Femtosecond Time-Resolved Fluorescence Study of TiO2-Coated ZnO Nanorods/P3HT Photovoltaic Films

The photovoltaic performance of organic-inorganic hybrid films based on regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and ZnO nanorods depends strongly on the quality of the interfaces between the organic polymer and the inorganic nanostructures. A thin, solution-processed TiO2 interlayer was introduced on the surface of ZnO nanorods to improve the interface between ZnO and P3HT. In order to understand the effects of the TiO2 interlayer on the initial photophysical processes after photoexcitation, femtosecond time-resolved fluorescence dynamics of the hybrid ZnO nanorods/P3HT (ZnO/P3HT) films with or without TiO2 interlayer coating were comparatively studied using a fluorescence up-conversion technique. Because of the well-aligned P3HT nanodomains in these annealed hybrid films, energetically downhill excitation energy transfer was found to be the dominant process in the subps time scale with an excitation at 400 nm. The fluorescence decay times for the ZnO-TiO2/P3HT film were significantly extended, which is likely due to the improved interface quality with fewer traps and defects when TiO2 interlayer was introduced. Moreover, ultrafast anisotropy decay and a dramatic decrease in residual anisotropy at the emission of 660 nm were observed for the ZnO-TiO2/P3HT film in comparison to both P3HT pristine film and the ZnO/P3HT film. This suggests a change in the disorder of energy, the local field, and an efficient energy-transfer process, resulting in a faster exciton reaction (dissociation) within the ZnO-TiO2/P3HT film. Solar cells fabricated using TiO2-coated ZnO nanorods and P3HT showed efficiencies of up to 0.76%, a 58% improvement over the TiO2-free equivalent devices.