Exciton dissociation at a poly(p-phenylenevinylene) C-60 heterojunction

We report measurements of the photovoltaic effect in heterojunction structures consisting of a layer of C-60 thermally evaporated onto a film of the conjugated polymer poly (p-phenylenevinylene) (PPV) sandwiched between electrodes of indium-tin oxide and Al. Peak quantum efficiencies of up to about 9% (electrons collected per incident photon) were measured under short-circuit conditions, with corresponding open-circuit voltages approaching 0.9 V. Increased quantum yields were obtained under forward and reverse biases. We model the response as arising from the dissociation of photogenerated excitons at the PPV/C-60 interface, with electron transfer to the C-60. The C-60 layer acts as a dielectric spacer, providing a constructive enhancement of the optical fields at the PPV/C-60 interface. We have applied this model to estimate an exciton diffusion range of 60 to 80 Angstrom.