Robust A‐site Cation Engineering for Stable 2D High‐ <i>n</i> Tin Perovskite Homologs with Bridged Lasing Emission Gaps

Abstract Two‐dimensional high‐ n halide perovskites (e.g., n = 3) offer a unique platform for stable, efficient optoelectronics by synergizing improved stability with bulk‐like carrier transport. However, their development is hindered by the intrinsic trade‐off between continuous bandgap tunability and structural integrity. Here, we report robust A‐site cation engineering to overcome these limitations in n = 3 tin perovskites. By incorporating distinct cations, we synthesized a library of homologous single crystals, including metastable (BA) 2 Cs 2 Sn 3 I 10 (BA + : butylammonium) via growth kinetics control. It is revealed that A‐site cations critically govern structural symmetry, exciton‐phonon coupling, lasing, etc. Tailoring cation composition enables the continuous bandgap tuning (1.62–2.01 eV) with minimal lattice mismatch (&lt;3.6%). A‐site cation engineering idealized the perovskite lattice and improved the crystal quality, e.g., the multication (BA) 2 Cs 0.7 MA 0.4 EA 0.5 GA 0.4 Sn 3 I 10 (MA + : methylammonium, EA + : ethylammonium, GA + : guanidinium) achieves a long carrier lifetime (15.1 ns), nearly three times that of containing single A‐site, and then increases diffusion length to &gt;1 µm. The n = 3 tin perovskites with multication exhibited exceptional phase stability and the corresponding nanolaser bridged the emission gaps between single‐A‐cation analogs, delivering a low threshold and unprecedented stability.