Understanding and Exploiting Interfacial Interactions Between Phosphonic Acid Functional Groups and Co-Evaporated Perovskites

Recent development of p-i-n perovskite solar cells (PSCs) has been fueled by the near lossless substrate interfaces afforded by self-assembled monolayer-based hole transport layers (SAM-HTLs). Substrate interfaces are of particular importance for the deposition of co-evaporated PSCs, substantially influencing the crystal quality and the resultant device power conversion efficiency. This influence adds a layer of complexity for utilizing SAM-HTLs with co-evaporated perovskites due to the underexplored interaction of such perovskites with phosphonic acid functional groups from the SAM-HTLs. In this work., we highlight how exposed phosphonic acid functional groups impact the initial phase, final bulk crystal properties and photovoltaic parameters of co-evaporated perovskites. Our work combines experimental results with density functional theory calculations to demonstrate that the surface interaction is mediated by hydrogen bonding of phosphonic acids with interfacial iodine. Notably, this leads to increased formamidinium iodide (FAI) adsorption, changing the required FAI rate to achieve optimum bulk perovskites. Furthermore, we report on persistent changes in perovskite structure, and stabilization of bulk α-FAPbI3 from these phosphonic acid functional groups. Our work demonstrates the potential for rational design of substrates to increase control of co-evaporated perovskite growth by modification of the HTL.