Organic Semiconductors for Next Generation Organic Photovoltaics
Torabi, Solmaz
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Publication date: 2018
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Torabi, S. (2018). Organic Semiconductors for Next Generation Organic Photovoltaics. University of Groningen.
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Organic Semiconductors
for Next Generation Organic Photovoltaics
Solmaz Torabi PhD thesis
University of Groningen, The Netherlands
Zernike Institute PhD Thesis Series 2018-07 ISSN:1570-1530
ISBN:978-94-034-0399-1 (printed book) ISBN:978-94-034-0398-4 (ebook)
The research presented in this thesis has been carried out in the research group Pho-tophysics & Optoelectronics of the Zernike Institute for Advanced Materials at the University of Groningen, The Netherlands. This work is part of the research program of the Foundation for Fundamental Research on Matter (FOM), which is part of the Netherlands Organization for Scientific Research (NWO).
Cover design: Solmaz Torabi
The maze resembles the interpenetrating network of donor and acceptor molecules in a bulk heterojunction organic solar cells. It is a real maze with a solution. Enjoy negotiating it!
Organic Semiconductors
for Next Generation Organic Photovoltaics
PhD thesis
to obtain the degree of PhD at the University of Groningen
on the authority of the Rector Magnificus Prof. E. Sterken
and in accordance with the decision by the College of Deans.
This thesis will be defended in public on
Friday 26 January 2018 at 14:30 hours
by
Solmaz Torabi
born on 8 March 1982 in Tabriz, Iran
Prof. dr. L. J. A. Koster Prof. dr. M. A. Loi Prof. dr. J. C. Hummelen Assessment Committee Prof. dr. J. Nelson Prof. dr. W. Br ¨utting Prof. dr. K. U. Loos
Contents
1 Introduction 1
1.1 Renewable energies . . . 2
1.2 Solar energy . . . 2
1.3 Solar cell efficiency and characteristic parameters . . . 3
1.4 Organic photovoltaics . . . 4
1.5 Efficiency limits of OPV . . . 5
1.5.1 Enhancement of the dielectric constant . . . 7
1.6 Objective and outline of this thesis . . . 7
2 Theory, Methods 15 2.1 Definition of the dielectric constant . . . 16
2.1.1 Static dielectric constant . . . 17
2.1.2 Dielectric constant in alternating field . . . 18
2.2 Experimental determination of the dielectric constant . . . 19
2.2.1 Impedance spectroscopy . . . 20
2.2.2 Definitions, notations and representations . . . 20
2.3 Determining the dielectric constant from IS data . . . 21
2.3.1 Equivalent circuits for a simple capacitor . . . 23
2.3.2 Biasing and contacts . . . 23
2.4 Determining the charge carrier mobility . . . 25
2.5 Layout of capacitors . . . 26
2.6 Materials . . . 27
3 Strategy for enhancing the dielectric constant 31 3.1 Introduction . . . 32
3.2 Materials . . . 33
3.3 Results and Discussions . . . 37
3.4 Conclusion . . . 40
3.5 Experimental . . . 42
4.1 Introduction . . . 48
4.2 Capacitance measurements reveal doping . . . 49
4.3 Current-voltage characterizations reveal doping . . . 52
4.3.1 Diodes with different cathodes . . . 52
4.3.2 The role of an Al capping layer in the doping effect of LiF . . . 53
4.3.3 The conductivity of PTEG-1 doped with a LiF interlayer . . . 56
4.4 Deposition of LiF onto films of fullerene/polymer blend can lead to doping 56 4.5 Possible doping mechanisms . . . 58
4.6 Conclusions . . . 59
4.7 Experimental . . . 59
5 A rough electrode creates excess capacitance in thin film capacitors 63 5.1 Introduction . . . 64
5.2 Theory . . . 65
5.3 Results and discussion . . . 68
5.3.1 Smooth capacitors . . . 68
5.3.2 Rough capacitors: determining excess capacitance . . . 70
5.3.3 Rough capacitors: polymers . . . 72
5.3.4 Dependence of excess capacitance on the roughness parameters . . 72
5.3.5 Accuracy of the parameters derived from capacitor equation . . . . 74
5.4 Conclusions . . . 75
5.5 Experimental . . . 75
6 Improving the efficiency of bulk heterojunction solar cells 83 6.1 Introduction . . . 84
6.1.1 Electrochemical properties . . . 84
6.1.2 Electron mobility . . . 85
6.2 Optimizing solar cells . . . 87
6.2.1 Device configuration . . . 87 6.3 Morphology . . . 88 6.4 Conclusion . . . 91 6.5 Experimental . . . 91 Publications 97 Samenvatting 103 Acknowledgenments 107
