Optimization of semitransparent inverted organic photovoltaic cells using optical modeling

Abstract

Solar cells generate electricity from sunlight. It is one of the promising renewable energy technologies which have become important in recent years due to environmental issues, such as global warming and air pollution. With the worries over the depletion of conventional fossil fuels it is gaining ever more interest. This thesis describes the study conducted on organic thin-film solar cells, particularly investigating the possibility of semitransparent photovoltaic cells which can be used for building integrated photovoltaics (BIPV). Normal and inverted solar cells are fabricated following the steps in the literature and their performances are compared. As buffer layers, PEDOT:PSS is used for normal structure and a thin layer of $Cs_2CO_3$ is used for inverted cells. Efficiency up to 4.2% is achieved, which is the highest among the evaporated $Cs_2CO_3$ based cells reported in the literatures. Efficient semitransparent organic photovoltaic (ST-OPV) cells are presented in an inverted geometry employing $ZnS/Ag/WO_3$ (ZAW) as a top anode and $ITO/Cs_2CO_3$ as a bottom cathode. Upon identification of the light absorption that differs depending on the illumination direction, the degree of the absorption asymmetry is tuned by varying the ZAW structure to maximize the efficiency for one direction or to balance it for both directions. Optical simulation is performed to find the appropriate thickness for each design scheme. $WO_3$ has relatively small effect on the optical characteristics of the whole device and is mainly used as a buffer layer. Power conversion efficiency close to that of conventional opaque OPV cells is demonstrated in semitransparent cells for the ITO side illumination by taking advantage of the internal reflection occurring at the organic/ ZAW interface. Cells with efficiencies that are reduced but balanced for both illumination directions are also demonstrated. The EQE spectra of the cells are shown for both illumination directions to verify the v...