Interfacial Engineering Based on Polyethylene Glycol and Cuprous Thiocyanate Layers for Efficient and Stable Perovskite Solar Cells

Abstract Cuprous thiocyanate (CuSCN) is an ideal inorganic hole transport material for perovskite solar cells (PSCs). However, polar solvents for film deposition of CuSCN run a risk of destroying the perovskite light‐absorbing layer. There is also a poor contact between perovskites and CuSCN, restricting photovoltaic performance and operational stability of PSCs. In this work, a polyethylene glycol (PEG‐10000) is employed as an interlayer to effectively overcome the obstacles mentioned above. By introducing this ultra‐thin layer at the interface between (MAFA)Pb(IBr) 3 and CuSCN, the power conversion efficiency (PCE) and operational stability of the related PSCs are both greatly improved. The results confirm that the insertion of PEG cannot only prevent the destruction of bulk perovskites but also reduce the traps/defects at the interface through its strong chemical bonding with undercoordinated ions, which significantly lowers the potential barrier between the perovskite and the hole transport layer. An improved PCE of 19.2% can be achieved in the CuSCN‐based PSCs by interface engineering with PEG. The introduction of PEG can also protect the perovskite film from moisture attack, and greatly enhance device stability. The PEG‐treated PSCs without encapsulation maintain more than 90% of the initial efficiency after 1860 h in the ambient air.