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

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)₂Cs₂Sn₃I₁₀ (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 (<3.6%). A-site cation engineering idealized the perovskite lattice and improved the crystal quality, e.g., the multication (BA)₂Cs_0.7MA_0.4EA_0.5GA_0.4Sn₃I₁₀ (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 >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.