Index

Preface

List of abbreviations

1.1 Human Energy Consumption Now And In The Future

1.2 Renewable Energy Sources

1.3 Important Facts About Energy, Energy Conversion, The Earth And The Sun

1.4 Solar Energy

1.5 The Storage And Relocation Problem

1.6 Types Of Solar Cells

1.7 Current Challenges

References

2.1 Introduction

2.2 Materials

2.2.1 Polymers

2.2.1.1 Poly(phenylenevinylenes)

2.2.1.2 Poly(thiophenes)

2.2.1.3 Low bandgap polymers

2.2.1.4 Other polymers

2.2.1.5 Thermocleavable materials

2.2.2 Molecules and oligomers

2.2.2.1 Small molecules

2.2.2.2 Fullerenes

2.2.2.3 Oligomers

2.3 Fast And Simple Guide To A Polymer Solar Cell From Scratch

2.3.1 Equipment

2.3.2 The substrate

2.3.3 The PEDOT:PSS layer

2.3.4 The active layer

2.3.4.1 Materials

2.3.4.2 Spin coating of the active layer

2.3.5 Evaporating the electrode

2.3.6 Applying electrodes and measuring the electrical properties of the devices

2.3.7 Device preparation and performance

2.3.7.1 Device preparation and performance using regiorandom and regioregular P3HT at Risø National Laboratory (Denmark) in air

2.3.7.2 Comparison between cells prepared at Risø National Laboratory (Denmark) in air and in a glove box

2.3.7.3 Device preparation and performance using regiorandom and regioregular P3HT in CEA (France) in a glove box

2.3.7.4 Comparison between cells prepared under glove box conditions at Risø National Laboratory and CEA using the same materials

References

3.1 Taking the Sun Inside

3.1.1 Air mass

3.1.1.1 The AM 1.5D (or simply AM 1.5) versus the AM 1.5G spectrum

3.1.2 The ASTM E 927-05 standard and the IEC 904-9 standard

3.1.3 Types of simulators

3.1.4 Halogen lamps

3.1.5 Recording the spectrum

3.1.6 Applying filters to improve the spectrum

3.1.7 Spectral, temporal, and spatial homogeneity of the light field

3.1.8 Calibration of the sun simulator

3.2 IV-Curves And Efficiencies

3.2.1 The Source meter

3.2.2 Where the electrons are and how to connect your cell to the outside world

3.2.3 Speed of IV-curve measurement, dielectric relaxation, and capacitive loading

3.2.4 Action spectra using a high-power spectrometer

3.2.5 IPCE measurements using a simple high-power spectrometer

3.2.6 Environmental effects

3.3. Outdoor Measurements

3.3.1 Why outdoor photovoltaic characterization is necessary for organic solar cells

3.3.2 Experimental procedure

3.3.3 Temperature dependence of the photovoltaic parameters of BHJ solar cells

3.3.4 Example of long-term outdoor testing of stability of organic solar cells3.3.5. 3.3.5 Some new experimental possibilities and suggestions for future studies

3.4 Methods For Preparation And Characterization Of Thin Films

3.4.1 Controlling morphological properties

3.4.1.1 Spin coating

3.4.1.2 Thermal evaporation

3.4.1.3 Annealing

3.4.2 Techniques For Monitoring Morphology

3.4.2.1 Electron microscopy: SEM and TEM

3.4.2.2 Atomic force microscopy (AFM)

References

4.1 Overview

4.2 Studies Of Degradation Mechanisms Using TOF-SIMS

4.2.1 Principle of TOF-SIMS

4.2.2 Isotopic labeling

4.2.3 TOF-SIMS depth profiling

4.2.4 Gaining access to the various layers in the photovoltaic device

4.2.5 TOF-SIMS imaging

4.2.6 Chemical structure elucidation based on mass spectral information

4.2.7 Monitoring photooxidation in time—mapping the "history" of degradation

4.3 Studies of Degradation Mechanisms Using XPS

4.3.1 Principle

4.3.2 Chemical shifts

4.3.3 Angle-dependent studies

4.3.4 Experimental details

4.3.5 Device aging and IV measurement

4.3.6 XPS overall observations

4.3.7 Li and F distribution

4.4 Studies Of Degradation Mechanisms Using RBS

4.4.1 Principles and quantitative depth profile of the composition

4.4.1.1 Sensitivity

4.4.1.2 Technical facts and sample preparation

4.4.1.3 ERDA

4.4.1.4 NRA

4.4.2 Studies of cathode degradation using RBS

4.4.2.1 Ca cathode degradation

4.4.2.2 Methods

4.4.2.3 Results and discussion

4.4.2.4 Ca/Ag or Ca/Al cathode degradation

4.4.2.5 Influence of the nature of the electrode on aging

4.5 Studies of Degradation Mechanisms Using Physical And/Or Spectroscopic Techniques

4.5.1 Interference microscopy

4.5.2 Atomic force microscopy (AFM)

4.5.3 Scanning electron microscopy (SEM)

4.5.4 Fluorescence microscopy

4.6 Accelerated Lifetime Measurements For Extended Periods Of Time

4.7 Apparatus For Lifetime Measurements And For Isotope Labeling

References

5.1 Printing and Coating Methods

5.1.1 R2R coating

5.1.2 Screen printing

5.1.3 Pad printing

5.1.4 Doctor blading

5.1.5 Other printing methods

5.2 Printing the Active Layer

5.2.1 Screen printing

5.2.1.1 Process optimization

5.2.1.2 Printing speed

5.2.1.3 Mesh size

5.2.1.4 Squeegee pressure and snap-off distance

5.2.1.5 Evaluation of the printed film as an active layer in a light-emitting diode

5.2.1.6 Rheological characterization

5.2.1.7 Rheology measurement setup

5.2.1.8 Temperature-dependent viscosity measurements

5.2.1.9 Shear rate-dependent viscosity measurements

5.2.1.10 Photovoltaic devices with screen printed active layer

5.2.1.11 Screen printed layers of donor/acceptor blends

5.2.1.12 A screen printed photovoltaic module based on MEH-PPV- [60]PCBM

5.2.1.13 Larger screen printed photovoltaic module based on MEH-PPV

5.2.1.14 An even larger screen printed photovoltaic module based on MEH- PPV

5.3 Carrier Substrates

5.4 Anodes and Cathodes

5.5 Processing of The Transparent Front-Side Contact

5.5.1 PEDOT as transparent contact

5.5.2 Introduction of a conductive grid into photovoltaic devices

5.6 Processing of the Opaque Back-Side Contact

5.6.1 Ag-based pastes as back-side contacts

5.7 Encapsulation and Permeability

5.7.1 Measurement of permeability

5.7.2 Measurement of the diffusion coefficient D

5.7.4 Apparatus

5.7.5 An example of a commercial instrument

5.7.6 The calcium test

5.7.7 Mass spectrometry

5.7.8 Tritied water

5.7.9 Oxygen permeation in PEDOT

5.8 Practical Encapsulation Techniques

5.8.1 Rigid encasement at IMEC (Belgium)

5.8.2 Small rigid encasement at Risø National Laboratory (Denmark)

5.8.3 Large rigid encasement at Risø National Laboratory (Denmark)

5.8.4 Flexible encasement

5.9 Production and Companies 2007

5.9.1 Intellectual property rights in Europe, the United States, and Asia 2007

5.9.2 A road map for setting up a company producing OPVs in Europe

5.9.3 What production equipment is available in 2007

References

6.1 Where is the Technology Now?

6.2 Where is it Suitable?

6.3 Where Could It Be in the Next Decades?

References

Preface

Polymer photovoltaics is a discovery that potentially houses the solutions to many of the problems currently encountered with traditional photovoltaic technologies. Most notably, the technology offers the possibility for ultrafast processing, low cost, light weight, flexibility, and a very low thermal budget. The technology rests on a moderately solid base of scientific literature spanning from the first prototypical literature reports. Among the most prominent contributors are the groups of C.W. Tang, Richard Friend, and Alan J. Heeger through an impressive number of original research papers documenting a steady increase in the performance at the level of very small devices with power- conversion efficiencies of up to around 5% for so-called bulk heterojunctions, which today represent the state of the art. This base of research reports, conference proceedings, reviews, and even many books makes the topic highly accessible to the newcomer and as such there is no need for a new book on the topic from a theoretical or explanatory point of view. One of the problems when entering the field of organic photovoltaics is getting a good idea of how to actually make devices, how to study them, and how to characterize them. The ambition with this book is that it should be a practical guide in the laboratory for the experimental solar cell scientist whether he or she is involved with synthesis, device preparation, processing, or device characterization. Our feeling is that such an experimental guide will be useful to all scientists working practically in the field. The book presents the process of creating a polymer solar cell device starting with a description of materials including how they are made and characterized, followed by how the materials are processed into devices and films and how these are characterized. Following on from this, the status of two emerging fields of polymer solar cells are described, namely, degradation/stability and large-area processing.

Frederik Krebs
December 2007