Thermodynamic stability and electronic and optical properties of lead-free hybrid double perovskite alloys MA2B+B3+Br6 (B+ = Ag, K, Tl, B3+ = Bi, Sb, In)

Abstract

Organic–inorganic lead-free halide double perovskites of the general formula A₂B⁺B³⁺X₆ have emerged as compelling candidates to replace toxic lead-based perovskite absorbers in optoelectronic applications. Taking MA₂AgBiBr₆ as the host lattice, we employ first-principles calculations combined with statistical thermodynamic modeling to systematically investigate the phase stability, electronic properties, and optical absorption characteristics of four B-site alloyed systems: MA₂AgSb_xBi_1−xBr₆, MA₂AgIn_xBi_1−xBr₆, MA₂K_xAg_1−xBiBr₆ and MA₂Tl_xAg_1−xBiBr₆. Thermodynamic phase diagram analysis identifies critical temperatures of 395 K (Sb), 281 K (In), 391 K (K), and 417 K (Tl), respectively. Notably, MA₂AgIn_xBi_1−xBr₆ is thermodynamically stable across the entire composition range at 300 K. B-site cation alloying affords precise bidirectional bandgap engineering: substitution with In³⁺ or K⁺ induces systematic bandgap widening, whereas Sb³⁺ or Tl⁺ incorporation results in progressive bandgap narrowing, thereby achieving a tunable optical bandgap spanning 2.07–2.52 eV. Importantly, this bandgap reduction is quantitatively correlated with enhanced absorption across the visible spectrum, thereby establishing a direct structure–property relationship rooted in electronic band alignment. Furthermore, In-doping drives an indirect-to-direct bandgap transition, demonstrating that B-site compositional control simultaneously governs both the magnitude and the nature (direct vs. indirect) of the fundamental bandgap. These combined attributes position the investigated lead-free double perovskites as highly promising materials for wide-bandgap top-cell absorbers in tandem solar cells and for efficient blue–green light-emitting diodes.