Bandgap Engineering of Lead-Free Double Perovskite Cs₂ AgBiBr₆ through Trivalent Metal Alloying

The double perovskite family, A₂ M^I M^III X₆ , is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH₃ NH₃ PbI₃ . Given the generally large indirect band gap within most known double perovskites, band-gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs₂ AgBiBr₆ as host, band-gap engineering through alloying of In^III /Sb^III has been demonstrated in the current work. Cs₂ Ag(Bi_1-x M_x )Br₆ (M=In, Sb) accommodates up to 75 % In^III with increased band gap, and up to 37.5 % Sb^III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs₂ Ag(Bi_0.625 Sb_0.375 )Br₆ . Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three-metal systems are also assessed.