Solution-processed CuI as a hole transport layer for Sn–Pb perovskite solar cells

Abstract

Mixed tin–lead (Sn–Pb) perovskites have received a lot of attention as active layers for both single-junction and tandem solar cells because of their optimal band gap. Generally, Sn–Pb perovskite solar cells are fabricated in a p–i–n structure using PEDOT:PSS as the hole transport layer (HTL), though its hygroscopic nature adversely affects the long-term stability of the perovskite layer. Here, we propose copper iodide (CuI), with high conductivity, wide bandgap, and optical transparency, as a low-cost and earth-abundant HTL candidate for Sn–Pb perovskite solar cells. Utilizing CuI as the HTL and an active layer composed of EACl_0.015Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ passivated with sodium fluoride (NaF), we obtained a remarkable short circuit current of 32.85 mA cm⁻² with a champion power conversion efficiency of 20.1%, while control devices with PEDOT:PSS showed an efficiency of 19.03% and a J_sc value of 30.59 mA cm⁻². In addition, the implemented CuI layer improved the device stability when stored in N₂ and under thermal stress conditions at 85 °C.