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Thin films of conjugated polymers blended with fullerene derivatives are frequently used as the active material in photovoltaic cells, where the blends are spin-coated from solution directly onto the bottom electrode. For such devices, so-called bulk-heterojunction solar cells, the blend film morphology has been shown to have a strong effect on the performance of the device.
Here we present performance data for solar cells based on spin-coated thin films of the low-bandgap alternating polyfluorene copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), with different morphologies.
Blend films were spin-coated from chloroform and chlorobenzene solutions. We have previously found that the morphology can be controlled from a vertical multilayer structure to a homogeneous distribution, by adjusting the volume ratio of the solvents. Rapid evaporation results in vertical variations in composition, expressed as multilayers of polymer-rich and PCBM-rich phases. In this spontaneously formed structure the surface of the film is strongly enriched with the polymer, which is the low surface energy component. We have also been able to fabricate bilayers with reversed structures as compared to the spontaneously formed multilayers.
Solar cells based on active layers with these different morphologies were fabricated and the current-voltage characteristics under illumination were analysed. We found that the multilayer structure shows the highest energy conversion efficiency, despite of having an excess of the hole-transporting material (APFO-3) near the low-workfunction metal electrode. This is mainly due to a higher fill factor in these multilayer structure devices compared to the devices having the more homogeneous blend structure or to the bilayer devices. This finding indicates that the present models for device operation of bulk heterojunction solar cells are not fully adequate and calls for more systematic studies of the relation between morphology and device performance, in particular including transport phenomena.
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