  We investigated the charge carrier mobility in pristine poly(3-hexyl
thiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM)
blend devices by applying the time resolved photoconductivity experiment in
dependence on the donor:acceptor ratio. We observe a bipolar transport in all
studied samples ranging from pure polymer to polymer:fullerene with 90% PCBM
content. For the ratios P3HT:PCBM 1:4 and 1:1 we observe two transit times in
the electron current transients, as well as hole double transients for
P3HT:PCBM 1:2. We find high hole and electron mobilities in the order of
10^(-3) - 10^(-2) cm^2/Vs for a concentration of 90% PCBM in the blend.
