Stability and Degradation of Organic and Polymer Solar Cells

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.

Autorentext

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.

Inhalt

Preface xi

Acknowledgements xiii

List of Contributors xv

**1. The Different PV Technologies and How They Degrade 1
Frederik C. Krebs

1.1 The Photovoltaic Effect and the Overview 1

1.2 The Photovoltaic Technologies 2

1.3 Intrinsic Versus Extrinsic Stability 3

1.3.1 Intrinsic Stability 3

1.3.2 Extrinsic Stability 3

1.4 Degradation The Culprits, the What, the Why and the How 3

1.5 Some Representative Technologies and How They Degrade 4

1.5.1 Mono- and Polycrystalline Silicon Solar Cells 5

1.5.2 Amorphous, Micro- and Nanocrystalline Silicon Solar Cells 6

1.5.3 CIS/CIGS Solar Cells 8

1.5.4 CdS/CdTe Solar Cells 9

1.5.5 Dye-Sensitized Solar Cells (DSSC) 10

1.5.6 Organic and Polymer Solar Cells (OPV) 11

**2. Chemical and Physical Probes for Studying Degradation 17
Birgitta Andreasen and Kion Norrman

2.1 Introduction 17

2.2 Physical Probes 18

2.2.1 UV-vis Spectroscopy 18

2.2.2 Atomic Force Microscopy (AFM) 18

2.2.3 Interference Microscopy 20

2.2.4 Scanning Electron Microscopy (SEM) 21

2.2.5 Fluorescence Microscopy 23

2.2.6 Light-Beam Induced-Current Microscopy (LBIC) 24

2.2.7 Electroluminescence and Photoluminescence Imaging Microscopy (ELI and PLI) 25

2.2.8 X-ray Reflectometry 26

2.3 Chemical Probes 27

2.3.1 Infrared Spectroscopy (IR) 27

2.3.2 Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) 28

2.3.3 X-ray Photoelectron Spectroscopy (XPS) 32

2.4 Summary and Outlook 35

**3. Imaging Techniques for Studying OPV Stability and Degradation 39
Marco Seeland, Roland R¨osch and Harald Hoppe

3.1 Introduction to Imaging Techniques 39

3.1.1 Microscopy and Optical Scanning 39

3.1.2 Luminescence Imaging 40

3.1.3 Lock-In Thermography 43

3.1.4 Light-Beam Induced Current 45

3.2 Reports 46

3.2.1 Background: Degradation of OLED Devices 46

3.2.2 Light-Beam Induced Current 50

3.2.3 Luminescence Imaging 54

3.2.4 Optical Microscopy 57

3.2.5 Dark Lock-In Thermography and LBIC 58

3.2.6 Dark Lock-In Thermography and Optical Scanning for Failure Analysis 62

3.3 Discussion: Comparison of Imaging Techniques 63

3.4 Summary 66

**4. Photochemical Stability of Materials for OPV 71
Matthieu Manceau, Agnes Rivaton and Jean-Luc Gardette

4.1 Introduction 71

4.2 Methods 72

4.2.1 Aging Condition 72

4.2.1.1 Natural and Artificial Accelerated Aging 72

4.2.1.2 Temperature Effect 73

4.2.1.3 Atmosphere Composition 74

4.2.2 Degradation Monitoring 74

4.2.2.1 Spectroscopies 75

4.2.2.2 Microscopies 80

4.3 State-of-the-Art 82

4.3.1 Degradation of the -Conjugated Polymer 82

4.3.1.1 Study of the Pioneers: MDMO-PPV and P3HT 82

4.3.1.2 Material Chemical Structure vs. Material Stability 90

4.3.2 Acceptor Material Aging and Blend Degradation 99

4.3.2.1 Acceptor Degradation 99

4.3.2.2 Blend Degradation 99

**5. Degradation of Small-Molecule-Based OPV 109
Martin Hermenau, Moritz Riede and Karl Leo

5.1 Comparison to Small-Molecule OLEDs 110

5.1.1 Number of Photoexcitations per Molecule 113

5.2 Comparison to Polymer Solar Cells 115

5.2.1 Sensitivity to Air 115

5.2.2 Temperature Stability 115

5.3 Small-Molecule Organic Materials 116

5.3.1 Active Materials 116

5.3.1.1 Fullerene C60 116

5.3.1.2 Phthalocyanines 117

5.3.1.3 Pentacene 117

5.3.2 Transport- and Exciton-Blocking Materials 119

5.3.2.1 Electron-Transport Materials 119 5.3.2.2 Hole-Trans...