Do Chalcogenide Double Perovskites Work as Solar Cell Absorbers: A First-Principles Study

Organic-inorganic hybrid perovskite solar cells have recently been developed at an unprecedented rate as an emerging solar cell technology, with its certified power conversion efficiency (PCE) (23.7%) surpassing conventional thin-film contenders. However, the poor long-term stability and toxicity of Pb pose major setbacks to its commercialization. Theoretical calculations and experimental trail-and-error processes have recently aimed to find alternative perovskites, including inorganic halide perovskites (CsPbI3, CsPbIBr2, etc.), inorganic halide double perovskites (Cs2AgBiBr6, etc.), and chalcogenide single perovskites (BaZrS3, etc.). However, their material properties are inferior to hybrid perovskite in terms of cell performance and material toxicity. Here, a class of lead-free chalcogenide double perovskites A(2)M(III)M(V)X-6 [A = Ca2+, Sr2+, Ba2+; M(III) = Bi3+ or Sb3+; M(V) = V5+, Nb5+, Ta5+; X = S2-, Se2-] are comprehensively investigated with respect to its stability and electronic and optical properties. First-principles calculations on bandgaps, effective masses, optical absorption, and ideal power conversion efficiencies led to the selection of nine stable double chalcogenide perovskites that exhibit superior optoelectronic properties, i.e., quasi-direct bandgaps, balanced electron and hole effective masses, and strong optical absorption owing to the strong antibonding character both at the valence band maximum (VBM) and conduction band minimum (CBM). Unfortunately, thermodynamic stability calculations on massive decomposition pathways show negative decomposition energies ranging from 0 (-0.37) to -66 meV/atom, indicating the difficulty for a thin-film phase. The most likely compound is Ba2BiNbS6, with its decomposition energies (0 and -22 meV/atom for P2(1)/n and R (3) over bar phases, respectively) within the computational errors, which may be further stabilized by the confinement effect in nanocrystal form.