Arne Müller

• This thesis covers four topics about organic solar cells to improve the understanding of cell behaviour with models based on physical processes and the influence of degradation. First, this thesis presents a new model to describe the electrical behavior of solar cells based on physical parameters. This model is based on the assumption of a constant electric field and a linear increasing current in the intrinsic layer (CELIC). As a result, the model describes nicely the currentvoltage (JV) behavior and thereby covers a large range of charge carrier mobilities and illumination intensities. In addition, the crossing point of JV-curves under different illumination intensities can be modeled and correlated to the built-in potential for ohmic contact properties at the interface to the intrinsic layer. Second, organic solar cell behavior under different illumination levels was tested with the JscVoc method, which allows an investigation of the JV curve without a series resistance. An expression for the series and shunt resistance was developed combining JscVoc and JV analysis. It is shown that the conductivity of the intrinsic layer is associated with recombination processes and is dependent on illumination power. From the findings, a model is presented which splits the recombination and extraction into four illumination dependent regimes. Third, the influence of the electron and hole blocking layers on the current-voltage characteristics was tested. It was found that the forward current of the solar cells is originating from an exclusive surface recombination current at the semiconductor-ZnO interface. Forth, the degradation processes in organic solar cells were tested under different external conditions. It was found that the materials and the solar cell do degrade under these conditions by phase separation of the semiconductor mixture, p-doping of the semiconductor, photo-bleaching, and dedoping of the zinc oxide layer.