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.

A facile inside-out Ostwald ripening route to the morphology-controlled preparation of TiO₂ microspheres is developed. Here, TiO₂ hollow microspheres (HM) and solid microspheres (SM) are prepared by adjusting the volume ratio of isopropanol (IPA) to acetylacetone (Acac) in the solvothermal process. During the formation process of HM, precipitation of solid cores, subsequent deposition of outer shells on the surface of cores, and simultaneous core dissolution and shell recrystallization are observed, which validate the inside-out Ostwald ripening mechanism. Design and optimization of the properties (pore size, surface area, and trap state) of TiO₂ microspheres are vital to the high performance of dyesensitized solar cells (DSSCs). The optimized TiO₂ microspheres (rHM and rSM) obtained by post-processing on recrystallization, possess large pore sizes, high surface areas and reduced trap states (Ti³⁺ and oxygen vacancy), and are thus ideal materials for photovoltaic devices. The power conversion efficiency of DSSCs fabricated using rHM photoanode is 11.22%, which is significantly improved compared with the 10.54% efficiency of the rSM-based DSSC. Our work provides a strategy for synthesizing TiO₂ microspheres that simultaneously accommodate different physical properties, in terms of surface area, crystallinity, morphology, and mesoporosity.