Lead-free halide double perovskites have attracted considerable attention as environmentally benign alternatives to conventional lead-based perovskite semiconductors for photonic and optoelectronic applications. Their enhanced chemical stability, structural versatility, and compositional flexibility make them promising candidates for light-emitting devices, photodetectors, and integrated photonic components [1]. In particular, layered double perovskites exhibit strong quantum confinement and enhanced excitonic effects arising from their quasi-two-dimensional crystal structure, providing an attractive platform for tailoring light–matter interactions. However, effective strategies for engineering their electronic structure and optical properties remain relatively unexplored.

Here, we investigate heterovalent cation alloying as an approach to bandgap engineering in lead-free layered double perovskites. Sn-alloyed Ag–Bi-based layered double perovskite microcrystals were synthesized over a broad range of alloy compositions by partially substituting Bi³⁺ with Sn²⁺. Structural, optical, and vibrational characterization was performed to elucidate the influence of heterovalent alloying on the crystal lattice and electronic structure. Particular attention was paid to the evolution of the optical absorption edge and the corresponding bandgap as a function of Sn content.

The alloyed materials exhibit a pronounced composition-dependent evolution of their optical properties. Optical absorption measurements reveal a systematic red shift of the absorption edge with increasing Sn incorporation at intermediate alloying levels, corresponding to a significant narrowing of the optical bandgap. This behavior indicates that heterovalent substitution induces substantial modification of the electronic states rather than a simple interpolation between the parent compounds. In addition to bandgap tuning, incorporation of Sn²⁺ is expected to influence lattice distortions and vibrational properties, further affecting carrier relaxation and energy transfer processes within the material.

These results demonstrate that heterovalent cation alloying provides an effective route for continuously tuning the electronic structure of layered double perovskites. The observed composition-dependent bandgap modulation highlights the potential of this material platform for designing lead-free perovskites with tailored optical responses and controllable light–matter interactions. Such tunability makes Sn-alloyed layered double perovskites promising candidates for next-generation photonic and optoelectronic technologies [2–4].