Enhancing the stability of methylammonium-based perovskite solar cells prepared in ambient conditions by adding formamidinium cations

Abstract

Perovskite solar cells (PSCs) represent an up-and-coming emerging technology for the future of clean and renewable energy generation. However, these devices still suffer stability and durability issues due to moisture when exposed and/or prepared under ambient conditions. Here, we investigate the influence of incorporating formamidinium cation into methylammonium-based lead iodide perovskite films to improve the stability of PSCs. The amount of FA⁺ ranges from 0 (pure MAPI) to 1 (pure FAPI) perovskite passing by different amounts (FA_0.125MA_0.875PI, FA_0.25MA_0.75PI, FA_0.5MA_0.5PI). All the solutions and deposition of each film are performed in an uncontrolled ambient atmosphere (relative humidity ranging from 40 % to 60 %), aiming to replicate scalable manufacturing environments. The perovskite films present high crystallinity and uniformity, confirmed by the X-ray patterns, corroborating the predominantly photoactive FAPI α-phase or the MAPI cubic one. Scanning electron microscopy (SEM) images reveal that increasing FA⁺ content leads to larger grain sizes, reducing grain boundaries. Absorption spectra present a gradual red shift from MAPI to FAPI perovskites due to increased FA⁺ content, corresponding with the bandgap energy analysis. PSCs present photocurrent conversion efficiencies up to 13.4 %, and the photocurrent action spectra confirm the absorption observation, presenting energy conversion in higher wavelengths as the FA⁺ contents increase. A long-term stability experiment reveals that perovskites with more than 25 % FA⁺ maintain their efficiency over 80 % after 2160 h, a longer durability than MAPI PSCs. The durability improvement is ascribed to incorporating FA⁺ into the MAPI perovskite crystal lattice, which changes the grain size, surface area, and grain boundaries.