Lead halide perovskites are widely studied for optoelectronic devices due to their excellent properties such as tunable emission wavelengths, high photoluminescence quantum yield and remarkable color purity. However, a major drawback is the presence of toxic lead in these materials. In recent years, tin halide perovskites have emerged as promising alternatives for lead halide perovskites due to their lower toxicity. Nevertheless, the rapid oxidation of Sn²⁺ to Sn⁴⁺ makes the stabilization of these materials a big challenge [1]. An alternative strategy is to synthesize low-dimensional tin halide perovskites, which exhibit improved stability compared to three-dimensional (3D) counterparts. Two-dimensional (2D) perovskite can be prepared by replacing the small monovalent cation, such as Cs⁺, methylammonium or formamidinium, with a bigger organic cations, which isolates the inorganic part of the perovskite, thereby improving stability [1].  

Furthermore, although tin iodide perovskites have the best absorption characteristics for efficient devices, as they absorb almost all visible light wavelengths, their instability remains an obstacle due to weaker bonds. Replacing iodide with bromide can help to generate more stable materials, but also blue-shift absorption edge and emission peak. 

Herein, we present 2D tin halide perovskites (Sn-HPs) with mixed halide composition. Using the hot injection method, we prepared perovskites based on 4-fluorophenethylamine (4F-PEA). By comparing 4F-PEA₂SnI₄, 4F-PEA₂SnBr₂I₂ and 4F-PEA₂SnBr₄ we observed that the last two photoactive materials show dual emission, band-to-band (b-b) and self-trapped exciton emission dynamics, depending on the excitation wavelength. This means that the Br-contained Sn HPs show both mobile and localized electrons through the [SnX₆]^4- octahedra layers, deducing the emergence of intraband and STE energy levels where carrier transfer is favored [2]. Nevertheless, only b-b PL emission is only evidenced by pure iodide Sn-HPs, indicating a better octahedra connectivity. This last can be detailed through the operational performance of LEDs, providing the device composed of 4F-PEA₂SnI₄ layer, the highest external quantum efficiency around 0.3%, being this value lower in presence of Br species. At this point, we conclude that the presence of Br and I into Sn-HPs can generate two different types of optoelectronic features, opening the gamut of alternatives to fabricate several LED technologies