B-site co-doping-induced symmetry breaking for enhanced optoelectronic properties in vacancy-ordered double perovskites

Vacancy-ordered double perovskites (i.e., Cs₂SnBr₆) have attracted considerable attention due to their intrinsic stability and lead-free composition. However, their high-symmetry cubic framework results in parity-forbidden band-edge transitions, severely suppressing optical absorption and thereby constraining their optoelectronic potential. Herein, we introduce a heterovalent B-site co-doping strategy, wherein Sn⁴⁺ is replaced by a paired M(III)–M(V) cation combination (Sn⁴⁺ → M(III) + M(V)). First-principles calculations reveal that this approach induces a series of low-symmetry tetragonal phases that are thermodynamically, dynamically, mechanically, and thermally stable. The symmetry reduction transforms the intrinsically parity-forbidden direct gap into an indirect gap. Despite the indirect nature, optical absorption is significantly enhanced through the emergence of a strong, low-lying, p → d orbital-allowed transition. Consequently, the absorption coefficient in the visible region exceeds 10⁵ cm⁻¹, and the spectroscopic limited maximum efficiency (SLME) for the optimized Cs₂(Bi_0.5Nb_0.5)Br₆ system increases remarkably from 8% in the pristine phase to 28.57%. Moreover, this strategy enables effective carrier transport modulation. The parent compound, which exhibits dominant p-type behavior (µ_h ≈ 10 cm² V⁻¹ s⁻¹) with negligible electron mobility (µ_e < 1 cm² V⁻¹ s⁻¹), is transformed into an n-type system with substantially enhanced electron transport (µ_e ≈ 10 cm² V⁻¹ s⁻¹). Our study not only overcomes the intrinsic light-absorption bottleneck in vacancy-ordered double perovskites, but also establishes a generalizable strategy for developing optoelectronic materials via atomic-scale symmetry breaking.