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(Bi1-x Mx )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(Bi0.625 Sb0.375 )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.