Metal oxide layer in organic solar cells Doctoral Thesis vorgelegt von Haiyan Sun angefertigt in der Arbeitsgruppe Hybrid Nanostructures Prof. Dr. Lukas Schmidt-Mende Fakultät für Physik Ludwig-Maximilians-Universität München Juni 19, 2012 Erstgutachter: Prof. Dr. Lukas Schmidt-Mende Zweitgutachter:Prof. Dr. Alexander Högele Datum der muendlichen Pruefung: 09.08.2012 Contents Abstract ............................................................................................................................................. v List of Abbreviations ........................................................................................................................ ix 1 Introduction .................................................................................................................................... 1 1.1 Solar cells working principles ............................................................................................. 3 1.1.1 Organic photovoltaic materials ................................................................................ 3 1.1.2 Donor-acceptor concept ........................................................................................... 4 1.1.3 Charge generation .................................................................................................... 5 1.1.4 Morphology in organic solar cells ............................................................................ 8 1.1.5 Recombination mechanisms ................................................................................... 11 1.2 The performance of OPVs................................................................................................. 13 1.2.1 Efficiencies of devices ........................................................................................... 14 1.2.2 Stability of the devices ........................................................................................... 15 1.2.3 To improve the performance of devices ................................................................. 17 1.3 Organic photovoltaic devices ............................................................................................ 19 1.3.1 Organic solar cells .................................................................................................. 19 1.3.2 Dye-sensitized solar cells ....................................................................................... 20 1.3.3 Hybrid solar cells ................................................................................................... 22 1.4 Metal oxides in hybrid solar cells ..................................................................................... 24 1.4.1 TiO2 and ZnO metal oxides .................................................................................... 25 1.4.2 Applications in hybrid solar cells ........................................................................... 26 1.4.3 Nanostructures of metal oxides .............................................................................. 27 2 Experimental Methods ................................................................................................................. 30 2.1 Solar cell architecture ........................................................................................................ 30 2.1.1 Solar cell design ..................................................................................................... 30 2.1.2 Non-inverted solar cells ......................................................................................... 31 2.1.3 Inverted solar cells ................................................................................................. 32 2.1.4 Inverted-inverted solar cells architecture ............................................................... 33 2.1.5 Flexible devices architecture .................................................................................. 34 2.1.6 Transparent devices architecture ............................................................................ 35 2.2 Materials and fabrication methods .................................................................................... 36 2.2.1 Photoactive materials and blend layer .................................................................... 36 i Contents 2.2.2 Syntheses of the TiO2 blocking layers .................................................................... 37 2.2.3 Syntheses of the ZnO blocking layer ..................................................................... 41 2.2.4 Fabrication of buffer layers .................................................................................... 43 2.2.5 Deposition of electrodes ......................................................................................... 44 2.3 Characterization ................................................................................................................ 45 2.3.1 I-V Characterization ............................................................................................... 45 2.3.2 EQE characterization ............................................................................................. 48 2.3.3 Absorption measurement ........................................................................................ 49 2.3.4 Microscopy techniques ........................................................................................... 50 2.3.5 XRD measurements ............................................................................................... 50 2.3.6 Van der Pauw method and Hall Effect measurement ............................................. 51 2.3.7 Intensity dependence measurement ........................................................................ 53 2.3.8 Impedance spectroscopy measurement .................................................................. 54 2.3.9 Photo-CELIV method ............................................................................................ 55 3. Device Optimization ................................................................................................................... 57 3.1 Blocking layers improvement ........................................................................................... 57 3.1.1 TiO2 Surface Cleaning ............................................................................................ 57 3.1.2 N-Doped TiO2 to increase the absorption .............................................................. 61 3.1.3 Mg-doped ZnO to enhance the VOC ....................................................................... 66 3.1.4 TiO2-ZnO co-blocking layer .................................................................................. 69 3.2 P3HT:PCBM layer improvement ...................................................................................... 71 3.2.1 P3HT:PCBM blend solutions ................................................................................. 72 3.2.2 P3HT:PCBM ratios ................................................................................................ 74 3.2.3 P3HT:PCBM layer thickness ................................................................................. 76 3.2.4 Post-deposition treatment process on blend layer .................................................. 78 3.3 Spray-deposited PEDOT: PSS for air stable inverted organic solar cells ......................... 81 3.3.1 Background ............................................................................................................ 81 3.3.2 PEDOT:PSS layer deposition ................................................................................. 83 3.3.3 Device performance ............................................................................................... 83 3.3.4 Time evolution of the VOC ...................................................................................... 87 3.3.5 Summary ................................................................................................................ 88 3.4 Conclusion ........................................................................................................................ 89 4 Effect of Blocking Layer .............................................................................................................. 91 4.1 Introduction ....................................................................................................................... 91 4.2 Device structure and experimental .................................................................................... 92 4.3 Properties of TiO2 and ZnO layers .................................................................................... 93 4.3.1 Crystallinity ............................................................................................................ 93 ii Contents 4.3.2 Conductivity and mobility ...................................................................................... 94 4.3.3 Surface morphology ............................................................................................... 95 4.3.4 Absorption .............................................................................................................. 97 4.3.5 Summary ................................................................................................................ 99 4.4 Performances of inverted devices based on TiO2 and ZnO ............................................... 99 4.4.1 EQE characterization of TiO2 and ZnO devices ................................................... 100 4.4.2