Surface states in TiO₂ submicrosphere films and their effect on electron transport

Owing to their special three-dimensional network structure and high specific surface area, TiO₂ submicrospheres have been widely used as electron conductors in photoanodes for solar cells. In recent years, utilization of TiO₂ submicrospheres in solar cells has greatly boosted the photovoltaic performance. Inevitably, however, numerous surface states in the TiO₂ network affect electron transport. In this work, the surface states in TiO₂ submicrospheres were thoroughly investigated by charge extraction methods, and the results were confirmed by the cyclic voltammetry method. The results showed that ammonia can effectively reduce the number of surface states in TiO₂ submicrospheres. Furthermore, in-depth characterizations indicate that ammonia shifts the conduction band toward a more positive potential and improves the interfacial charge transfer. Moreover, charge recombination is effectively prevented. Overall, the cell performance is essentially dependent on the effect of the surface states, which affects the electron transfer and recombination process.