Measurement of diffusion and drift of charge carriers from photocurrent transients

A theoretical equation has been fitted to time-of-flight photocurrent transients of molecularly doped polymers in order to obtain the diffusion coefficient (D) and drift mobility (mu) simultaneously. D and mu did not show the sample thickness dependence such as was previously reported. The logarithm of D and mu. increased linearly with the square root of the electric field and decreased linearly with T-2. The negative field dependence of the mobility in low electric field, obtained from the intersection time of asymptotes of the plateau and the trailing edge of the photocurrent transients, can be interpreted to be a result of the combination of drift and diffusion.