Effects of Chemical Valences of Sulfur on the Performance of CsFAMA Perovskite Solar Cells.

The low electrical conductivity and the high surface defect density of the TiO 2 electron transport layer (ETL) limit the quality of the following perovskite (PVK) layers and the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Sulfur was reported as an effective element to passivate the TiO 2 layer and improve the PCE of PSCs. In this work, we further investigate the effect of chemical valences of sulfur on the performance of TiO 2 /PVK interfaces, CsFAMA PVK layers, and solar cells using TiO 2 ETL layers treated with Na 2 S, Na 2 S 2 O 3 , and Na 2 SO 4 , respectively. Experimental results show that the Na 2 S and Na 2 S 2 O 3 interfacial layers can enlarge the grain size of PVK layers, reduce the defect density at the TiO 2 /PVK interface, and improve the device efficiency and stability. Meanwhile, the Na 2 SO 4 interfacial layer leads to a smaller perovskite grain size and a slightly degraded TiO 2 /PVK interface and device performance. These results indicate that S 2- can obviously improve the quality of TiO 2 and PVK layers and TiO 2 /PVK interfaces, while SO 4 2- has little effects, even negative effects, on PSCs. This work can deepen the understanding of the interaction between sulfur and the PVK layer and may inspire further progress in the surface passivation field.