Accelerated electron extraction and improved UV stability of TiO2 based perovskite solar cells by SnO2 based surface passivation

TiO2 is a famous electron-transport-material (ETM) that used widely in perovskite solar cells. Photo-catalysis effect is a highly concerned issue for such TiO2 based devices because of its impact on device stability. To deal with the problem, a strategy was proposed to passivate the surface of TiO2 by a thin layer of in-situ formed SnO2, and then perovskite solar cells (PSCs) were prepared using such modified TiO2. It was observed that, after the surface passivation, photo-to-electric power conversion efficiency, hysteresis as well as stability of the TiO2 based PSCs were all improved. For example, efficiency increased from 14.92 (±1.86)% (AM 1.5G, 100 mW/cm2) to 16.33 (±0.26)%; hysteresis index decreased from 20.49 (±11.85)% to 1.40 (±0.01)%. As for stability, after being stored for 25 days (relative humidity of about 45%, no encapsulation), efficiency could preserve 73.99% of the original value, comparing to 47.55% for device based on bare TiO2. Moreover, the passivation was found to accelerate electron extraction, while retard charge recombination between perovskite and TiO2. The improvement was ascribed to the enhanced contact between perovskite and bottom electron-transport-material, passivated surface defects of TiO2 by SnO2, as well as the high mobility of SnO2. Finally, designated ultraviolet (UV) illumination experiment showed that SnO2 passivation could obviously retard the photo-catalysis activity of TiO2, thus improve the UV stability of the devices.