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

C60 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 C60 with the electrodeless time-resolved microwave conductance (TRMC) technique. Bilayers of 30 nm Zn-phthalocyanine with a C60 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 C60 molecules exceeds 8 × 10(10) s(-1). Exciton diffusion cannot be described by the Förster model due to the close proximity of the molecules.