of both academic and commercial technological views, this is an optimal source of information for scientists, engineers, and graduate students already actively working in this field, and looking for comprehensive summaries on specific topics.

Leseprobe

CONTENTS

Cover

Related Titles

Title Page

Copyright

List of Contributors

Part One: Materials for Thin Film Organic Photovoltaics

Chapter 1: Overview of Polymer and Copolymer Materials for Organic Photovoltaics

1.1 Introduction

1.2 Early Efforts

1.3 Toward Devices with 5% Efficiencies

1.4 Novel Thiophene-Containing Polymers

1.5 Fluorene-Containing Molecules

1.6 Carbazole-Based Copolymers

1.7 New Heterocyclic Polymers

1.8 Polymers Based on Other Types of Building Blocks

1.9 Conclusions

References

Chapter 2: Thiophene-Based High-Performance Donor Polymers for Organic Solar Cells

2.1 Introduction

2.2 Bandgap Engineering

2.3 Charge Generation in Bulk Heterojunction Organic Solar Cells

2.4 Polyalkylthiophenes

2.5 Polyalkylthiophene/PCBM Blends

2.6 Polythiophene Copolymers

2.7 Side Chain Functionalized P3AT Derivatives

2.8 Third-Generation Polythiophenes

2.9 Thiophene-Based Push–Pull Copolymers

2.10 Benzo[1,2-b:4,5-b′]dithiophene-Based Polymers

2.11 Cyclopenta[2,1-b:3,4-b′]dithiophene-Based Polymers

2.12 Indacenodithiophene-Based Polymers

2.13 Conclusion and Outlook

References

Chapter 3: Molecular Design of Conjugated Polymers for High-Efficiency Solar Cells

3.1 Introduction

3.2 Structural Features of Conjugated Polymers

3.3 “D–A” Approach

3.4 Quinoid Approach

3.5 Summary and Outlook

References

Chapter 4: Solution-Processed Molecular Bulk Heterojunction Solar Cells

4.1 Introduction

4.2 Monochromophoric Molecules

4.3 Multichromophoric Molecules

4.4 Summary and Future Directions

References

Chapter 5: Vacuum-Processed Donor Materials for Organic Photovoltaics

5.1 Introduction

5.2 Planar and Bulk Heterojunction Solar Cells

5.3 Summary and Future Prospects

Acknowledgments

References

Chapter 6: Polymer–Nanocrystal Hybrid Solar Cells

6.1 Introduction

6.2 Semiconductor Nanocrystals

6.3 Working Principles and Device Structure

6.4 Evolution of Polymer–NC Hybrid Solar Cells

6.5 Recent Approaches for Overcoming Current Limitations

6.6 Novel Concepts and Perspectives

6.7 Summary and Outlook

Acknowledgments

References

Chapter 7: Fullerene-Based Acceptor Materials

7.1 Introduction and Overview

7.2 Fullerenes as n-Type Semiconductors

7.3 Fullerene Derivatives

7.4 Derivatives of C70 and C84

7.5 Fullerene Bisadducts

7.6 Endohedral Compounds

7.7 Commercialization of Fullerene Derivatives

References

Chapter 8: Polymeric Acceptor Semiconductors for Organic Solar Cells

8.1 Introduction

8.2 Basics Principles and Operation for Organic Solar Cells

8.3 Polymeric Acceptor Semiconductors

8.4 Conclusions and Perspective

References

Chapter 9: Water/Alcohol-Soluble Conjugated Polymer-Based Interlayers for Polymer Solar Cells

9.1 Introduction

9.2 The Development of Water/Alcohol-Soluble Conjugated Polymers as Interlayer Materials

9.3 Interface Engineering for Polymer Solar Cells

9.4 Discussion of the Working Mechanism

9.5 Summary

References

Chapter 10: Metal Oxide Interlayers for Polymer Solar Cells

10.1 Introduction

10.2 Conventional Structure

10.3 Inverted Structure

10.4 Tandem Structure

10.5 Additional Oxides (Cr2O3, CuOx, PbO)

10.6 Conclusions

References

Part Two: Device Physics of Thin Film Organic Photovoltaics

Chapter 11: Bimolecular and Trap-Assisted Recombination in Organic Bulk Heterojunction Solar Cells

11.1 Introduction

11.2 Recombination at Open Circuit

11.3 Trap-Assisted Recombination at Open Circuit

11.4 Investigation of the Nature Recombination by Electroluminescence Measurements

11.5 Bimolecular Recombination Strength in Organic BHJ Solar Cells

11.6 Bimolecular Recombination Losses Under Short-Circuit Conditions

11.7 Effect of Bimolecular Recombination on Fill Factor and Efficiency

11.8 Conclusions

References

Chapter 12: Organic Photovoltaic Morphology

12.1 Introduction

12.2 Order in Bulk Heterojunctions

12.3 Nanoscale Morphology in Bulk Heterojunctions

12.4 Phases in a Bulk Heterojunction

12.5 Structure of the Interface between Phases

12.6 In Situ Measurements of Morphology Development

References

Chapter 13: Intercalation in Polymer:Fullerene Blends

13.1 Introduction

13.2 Methods for Detecting Molecular Mixing

13.3 Factors Affecting Molecular Mixing

13.4 The Effect of Molecular Mixing on Electronic Properties and Solar Cells

13.5 Conclusions

References

Chapter 14: Organic Tandem Solar Cells

14.1 Introduction and Working Principle

14.2 Measurement Techniques

14.3 Efficient Intermediate Charge Carrier Recombination

14.4 Light Management

14.5 Choice of Materials

14.6 Parallel Tandem Architectures

14.7 New Tandem Solar Cell Concepts

14.8 Conclusions

Acknowledgments

References

Chapter 15: Solid-State Dye-Sensitized Solar Cells

15.1 Introduction

15.2 Working Principles of Solid-State Dye-Sensitized Solar Cells

15.3 Loss Mechanisms in Solid-State Dye-Sensitized Solar Cells

15.4 Solid-State Dye-Sensitized Solar Cells with Spiro-OMeTAD as Hole Conductor

15.5 Hybrid Solar Cells with Absorbing Hole Conductors

15.6 Ordered Nanostructures for Solid-State Dye-Sensitized Solar Cells

15.7 Summary and Outlook

References

Part Three: Technology for Thin Film Organic PV

Chapter 16: Reel-to-Reel Processing of Highly Conductive Metal Oxides

16.1 Introduction

16.2 Materials

16.3 Deposition Technology

16.4 Equipment

16.5 Alternative Approaches

References

Chapter 17: Flexible Substrate Requirements for Organic Photovoltaics

17.1 Introduction

17.2 Polyester Substrates

17.3 Properties of Base Substrates

17.4 Concluding Remarks

Acknowledgments

References

Chapter 18: Adhesives for Organic Photovoltaic Packaging

18.1 Introduction

18.2 Encapsulation Process for Organic Photovoltaics

18.3 Chemistry Aspects of Barrier Adhesives

18.4 Barrier Performance of OPV Adhesives

18.5 Conclusions

References

Chapter 19: Roll-to-Roll Processing of Polymer Solar Cells

19.1 Introduction

19.2 The Roll-to-Roll Process

19.3 Structure of Modules

19.4 Coating and Printing Techniques for PSC Materials

19.5 Roll-to-Roll Printing of Electrodes

19.6 R2R Encapsulation

19.7 Roll-to-Roll Characterization

19.8 Future and Outlook

References

Chapter 20: Current and Future Directions in Organic Photovoltaics

20.1 Scientific and Technological Aspects

20.2 Commercial Applications

20.3 Challenges and Major Hurdles

Acknowledgments

References

Index

End User License Agreement

List of Tables

Table 1.1

Table 1.2

Table 3.1

Table 3.2

Table 3.3

Table 3.4

Table 3.5

Table 3.6

Table 3.7

Table 3.8

Table 4.1

Table 4.2

Table 4.3

Table 4.4

Table 4.5

Table 4.6

Table 4.7

Table 4.8

Table 4.9

Table 5.1

Table 5.2

Table 5.3

Table 6.1

Table 6.2

Table 6.3

Table 6.4

Table 7.1

Table 7.2

Table 9.1

Table 10.1

Table 10.2

Table 16.1

Table 16.2

Table 17.1

Table 17.2

Table 17.3

Table 19.1

Table 20.1

Table 20.2

List of Illustrations

Figure 1.1

Figure 1.2

Figure 1.3

Figure 1.4

Figure 1.5

Figure 2.1

Figure 2.2

Figure 2.3

Scheme 2.1

Scheme 2.2

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.11

Figure 2.12

Scheme 2.3

Scheme 2.4

Figure 2.13

Figure 2.14

Scheme 2.5

Figure 2.15

Scheme 2.6

Figure 2.16

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

Figure 3.10

Figure 3.11

Figure 3.12

Figure 3.13

Figure 3.14

Figure 3.15

Figure 3.16

Figure 3.17

Figure 3.18

Figure 3.19

Figure 3.20

Figure 3.21

Figure 3.22

Figure 3.23

Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Figure 4.8

Figure 4.9

Figure 4.10

Figure 4.11

Figure 4.12

Figure 4.13

Figure 4.14

Figure 4.15

Figure 5.1

Figure 5.2

Figure 5.3

Figure 6.1

Figure 6.2

Figure 6.3

Figure 6.4

Figure 6.5

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9

Figure 6.10

Figure 6.11

Figure 6.12

Figure 6.13

Figure 6.14

Figure 6.15

Figure 6.16

Figure 6.17

Figure 6.18

Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 8.1

Figure 8.2

Figure 8.3

Figure 8.4

Figure 8.5

Figure 8.6

Figure 8.7

Figure 8.8

Figure 8.9

Figure 8.10

Figure 8.11

Figure 8.12

Figure 8.13

Figure 8.14

Figure 8.15

Figure 8.16

Figure 8.17

Figure 8.18

Figure 8.19

Figure 8.20

Figure 8.21

Figure 8.22

Figure 8.23

Figure 8.24

Figure 8.25

Figure 8.26

Figure 8.27

Figure 8.28

Figure 8.29

Figure 8.30

Figure 8.31

Figure 8.32

Figure 8.33

Figure 8.34

Figure 8.35

Figure 9.1

Scheme 9.1

Scheme 9.2

Figure 9.2

Figure 9.3

Figure 10.1

Figure 10.2

Figure 10.3

Figure 10.4

Figure 10.5

Figure 10.6

Figure 10.7

Figure 10.8

Figure 10.9

Figure 10.10

Figure 10.11

Figure 10.12

Figure 10.13

Figure 10.14

Figure 11.1

Figure 11.2

Figure 11.3

Figure 11.4

Figure 11.5

Figure 11.6

Figure 11.7

Figure 11.8

Figure 11.9

Figure 11.10

Figure 11.11

Figure 11.12

Figure 11.13

Figure 11.14

Figure 11.15

Figure 11.16

Figure 12.1

Figure 12.2

Figure 12.3

Figure 12.4

Figure 12.5

Figure 12.6

Figure 12.7

Figure 12.8

Figure 12.9

Figure 12.10

Figure 12.11

Figure 12.12

Figure 12.13

Figure 12.14

Figure 12.15

Figure 12.16

Figure 12.17

Figure 12.18

Figure 12.19

Figure 13.1

Figure 13.2

Figure 13.3

Figure 13.4

Figure 13.5

Figure 13.6

Figure 13.7

Figure 13.8

Figure 13.9

Figure 13.10

Figure 13.11

Figure 14.1

Figure 14.2

Figure 14.3

Figure 14.4

Figure 14.5

Figure 14.6

Figure 15.1

Figure 15.2

Figure 15.3

Figure 15.4

Figure 15.5

Figure 15.6

Figure 15.7

Figure 15.8

Figure 15.9

Figure 15.10

Figure 15.11

Figure 15.12

Figure 15.13

Figure 16.1

Figure 16.2

Figure 16.3

Figure 16.4

Figure 16.5

Figure 16.6

Figure 16.7

Figure 16.8

Figure 16.9

Figure 17.1

Figure 17.2

Figure 17.3

Figure 17.4

Figure 17.5

Figure 17.6

Figure 17.7

Figure 17.8

Figure 17.9

Figure 17.10

Figure 17.11

Figure 17.12

Figure 17.13

Figure 17.14

Figure 17.15

Figure 17.16

Figure 17.17

Figure 17.18

Figure 18.1

Figure 18.2

Figure 18.3

Figure 18.4

Figure 18.5

Figure 18.6

Figure 18.7

Figure 18.8

Figure 18.9

Figure 18.10

Figure 18.11

Figure 18.12

Figure 18.13

Figure 18.14

Figure 19.1

Figure 19.2

Figure 19.3

Figure 19.4

Figure 19.5

Figure 19.6

Figure 19.7

Figure 19.8

Figure 19.9

Figure 19.10

Figure 19.11

Figure 19.12

Figure 19.13

Figure 20.1

Figure 20.2

Figure 20.3

Guide

Cover

Table of Contents

List of Contributors

Part 1

Chapter 1

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Edited by

Christoph Brabec

Ullrich Scherf

Vladimir Dyakonov

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Cover Design Grafik-Design Schulz, Fußgönheim, Germany

List of Contributors

Dechan Angmo

Technical University of Denmark

Department of Energy Conversion and Storage

Frederiksborgvej 399

4000 Roskilde

Denmark

Raja Shahid Ashraf

Imperial College London

Department of Chemistry and

Centre for Plastic Electronics

South Kensington Campus

London SW7 2AZ

UK

Daniel Bahro

Karlsruhe Institute of Technology (KIT)

Light Technology Institute

Engesserstrasse 13

76131 Karlsruhe

Germany

Peter Bäuerle

University of Ulm

Institute of Organic Chemistry II and Advanced Materials

Albert-Einstein-Allee 11

89081 Ulm

Germany

Paul W.M. Blom

University of Groningen

Zernike Institute for

Advanced Materials

Molecular Electronics

Nijenborgh 4

9747 AG Groningen

The Netherlands

and

Max-Planck Institute for

Polymer Research

Ackermannweg 10

55128 Mainz

Germany

Felicia A. Bokel

National Institute of Standards and Technology

Organic Electronics & Photovoltaics Polymer Division

Electronics Materials Group

100 Bureau Drive

Gaithersburg, MD 20899

USA

Yong Cao

South China University of Technology

Institute of Polymer Optoelectronic Materials and Devices

State Key Laboratory of Luminescent Materials and Devices

No. 381, Wushan Road

Tianhe district

Guangzhou 510640

China

Stelios A. Choulis

Cyprus University of Technology

Department of Mechanical Engineering and Materials Science and Engineering

Molecular Electronics and Photonics Research Unit

45 Kitiou Kyprianou Str.

3603 Limassol

Cyprus

Brian A. Collins

National Institute of Standards and Technology

Organic Electronics & Photovoltaics Polymer Division

Electronics Materials Group

100 Bureau Drive

Gaithersburg, MD 20899

USA

Alexander Colsmann

Karlsruhe Institute of Technology (KIT)

Light Technology Institute

Engesserstrasse 13

76131 Karlsruhe

Germany

Dean M. DeLongchamp

National Institute of Standards and Technology

Organic Electronics & Photovoltaics Polymer Division

Electronics Materials Group

100 Bureau Drive

Gaithersburg, MD 20899

USA

Michael Eck

University of Freiburg

Freiburg Materials Research Centre (FMF)

Laboratory for Nanosciences

Stefan-Meier-Straße 21

79104 Freiburg

Germany

Solon P. Economopoulos

Cyprus University of Technology

Department of Mechanical Engineering and Materials Science and Engineering

Molecular Electronics and Photonics Research Unit

45 Kitiou Kyprianou Str.

3603 Limassol

Cyprus

Antonio Facchetti

Northwestern University

Department of Chemistry and the Materials Research Center

2145 Sheridan Road

Evanston, IL 60208-3113

USA

Matthias Fahland

Department Coating of Flexible Products

Fraunhofer FEP

Winterbergstrasse 28

01277 Dresden

Germany

Konstantin Glaser

Karlsruhe Institute of Technology (KIT)

Light Technology Institute

Engesserstrasse 13

76131 Karlsruhe

Germany

Jens Hauch

Entwicklungszentrum für Polytronics

Energie Campus Nürnberg

Tramstrasse 99

4132 Muttenz

Switzerland

Eric T. Hoke

Stanford University

Department of Applied Physics

476 Lomita Mall

Stanford, CA 94305-4045

USA

Markus Hösel

Technical University of Denmark

Department of Energy Conversion and Storage

Frederiksborgvej 399

4000 Roskilde

Denmark

Fei Huang

South China University of Technology

Institute of Polymer Optoelectronic Materials and Devices

State Key Laboratory of Luminescent Materials and Devices

No. 381, Wushan Road

Tianhe district

Guangzhou 510640

China

Jan C. Hummelen

University of Groningen

Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials

Nijenborgh 4

9747 AG Groningen

The Netherlands

Grigorios Itskos

University of Cyprus

Department of Physics

Experimental Condensed Matter Physics Laboratory

1678 Nicosia

Cyprus

Alex K.-Y. Jen

University of Washington

Department of Chemistry

302 Roberts Hall

Seattle, WA 98195-2120

USA

and

University of Washington

Department of Materials Science & Engineering

302 Roberts Hall

Seattle, WA 98195-2120

USA

L. Jan Anton Koster

University of Groningen

Zernike Institute for

Advanced Materials

Molecular Electronics

Nijenborgh 4

9747 AG Groningen

The Netherlands

Panayiotis A. Koutentis

University of Cyprus

Department of Chemistry

1678 Nicosia

Cyprus

Frederik C. Krebs

Technical University of Denmark

Department of Energy Conversion and Storage

Frederiksborgvej 399

4000 Roskilde

Denmark

Kilian Kreul

DELO Industrie Klebstoffe

Engineering Department/Process Management OPV/CCM/Display

DELO-Allee 1

86949 Windach

Germany

Michael Krueger

University of Freiburg

Freiburg Materials Research Centre (FMF)

Laboratory for Nanosciences

Stefan-Meier-Straße 21

79104 Freiburg

Germany

Jianhua Liu

University of California

Department of Chemistry & Biochemistry and Center for Polymers and Organic Solids

Santa Barbara, CA 93106-9510

USA

William A. MacDonald

DuPont Teijin Films (UK) Limited

The Wilton Centre

Redcar TS10 4RF

UK

Julian M. Mace

DuPont Teijin Films (UK) Limited

The Wilton Centre

Redcar TS10 4RF

UK

Iain McCulloch

Imperial College London

Department of Chemistry and Centre for Plastic Electronics

South Kensington Campus

London SW7 2AZ

UK

Michael D. McGehee

Stanford University

Department of Materials Science & Engineering

476 Lomita Mall

Stanford, CA 94305-4045

USA

Jan Mescher

Karlsruhe Institute of Technology (KIT)

Light Technology Institute

Engesserstrasse 13

76131 Karlsruhe

Germany

Nichole Cates Miller

Stanford University

Department of Materials Science & Engineering

476 Lomita Mall

Stanford, CA 94305-4045

USA

Amaresh Mishra

University of Ulm

Institute of Organic Chemistry II and Advanced Materials

Albert-Einstein-Allee 11

89081 Ulm

Germany

Thuc-Quyen Nguyen

University of California

Center for Polymers and Organic Solids (CPOS)

Santa Barbara, CA 93106-9510

USA

Giovanni Nisato

Business and technology development senior manager

CSEM SA

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4132 Muttenz

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Kevin M. O'Malley

University of Washington

Department of Chemistry

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USA

Andreas Pütz

Karlsruhe Institute of Technology (KIT)

Light Technology Institute

Engesserstrasse 13

76131 Karlsruhe

Germany

Markus Rojahn

DELO Industrie Klebstoffe

Department of Chemistry/R&D and Analytical Chemistry

DELO-Allee 1

86949 Windach

Germany

Marion Schmidt

DELO Industrie Klebstoffe

Engineering Department/Training Management

DELO-Allee 1

86949 Windach

Germany

Lukas Schmidt-Mende

University of Konstanz

Department of Physics

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78457 Konstanz

Germany

Bob C. Schroeder

Imperial College London

Department of Chemistry and Centre for Plastic Electronics

South Kensington Campus

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Alexander B. Sieval

Solenne BV

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9747 AN Groningen

The Netherlands

Andrew C. Stuart

University of North Carolina at Chapel Hill

Department of Chemistry

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USA

Bright Walker

University of California

Department of Chemistry & Biochemistry and Center for Polymers and Organic Solids

Santa Barbara, CA 93106-9510

USA

Jonas Weickert

University of Konstanz

Department of Physics

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Germany

Gert-Jan A.H. Wetzelaer

University of Groningen

Zernike Institute for Advanced Materials

Molecular Electronics

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Hongbin Wu

South China University of Technology

Institute of Polymer Optoelectronic Materials and Devices

State Key Laboratory of Luminescent Materials and Devices

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Tianhe district

Guangzhou 510640

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Liqiang Yang

University of North Carolina at Chapel Hill

Curriculum in Applied Sciences and Engineering

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USA

Hin-Lap Yip

University of Washington

Department of Materials Science & Engineering

302 Roberts Hall

Seattle, WA 98195-2120

USA

Wei You

University of North Carolina at Chapel Hill

Curriculum in Applied Sciences and Engineering

125 South Road

Chapel Hill, NC 27599-3287

USA

and

University of North Carolina at Chapel Hill

Department of Chemistry

125 South Road

Chapel Hill, NC 27599-3290

USA

Chengmei Zhong

South China University of Technology

Institute of Polymer Optoelectronic Materials and Devices

State Key Laboratory of Luminescent Materials and Devices

No. 381, Wushan Road

Tianhe district

Guangzhou 510640

China

Huaxing Zhou

University of North Carolina at Chapel Hill

Department of Chemistry

125 South Road

Chapel Hill, NC 27599-3290

USA

Part OneMaterials for Thin Film Organic Photovoltaics

1Overview of Polymer and Copolymer Materials for Organic Photovoltaics

Solon P. Economopoulos, Grigorios Itskos, Panayiotis A. Koutentis, and Stelios A. Choulis

1.1 Introduction

Predictions of limited fossil fuels and issues associated with their environmental impact have led to a rapid growth of research on photovoltaics (PVs). Until recently, the majority of PVs were silicon-based conventional p–n junction devices; however, the dominance of these solar cells is being challenged by the emergence of third-generation PV technologies based on new materials and device approaches. Among these, are PV technologies based on solution processing methods that enable the low-cost fabrication of solar cell devices. These processes allow the incorporation of different semiconductor materials into single devices that are not necessarily lattice matched. Organic semiconductors are of particular interest as PV materials owing to their unique combination of properties: ease of fabrication, flexibility, tunability, lightweight, and the possibility of large surface coverage [1]. Organic photovoltaics (OPVs) refer to solar cells that contain at least one organic semiconductor in the cell active region [1]; as such, the term includes both all-organic and hybrid PV material approaches. Various OPV approaches that include combinations of organics such as conjugated polymers, fullerenes, small molecules, dyes and inorganics such as porous semiconductors, oxides, and colloidal nanocrystals have been successfully used [1]. In light of the above, OPVs based on light absorbers deposited by solution-processed techniques, in contrast to more involved processing of materials requiring vacuum- or vapor-phase deposition, are of particular interest.

Of the various OPVs, polymer–fullerene solar cells represent a unique category that has seen remarkable progress during the last 15 years, overcoming several key obstacles toward the anticipated OPV milestone efficiency of 10% (Figure 1.1). This chapter will focus on conjugated polymers used for polymer–fullerene bulk heterojunction OPVs with occasional references to all-polymer OPVs. In the first part, key developments over the past 15 years on the characteristics and understanding of such devices will be presented. This will be followed by a review of the effort to improve the performance of such solar cells via optimization of the material donor (polymer) part of the device, guided by progress in the material design, and improvements on the physical and chemical properties of the conjugated polymers used. The most prominent state-of-the-art conjugated polymer families used in solar cells will be summarized, and their future potential will be discussed. Breakthrough ideas that contributed to the understanding of polymer physical chemistry in devices will be highlighted along with insights that can guide future efforts.

Figure 1.1 Timeline of power conversion efficiencies in OPVs since 1992 to the current best efficiency demonstrated by Konarka Technologies.

1.2 Early Efforts

The first major development in the field came from the experimental observation that conjugated polymer photoexcited excitons efficiently dissociate from the conjugated polymer to the fullerene interfaces via ultrafast electron transfer processes [2]. This was directly exploited in organic solar cell devices using blends of the two material families (electron donor and electron acceptor) with length scale of heterojunctions within the blend approximately equal to the exciton diffusion length. The proposed structure resulted in the invention of the bulk heterojunction (BHJ) architecture in 1995 [3]. Prior to this, the low conversion yield of photoexcited Frenkel excitons into mobile carriers heavily limited the efficiencies of polymer–fullerene bilayer OPVs structures; this was attributable to the large binding energies of the former. Yu et al. [3] introduced the notion of a bulk mixture of the polymer donor and the fullerene acceptor with phase-separated domain regions of the order of the exciton diffusion length to allow efficient interfacial polymer exciton dissociation. Bulk heterojunction device structures allowed the implementation of thick active layers for efficient light harvesting without compromising the efficiency of the charge separation process and are still the basis for today's best performing organic solar cells. Our review focuses on conjugated polymers used in this type of PV geometry. The success of BHJs was partly attributable to the early use of the highly soluble fullerene phenyl-C61-butyric acid methyl ester ([60]PCBM) [4], which has good electron transport properties and is still the acceptor material of choice in OPVs. Recently, C70 [5] and C84 [6] fullerene adducts have been introduced and may offer additional advantages. In the same publication introducing the BHJ architecture, Yu et al. also investigated the effect of blend morphology on device performance by exploring parameters such as fullerene content, solvent type, and electrode material. Six years later in 2001, Shaheen et al. [7] convincingly demonstrated that the nanostructure morphology of the polymer–fullerene blend profoundly affected the PV device performance: The careful selection of solvent gave a better polymer–fullerene blend with smaller phase-separated fullerene domains. In addition, it was later shown that the addition of PCBM to the blend improved the hole mobility in the polymer due to enhanced intermolecular interactions of the polymer chains induced by the fullerene molecules [8]. The heterojunction morphology and its effect on exciton dissociation, charge recombination, and transport in polymer–fullerene BHJ solar cells is still an intense topic of research in polymer–fullerene OPVs [9,10].

1.3 Toward Devices with 5% Efficiencies

Polymer solar cells [2] and polymer light-emitting diodes (PLEDs) [11] were invented in the early 1990s alongside the precursor to polymer field-effect transistors (PFETs) [12]. As such, these branches of polymer electronics shared common grounds and materials that when found to work well in one area would be tried and tested in another. For example, the PPV analog poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV), which was used as a proprietary material in the first PLEDs, was also used in early polymer PVs. MEH-PPV and the similar poly[2-methoxy-5-(3,7-dimethyloctyl-oxy)-p-phenylenevinylene] (MDMO-PPV) (Figure 1.1) dominated polymer-based solar cells for most of the 1990s. A large part of the aforementioned progress in understanding the basic physics underlying polymer fullerene solar cells was made using MDMO-PPV (Figure 1.2). Power conversion efficiencies at the start of the century were standing at the level of ∼1%. Optimization of the blend morphology introduced by Shaheen et al. [7] led to a threefold increase. By that time, polymer groups worldwide were able to synthesize polymers more efficiently, and the factor of purity came into play in device performance. A research team at Linz Institute for Organic Solar Cells (LIOS) showed that high-purity samples were needed to reach higher efficiencies, pushing PPV-based devices to 3% efficiencies [13]. It is worth noting that another PPV analog synthesized in Cambridge [14] was targeted as a fullerene substitute in creating all-polymer solar cells, but efficiencies failed to compete with the fullerene counterparts.

Figure 1.2 Illustration of prominent polymers used in solar cell devices with power conversion efficiencies up to 5%.

Optimization of PPV-based polymers brought OPVs based on this polymer family to their performance limits (∼3%). Limitations included the relative large energy bandgap (2.5 eV) and low hole mobility of such polymers [15]. Efforts directed toward new polymer families quickly identified thiophene-based polymers [16] as promising materials owing to their good charge transport properties [17]. Alkyl-substituted polythiophenes, in addition to exhibiting good hole mobilities, had increased solubility as well as high regioregularity, a material property that was identified by the PV community at the time as being important. Regioregularity can be controlled during polymerization, and a breakthrough in obtaining high regioregular (>98%) head-to-tail poly(3-hexylthiophene) (P3HT) was first reported separately by Chen and Rieke [18] and McCullough et al. [19]. Brabec and coworkers used the optimized material in P3HT/[60]PCBM solar cells [20] and obtained a record high internal quantum efficiency approaching unity. Further progress was achieved by Sariciftci's group that first introduced postproduction thermal annealing treatments (Figure 1.1) to demonstrate the best OPV efficiency of 3.5% at the time (Figure 1.2) [21]. Hosting a number of improvements over PPV analogs, P3HT took over as the workhorse of polymer–fullerene solar cell research for many years, pushing efficiencies forward [22–25], being a scaffold for interesting structures [26,27], and at the same time providing a prototype system enabling scientists to understand many aspects of the operation of BHJ polymer–fullerene solar cells.

Progress toward OPVs with a landmark of 5% efficiency was achieved by further optimization of PPV- and P3HT-based devices along with synthetic efforts to produce new polymer donors based on new building blocks such as fluorene (APFO3) and carbazole (carbazole–triphenylamine (TPA)) molecules or noble metal-based molecules (Figure 1.2).

The synthetic efforts to create new electron donor polymers were guided by the following desired attributes of the material:

Broad absorption covering most of the visible and extending to near-IR up to the predicted optimum gap for single-junction cells of 1.1 μm [28].

High hole mobilities for efficient charge transport with values within an optimum range matching electron mobilities in the fullerenes.

Optimum leveling of energy states of donor, acceptor, and electrode materials, allowing efficient charge separation with minimum losses to thermal energy while minimizing the energy barrier to the collecting electrodes.

A recently reported model [29] has shown that the maximum power conversion efficiency of a BHJ solar cell can be predicted by the aforementioned properties, namely, the energy bandgap and the lowest unoccupied molecular orbital (LUMO) level of the polymer donor, and by taking into consideration the need to optimize morphological properties controlling transport and recombination within the blend. Keeping this in mind, the LUMO level of an ideal donor polymer should be around 3.7–4.0 eV, considering that the LUMO level of the soluble fullerene (PCBM) is 4.3 eV, to provide the minimum energy difference of approximately 0.3 eV required for efficient Frenkel exciton dissociation. As the optimized bandgap of the ideal light harvesting material (polymer in this case) should be around 1.2–1.5 eV, the highest occupied molecular orbital (HOMO) level value should be adjusted between 5.2 and 5.5 eV. This range of values for the HOMO level of the donor polymer has the additional benefit to ensure a relatively high Voc and air stability in the final devices. Electron and hole mobilities are also crucial parameters for OPV power conversion efficiencies. High mobilities for electrons and holes within the active region are necessary to favorably compete with losses due to geminate and nongeminate charge recombination. Recent theoretical prediction models [30] place desired mobility values, in BHJ cells, on the order of 10−2 cm2 V−1 s−1 for one and within a tolerance range of 10−1 to 10−3 cm2 V−1 s−1 for the other, irrespective of the type of carriers in each case. We note, however, that a balanced ambipolar transport within BHJs is the ideal condition to eliminate space charge effects and recombination, both of which are essential parameters for optimized solar cell power conversion efficiency values.

Several synthetic strategies have been used [31] to engineer the desired optical absorption characteristics. Among these include increasing the quinoid character of the ground state of polyaromatic conjugated polymers [32,33] and introducing molecular rigidity to increase planarity between adjacent aromatic units, thus extending conjugation and facilitating delocalization [34–36]. An additional and widespread strategy includes the incorporation of electron-withdrawing and electron-accepting moieties, either in the aromatic unit [37] or, more effectively, in the same polymer backbone; this approach will be discussed in more detail later. Other efforts include the optimization of the charge transport properties such as that recently reported by Ying and coworkers who used pyridal[2,1,3]thiadiazole as a building block for conjugated polymers and reported on the increased regioregularity of the thiadiazole moiety, resulting in an increase in hole mobilities by two orders of magnitude compared with their regiorandom counterparts [38]. As more and better syntheses are being explored to tailor physical and chemical properties of donor materials to the desired functionalities mentioned, the current crop of polymers used in polymer–fullerene PVs exceeding 7% solar cell efficiencies (Figure 1.3) will be enriched. Such high-performance polymers are discussed in the following sections.

Figure 1.3 Various polymer electron donors with some of the highest device efficiencies reported to date.

1.4 Novel Thiophene-Containing Polymers

Moving away from P3HT homopolymers, the community realized that the chemical tailoring required to attain the variety of properties for this particular technology would necessarily involve copolymerization. This meant targeting the incorporation of various molecular units, each enriching the final material with specific properties. Thiophenes and their analogs are still encountered as basic building blocks in many copolymers in new polymer–fullerene solar cells with record high efficiencies. In part, this is because thiophenes are good electron donators in donor–acceptor copolymers, and their facile incorporation was attributable to the wide commercial availability of analogs suitable for copolymerization. In light of thiophenes' widespread use in low-bandgap copolymers, a reference to specific outstanding thiophene-based copolymers follows.

A copolymer specifically designed for solar cells was introduced based on the 4,4-dialkylcyclopentadithiophene-2,6-diyl (CPDT) [39,40]. Copolymerization of CPDT with benzothiazole afforded the copolymer PCPDTBT (Figure 1.2) [41] exhibiting desirable optical characteristics and charge transport mobilities, leading to initial device efficiencies of ∼3%. PCPDTBT, however, seemed to underperform based on theoretical estimations (Figure 1.4) [29]. The main drawback was attributable to poor morphological characteristics of PCPDTBT–PCBM blends. A year later, Bazan and coworkers introduced additives to control the blend morphology, which allowed power conversion efficiency to reach 5.5% (Figure 1.1) [42]. Improved polymer orientation and morphology induced by the use of mixed solvents was also concurrently being studied, and these are still the subject of intense research today [43,44]. Benzothiazoles “sandwiched” between two thiophenes (PCPDTTBTT) also yield efficient solar cells [45]. Furthermore, cyclopenta–dithiophene copolymers were also synthesized using nonconventional microwave-assisted synthetic procedures [46].

Figure 1.4 Schematic illustration of the theoretical prediction by Scharber et al. [29] of the efficiency of polymer–PCBM solar cells based on the polymer energy gap and the polymer–PCBM LUMO band offsets. Various high-performance polymer donors synthesized are placed on the prediction graph. In publications in which the exact LUMO of the acceptor is not mentioned, a value of 4.0 eV for [60]PCBM and 4.1 eV for [70]PCBM is assumed.

It is presumed that fused thiophenes display their desirable characteristics partly because of increased rigidity. As such, extended fused systems such as the indacene mimicking dithiophene were developed. Benzo[1,2-b:4,5-b′]dithiophenes (BDTs) copolymerized with other appropriate units [47–52] are promising building blocks for polymer solar cells (Table 1.1) and when used by Solarmer in BHJ devices led to a significant enhancement in the solar cell efficiency of 6.58% [53]. Fine-tuning of these copolymers led to increased record efficiencies [54–56]. Modifying either the type [57] or the orientation [58] of alkyl side chains affected device performance and improved efficiencies close to 7% as reported by Fréchet's group [59]. Two-dimensional polymer structures were synthesized with the alkoxy side chains of the BDT moiety replaced by alkylthienyl groups. The resulting PBDTTT-C-T polymer (Figures 1.3 and 1.4) exhibited lower energy bandgap, a deeper HOMO level, and increased efficiency up to a high value of 7.6% [60]. Coupling with naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole also produced solar cells with high efficiencies (6%) [61], while the use of thiazolo[5,4-d]thiazole (TTZ) as acceptor yielded comparable efficiencies (5.2%) (Table 1.1) [62]. Fluorine atoms decorating a benzothiazole (PBnDT-DTffBT) [63] or benzotriazole (PBnDT-FTAZ) [64] acceptor unit as the comonomer were found to increase hole mobility of the end-polymer blend and also exhibited, in both cases, power conversion efficiencies >7% (Figure 1.3).

Table 1.1 Diverse polymer structures based on benzo[1,2-b:4,5-b′]dithiophenes (BDT) and their device characteristics when blended with fullerene adducts as the electron acceptor.

PCE%

V

oc

(V)

J

sc

(mA cm

−2

)

Mobility (cm

2

V

−1

s

−1

)

E

g

(eV)

HOMO (eV)

LUMO (eV)

Reference

6.58

0.70

14.7

2 × 10

−4

Device

1.57

5.12

3.55

[53]

6.32

0.85

12.78

5.91 × 10

−6

Device

1.51 (Optical)

5.47

3.44

[51]

5.22

0.82

10.3

1.67 × 10

−5

Device

2.0 (Optical)

5.3

3.2

[62]

2.54

0.82

6.64

0.01 Pristine

1.94 (Optical)

5.48

2.92

[52]

R

sc

denotes various solubilizing chains and

*

denotes the copolymerization functional ends.

Recently, Zhang et al. synthesized and used an even more rigid structure than BDT, the indacenodithiophene (IDT), which had four dodecyl groups introduced for solubility. This new donor was copolymerized with a standard thiophene–benzothiazole–thiophene group and when used with [70]PCBM as the fullerene acceptor gave an impressive 6.17% efficiency [65].

1.5 Fluorene-Containing Molecules

Fluorene-containing polymers represent an important class of arenes that have received considerable attention because of their unique photophysical properties and ease of chemical modification. Fluorenes are rigid, planar molecules that are usually associated with relatively large bandgaps and low HOMO energy levels, rendering them highly stable toward photodegradation and thermal oxidation during device operation. They exhibit high fluorescence quantum yields, excellent hole-transporting and film-forming properties, and have been extensively studied and used in organic light-emitting diodes (OLEDs) [66–68]. To counter the inherently large energy bandgaps of fluorene polymers, synthetic strategies involving the donor–acceptor approach have been used. Perhaps the most promising approach, adopted by Inganäs, introduced alternating polyfluorene copolymers known as alternating polyfluorene structures (APFOs) [69]. Although initial device efficiencies were low, APFOs showed promise owing to their low bandgaps, deep HOMO levels, and good ambipolar charge transport properties [70–72]. However, power conversion efficiencies in APFO BHJ solar cells remained predominantly below 5% (Figure 1.2) [73,74]. Notably, morphology optimization [75] and side chain engineering [74] are relatively unexplored parameters in APFO-containing solar cells. Recently, the introduction of an alkylidene spacer on the alkyl substituent at the 9-position of the fluorene increased power conversion efficiencies to above 6%; presumably this was due to improved planarity of the copolymer and further lowering the bandgap [76].

1.6 Carbazole-Based Copolymers

Carbazole (9-azafluorene) is one of the most attractive heteroarenes for electronic applications in the field of conjugated polymers. Although structurally analogous to fluorene, the central fused pyrrole, with its donating nitrogen, makes tricyclic carbazole fully aromatic and electron rich. Replacement of the fluorene C-9 carbon by nitrogen avoids the formation of fluorenone via oxidation at C-9 that leads to undesirable electronic and optical properties. The availability of nitrogen in carbazole aids functionalization with alkyl chains that can improve solubility. As such, carbazoles have been widely used as p-type semiconductors owing to their ability to form stable radical cations, excellent thermal and photochemical stabilities, relatively high charge mobility, and material availability. For photovoltaic applications, carbazoles have been under intense synthetic scrutiny. Typically, the HOMO energy level of copolymers seems to be governed by the carbazole, whereas the LUMO energy levels are directly related to the nature of the electron-withdrawing unit [77]. Their “dimer” indolo[3,2-b]carbazole consisting of two carbazoles fused together [78] also shows promise. LeClerc's group has been on the forefront of synthetic efforts for a variety of interesting carbazole-containing copolymers. Solar cells based on poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) (Figure 1.4), which consists of a carbazole–thiophene–benzothiazole–thiophene series along the main polymer axis, have achieved efficiencies as high as 6.1% (Table 1.2) [79]. High internal quantum efficiency and high Voc are responsible for the high power conversion efficiencies because the bandgap of PCDTBT is similar to that of P3HT. Exchanging the thiophene–benzothiazole–thiophene unit with a modified TPA moiety where the acceptor unit is not along the main chain but rather located as a pendant side chain on the phenyl group of TPA yields low-bandgap copolymers, which when blended with [70]PCBM give a respectable power efficiency of 4.47% [80]. Other similar attempts using the “main chain donor–side chain acceptor” approach have yielded efficiencies of 2.16% (Table 1.2) [81]. Synthetic efforts bearing the carbazole moiety and pendant TPA units have yielded devices with efficiencies 2.4% [82]; copolymers with bandgaps as low as 1 eV, optimized for solar light absorbance, have also been reported, but device efficiencies were very low [83].

Table 1.2 Polymer structures based on the carbazole moiety and their device characteristics when blended with fullerene adducts or when the carbazole copolymer acts as an electron acceptor.

PCE%

V

oc

(V)

J

sc

(mA cm

−2

)

Mobility (cm

2

V

−1

s

−1

)

E

g

(eV)

HOMO (eV)

LUMO (eV)

Reference

6.1

0.88

10.6

10

−3

Pristine

1.85

5.45

3.60

[79]

2.23

0.7

6.35

1.7 × 10

−4

(Electron mobilities)

2.17

5.83

3.66

[118]

2.25

5.08

2.83

2.16

1.03

5.75

2.9 × 10

−5

Device

2.05

5.60

3.55

[81]

1.7 New Heterocyclic Polymers

Advances in organic synthesis have led to increased interest in exploring unusual heterocycles. Similar to the carbazole–fluorene analogy, different linkers are used to enrich and dramatically alter the properties of the material. Nowadays, it is not uncommon for Si, Ge, or Se to decorate the structures of polymers, replacing carbon atoms and offering a host of new possibilities. In fact, the importance of the bridging atom is a topic that is currently being explored in polymer-based solar cells [84–88].

Poly(3-hexylselenophene) (P3HS) is the selenium analog of P3HT and was synthesized by Heeney et al. [89]. P3HS has a smaller bandgap (1.6 eV) compared with P3HT (1.9 eV), which is achieved via lowering of the LUMO but not the HOMO level. The lower LUMO can be predominantly attributed to the smaller ionization potential of selenium, a prime example of the variation that the atom infers on the polymer properties. These observations classify P3HS as promising for light harvesting while maintaining an open-circuit voltage comparable to that of P3HT. P3HS displays crystalline morphology and thus has a field-effect transistor (FET) charge mobility similar to that of P3HT under the same measurement conditions. Solar cell devices based on P3HS/PCBM blends (1 : 1 w/w) produced a PCE of 2.7% after optimal thermal annealing [90].

Similar to the previously discussed fused thiophene–benzothiazole copolymer PCPDTBT, the replacement of the bridging atom with Si gave the poly{[4,4′-bis(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole]-2,6-diyl-alt-[4,7-bis(2-thienyl)-2,1,3-benzothiadiazole]-5,5′-diyl} (Si-PCPDTBT) [84]. Although the absorption spectrum of the former is similar to that of the latter, the substitution of the silicon atom improves the hole mobility by a factor of 3 compared with that of PCPDTBT. The result was the first low-bandgap National Renewable Energy Laboratory (NREL)-certified polymer–fullerene solar cell with efficiencies of over 5% using either C60 or C70 adducts [85,91,92].

Other fullerenes have also been used successfully with Si-PCPDTBT, achieving higher open-circuit voltages [93]. Attempts at synthesizing and incorporating silole-based polymers [94–96] resulted in the enhancement of the power conversion efficiencies of solar cells, with efficiencies over 7% being reported [97]. Furthermore, So and coworkers reported on the advantages of Ge as the bridging atom. On a copolymer, based on the fused thiophene unit (P–Si) that exhibited a power conversion efficiency of around 6.6% with Si as the bridging atom, when replaced by Ge (P–Ge), the efficiencies rose to 7.3% [98] with an increase in both Voc and Jsc (Figure 1.5). Similar structures based on the Ge-bridged dithiophenes have been reported by Leclerc's group [99].

Figure 1.5 Illustration of the influence of the bridging atom of the dithiophene moiety on the resulting efficiencies, when copolymerized with the thieno[3,4-c]pyrrole-4,6-dione. Rsc denotes 2 ethyl-hexyl solubilizing chains, and * denotes the copolymerization functional ends. The Si-bridged derivative efficiency is not the highest reported, but rather is taken from a report of the same group as the Ge-bridged to ensure the same experimental conditions are kept during fabrication and measurement for comparison purposes. Similarly, PCPDTTPD with different Rsc chains has exhibited efficiencies as high as 6.4% [100].

Recently, more silole derivatives have emerged such as benzotriazole [101] and thiazolothiazoles [102]. The effect of side chains has also been investigated on this class of polymers [103]. APFOs have also been modified by replacement of the fluorene 9-position with silicon, which led to an overall lowering of the bandgap and increased efficiencies over 5% [96].

1.8 Polymers Based on Other Types of Building Blocks

Besides the main polymer classes that are currently being used in solar cells, synthetic efforts have also been directed toward newer building blocks, some of which have appeared as comonomers in materials already discussed. Diketopyrrolopyrrole (DPP) is an excellent electron-deficient unit and a prime candidate for coupling with various electron donors for push–pull-type low-bandgap copolymers that display excellent optical characteristics [104]. For example, coupling DPP with a simple terthiophene unit afforded a copolymer with an energy bandgap of 1.3 eV and efficiencies bordering 5% [105]. Janssen and coworkers achieved efficiencies of ∼5.5% with mobilities of ∼0.02 cm2 V−1 s−1 [106], and other efforts reported similar results [107,108]. Of related interest were DPP-based copolymers that exhibited the highest hole mobility reported for a PFET [109] along with a 5.4% device efficiency for BHJ solar cells using [70]PCBM; moreover, DPP-based donor–acceptor copolymers exhibit excellent ambipolar charge transport properties [110].

Dyes are also attractive building blocks for the preparation of solar cell copolymers. Squaraine-containing copolymers have afforded materials with very low bandgaps (<1 eV), allowing the possibility for two-photon absorption light harvesting [111]. 4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) dyes are also promising materials for optoelectronics, but have yet to realize their potential in solar cells. Ziessel and coworkers [112] have pioneered work on such materials, showing a variety of interesting properties [113] and the potential of forming useful photoactive units as fullerene adducts [114]. Polymers based on BODIPY have slowly emerged [115,116] and in some cases exhibit lower bandgaps and higher absorption coefficients than P3HT; however, they are plagued by mediocre hole mobilities, resulting in devices exhibiting efficiencies on the order of 2% [117]. Perylenes are another well-studied class of dyes, and perylene was used as the electron acceptor in the very first organic solar cells reported in the 1980s. Perylene-based polymers have been tested as acceptors in all polymer-based solar cells with thiophene-based polymers and exhibited efficiencies of 2% (Table 1.2) [118].

Isoindigo-based copolymers are also promising materials. Homopolymers of this low-bandgap electron-deficient unit were examined in all polymer solar cells with P3HT as the donor unit, yielding <1% efficiencies [119]. Numerous synthetic efforts yielding interesting copolymers of isoindigo with BDT [120] or with thiophene [121] comonomers gave efficiencies up to 5%. However, very recently, Inganäs reported efficiencies as high as 6.3% with terthiophene as an electron-rich comonomer of the isoindigo moiety [122], breathing new life into this unit.

Finally, reports on noble metal atoms incorporated into polymer chains, with intriguing optical properties for the resulting material, have surfaced. Low-bandgap polymers bearing platinum have been reported, with promising preliminary results of power conversion efficiencies over 4% (Figure 1.2) [123]. However, the optimization of such materials for use in high-performance devices will require significant effort [124–126].

1.9 Conclusions

Among the various thin film PV approaches, conjugated polymer-based solar cells have one of the highest potentials for becoming a true low-cost PV technology because their production demands only solution-based low-temperature deposition methods. At present, bulk heterojunction structures based on blends of a conjugated polymer as donor and soluble fullerene derivatives as electron acceptors exhibit the highest power conversion efficiencies. Over the past 5 years, remarkable progress has been achieved, placing power conversion efficiencies of such solar cells within striking distance to the 10% efficiency milestone, viewed as the landmark for broader commercial PV exploitation. A great part of this success was attributable to research on the design and synthesis of the conjugated polymer component of such PVs. It should be emphasized, however, that enhancement of the power conversion efficiency is a combination of parameters, including improvements of the acceptor, the selectivity of the electrodes, optimization of the morphology, and device processing. To capitalize on the progress achieved, further advances must be made, including new material concepts, a better understanding of the device operation mechanisms, and addressing lifetime issues. Furthermore, the application of printing technology as a fabrication tool for organic photovoltaics [127] indicates the potential of these novel materials for low-cost future light-activated power plastic sources.

References

1.

Brabec, C.J., Gowrisanker, S., Halls, J.J.M., Laird, D., Jia, S., and Williams, S.P. (2010) Polymer–fullerene bulk-heterojunction solar cells.

Adv. Mater.

,

22

, 3839–3856.

2.

Sariciftci, N.S., Smilowitz, L., Heeger, A.J., and Wudl, F. (1992) Photoinduced electron transfer from a conducting polymer to buckminsterfullerene.

Science

,

258

, 1474–1476.

3.

Yu, G., Gao, J., Hummelen, J.C., Wudl, F., and Heeger, A.J. (1995) Polymer photobtaic cells: enhanced efficiencies via a network of internal donor–acceptor heterojunctions.

Science

,

270

, 1789–1791.

4.

Hummelen, J.C., Knight, B.W., LePeq, F., Wudl, F., Yao, J., and Wilkins, C.L. (1995) Preparation and characterization of fulleroid and methanofullerene derivatives.

J. Org. Chem.

,

60

, 532–538.

5.

Wienk, M.M., Kroon, J.M., Verhees, W.J.H., Knol, J., Hummelen, J.C., vanHal, P.A., and Janssen, R.A.J. (2003) Efficient methano[70]fullerene/MDMO-PPV bulk heterojunction photobtaic cells.

Angew. Chem., Int. Ed.

,

42

, 3371–3375.

6.

Kooistra, F.B., Mihailetchi, V.D., Popescu, L.M., Kronholm, D., Blom, P.W.M., and Hummelen, J.C. (2006) New C84 derivative and its application in a bulk heterojunction solar cell.

Chem. Mater.

,

18

, 3068–3073.

7.

Shaheen, S., Brabec, C., Sariciftci, N., Padinger, F., Fromherz, T., and Hummelen, J. (2001) 2.5% Efficient organic plastic solar cells.

Appl. Phys. Lett.

,

78

, 841.

8.

Pacios, R., Nelson, J., Bradley, D.D.C., and Brabec, C. (2003) Composition dependence of electron and hole transport in polyfluorene:[6,6]-phenyl C61-butyric acid methyl ester blend films.

Appl. Phys. Lett.

,

83

, 4764.

9.

Cates, N.C., Gysel, R., Dahl, J.E.P., Sellinger, A., and McGehee, M.D. (2010) Effects of intercalation on the hole mobility of amorphous semiconducting polymer blends.

Chem. Mater.

,

22

, 3543–3548.

10.

Collins, B.A., Gann, E., Guignard, L., He, X., McNeill, C.R., and Ade, H. (2010) Molecular miscibility of polymer–fullerene blends.

J. Phys. Chem. Lett.

,

1

, 3160–3166.

11.

Burroughes, J.H., Bradley, D.D.C., Brown, A.R., Marks, R.N., Mackay, K., Friend, R.H., Burns, P.L., and Holmes, A.B. (1990) Light-emitting diodes based on conjugated polymers.

Nature

,

347

, 539–541.

12.

Burroughes, J.H., Jones, C.A., and Friend, R.H. (1988) New semiconductor device physics in polymer diodes and transistors.

Nature

,

335

, 137–141.

13.

Munters, T., Martens, T., Goris, L., Vrindts, V., Manca, J., Lutsen, L., De Ceuninck, W., Vanderzande, D., De Schepper, L., Gelan, J., Sariciftci, N.S., and Brabec, C.J. (2002) A comparison between state-of-the-art “ gilch” and “sulphinyl” synthesised MDMO-PPV/PCBM bulk hetero-junction solar cells.

Thin Solid Films

,

403–404

, 247–251.

14.

Halls, J.J.M., Walsh, C.A., Greenham, N.C., Marseglia, E.A., Friend, R.H., Moratti, S.C., and Holmes, A.B. (1995) Efficient photodiodes from interpenetrating polymer networks.

Nature

,

376

, 498–500.

15.

Geens, W., Shaheen, S.E., Wessling, B., Brabec, C.J., Poortmans, J., and Serdar Sariciftci, N. (2002) Dependence of field-effect hole mobility of PPV-based polymer films on the spin-casting solvent.

Org. Electron.

,

3

, 105–110.

16.

McCullough, R.D. (1998) The chemistry of conducting polythiophenes.

Adv. Mater.

,

10

, 93–116.

17.

Sirringhaus, H., Brown, P.J., Friend, R.H., Nielsen, M.M., Bechgaard, K., Langeveld-Voss, B.M.W., Spiering, A.J.H., Janssen, R.A.J., Meijer, E.W., Herwig, P., and deLeeuw, D.M. (1999) Two-dimensional charge transport in self-organized, high-mobility conjugated polymers.

Nature

,

401

, 685–688.

18.

Chen, T.A. and Rieke, R.D. (1992) The first regioregular head-to-tail poly(3-hexylthiophene-2,5-diyl) and a regiorandom isopolymer: nickel versus palladium catalysis of 2(5)-bromo-5(2)-(bromozincio)-3-hexylthiophene polymerization.

J. Am. Chem. Soc.

,

114

, 10087–10088.

19.

McCullough, R.D., Lowe, R.D., Jayaraman, M., and Anderson, D.L. (1993) Design, synthesis, and control of conducting polymer architectures: structurally homogeneous poly(3-alkylthiophenes).

J. Org. Chem.

,

58

, 904–912.

20.

Schilinsky, P., Waldauf, C., and Brabec, C. (2002) Recombination and loss analysis in polythiophene based bulk heterojunction photodetectors.

Appl. Phys. Lett.

,

81

, 3885.

21.

Padinger, F., Rittberger, R.S., and Sariciftci, N.S. (2003) Effects of postproduction treatment on plastic solar cells.

Adv. Funct. Mater.

,

13

, 85–88.

22.

Ma, W., Yang, C., Gong, X., Lee, K., and Heeger, A.J. (2005) Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology.

Adv. Funct. Mater.

,

15

, 1617–1622.

23.

Li, G., Shrotriya, V., Yao, Y., and Yang, Y. (2005) Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene).

J. Appl. Phys.

,

98

, 043704–043705.

24.

Zhao, G., He, Y., and Li, Y. (2010) 6.5% Efficiency of polymer solar cells based on poly(3-hexylthiophene) and indene-C60 bisadduct by device optimization.

Adv. Mater.

,

22

, 4355–4358.

25.

Kim, Y., Cook, S., Tuladhar, S.M., Choulis, S.A., Nelson, J., Durrant, J.R., Bradley, D.D.C., Giles, M., McCulloch, I., Ha, C.-S., and Ree, M. (2006) A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells.

Nat. Mater.

,

5

, 197–203.

26.

Hou, J., Tan, Z., Yan, Y., He, Y., Yang, C., and Li, Y. (2006) Synthesis and photobtaic properties of two-dimensional conjugated polythiophenes with bi(thienylenevinylene) side chains.

J. Am. Chem. Soc.

,

128

, 4911–4916.

27.

Dante, M., Yang, C., Walker, B., Wudl, F., and Nguyen, T.-Q. (2010) Self-assembly and charge-transport properties of a polythiophene–fullerene triblock copolymer.

Adv. Mater.

,

22

, 1835–1839.

28.

Sargent, E.H. (2009) Infrared photobtaics made by solution processing.

Nat. Photon.

,

3

, 325–331.

29.

Scharber, M.C., Mühlbacher, D., Koppe, M., Denk, P., Waldauf, C., Heeger, A.J., and Brabec, C.J. (2006) Design rules for donors in bulk-heterojunction solar cells: towards 10% energy-conversion efficiency.

Adv. Mater.

,

18

, 789–794.

30.

Shieh, J., Liu, C., Meng, H., Tseng, S., Chao, Y., and Horng, S. (2010) The effect of carrier mobility in organic solar cells.

J. Appl. Phys.

,

107

, 084503.

31.

Cheng, Y.-J., Yang, S.-H., and Hsu, C.-S. (2009) Synthesis of conjugated polymers for organic solar cell applications.

Chem. Rev.

,

109

, 5868–5923.

32.

Brédas, J.L. (1985) Relationship between band gap and bond length alternation in organic conjugated polymers.

J. Chem. Phys.

,

82

, 3808.

33.

Roncali, J. (2007) Molecular engineering of the band gap of π-conjugated systems: facing technological applications.

Macromol. Rapid Commun.

,

28

, 1761–1775.

34.