Thiol-bearing Tertiary Alkylammonium Chloride for Regulation of PbI₂ Excess in FAPbI₃ Perovskite Solar Cells

One of the key strategies for record photovoltaic efficiencies in metal halide perovskite solar cells is the addition of PbI₂ excess in a stoichiometric perovskite solution, which controls crystallization, passivates defects, and induces a preferred orientation in the perovskite layer. However, residual PbI₂, typically found in the perovskite layer after crystallization, generates nonradiative recombination centers and promotes ion migration under light and heating stress, thus accelerating performance loss. To mitigate the above issues, a common strategy is the postdeposition of organic ammonium salts, which interact in situ with residual PbI₂. A special case is the effective use of 2-aminoethanethiol hydrochloride (CYS-HCl), where the thiol (-SH) group forms Lewis acid-base adducts with PbI₂. Here, we adopt a similar alkylammonium salt, 2-diethylaminoethanethiol hydrochloride (DEAET), where the two hydrogens of CYC-HCl are replaced by two ethyl groups, endowing the molecule with a protonated tertiary amine with the ability to more strongly bind to PbI₂. Upon deposition of DEAET on top of the FAPbI₃ film, we show that DEAET decreases the percentage of residual PbI₂ by 40% and totally eliminates Pb⁰, which is produced from photolysis of PbI₂ during illumination from X-rays during the XPS analysis. These two effects lead to enhanced radiative recombination, proving a net passivation effect, while chemical analysis (FTIR and liquid-state NMR) explains that this is due to strong interactions between tertiary protonated ammonium (-NH⁺) and thiol (-SH) groups of DEAET with under-coordinated Pb²⁺. As a result, DEAET-treated devices show enhanced open-circuit voltages, reduced hysteresis, and improved stabilized power conversion efficiencies, approaching 19%. In addition, DEAET significantly enhances device stability, with treated solar cells retaining their initial performance for over 350 h under shelf-storage conditions and maintaining 80% of their initial PCE after more than 2 h of continuous operation under ISOS-L-1 conditions. The encouraging findings of this study lay the foundation for the utilization of tertiary ammonium-thiol-based salts as efficient agents for interface engineering in perovskite solar cells.