A-site alkali engineering enables decoupled property tunability in lead-free Bi-based chloride double perovskites: A DFT study of Cs₂NaBiCl₆/Rb₂NaBiCl₆ as p-type UWBG transparent conductors

The development of high-performance p-type ultra-wide bandgap (UWBG) transparent conductors is fundamentally hindered by the inherent “performance coupling” between optical transparency and charge transport: strategies that enhance conductivity typically compromise the wide bandgap essential for transparency, and vice versa. Herein, we propose and computationally validate a novel alkali metal A-site engineering strategy to achieve decoupled property tuning in lead-free bismuth-based chloride double perovskites. Through hybrid-functional density functional theory studies of Cs₂NaBiCl₆ and Rb₂NaBiCl₆, we demonstrate that substituting Cs⁺ with the smaller Rb⁺ induces a subtle lattice strain. This strain selectively strengthens the Bi 6s/Cl 3p antibonding hybridization at the valence band maximum, thereby enhancing hole mobility by ∼40% (from 39.90 to 56.12 cm² V⁻¹ s⁻¹) and reducing the hole effective mass (0.441 m₀ → 0.313 m₀), while leaving the UWBG virtually unchanged (ΔE_g ≈ 0.018 eV; ∼4.67 eV for both). Crucially, this decoupling of transport from the optical gap enables both compounds to retain excellent visible-light transmittance (>90%) and favorable mechanical flexibility (Young's modulus < 30 GPa) for thin-film processing. The work establishes A-site cation substitution as a general design principle for independently tuning electronic transport and optical properties in A₂BB′X₆-type halide double perovskites, providing a clear pathway to overcome the longstanding performance-coupling bottleneck and advance the development of efficient p-type UWBG transparent conductors for optoelectronic applications.