Interfacial electron transfer dynamics and photovoltaic performance of TiO2 and ZnO solar cells sensitized with Coumarin 343

In this work, the photovoltaic performance and interfacial electron transfer (IET) dynamics of Coumarin 343 (C343) on ZnO and TiO2 nanocrystalline semiconductors (sensitized films) were investigated using femtosecond upconversion spectroscopy. Inert Al2O3 nanostructured films were also used for comparison. In addition, electron transfer dynamics were studied in complete solar cells in the presence of a redox electrolyte in order to investigate the relation between electron transfer and solar cell performance. For the sensitized films, a more rapid IET was observed from C343 towards ZnO compared to TiO2. An IET efficiency of 0.90 and 0.81 from C343 to ZnO and TiO2 films respectively, was found at a transition corresponding to an emission wavelength of 480 nm. In the case of complete cells, electron transfer dynamics showed some retardation compared to the sensitized films. It is noteworthy that, in complete TiO2 cells the injection dynamics were much faster than in ZnO ones, in contrast to the results obtained for sensitized semiconductor films. However, TiO2 cells sensitized with C343 exhibited lower energy conversion efficiency than ZnO ones, indicating that faster injection is not always associated with better solar cell efficiency and that other factors, such as electron recombination, affect significantly the performance of the cells. Indeed, dark current measurements for ZnO and TiO2 cells have shown that the recombination of the injected electrons with accepting sites in the electrolyte occurs more rapidly in TiO2 cells, thus reducing their efficiency.