Electron diffusion in polymer:fullerene bulk heterojunctions

A method to study the bulk electron diffusion dynamics in poly2-methoxy-5-3 ,7 -dimethyloctyloxy1,4-phenylene-vinylene :1 -3-methoxycarbonyl-propyl-1-phenyl-6,6 C61 PCBM blend films with varying PCBM content is presented. The blends are spin coated on top of dense smooth TiO2 and excited with a visible laser pulse. The photogenerated electrons on PCBM diffuse through the blend film until they reach the interface with TiO2, which serves to collect the electrons. The resulting change in conductance of TiO2 is measured using the time-resolved microwave conductivity technique. Theoretical analysis of the timedependent photoconductance gives the electron diffusion constant De and the lower limit of the time for electrons to decay by trapping or recombination e. De increases from 7.010 13 m 2 /s at a weight fraction PCBM WPCBM=0.11 to 3.510 10 m 2 /s at WPCBM=0.50, while, interestingly, at WPCBM=0.75, a smaller value of 1.210 10 m 2 /s is found. The electron diffusion length increases with PCBM content. The presence of PCBM clusters in blends with a high PCBM content extends the time for electrons to decay, so that the lower limit of the electron diffusion length exceeds the film thickness of 100 nm, which is required for sufficient light absorption in a photovoltaic device.