Bridging Effects of Sulfur Anions at Titanium Oxide and Perovskite Interfaces on Interfacial Defect Passivation and Performance Enhancement of Perovskite Solar Cells.

Interfacial defects at the electron transport layer (ETL) and perovskite (PVK) interface are critical to the power conversion efficiency (PCE) and stabilities of the perovskite solar cells (PSCs) via significantly affecting the quality of both interface contacts and PVK layers. Here, we demonstrate a simple ionic bond passivation method, employing NaS solution treatment of the surface of titanium dioxide (TiO) layers, to effectively passivate the traps at the TiO/Cs(MAFA)Pb(BrI) PVK interface and enhance the performance of PSCs. X-ray photoelectron spectroscopy and other characterizations show that the NaS treatment introduced S ions at the TiO/PVK interface, where S ions effectively bridged the TiO ETL and the PVK layer via forming chemical bonds with Ti atoms and with uncoordinated Pb atoms and resulted in the reduced defect density and improved the crystallinity of PVK layers. In addition, the S ions can effectively enlarge the grain size of the PVK layers. The average PCE of solar cells is improved from 15.77 to 19.06% via employing the NaS-treated TiO layers. This work demonstrates a simple and facile interface passivation method using ionic bond passivation to afford high-performance PSCs. The bridging effect of S ions may inspire the further exploration of the ionic bond passivation and sulfur-based passivation materials.