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Tin halide perovskite (Sn-HPs) photovoltaics could potentially equal or exceed the performance of their more toxic Pb-based analogues if defect state densities, particularly originating from the presence of Sn4+, can be significantly decreased. Numerous additives are incorporated into Sn-HPs to minimize the amount of Sn4+, including SnF2, reducing agents such as hydrazine derivatives, and various antioxidants. Despite the frequent use of additives to reduce Sn4+ content, there is limited understanding of how they function and consequently limited guidance for the development of new additives. Herein, we use cyclic voltammetry to probe the redox behavior of SnI2, SnI4, Sn-HP precursor solutions, and 18 different additives. Through 119Sn NMR measurements we show that hydrochloride containing additives undergo halide exchange with SnI4 to form SnIxCly, which results in decreased Sn4+ concentrations and less p-type character in the Sn-HP films. We find that the most effective additive at lowering the Sn4+ content in FASnI3 is not capable of reducing SnI4 or forming SnIxCly, but rather it acts as a sacrificial and coordinating antioxidant. In general, when selecting additives for Sn-HPs it is important to account for the redox potential, coordination with Sn species, ability to react with oxygen, and the potential for halide exchange. [ABSTRACT FROM AUTHOR]

Unlike single‐component 2D metal halide perovskites (MHPs) exhibiting sharp excitonic photoluminescence (PL), a broadband PL emerges in mixed Pb‐Sn 2D lattices. Two physical models –self‐trapped exciton and defect‐induced Stokes‐shift – are proposed to explain this unconventional phenomenon. However, the explanations provide limited rationalizations without consideration of the formidable compositional space, and thus, the fundamental origin of broadband PL remains elusive. Herein, the high‐throughput automated experimental workflow is established to systematically explore the broadband PL in mixed Pb‐Sn 2D MHPs, employing PEA (Phenethylammonium) as a model cation known to work as a rigid organic spacer. Spectrally, the broadband PL becomes further broadened with rapid PEA2PbI4 phase segregation with increasing Pb concentrations during early‐stage crystallization. Counterintuitively, MHPs with high Pb concentrations exhibit prolonged PL lifetimes. Hyperspectral microscopy identifies substantial PEA2PbI4 phase segregation in those films, hypothesizing that the establishment of charge transfer excitons by the phase segregation upon crystallization at high‐Pb compositions results in distinctive PL properties. These results indicate that two independent mechanisms—defect‐induced Stoke‐shifts and the establishment of charge transfer excitons by phase segregation—coexist which significantly correlates with the Pb:Sn ratio, thereby simultaneously contributing to the broadband PL emission in 2D mixed Pb‐Sn HPs. [ABSTRACT FROM AUTHOR]

Tin halide perovskites suffer from high defect densities compared with their lead counterparts. To decrease defect densities, SnF 2 is commonly used as an additive in tin halide perovskites. Herein, we investigate how SnF 2 compares to other SnX 2 additives (X = F, Cl, Br) in terms of electronic and ionic defect properties in FASnI 3 . We find that FASnI 3 films with SnF 2 show the lowest Urbach energies ( E U ) of 19 meV and a decreased p-type character, as probed with ultraviolet photoemission spectroscopy. The activation energy of ion migration, as probed with thermal admittance spectroscopy, for FASnI 3 with SnF 2 is 1.33 eV, which is higher than with SnCl 2 and SnBr 2 , which are 1.22 and 0.79 eV, respectively, resulting in less ion migration. Because of improved defect passivation, the champion power conversion efficiency of FASnI 3 with SnF 2 is 7.47% and only 1.84% and 1.20% with SnCl 2 and SnBr 2 , respectively.