Ligand-exchange TiO2 nanocrystals induced formation of high-quality electron transporting layers at low temperature for efficient planar perovskite solar cells

Exploration of low-temperature solution-processed methodology for fabricating planar perovskite solar cells (PSCs) is meaningful for simplifying manufacturing, roll-to-roll industrial mass production on flexible substrates and designing perovskite tandem devices. However, some complicated, time consuming, or even high-cost methodologies such as atomic layer deposition, magnetron sputtering, and utilizing careful interface engineering are still needed for preparing efficient planar PSCs with TiO2 electron transport layers (ETLs) at low temperature. Here, we report a simple ligand-exchange strategy to overcome the problems. We use oleic acid (OA) molecules as surface ligands for synthesis well crystalline and monodisperse TiO2 nanocrystals. Subsequently, instead of high temperature decomposition, we find that the ligand-exchange strategy can also totally peel off these insulating ligands on the TiO2 nanocrystal surfaces and form high-quality TiO2 ETLs at low temperature. The OA-free TiO2 ETLs prepared at 150 degrees C show high conductivity, fast electron extraction and transport speeds, low series resistance and high shunt resistance in the assembled PSCs, contributing to high performance devices with slight hysteresis and good reproducibility.