Surface photovoltage method for evaluation of exciton diffusion length in fluorene–thiophene based copolymers

The surface photovoltage method was used for the study of polymer layers made of copolymers based on fluorene and thiophene derivatives, particularly for the determination of exciton diffusion lengths. The surface photovoltage (SPV) in a polymer layer is the result of dissociation of photogenerated free excitons in electric field of the space charge region (SCR), which forms spontaneously at the surface. Our conception of the SPV calculates both currents from the bulk and currents from the SCR where recombination is considered. Fitting theory to the experimental data allows estimation of the diffusion length and the thickness of the SCR regardless of the thickness of the layers. Photovoltage spectra were measured under illumination from both the free surface (SPVfs) and the substrate (SPVsub) side of the layers. For the copolymers under study the diffusion length was determined to be 20–23 nm.