Effect of annealing temperature on back electron transfer and distribution of deep trap sites in dye-sensitized TiO₂, studied by time-resolved infrared spectroscopy

The effect of annealing temperature on electron dynamics in Ru complex-sensitized TiO₂ films was studied by highly sensitive measurement of transient IR absorption. The amount of electron injection and the back electron transfer rate were not influenced to a large extent by annealing temperature, but the distribution of deep trap sites was considerably influenced. The difference in solar cell efficiency due to annealing temperature was mainly attributed to the difference in the nature of deep trap sites. By extending the probe light window from the mid-IR (1000-4000 cm^-1) to the near-IR (-10000 cm^-1) region, we investigated deep trap sites directly and found that electrons in deep trap sites have absorption in the near-IR region whose peak locates around 7500 cm^-1. As a whole, the electron dynamics in the films annealed at higher than 400°C were considerably different compared with those in the films annealed at lower than 400°C.