Enhanced α-phase stability of formamidinium lead iodide with addition of 5-ammonium valeric acid chloride†

Formamidinium lead iodide (FAPbI₃) is a metal halide perovskite composition that exhibits improved thermal stability and a more favorable band gap compared to the archetypical methylammonium lead iodide (MAPbI₃). However, the photoactive α-phase is not thermodynamically stable at operating temperatures, which is a challenge that must be overcome for the viability of FAPbI₃-based photovoltaics. This study explores the use of the ammonium acid additives 5-ammonium valeric acid iodide (5-AVAI) and 5-ammonium valeric acid chloride (5-AVACl), to stabilize the α-phase of FAPbI₃. While both additives stabilize the photoactive α-phase and suppress the formation of the photoinactive δ-phase, increase grain size, reduce non-radiative recombination, and improve carrier lifetimes, the addition of 5-AVACl results in superior performance. The improvements with 5-AVACl added are possibly due to its unique ability to initiate formation of the α-phase of FAPbI₃ prior to annealing. DFT calculations also show that the growth of moisture-stable (111) facets is more favorable with the addition of 5-AVACl. These property improvements result in a significant increase in the power conversion efficiency of solar cells, from 9.75 ± 0.61% for devices with pristine FAPbI₃ to 13.50 ± 0.81% for devices incorporating 1 mol% 5-AVACl.