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 non-radiative 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 post-deposition 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-Cl 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 FAPbI3 film, we show that DEAT 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 DEAT with under-coordinated Pb2+. The stabilization of FAPbI3 black phase along with the establishment of a solid barrier to impede the infiltration of moisture into the perovskite layer over time lead to enhanced operational stability for the as-fabricated solar cells. 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.