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Title: Performance Improvement of Organic Solar Cells by Metallic Nanoparticles (Prestatieverbetering van organische zonnecellen door metalen nanodeeltjes)
Other Titles: Performance Improvement of Organic Solar Cells by Metallic Nanoparticles
Authors: Niesen, Björn; S0201624
Issue Date: 3-Sep-2012
Abstract: The aim of this thesis is to investigate the utilization of plasmonic metal nanostructures for efficiency enhancement of organic solar cells. This efficiency enhancement strategy exploits the strong near-field enhancement and highly efficient light scattering that originates from localized surface plasmon resonances (LSPRs) excited in metal nanostructures and leads to an increased absorption in the solar cell active layer. In the first part of this thesis, the near-field enhancement is systematically studied by employing a model system of metal nanoparticles (NPs) covered by thin-films of organic molecules. Absorption and both steady-state and transient photoluminescence measurements are used to probe the interactions between the LSPRs excited in the NPs and the excitons in the organic thin-film. By introducing a transparent spacer layer, the range of these near-field plasmon-exciton interactions is determined. The absorption enhancement is found to be accompanied by a strong reduction in exciton lifetime, which is detrimental to the device efficiency. Since the range of both interactions is comparable, isolating the NPs with a layer thick enough to prevent exciton quenching may simultaneously eliminate any beneficial effect due to the near-field absorption enhancement. These results demonstrate that the exploitation of near-field enhancement in organic solar cells is quite challenging. Far-field light scattering, on the other hand, shows promise for producing plasmonically-enhanced organic solar cells. This is demonstrated by equipping an organic solar cell with a plasmonic nanostructured Ag rear electrode. The LSPR of this electrode is tuned to the red absorption tail of the active layer and generates enhanced absorption by light scattering, as verified by experiments and numerical simulations. Due to this absorption enhancement, the plasmonic nanostructured cell exhibits an enhanced power conversion efficiency compared to an optimized, high performance organic solar cell.
Table of Contents: Abstract v
Contents xiii
List of Figures xvii
List of Tables xxi

1 Introduction 1
1.1 Motivation 1
1.2 Organic molecules 3
1.2.1 Molecular orbitals 3
1.2.2 Optical transitions 5
1.3 Organic solar cells 8
1.4 Optical properties of metal nanoparticles 12
1.4.1 Localized surface plasmon resonances 12
1.4.2 Light extinction by metal NPs 14
1.4.3 Metal NP-embedding medium Interactions 19
1.5 Plasmonic solar cells 21
1.5.1 Plasmonic enhancement strategies 21
1.5.2 Literature review for plasmonic organic solar
cells 24
1.6 Thesis outline 26

2 Optical properties of metal nanoparticles covered by
organic thin-films 29
2.1 Introduction 29
2.2 Experiments 30
2.3 Three-dimensional finite-element numerical
simulations 32
2.4 Results and discussion 33
2.4.1 Morphology and optical properties of the Ag NP
layer 33
2.4.2 Excitation of multiple dipole resonances 37
2.5 Conclusions 41

3 Plasmon-exciton near-field interactions: absorption 43
3.1 Introduction 43
3.2 Results and discussion 44
3.2.1 Absorption enhancement in CuPc 44
3.2.2 Spectral dependence and influence of the
permittivity 51
3.2.3 Effect of a spacer layer 53
3.3 Conclusions 57

4 Plasmon-exciton near-field interactions: emission 59
4.1 Introduction 59
4.2 Experiments 60
4.3 Theory 62
4.4 Results and discussion 64
4.4.1 Analytical calculations 64
4.4.2 Experimental results 71
4.4.3 Comparison between experiment and theory 77
4.5 Conclusions 81

5 Organic solar cells with nanostructured metal rear
electrode 83
5.1 Introduction 83
5.2 Experiments 84
5.3 Results and discussion 87
5.3.1 Reference device optimization 89
5.3.2 Optical properties of the nanostructured Ag
layer 92
5.3.3 Organic solar cells with nanostructured Ag rear
electrode 97
5.4 Conclusions 101

6 General conclusions and outlook 103

Bibliography 107
Curriculum vitae 123
List of publications 125
ISBN: 978-94-6018-550-2
Publication status: published
KU Leuven publication type: TH
Appears in Collections:ESAT - MICAS, Microelectronics and Sensors

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