DIRECTED PHOTOCURRENTS IN NANOSTRUCTURED TIO2 SNO2 HETEROJUNCTION DIODES/

Time-resolved photocharge (TRPC) measurements are applied for the firs t time to demonstrate the presence of a contact potential between ligh tly sintered colloidal nanocrystalline TiO2 films deposited on transpa rent highly conductive tin oxide films degenerately doped with fluorin e, SnO2(F). By virtue of its contactless nature and absence of an exte rnally applied field, TRPC has previously been demonstrated to possess the capability to directly probe the electric field in semiconductor heterojunction particulate materials without the complications and amb iguities encountered with many other measurement techniques. Using TRP C, vectorial directions of photoelectron currents in glass/SnO2(F)/TiO 2 samples, as determined by signal polarity, are found to be dependent on their orientation with respect to the incident photon flux (i.e., negative polarity for exposure through the glass; positive polarity fo r exposure directly on the TiO2). This implies that photoelectrons alw ays flow toward the SnO2(F)/TiO2 interface. In a control experiment, i n the absence of SnO2(F), with the nanoporous TiO2 deposited directly onto the glass, the TRPC signal polarity is negative regardless of sam ple orientation. These results suggest the presence of a contact poten tial at the SnO2(F)/TiO2 interface, with downward band bending from th e TiO2 to the SnO2(F). The implications of these results in photovolta ic applications are discussed.