Origin of the dark-current ideality factor in polymer:fullerene bulk heterojunction solar cells

In organic bulk heterojunction solar cells, a deviation of the ideality factor of the dark current from unity is commonly put forward as evidence for the presence of trap-assisted recombination. We demonstrate that the non-ideality of the dark characteristics is determined by deeply trapped carriers in the transport-dominating constituent of the donor:acceptor blend, rather than a trap-assisted recombination mechanism. The light-intensity dependence of the open-circuit voltage confirms the absence of trap-assisted recombination and demonstrates that the dominant recombination mechanism in the investigated polymer:fullerene solar cells is bimolecular.