Cs 2 AgBiX 6 (X = Br, Cl): New Visible Light Absorbing, Lead-Free Halide Perovskite Semiconductors

The double perovskites Cs 2 AgBiBr 6 and Cs 2 AgBiCl 6 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 Fm 3̅ m, 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 CH 3 NH 3 PbBr 3 and 3.00 eV for CH 3 NH 3 PbCl 3 . 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 Cs 2 AgBiBr 6 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.