Stability and Degradation of Organic and

Organic photovoltaics (OPV) are a new generation of solar cells with the potential to offer very short energy pay back times, mechanical flexibility and significantly lower production costs compared to traditional crystalline photovoltaic systems. A weakness of OPV is their comparative instability during operation and this is a critical area of research towards the successful development and commercialization of these 3rd generation solar cells. Covering both small molecule and polymer solar cells, Stability and Degradation of Organic and Polymer Solar Cells summarizes the state of the art understanding of stability and provides a detailed analysis of the mechanisms by which degradation occurs. Following an introductory chapter which compares different photovoltaic technologies, the book focuses on OPV degradation, discussing the origin and characterization of the instability and describing measures for extending the duration of operation. Topics covered include: Chemical and physical probes for studying degradation Imaging techniques Photochemical stability of OPV materials Degradation mechanisms Testing methods Barrier technology and applications Stability and Degradation of Organic and Polymer Solar Cells is an essential reference source for researchers in academia and industry, engineers and manufacturers working on OPV design, development and implementation.

Professor Frederik Krebs is based at the Riso National Laboratory for Sustainable Energy in Denmark where he is part of the Solar Energy Program. The aim of the program is to synthesize new materials for light harvesting and to optimize the structure of the solar cell with regards to energy conversion, stability and cost.
Professor Krebs has been working in the field of polymer solar cells for over 10 years, focusing his efforts on the materials, manufacture and stability of solar cells. His research group are international pioneers, leading the way in their work on organic and polymeric solar cells. He has authored more than 200 publications, has written 2 books on organic photovoltaics (OPVs) and has hosted a conference on the stability of polymer solar cells (ISOS-3 International Summit on OPV Stabilty). He has also edited many journal special issues on organic photovoltaics, including one dedicated to their stability. In 2009, Professor Krebs received the Carlsberg Energy Research Prize in recognition of his work in polymer solar cells.

Preface	p. xi
Acknowledgements	p. xiii
List of Contributors	p. xv
The Different PV Technologies and How They Degrade	p. 1
The Photovoltaic Effect and the Overview	p. 1
The Photovoltaic Technologies	p. 2
Intrinsic Versus Extrinsic Stability	p. 3
Intrinsic Stability	p. 3
Extrinsic Stability	p. 3
Degradation - The Culprits, the What, the Why and the How	p. 3
Some Representative Technologies and How They Degrade	p. 4
Mono- and Polycrystalline Silicon Solar Cells	p. 5
Amorphous, Micro-and Nanocrystalline Silicon Solar Cells	p. 6
CIS/CIGS Solar Cells	p. 8
CdS/CdTe Solar Cells	p. 9
Dye-Sensitized Solar Cells (DSSC)	p. 10
Organic and Polymer Solar Cells (OPV)	p. 11
References	p. 12
Chemical and Physical Probes for Studying Degradation	p. 17
Introduction	p. 17
Physical Probes	p. 18
UV-vis Spectroscopy	p. 18
Atomic Force Microscopy (AFM)	p. 18
Interference Microscopy	p. 20
Scanning Electron Microscopy (SEM)	p. 21
Fluorescence Microscopy	p. 23
Light-Beam Induced-Current Microscopy (LBIC)	p. 24
Electroluminescence and Photoluminescence Imaging Microscopy (ELI and PLI)	p. 25
X-ray Reflectometry	p. 26
Chemical Probes	p. 27
Infrared Spectroscopy (IR)	p. 27
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SDVIS)	p. 28
X-ray Photoelectron Spectroscopy (XPS)	p. 32
Summary and Outlook	p. 35
References	p. 36
Imaging Techniques for Studying OPV Stability and Degradation	p. 39
Introduction to Imaging Techniques	p. 39
Microscopy and Optical Scanning	p. 39
Luminescence Imaging	p. 40
Lock-In Thermography	p. 43
Light-Beam Induced Current	p. 45
Reports	p. 46
Background: Degradation of OLED Devices	p. 46
Light-Beam Induced Current	p. 50
Luminescence Imaging	p. 54
Optical Microscopy	p. 57
Dark Lock-In Thermography and LBIC	p. 58
Dark Lock-In Thermography and Optical Scanning for Failure Analysis	p. 62
Discussion: Comparison of Imaging Techniques	p. 63
Summary	p. 66
Acknowledgement	p. 66
References	p. 66
Photochemical Stability of Materials for OPV	p. 71
Introduction	p. 71
Methods	p. 72
Aging Condition	p. 72
Degradation Monitoring	p. 74
State-of-the-Art	p. 82
Degradation of the ¿-Conjugated Polymer	p. 82
Acceptor Material Aging and Blend Degradation	p. 99
References	p. 102
Degradation of Small-Molecule-Based OPV	p. 109
Comparison to Small-Molecule OLEDs	p. 110
Number of Photoexcitations per Molecule	p. 113
Comparison to Polymer Solar Cells	p. 115
Sensitivity to Air	p. 115
Temperature Stability	p. 115
Small-Molecule Organic Materials	p. 116
Active Materials	p. 116
Transport-and Exciton-Blocking Materials	p. 119
Degradation Conditions	p. 125
Oxygen and Water	p. 125
UV Radiation	p. 132
State-of-the-Art in Lifetime Studies	p. 134
Summary and Outlook	p. 138
References	p. 139
Degradation of Polymer-Based OPV	p. 143
Focus on the Degradation and Stability of Polymer Solar Cells	p. 143
A Chart of Degradation and Stability of Polymer Solar Cells	p. 143
A Short Account of the OPV Stability/Degradation History	p. 144
The Divisions of Degradation Mechanisms	p. 146
The Methodologies	p. 148
Modus Operandi for Evolving OPV	p. 148
The Recent Developments	p. 149
The Photocatalytic Oxides	p. 149
Interlayers	p. 150
The Inverted Structure	p. 151
R2R Processing	p. 152
Lamination and Encapsulation	p. 153
Water Processing	p. 154
Mechanical Degradation - Delamination	p. 155
Interlaboratory Studies and Round Robins	p. 156
Outside Studies	p. 157
How Far Can OPV Be Taken in Terms of Stability?	p. 158
References	p. 158
Test Equipment for OPV Stability	p. 163
Introduction	p. 163
Reliability and Durability Testing of PV Products	p. 165
Reliability, a Function of Durability	p. 165
Environmental Durability	p. 166
Durability and Weathering Testing	p. 167
Laboratory Weathering Testing	p. 168
Acceleration	p. 168
Relevance	p. 169
Precision	p. 170
Introduction to Determination of Acceleration Factors	p. 170
Durability Testing Techniques	p. 172
Outdoor Weathering	p. 172
Laboratory Weathering	p. 175
Laboratory Photoaging	p. 180
Others	p. 183
Conclusion	p. 185
References	p. 186
Characterization and Reporting of OPV Device Lifetime	p. 193
Introduction	p. 193
Photoelectric Characterization of OPV Devices	p. 194
Photoelectric Characterization Tools	p. 194
Characterization in Controlled Environments	p. 197
Interlaboratory Studies of OPVs	p. 202
Introduction	p. 202
Interlaboratory Studies of Flexible Large-Area Roll-to-Roll Processed Polymer Solar Cell Modules	p. 203
Interlaboratory Stability Studies of OPVs	p. 204
Lifetime Testing and Reporting: Standardized Approach	p. 213
Introduction	p. 213
Procedures for Standard Lifetime Measurements	p. 214
Reporting Lifetime	p. 235
Conclusions	p. 238
List of Abbreviations	p. 238
References	p. 238
Concentrated Light for Organic Photovoltaics	p. 243
Introduction	p. 243
Light-Concentration Setups	p. 245
Refractive Sunlight Concentration	p. 245
Reflective Sunlight Concentration	p. 246
Concentrated Solar Simulation	p. 249
Experimental Work Performed with Concentrated Light	p. 251
IPV Response to Concentrated Sunlight	p. 251
Polymer Response to Concentrated Light	p. 253
Organic Solar Cell Response to Concentrated Light	p. 257
Physical Characterization by Concentrated Sunlight	p. 261
Conclusion	p. 265
References	p. 265
Barrier Technology and Applications	p. 269
Encapsulation Requirements	p. 269
Types of Encapsulation	p. 270
Glass/Glass Encapsulation	p. 271
Lamination of Barrier Films	p. 272
Thin-Film Encapsulation	p. 273
Perimeter Sealing	p. 273
Thin-Film Permeation Physics	p. 274
Solid-State Diffusion and Diffusion in Polymers	p. 274
Fick's First Law of Diffusion	p. 275
Sorption	p. 276
Permeation in Thin Films	p. 277
Models for the Permeation Barrier Coated Polymer Films	p. 278
Temperature Dependence of Permeation	p. 279
Dependence of Water Permeation on Layer Thickness	p. 280
Time Dependence of Permeation	p. 282
Permeation in Multilayer Barriers	p. 284
Pinholes in Multilayer Systems	p. 287
Measurement of Barrier Properties	p. 288
Gravimetric Cup	p. 289
Carrier-Gas-Based Coulometric Barrier Measurement	p. 289
Mass Spectrometer	p. 290
Direct Pressure Measurement	p. 291
Radioactive Isotopes	p. 292
Calcium Test (Optical or Electrical)	p. 293
Device Testing	p. 294
Standards and Typical Measurement Conditions	p. 295
Test Method Overview	p. 295
Barrier Technologies	p. 295
Single-Layer Technologies	p. 297
Multilayer Technologies	p. 309
Barrier Application in OPV	p. 315
Products	p. 316
Barrier Cost and Manufacturability	p. 318
Conclusion	p. 321
References	p. 322
Summary and Outlook	p. 331
Index	p. 333
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Stability and Degradation of Organic and Polymer Solar Cells

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