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 TiO₂ 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 TiO₂ layer and improve the PCE of PSCs. In this work, we further investigate the effect of chemical valences of sulfur on the performance of TiO₂/PVK interfaces, CsFAMA PVK layers, and solar cells using TiO₂ ETL layers treated with Na₂S, Na₂S₂O₃, and Na₂SO₄, respectively. Experimental results show that the Na₂S and Na₂S₂O₃ interfacial layers can enlarge the grain size of PVK layers, reduce the defect density at the TiO₂/PVK interface, and improve the device efficiency and stability. Meanwhile, the Na₂SO₄ interfacial layer leads to a smaller perovskite grain size and a slightly degraded TiO₂/PVK interface and device performance. These results indicate that S^2- can obviously improve the quality of TiO₂ and PVK layers and TiO₂/PVK interfaces, while SO₄^2- 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.