High-Performance Planar Perovskite Solar Cells with a Reduced Energy Barrier and Enhanced Charge Extraction via a Na 2 WO 4 -Modified SnO 2 Electron Transport Layer.

Tin oxide (SnO 2 ) has been commonly used as an electron transport layer (ETL) in planar perovskite solar cells (p-PSCs) because it can be prepared by a low-temperature solution-processed method. However, the device performance has been restricted due to the limited electrical performance of SnO 2 and its mismatched energy level alignment with the perovskite absorber. Considering these problems, sodium tungstate (Na 2 WO 4 ) has been employed to modify the SnO 2 ETL. The conduction band minimum of SnO 2 increases and the defects at the ETL/perovskite interface decrease by the modification of the SnO 2 ETL with Na 2 WO 4 , thus reducing the energy barrier between the ETL and perovskite. In addition, the electron extraction ability has been enhanced and the interface recombination between the ETL and perovskite has also been inhibited. As a result, the photovoltaic performance of p-PSCs based on the modified ETL has been improved, and a champion power conversion efficiency of 21.16% has been achieved compared with the control device of 17.30% with an open circuit voltage increased from 1.075 to 1.162 V.