Explanation of Effect of Added Water on Dye-Sensitized Nanocrystalline TiO₂ Solar Cell: Correlation between Performance and Carrier Relaxation Kinetics

Time-resolved mid-IR transient absorption spectroscopy is employed to explore the mechanism of improving the performance of dye-sensitized TiO₂ solar cell (DSSC) when a certain amount of H₂O is added into the electrolyte. The relaxation kinetics of dye-sensitized TiO₂ nanocrystalline film and the corresponding DSSC performance are investigated under different conditions. It is found that the interfacial charge recombination is retarded and electron injection efficiency is increased in the water vapour and in the electrolyte when D₂O is added. The values of open-circuit photovoltage V_oc and the short-circuit photocurrent J_sc of the cells are linearly correlated to the product of the two decay time constants. We also observed that V_oc well correlates with electron injection efficiency. It provides a preliminary microscopic account for the function of the added water in improving the performance of DSSCs.