Bandgap Engineering of Lead-Free Double Perovskite Cs2AgBiBr6 through Trivalent Metal Alloying

The double perovskite family, A(2)M(I)M(III)X(6), is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH3NH3PbI3. 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 Cs2AgBiBr6 as host, band-gap engineering through alloying of In-III/Sb-III has been demonstrated in the current work. Cs2Ag(Bi1-xMx)Br-6 (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 Cs2Ag(Bi0.625Sb0.375)Br-6. 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.