Synergistic Effect of Precursor and Interface Engineering Enables High Efficiencies in FAPbI Perovskite Solar Cells.

Formamidinium lead iodide (FAPbI)-based perovskite solar cells have gained immense popularity over the last few years within the perovskite research community due to their incredible opto-electronic properties and the record power conversion efficiencies (PCEs) achieved by the solar cells. However, FAPbI is vulnerable to phase transitions even at room temperature, which cause structural instability and eventual device failure during operation. We performed post-treatment of the FAPbI surface with octyl ammonium iodide (OAI) in order to stabilize the active phase and preserve the crystal structure of FAPbI. The formation of a 2D perovskite at the interface depends on the stoichiometry of the precursor. By optimizing the precursor stoichiometry and the concentration of OAI, we observe a synergistic effect, which results in improved power conversion efficiencies, reaching the best values of 22% on a glass substrate. Using physical and detailed optical analysis, we verify the presence of the 2D layer on the top of the 3D surface of the perovskite film.