Highly Efficient Perovskite Solar Cells with Gradient Bilayer Electron Transport Materials

Electron transport layers (ETLs) with suitable energy level alignment for facilitating charge carrier transport as well as electron extraction are essential for planar heterojunction perovskite solar cells (PSCs) to achieve high open-circuit voltage (<i>V</i><sub>OC</sub>) and short-circuit current. Herein we systematically investigate band offset between ETL and perovskite absorber by tuning F doping level in SnO<sub>2</sub> nanocrystal. We demonstrate that gradual substitution of F<sup>–</sup> into the SnO<sub>2</sub> ETL can effectively reduce the band offset and result in a substantial increase in device <i>V</i><sub>OC</sub>. Consequently, a power conversion efficiency of 20.2% with <i>V</i><sub>OC</sub> of 1.13 V can be achieved under AM 1.5 G illumination for planar heterojunction PSCs using F-doped SnO<sub>2</sub> bilayer ETL. Our finding provides a simple pathway to tailor ETL/perovskite band offset to increase built-in electric field of planar heterojunction PSCs for maximizing <i>V</i><sub>OC</sub> and charge collection simultaneously.