Ru-Doping in TiO2 electron transport layers of planar heterojunction perovskite solar cells for enhanced performance

TiO2 is widely used as an electron transport layer (ETL) material for perovskite solar cells, and various methods have been used to engineer the properties of TiO2 ETLs for further improving the performance of perovskite solar cells. In this study, compact Ru-doped TiO2 films, prepared via a one-step spray pyrolysis method, have been employed as ETLs for planar perovskite solar cells. Compared to the pristine TiO2 film, the doped counterpart exhibits remarkably improved conductivity, as revealed by the conducting atomic force microscopy measurement. Consequently, the optimized device containing a 1% Ru-doped TiO2 ETL presents a power conversion efficiency (PCE) of up to 15.7% (with an average value of 14.74%), which is 17% higher than that of the device using the pristine TiO2 layer (13.42%, with an average value of 12.20%). The mechanism behind the enhancement in photovoltaic parameters has been investigated intensively via physicochemical characterization. A slight upshift of the conduction band minimum (CBM) is observed in the case of Ru-doped TiO2 films. More importantly, fast injection of the photo-generated electrons from a perovskite layer into an ETL is also found when Ru-doped TiO2 is applied as the ETL. Meanwhile, impedance spectroscopy suggests that the application of Ru-doped TiO2 films leads to an increase in recombination resistance and a decrease in selective contact resistance. The enhancement in PCE is attributed to the improved charge injection and transport properties of the Ru-doped TiO2 film, as well as its better band matching with the perovskite layer. These results demonstrate that doping TiO2 ETLs with Ru is an efficient approach to improve the photovoltaic performance of perovskite solar cells. The presented work will provide a potential approach for developing materials with high-quality electron transport layers for efficient perovskite-based photovoltaic devices.