Space-charge formation in thick MDMO-PPV:PCBM solar cells

One of the most studied systems in the field of organic photovoltaic devices are bulk heterojunction solar cells based on poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and methano[60] fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as electron acceptor. A striking feature of these types of solar cells is that at the optimal device thickness of typically 100 nm only 60% of the incident light is absorbed. It is evident that the absorption can be enhanced by increasing the thickness of the active layer. However, in spite of an increased absorption the overall power conversion efficiency shows a negative trend when increasing the device thickness beyond 100nm. From device point of view the reduced performance with increasing thickness mainly originates from a decrease of the fill factor. We demonstrate that this fill factor decrease is a result of the formation of space-charges in thick devices and charge recombination.