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

Abstract

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 Fm3̅ 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/4porbitals 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.

Cu–In Halide Perovskite Solar Absorbers

“…59,60 The electron-core interactions are described with the projected augmented wave pseudopotentials 60 6 and CX 6 octahedra ( Supplementary Fig. S1).…”

“…[10][11][12] The significance of this work is stepping out of conventional design principles of replacing Pb 2+ with other similar ns 2 cations and considering instead transmuting two Pb 2+ to the pair of a group IB (Cu + and Ag + ) and a group III (Ga 3+ and In 3+ ) cation, exemplified by Cs 2 [AgIn]Cl 6 . Not only are these halide perovskites free of unwanted toxic Pb, or easily oxidized Sn replacement of Pb, 1,2 as well as avoiding the Ag + Bi transmutation that causes indirect and oversized band gaps, 5,6,13 but they also benefit from the d 10 electronic motif dominated valence bands underlying the successful Cu-based chalcopyrites that enable a rather strong absorption curve and promise favorable doping and good materials stability. 10,14,15 Rapid progress in APbX 3 group hybrid perovskite solar cells research and the challenges it raises.…”

“…1b). 6,5,13 Unfortunately, in contrast with APbX 3 having a direct band gap at the R point of the Brillouin zone, 32 Cs 2 [AgBi]Cl 6 has an indirect gap between the valence band maximum (VBM) at X and the conduction band minimum (CBM) at L (nearly degenerate with the Γ state) (Fig. 1b).…”

Design of Lead-Free Inorganic Halide Perovskites for Solar Cells via Cation-Transmutation

“…Only until quite recently proposals of using Cs 2 AgBiCl 6 and Cs 2 AgBiBr 6 as potential solar absorbers were put forward. [61][62][63] Though exhibiting good stability when exposed to air, neither of them show superior photovoltaic performance due to their indirect band-gap feature and large gap values (above 2 eV). 61,63,64 Given the fact that the group of quaternary A 2 M + M 3+ X VII 6 perovskites is much broader owing to its multinary nature, one wonders if other members may have advantages.…”

Research Update: Bismuth-based perovskite-inspired photovoltaic materials

“…18,40 Cs 2 AgBiBr 6 , which has 3D crystallographic symmetry, has higher dispersion still, with the lowest effective masses of 0.14 for holes along the X-Γ direction or 0.37 along the R-X direction, and 0.37 for electrons along the L-W direction in reciprocal space. 41 However, these effective masses are not as low as found in CH 3 NH 3 PbI 3 , in part due to reduced orbital overlap. Recently, Yan et al introduced the concept of electronic dimensionality to describe the connectivity of the orbitals in the upper valence band and lower conduction band.…”