Due to their special properties, organic semiconductors enable the development of large-area, low-cost devices, paving the way for flexible and nomadic applications that advantageously replace those made with traditional semiconductors. In this second volume, we study the main applications of organic semiconductors, such as organic light-emitting diodes (OLEDs), solar cells (OPVs) and organic field-effect transistors (OFETs).

The commercialization of these new devices is then discussed within the Brabec triangle framework, in which yield, stability and production costs are the key factors. We also address the environmental impact of organic devices for their future development. This book presents the application side of organic electronics from a technological, economic and environmental perspective. It is intended for researchers and students in university programs or engineering schools specializing in electronics, energy and materials.

Thien-Phap Nguyen is Professor Emeritus at the University of Nantes, France. He has taught solid state physics and organic electronics. His research focuses on semiconductors and electronic components.

Introduction ix

Chapter 1 Organic Light-Emitting Diodes 1

1.1 Introduction 1

1.2 Reminders on optics 2

1.2.1 Photometry and radiometry 2

1.2.2 Colors 3

1.3 OLED operating principle 6

1.3.1 P–N junction LED 6

1.3.2 OLEDs 15

1.4 OLED applications 35

1.4.1 OLEDs for lighting 36

1.4.2 OLEDs for display 37

1.4.3 OLEDs for automotive equipment 39

1.5 Conclusion 40

Chapter 2 Organic Solar Cells 41

2.1 Introduction 41

2.2 Solar spectrum 42

2.3 Operating principle 44

2.3.1 Absorption of photons 45

2.3.2 Diffusion of excitons 46

2.3.3 Dissociation of excitons 47

2.3.4 Diffusion of carriers to electrodes 50

2.3.5 Collection of charges 50

2.3.6 Process optimization for an organic solar cell 50

2.4 Characteristic parameters of solar cells 52

2.4.1 Current–voltage characteristics 52

2.4.2 Photovoltaic parameters of a solar cell 54

2.4.3 Efficiency 57

2.5 Organic materials 59

2.5.1 Electron donor materials 59

2.5.2 Electron acceptor materials 61

2.6 P3HT:PCBM 63

2.7 Perovskite 65

2.7.1 Structure of perovskite 66

2.7.2 Solar cells based on perovskite 67

2.7.3 Conversion efficiency 69

2.7.4 Problems with the use of perovskite solar cells 69

2.8 Solar cells based on organic, hybrid and silicon materials 73

2.9 Strategies to improve the performance of organic and hybrid solar cells 75

2.9.1 Low bandgap semiconductors 76

2.9.2 Tandem cells 78

2.10 Conclusion 82

Chapter 3 Organic Transistors 85

3.1 Introduction 85

3.2 Operating principle 86

3.2.1 Transistor effect 87

3.2.2 Field effect 88

3.3 Principal OFET parameters 95

3.3.1 Charge carrier mobility 96

3.3.2 Contact resistance 97

3.3.3 Hysteresis 101

3.3.4 Gate-bias stress effects, VGS 102

3.3.5 I_on/I_off current ratio 102

3.4 Materials 103

3.4.1 Metals used for electrodes 103

3.4.2 Dielectric materials 104

3.4.3 Active organic materials 109

3.5 Ambipolar transistors and semiconductors 115

3.5.1 Ambipolar semiconductors 115

3.5.2 Ambipolar transistors 116

3.6 Light-emitting transistors 118

3.6.1 Ambipolar OLETs with BHJ structure 118

3.6.2 Single-semiconductor ambipolar OLETs 118

3.6.3 Vertical OLETs 120

3.7 OFET applications 121

3.7.1 RFID tags 121

3.7.2 Sensors 122

3.7.3 Active-matrix displays 123

3.8 Conclusion 124

Chapter 4 The Brabec Triangle 127

4.1 Introduction 127

4.2 Device efficiency 128

4.2.1 OLED efficiency 128

4.2.2 Solar cell efficiency 130

4.2.3 OFET performance 133

4.3 Stability of materials and devices 134

4.3.1 Process of degradation of organic materials and devices 135

4.3.2 Classification of device degradation mechanisms 137

4.3.3 Degradation of OFETs 143

4.3.4 Measuring the lifetime of devices 146

4.4 Organic device production cost and marketing 151

4.4.1 Production of OLEDs 152

4.4.2 Production of OSCs 153

4.4.3 Production of OFETs 155

4.5 Synthesis on Brabec’s criteria 156

4.6 Environmental dimension 157

4.6.1 Life-cycle assessment 158

4.6.2 Levelized cost of energy 160

4.6.3 Energy payback time 160

4.6.4 Life cycle of organic solar cells 161

4.6.5 Fate of released pollutants 162

4.6.6 Mass production and environment 165

4.7 Prospects and developments 167

List of Acronyms 173

References 181

Index 197