Diameter engineering on TiO₂ nanorod arrays for improved hole-conductor-free perovskite solar cells

TiO₂ nanorod arrays (NAs) have been successfully applied in mesoporous perovskite solar cells (PSCs) due to the direct electron transfer channels, and previous focus has been mainly on the length of TiO₂ NAs to improve the performance. To date, diameter engineering without changing length is still a big challenge, which is another key factor on the performance of TiO₂ NAs-based PSCs. Here, the diameter of TiO₂ NAs was effectively engineered by annealing temperatures from 100 °C to 500 °C to enhance the crystallinity and surface smoothness of TiO_x seed-layers, which restrained the transverse direction to decrease the average diameter of TiO₂ NAs from 74 to 31 nm via the Steric effect. In addition, the smooth TiO_x seed-layers improved the vertical orientation of the TiO₂ NAs. The TiO₂ NAs were further applied as photoanodes in carbon-based PSCs without hole conductor layers. The fine and vertical TiO₂ NAs enhance the performance of PSCs, which mainly results from the improved charge transfer and excellent transmission. The carbon-based PSCs with 31 nm TiO₂ NAs achieved the best performance with power conversion efficiency (PCE) up to 10.15%, which is higher than 7.18% of that based on the common TiO₂ nanoparticles. Therefore, our findings suggest that the annealing of seed-layers at different temperatures is a feasible and effective strategy to acquire diameter engineering on TiO₂ NAs for high performance PSCs.