ZrCl4 for energy level alignment at the perovskite/TiO2 interface

•Post-treatment of the electron transport layer with ZrCl4 in planar PSCs leads to enhanced short-circuit current.•Enhanced stabilized PCE is observed probably due to reduced charge accumulation at the interface.•UPS/XPS measurements reveal that energy band alignment between perovskite and transport layer is achieved and electrical experiments prove that charge extraction is facilitated. Planar lead halide perovskite solar cells (PSCs) with a n-i-p superstrate structure are now attaining certified efficiencies of over 25%, rivaling those of single crystal silicon photovoltaics. Nanoparticulate films of SnO2 or compact layers of TiO2 are usually adopted as the electron transporting layers (ETLs) due to their hole-blocking ability, simultaneously acting as a substrate to control perovskite film crystallization. Despite these advantages, non-radiative recombination, non-ideal band alignment and inefficient electron extraction dominate the interface between the perovskite and ETL, limiting the stabilized output power of the devices. To tackle these issues, in this work, the TiO2 compact layer was post-treated with ZrCl4. Optical, morphological and electronic properties of the ETL and the perovskite films were evaluated and the corresponding photovoltaic devices were electrically characterized. The obtained results indicate that ZrCl4 does not measurably affect the perovskite crystallization but favors energy band alignment at the interface, facilitating electron extraction. Consequently, a stabilized efficiency of 18.6% was attained by the modified devices, increased by ∼18% compared to the reference solar cells. Additionally, the long-term stability against thermal and humidity was improved. These results pave the way for the use of metal chlorides as efficient agents for interface engineering in perovskite-based optoelectronic devices. [Display omitted]