Bimolecular recombination and fill factor in crystalline polymer solar cells

Fill factor (FF) is one of the important photovoltaic parameters, which is limited mainly by charge recombination in solar cells. It is still a challenging issue to realize a high FF with a thick active layer, which is required for the development of efficient bulk-heterojunction polymer solar cells. In this study, we discussed how recombination losses limit charge collection efficiency in polymer solar cells with low FF by transient optoelectronic techniques. As a result, we found that geminate recombination is almost voltage-independent, that bimolecular recombination is suppressed significantly under open-circuit condition, and that hole and electron mobilities are low and imbalanced. We therefore concluded that the low collection efficiency is caused by the bimolecular recombination of accumulated charges due to low and imbalanced mobilities