Contactless determination of the photoconductivity action spectrum, exciton diffusion length, and charge separation efficiency in polythiophene-sensitized TiO2 bilayers

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study photoinduced charge separation in bilayers consisting of a smooth, transparent, 80 nm thick layer of anatase TiO2 onto which poly(3-hexylthiophene) (P3HT) sensitizer layers have been spin-coated. Interfacial charge separation, resulting from excitation of the polymer in the visible, is found to persist well into the millisecond time domain. Photoconductivity action spectra have been measured between 420 and 700 nm for P3HT layer thicknesses, L, from similar to2 to 200 nm. Using this electrodeless technique, the bilayers could be irradiated from either the polymer ("front") or semiconductor ("back") side. On front-side irradiation at 540 nm (close to the absorption maximum of the polymer), the efficiency of charge separation per incident photon (IPCSE) initially increased to a maximum value of 0.8% for L approximate to 10 nm. For thicker layers the IPCSE gradually decreased, eventually to 0.1% for L approximate to 170 nm. On back-side irradiation the IPCSE increased over the first 10 nm to a value close to the maximum found for front-side irradiation, and decreased only slightly for further increase in layer thickness. Analytical expressions for the thickness dependence based on exciton diffusion with a Lambert-Beer excitation profile have been used to fit the experimental data. Best fits were obtained for an exciton diffusion length, Lambda (=root(Dtau) with D the diffusion coefficient and T the natural lifetime), of 5.3 or 2.6 nm depending on whether excitons were taken to be reflected or quenched at the polymer/gas interface, respectively. The IPCSE decreased at high light intensities; an effect that is attributed to the occurrence of exciton-exciton annihilation within the polymer layer.