Neat bi-layer solar cells of a fullerene acceptor and a cyanine dye donor were prepared using meniscus coating.
Meniscus coating is very material efficient and leads to high quality pinhole-free films. The cells exhibit high open
circuit voltages of 1 volt, only 0.8 eV below the band gap of the cyanine dye. This is one of the smallest differences
reported for organic solar cells and illustrates an almost optimal donor-acceptor energy level alignment.
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.
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