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We developed finite element models of organic solar cells in order to investigate current pathways and dissipative losses under different geometries. The models are of purely resistive nature, as this is sufficient to describe the effects under consideration.
The overall behaviour of the current mostly steers the resistive behaviour of the device and is a delicate consequence of the interplay between the individual layer properties, namely the resistivities and layer thicknesses in combination. The model calculations solely based on external material parameters, i.e. without fitting, yield the spatial distribution of the current densities, potentials and the according resistive losses. In particular, the current pathways are spread out from the entire length of the top contact towards the entire width of the ground contact, running along the electric potential gradient. On the other hand, current crowding appears at the foremost part of the top electrode, resulting in a respective concentration of the resistive loss in this vicinity. The resistive loss in turn is the origin of the heat pattern, which is visible in DLIT/ILIT experiments. The comparison between experiment and simulation shows remarkable agreement.
Having established the description of defect free solar cells, defects were simulated. We utilized the micro-diode-model as another established simulation method to model shunt or blocking contact defects in combination with electro luminescence imaging methods. The respective heat patterns were calculated in FEM. Nice agreement is found between the various experimental and simulation methods.
The respective heat patterns then allow identifying several classes of defects such as shunt defects or blocking contact defects in accordance with their patterns from various imaging measurements, bridging the gap between theory and experiment to further the detailed analysis of organic solar cells.
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