Directed photocurrents in nanostructured TiO2/SnO2 heterojunction diodes

Time-resolved photocharge (TRPC) measurements are applied for the first time to demonstrate the presence of a contact potential between lightly sintered colloidal nanocrystalline TiO2 films deposited on transparent highly conductive tin oxide films degenerately doped with fluorine, SnO2(F). By virtue of its contactless nature and absence of an externally 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 ambiguities encountered with many other measurement techniques. Using TRPC, vectorial directions of photoelectron currents in glass/SnO2(F)/TiO2 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 for exposure directly on the TiO2). This implies that photoelectrons always flow toward the SnO2(F)/TiO2 interface. In a control experiment, in the absence of SnO2(F), with the nanoporous TiO2 deposited directly onto the glass, the TRPC signal polarity is negative regardless of sample orientation. These results suggest the presence of a contact potential at the SnO2(F)/TiO2 interface, with downward band bending from the TiO2 to the SnO2(F). The implications of these results in photovoltaic applications are discussed.