The Impact of the Electrical Nature of the Metal Oxide on the Performance in Dye-Sensitized Solar Cells: New Look at Old Paradigms

The process of primary charge separation in Ru-based dye-sensitized solar cells (DSSCs) based on both TiO2 and ZnO photoanodes has been studied in fully working devices by time-resolved measurements, including emission and transient absorption (from femtoseconds to microseconds), and electrochemical techniques (currentvoltage characteristics and impedance spectroscopy). By studying the effect of different electrolyte compositions using potential-determining additives (Li+ ions, 4-tert-butylpiridine), we have been able to provide novel insights into the mechanisms of electron injection and dye regeneration across the oxide/dye/electrolyte system. In this respect, the shift of the conduction band that is commonly reported for TiO2 in the presence of additives is not observed for ZnO. In addition, both injection and regeneration for ZnO-based cells are shown to be much slower than those for TiO2 and independent of the electrolyte composition. The slower injection and regeneration for ZnO and its lower sensitivity with respect to the addition of potential-determining additives strongly suggest that the electrical nature of the oxide is crucial to facilitate charge separation across the oxide/dye/electrolyte interface. In addition, electron injection from singlet and triplet states has been identified for both metal oxides. The former process occurs on the ultrafast time scale (<100 fs), while the latter extends over many time scales from single ps to single ns. The present work provides novel insights about the processes of electron injection and dye regeneration in DSSCs as well as the reasons for the lower performance of ZnO-based dye solar cells.