Inversion of activity in DSSC for TiO2 and ZnO photo-anodes depending on the choice of sensitizer and carrier dynamics

Abstract

In this study, we have demonstrated an inversion in the efficiency of TiO2 and ZnO based dye sensitized solar cells for an organic sensitizer C343 and another Ru based sensitizer N719. Despite of faster electron injection from sensitizer C343 to TiO2 than ZnO, the later shows a better photovoltaic efficiency. The higher photocurrent in C343-ZnO has been mechanistically justified by slower back electron recombination and higher dye loading obtained from open circuit potential decay, electrochemical impedance spectroscopy and absorption spectra of the sensitized photoanodes. The similar positive features are there for N719-ZnO system also. Still TiO2-N719 performs much better than ZnO-N719 in terms of device efficiency. We have found a signature of H-type aggregate formation of N719 dye on the ZnO photoanode surface which shows a higher energy absorption peak than the monomeric form of the dye in the diffuse reflectance mode absorption spectra of the photoanodes. These type of aggregates are unable to inject electrons into the semiconductor rather the dye aggregates can pull back the electrons from the CB of the ZnO and eventually undergoes ground state recombination. Ultrafast transient absorption spectroscopy and wavelength dependent photo-current spectra provide further insight into the effect of the dye aggregates on the photo-excited carrier dynamics which essentially reduces the light harvesting efficiency.