Problems With Solutions: Engineering Alkylammonium Additive Reactivity for Durable High-Quality Perovskite Films

Methylammonium chloride (MACl) is used as a volatile additive in most high-efficiency formamidinium lead iodide (FAPbI3) perovskite solar cells (PSCs). However, MA is known to irreversibly react with FA to form n-methyl formamidinium (nMFA), which is detrimental to the absorber quality. In this work, we explore a series of alternative primary alkylammonium chlorides (RACls) in an antisolvent-free deposition. We analyze ink reactivity toward FA, film crystallinity, photoluminescence quantum yield (PLQY), and durability under light and heat. We find that the alkyl chain has a significant effect on reaction rate with FA, with isopropylammonium (iPA) showing a 10 times slower reaction rate than MA. Diffraction intensity of the (001) perovskite peak is maximized with ethylammonium (EA) and iPA, then decreases with the increasing alkyl chain length of n-propyl (nPA) and n-butyl (nBA) ammonium. Importantly, films cast from aged solutions containing the alternative RACl additives all formed majority black perovskite α-phase, while the MACl controls did not, highlighting the negative impact of nMFA. PLQY of alternative additives were comparable to pristine MACl and was retained in films made after solution aging. Durability tests of unencapsulated films in inert atmosphere under 0.6-sun illumination and 85°C heating revealed increased stability of the black perovskite α-phase by replacing MA, with iPA and nBA offering the best phase stability. Unoptimized devices prepared with fresh inks containing iPACl deliver performance on par with the MACl control. The results presented herein demonstrate promising alternative volatile additives to replace MACl in FAPbI3-based perovskite solar cells.