Widmer J. Charge transport and energy levels in organic semiconductors (Dresden, 2014). - ОГЛАВЛЕНИЕ / CONTENTS

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ОбложкаWidmer J. Charge transport and energy levels in organic semiconductors: Diss. … Dr. rer. nat. - Dresden: Institut für Angewandte Photophysik : Technische Universität Dresden, 2014. - xi, 256 p.: ill. - Bibliogr.: p.233-243. - ISBN 978-7-5085-2634-8
 

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Оглавление / Contents
 
Abstract ........................................................ v

1    Introduction ............................................... 1

2    Organic semiconductors and devices ......................... 5
2.1  Organic semiconductors  .................................... 5
     2.1.1  Conjugated n system ................................. 7
     2.1.2  Small molecules and polymers ........................ 9
     2.1.3  Disorder in amorphous materials .................... 10
     2.1.4  Polarons  .......................................... 12
     2.1.5  Polaron hopping  ................................... 12
     2.1.6  Fermi-Dirac distribution and Fermi level ........... 12
     2.1.7  Quasi-Fermi levels ................................. 13
     2.1.8  Trap states ........................................ 14
     2.1.9  Doping ............................................. 15
     2.1.10 Excitons  .......................................... 16
2.2  Interfaces and blend layers ............................... 21
     2.2.1  Interface dipoles .................................. 22
     2.2.2  Energy level bending ............................... 22
     2.2.3  Injection from metal into semiconductor, and
            extraction ......................................... 24
     2.2.4  Excitons at interfaces ............................. 25
2.3  Charge transport and recombination in organic
     semiconductors ............................................ 26
     2.3.1  Drift transport .................................... 28
     2.3.2  Charge carrier mobility ............................ 28
     2.3.3  Thermally activated transport  ..................... 29
     2.3.4  Diffusion transport ................................ 32
     2.3.5  Drift-diffusion transport .......................... 33
     2.3.6  Space-charge limited current ....................... 33
     2.3.7  Recombination ...................................... 36
2.4  Mobility measurement ...................................... 38
     2.4.1  SCLC and TCLC ...................................... 38
     2.4.2  Time of flight ..................................... 39
     2.4.3  Organic field effect transistors ................... 40
     2.4.4  CELIV .............................................. 41
2.5  Organic solar cells  ...................................... 44
     2.5.1  Exciton diffusion towards the interface ............ 44
     2.5.2  Dissociation of CT states .......................... 44
     2.5.3  CT recombination  .................................. 45
     2.5.4  Flat and bulk heterojunction ....................... 46
     2.5.5  Transport layers ................................... 46
     2.5.6  Thin film optics ................................... 50
     2.5.7  Current-voltage characteristics and equivalent
            circuit ............................................ 50
     2.5.8  Solar cell efficiency .............................. 57
     2.5.9  Limits of efficiency ............................... 57
     2.5.10 Correct solar cell characterization ................ 58
     2.5.11 The "O-Factor" ..................................... 60

3    Materials and experimental methods ........................ 61
3.1  Materials  ................................................ 61
3.2  Device fabrication and layout ............................. 65
     3.2.1  Layerdeposition .................................... 65
     3.2.2  Encapsulation ...................................... 67
     3.2.3  Homogeneity of layer thickness on a wafer .......... 68
     3.2.4  Device layout ...................................... 69
3.3  Characterization .......................................... 71
     3.3.1  Electrical characterization ........................ 71
     3.3.2  Sample illumination ................................ 71
     3.3.3  Temperature dependent characterization ............. 72
     3.3.4  UPS ................................................ 74

4    Simulations ............................................... 75
4.1  SCLCtool .................................................. 75
     4.1.1  Model .............................................. 75
     4.1.2  Calculation strategy and algorithm ................. 76
     4.1.3  Discussion of the algorithm  ....................... 79
     4.1.4  Test calculations .................................. 79
     4.1.5  Conclusion and outlook  ............................ 80
4.2  OSOL simulation - a comprehensive multilayer drift-
     diffusion simulation ...................................... 80

5    Charge transport characterization in organic
     semiconductors ............................................ 85
5.1  Design of single carrier devices .......................... 85
     5.1.1  General design requirements ........................ 86
     5.1.2  Single carrier devices for space-charge limited
            current .............................................
     5.1.3  Ohmic regime ....................................... 87
     5.1.4  Design of injection and extraction layers .......... 88
5.2  Advanced evaluation of SCLC - potential mapping ........... 88
     5.2.1  Potential mapping by thickness variation .......... 103
     5.2.2  Further evaluation of the transport profile ....... 104
     5.2.3  Injection into and extraction from single carrier
            devices ........................................... 105
     5.2.4  Majority carrier approximation .................... 109
5.3  Proof of principle: POEM on simulated data ............... 111
     5.3.1  Constant mobility ................................. 111
     5.3.2  Field dependent mobility .......................... 116
     5.3.3  Field and charge density activated mobility ....... 120
     5.3.4  Conclusion ........................................ 120
5.4  Application: Transport characterization in organic
     semiconductors ........................................... 125
     5.4.1  Hole transport in ZnPc:C60 ........................ 125
     5.4.2  Hole transport in ZnPc:C60 - temperature
            variation ......................................... 128
     5.4.3  Hole transport in ZnPc:C60 - blend ratio
            variation ......................................... 139
     5.4.4  Hole transport in ZnPc:C70 ........................ 148
     5.4.5  Hole transport in neat ZnPc ....................... 148
     5.4.6  Hole transport in F4-ZnPc:C60 ..................... 153
     5.4.7  Hole transport in DCV-5T-Me33:C60 ................. 153
     5.4.8  Electron transport in ZnPc:C60 .................... 157
     5.4.9  Electron transport in neat Bis-HFl-NTCDI .......... 158
5.5  Summary and discussion of the results .................... 162
     5.5.1  Phthalocyanine: C60 blends ........................ 162
     5.5.2  DCV-5T-МеЗЗ:C60 ................................... 170
     5.5.3  Conclusion	 ....................................... 171

6    Organic solar cell characteristics: the influence of
temperature ................................................... 173
6.1  ZnPc:C60 solar cells ..................................... 173
     6.1.1  Temperature variation ............................. 173
     6.1.2  Illumination intensity variation .................. 180
6.2  VOC in flat ar)d bulk heterojunction organic solar 
     cells .................................................... 188
     6.2.1  Qualitative difference in VOC(I, T) ............... 189
     6.2.2  Interpretation of VOC(I, T) ....................... 191
6.3  BHJ stoichiometry variation .............................. 194
     6.3.1  VOC upon variation of stoichiometry and contact
            layer ............................................. 194
     6.3.2  V0 upon stoichiometry variation ................... 196
     6.3.3  Low donor content stoichiometry ................... 198
     6.3.4  Conclusion from stoichiometry variation ........... 199
6.4  Transport material variation ............................. 200
     6.4.1  HTM variation ..................................... 200
     6.4.2  ETM variation ..................................... 201
6.5  Donor:acceptor material variation ........................ 202
     6.5.1  Donor variation ................................... 202
     6.5.2  Acceptor variation ................................ 204
6.6  Conclusion ............................................... 208

7    Summary and outlook ...................................... 211
7.1  Summary .................................................. 211
7.2  Outlook .................................................. 212

A    Appendix ................................................. 217
     A.1  Energy pay-back of this thesis ...................... 217
     A.2  Tables and registers ................................ 222

Sources and references ........................................ 233


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