Determination of Singlet Exciton Diffusion Length in Thin Evaporated C60 Films for Photovoltaics

C-60 is used as an electron acceptor in small molecule photovoltaic devices in combination with various electron donors. The transport of excitons, i.e., bound electron/hole pairs, is an important factor determining the efficiency of such devices. Here we investigate the exciton diffusion length in C-60 with the electrodeless time-resolved microwave conductance (TRMC) technique. Bilayers of 30 nm Zn-phthalocyanine with a C-60, layer with variable thickness are prepared by physical vapor deposition. Analysis of the photoconductance with an exciton diffusion model yields a diffusion length of 7 nm, and the mobility of holes along Zn-Phthalocyanine stacks is close to 1 cm(2)/(V s). From analysis of the rise and decay of the TRMC transients, we attribute the photoconductance to diffusion and dissociation of singlet excitons rather than triplets. The energy transfer rate between C-60 molecules exceeds 8 X 10(10) s(-1). Exciton diffusion cannot be described by the Forster model due to the close proximity of the molecules.