Where does photocurrent flow in polycrystalline CdS?

We have studied the local photoconductance in polycrystalline CdS films using conductance atomic force microscopy under illumination, and found that photoconductivity along the grain boundaries is excited at photon energies significantly smaller than the CdS band gap, Eg, whereas phototransport through the grains is detected only above Eg. In addition, we observed a rather strong persistent photoconductivity effect at both conduction channels. The implications of these findings regarding the band tails in CdS films and photovoltaic applications are discussed.