B-site Co-doping-Induced Symmetry Breaking for Enhanced Optoelectronic Properties in Vacancy-Ordered Double Perovskites

Abstract

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(Bi0.5Nb0.5)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 (μh ≈ 10 cm2 V-1 s-1) with negligible electron mobility (μe < 1 cm2 V-1 s-1), is transformed into an n-type system with substantially enhanced electron transport (μe ≈ 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.