Cs2AgBiX6 (X = Br, Cl): New Visible Light Absorbing, Lead-Free Halide Perovskite Semiconductors

The double perovskites Cs2AgBiBr6 and Cs2AgBiCl6 have been synthesized from both solid state and solution routes. X-ray diffraction measurements show that both compounds adopt the cubic double perovskite structure, space group Fm3̅m, with lattice parameters of 11.2711(1) Å (X = Br) and 10.7774(2) Å (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = Cl) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH3NH3PbBr3 and 3.00 eV for CH3NH3PbCl3. Band structure calculations indicate that the interaction between the Ag 4d-orbitals and the 3p/4p-orbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compounds are stable when exposed to air, but Cs2AgBiBr6 degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semiconductors.