Quantitative Analyses of Competing Photocurrent Generation Mechanisms in Fullerene-Based Organic Photovoltaics

The performance of fullerene-based organic photovoltaic devices (OPVs) with low donor concentrations is not limited by the trade-off between short-circuit current density (<i>J</i>_sc) and open-circuit voltage (<i>V</i>_oc), unlike bulk heterojunction OPVs. While the high <i>V</i>_oc in this novel type of OPVs has been studied, here we investigate the mechanisms that govern <i>J</i>_sc, which are not well understood. Three mechanisms, diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and nongeminate recombination during charge transport, are studied analytically by combining various experimental techniques and transfer matrix simulation. We find that exciton dissociation at donor/acceptor interfaces is the dominant factor to produce high <i>J</i>_sc, and at low P3HT concentrations exciton relaxation limits photocurrent generation. With more P3HT inclusion, the creation of interfaces promotes exciton dissocation but also reduces fullerene crystallinity, weakening the driving force for charge separation, and introduces nongeminate recombination sites. Quantitative analyses show that the magnitude of measured <i>J</i>_sc and the donor concentration dependence are well accounted for by these three competing mechanisms