Effect of iodine doping of phthalocyanine on the photocurrent generation in a phthalocyanine/C_<60> heterojunction

Photocurrent generation in an indium?tin oxide (ITO)/iodine-doped Ni-phthalocyanine (NiPc-I_x)/C_<60>/In/Al heterojunction device with x～1 was studied. By keeping the device in air after preparation, the device slowly reached a stationary state in which the sign of the photocurrent is opposite to that of a non-doped ITO/NiPc/C_<60>/In/Al device although the rectification direction for the dark current is the same. By a simulation of incident photon-to-current conversion efficiency spectra and a measurement of internal electric field by electroabsorption spectroscopy, it was elucidated that, in the doped device, the band bending near the phthalocyanine/C_<60> interface is absent and the photocurrent is generated by a weak Schottky barrier at the C_<60>/In interface. It is also shown that the C_<60> film encapsulates the doped iodine into the NiPc-I_x layer to stabilize the doping level and prevent the reaction of iodine with In.