In situ surface modification of TiO2 by CaTiO3 to improve the UV stability and power conversion efficiency of perovskite solar cells

The usage of TiO2 in perovskite solar cells is always faced with the risk of device decomposition due to its high photocatalysis activity. To deal with this problem, here in this work, a strategy of in situ surface passivation was proposed and performed on TiO2. After spin-coating Ca(OH)2 solution on a TiO2 mesoporous scaffold and annealing, the surface layer of the scaffold was converted into CaTiO3, as confirmed by X-ray photoemission spectroscopy and X-ray diffraction studies. The modified TiO2 scaffold was then used as an electron-transport-material in perovskite solar cells. It was observed that after moderate modification, the short-circuit current density increased from 22.32 (±0.25) to 23.19 (±0.28) mA/cm2, the open-circuit voltage rose from 1.042 (±0.009) to 1.080 (±0.011) V, and the fill factor increased from 63.89 (±1.95)% to 71.37 (±0.43)%, leading to an improvement from 14.92 (±0.36)% to 17.88 (±0.37)% of the power conversion efficiency. Transient photocurrent/photovoltage decay curves and impedance spectroscopy tests showed that moderate modification accelerated charge extraction while it retarded charge recombination. Besides, the in situ CaTiO3 layer prolonged the device stability. After being stored in the dark for 46 days (relative humidity of 30%), 92.6% of the initial efficiency was reserved, compared to that of 68.4% for the control devices. Designated UV irradiation showed that the surface passivation retarded the photocatalysis activity of TiO2, which contributed to the prolonged device stability.