Charge carrier lifetime and charge carrier density in organic photovoltaic solar cells

International audience; Aim is to study the behavior of photo-generated charge carrier recombination processes in polymer-fullerene solar cells. Optimal morphology of OPV active layer results in an interpenetrated network of donor/acceptor materials. Optimal size domains are about exciton diffusion length. Oversized domains would result in a decrease of exciton separation efficiency and smaller domains would increase collisions between electrons and holes and that take place at the donor/acceptor (D/A) interfaces. In this study, poly(3-hexyl-thiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have been used as electron donor and acceptor materials respectively since they have been widely studied during last ten years. The blend of P3HT: PCBM has been considered as meta-material in which electrons and holes are transported in the Lowest Unoccupied Molecular Orbital (LUMO) of PCBM and Highest Occupied Molecular Orbital (HOMO) of P3HT respectively. We have demonstrated that the transition between bimolecular and monomolecular recombination mechanisms in BHJ solar cells is driven by the residual doping level. This transition has been observed by two different techniques. The transient open-circuit voltage decay method and illumination intensity dependence of open-circuit voltage confirm a similar nature of the transition. We found that this temperature-dependent transition is related to a trap delimitation energy level, which differentiates mobile from trapped charge carriers. Finally, the density and lifetimes of charge carriers were correlated and experimentally shown to have a one-to-one correspondence at room temperature