Scherer Correlating structure and function in small molecule organic solar cells by means of scanning probe and electron microscopy

1. Auflage 2016
ISBN: 978-3-7412-5911-1
Verlag: BoD - Books on Demand
Format: PDF
Kopierschutz: 1 - PDF Watermark

E-Book, Englisch, 200 Seiten

In this work nanoscale properties in active layers of small molecule organic solar cells are studied regarding their impact on device performance. For this, the effect of variations in stack design and process conditions is examined both electrically and with high resolution imaging techniques. Two topics are addressed: (i) the visualization of charge extraction/injection properties of solar cell contacts and (ii) the tailoring of structural properties of co-evaporated material blends for bulk heterojunction (BHJ) organic solar cells. (i) We study the impact of controlled contact manipulation on the internal electric potential distribution of fluorinated zincphtalocyanine (F4ZnPc)/fullerene (C60) organic solar cells under operating conditions. In a detailed analytical study using photoelectron spectroscopy and in-operando scanning Kelvin probe microscopy it is demonstrated that the electric field distribution of organic solar cells at the maximum power point depends in an overproportional manner on contact properties and ranges from bulk to contact dominated even for solar cells with decent device performance. (ii) The morphology of co-evaporated active layer blends depends on both substrate and substrate temperature. Here we study the morphology of F4ZnPc:C60 blends with analytical transmission electron microscopy. For all substrates used is found that co-evaporation of the materials at elevated substrate temperature (100° Cel) induces a distinct phase segregation of F4ZnPc and C60. However, only when using a C60 underlayer, as in inverted devices, also the crystallinity of the segregated C60 phase increases. There is only a slight increase in crystallinity when F4ZnPc acts as an underlayer, as typically for non-inverted devices. Solar cell characterization reveals that the crystalline C60 domains are the main driving force for enhanced free charge carrier generation and higher power conversion efficiencies. With this we could provide a novel explanation why record efficiencies of small molecule organic solar cells are realized in inverted device architecture only.

Autoren/Hrsg.

Scherer, Michael

Inhaltsverzeichnis

1;Title Page;3
2;Copyright;4
3;Table of Contents;7
4;1 Introduction;13
5;2 Fundamentals;17
5.1;2.1 Charge transport in organic semiconductors;17
5.1.1;2.1.1 Hopping transport;18
5.1.2;2.1.2 Polarization effects on the band gap;21
5.2;2.2 Organic solar cells;24
5.2.1;2.2.1 Solar cell characteristics;25
5.2.2;2.2.2 Charge carrier separation in OPV;27
5.2.3;2.2.3 Impact of material properties on OPV device design;31
5.2.4;2.2.4 The role of the built-in potential Vbi;32
6;3 Experimental details;35
6.1;3.1 Analytical methods;35
6.1.1;3.1.1 Atomic force microscopy;35
6.1.2;3.1.2 Scanning Kelvin probe microscopy;39
6.1.2.1;3.1.2.1 In-operando SKPM studies;44
6.1.3;3.1.3 Scanning electron microscopy;47
6.1.4;3.1.4 Focused ion beam microscopy;48
6.1.5;3.1.5 BRR integrated SEM-AFM;49
6.1.6;3.1.6 Transmission electron microscopy;50
6.1.7;3.1.7 Photoelectron spectroscopy;53
6.2;3.2 Vacuum preparation of small molecule solar cells;55
6.2.1;3.2.1 Materials and stacks;57
6.2.2;3.2.2 Experimentals- deposition parameter;60
6.2.3;3.2.3 Vacuum thin film deposition;61
6.2.4;3.2.4 Solar cell characterization;63
7;4 Electric potential distribution of F4ZnPc/C60 small molecule organic solar cells;65
7.1;4.1 Electric potential distribution of the OPV stack under short circuit conditions;66
7.2;4.2 UPS/XPS study on the hole extracting contact;73
7.2.1;4.2.1 Electronic properties;76
7.2.2;4.2.2 Chemical properties;78
7.3;4.3 The influence of FIB preparation on SKPM results;82
7.3.1;4.3.1 State of the art;83
7.3.2;4.3.2 Experimental details;85
7.3.3;4.3.3 Results of SRIM simulations: ion implantation profiles;88
7.3.4;4.3.4 Results of SKPM studies: electric potential profiles;94
7.4;4.4 In-operando SKPM studies on OPV cells with varied hole extracting contacts;97
7.4.1;4.4.1 State of the art;98
7.4.2;4.4.2 Experimental details;101
7.4.3;4.4.3 Results: In-operando SKPM studies;102
7.4.3.1;4.4.3.1 Results 1: Studies on illuminated devices;103
7.4.3.2;4.4.3.2 Results 2: Studies on devices under applied bias voltages;109
7.4.4;4.4.4 Discussion: Prospects and limits for in-operando SKPM studies;115
8;5 Structure-function relationship in F4ZnPc/C60 solar cells;119
8.1;5.1 Bilayer solar cells with varied hole extracting contact;120
8.1.1;5.1.1 State of the art;120
8.1.2;5.1.2 Solar cell results on F4ZnPc/C60 bilayer devices;121
8.1.3;5.1.3 In-situ monitoring of bilayer thin film growth;124
8.1.3.1;5.1.3.1 Growth and coverage studied with AFM;125
8.1.3.2;5.1.3.2 F4ZnPc growth studied with XPS;128
8.1.3.3;5.1.3.3 Conclusion: in-situ monitoring of bilayer stack growth;130
8.2;5.2 C60 crystallinity dictates device efficiency in F4ZnPc:C60 BHJ solar cells;130
8.2.1;5.2.1 State of the art;131
8.2.2;5.2.2 Solar cell results on F4ZnPc:C60 BHJ devices;133
8.2.2.1;5.2.2.1 Conventional BHJ devices;133
8.2.2.2;5.2.2.2 Inverted BHJ devices;135
8.2.2.3;5.2.2.3 Conclusion: solar cell results;137
8.2.3;5.2.3 Revealing the BHJ nanostructure with analytical TEM;138
8.2.3.1;5.2.3.1 Phase separation of F4ZnPc:C60 BHJs;140
8.2.3.2;5.2.3.2 Crystallinity in F4ZnPc:C60 BHJs;144
8.2.4;5.2.4 Prospect on the 3rd dimension-AFM studies on BHJ blends;147
8.2.4.1;5.2.4.1 Conclusion: AFM studies on BHJ blends;150
8.2.5;5.2.5 Discussion: Structure-function relationship;151
9;6 Conclusion and outlook;155
10;Bibliography;159
11;7 Appendix;192
12;Danksagung;199