Solid solution phenomenon in the amorphous conjugated polymer:fullerene bulk heterojunction structure

The solid solution phenomenon in the amorphous PCDTBT:PC71BM blend was investigated. It has been disclosed that the PCDTBT:PC71BM blend with less than 20% PC71BM may form a kind of solid solution structure, which demonstrates a deeper HOMO level, a higher hole mobility, more free charges and less trap density than those of the pure PCDTBT film. It may well explain why the typical PCDTBT:PC71BM bulk heterojunction solar cells require higher PC71BM ratio to achieve a high power conversion efficiency. In that case, a portion of PC71BM is well intermixed with PCDTBT to form a solid solution phase and others form a pure PC71BM phase. It is the PCDTBT:PC71BM solid solution but not the pure PCDTBT to serve as the donor phases and to form interpenetrating networks with PC71BM phases. The PCDTBT:PC71BM solid solution phases favor to increase the VOC and enhance the charge collection leading to a high photovoltaic performance for the PCDTBT:PC71BM bulk heterojunction solar cells.