Problems with solutions: manipulating alkylammonium additive reactivity for durable high-quality perovskite films

Abstract

Methylammonium chloride (MACl) is used as a volatile additive in many high-efficiency formamidinium lead iodide (FAPbI₃) perovskite solar cells. However, MA is known to irreversibly react with FA to form n-methyl formamidinium (nMFA), which is detrimental to absorber quality. In this work, we explore a series of alternative alkylammonium chlorides (RACls) in an antisolvent-free deposition. We analyze their reactivity toward FA in precursor inks and their effect on perovskite thin film crystallinity, photoluminescence quantum yield (PLQY), and durability under light and heat. We find that the length and branching of the alkyl chain has a significant effect on reaction rate with FA, with isopropylammonium (iPA) showing a 10 times slower reaction rate than MA. For films cast with the RACl additives, diffraction intensity of the (001) perovskite peak is maximized with ethylammonium (EA) and iPA, then decreases with further increases in alkyl chain length. Full width at half maximum of the (001) perovskite diffraction peak is smaller for all additives relative to the MACl control, with iPA maintaining a low FWHM after a week of precursor solution aging. Importantly, films cast from solutions aged for one week containing the alternative RACl additives all formed black perovskite α-phase, while the MACl controls did not, highlighting the negative impact of nMFA. PLQY of films made with alternative additives both before and after solution aging is comparable to pristine MACl. Durability tests of unencapsulated films in inert atmosphere under white light 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. The results presented herein demonstrate promising alternative volatile additives to replace MACl in FAPbI₃-based perovskite solar cells.