The application of a low-bandgap conjugated oligomer for the sensitization of SnO2 and TiO2

We have studied the photoinduced charge separation in a double-layer of a low-bandgap oligomer (PTPTB) spin-coated onto a smooth layer of SnO2 or TiO2, using the time-resolved microwave conductivity technique. This technique allows the intrinsic properties such as the charge separation efficiency of the sensitizer and the semiconductor (SC) to be measured without the necessity of applying conductive electrode layers that can introduce additional complications. Although the electron affinity of SnO2 is approximately 0.5 eV larger than that of TiO2, interfacial charge separation is possible for both systems. Photoconductivity transients resulting from mobile, conduction band electrons in the SC were measured on pulsed excitation with visible or UV light. For both double-layer systems the photoconductivity action spectrum closely resembles the optical absorption of the PTPTB layer, with a maximum at 560 rim. By relating the observed signals in the visible with those obtained in the UV, we find for the SnO2/PTPTB double-layer a 30 times higher efficiency than that for the TiO2/PTPTB combination. We propose that the lower-lying SnO2 CB edge, and consequently the increased driving force for photoinduced charge transfer, results in a larger forward rate constant. (C) 2003 Elsevier B.V All rights reserved.