Lead-free, stable mixed halide double perovskites Cs2AgBiBr6 and Cs2AgBiBr6-xClx - A detailed theoretical and experimental study

Recently, lead-free halide-based double perovskites (DPs) have emerged as a promising candidate for photovoltaic and optoelectronic applications. Here we report the synthesis, characterization and electronic structure calculations on lead-free Bi-based mixed-halide double perovskites of Cs2AgBiClxBr6-x stoichiometry. The introduction of Cl- dopant converts these DPs from cubic to orthorhombic or tetragonal crystal structures. Electronic structure calculations indicate that both the bandgap and the nature of the band edges of these materials are intimately related to the kind of halides constituting the BiX6 octahedra. When the BiX6 octahedra contains both Cl- and Br- ions, the band gap increases linearly with increase in chloride content. On the other hand, when the BiX6 octahedra comprise of only one halide (Br(- )or Cl-) ion, the band gap remains nearly equal to that of Cs2AgBiBr6 until the Cl- dopant concentration reaches 50%. Experimental characterization of the synthesized mixed-halide DPs show that the band gap value of 1.98 eV and 1.97 eV, obtained for Cs2AgBiBr6 and Cs2AgBiClxBr6-x, respectively, matches with our theoretical calculations. The PXRD spectrum indicates polycrystallinity in these DPs and changes slightly after one month, showing reasonably good stability. The elemental analysis showed incorporation of a small amount of Cl- (7 wt%) in the perovskite composition. In summary, we have performed detailed computational studies and synthesized and characterized mixed halide-based DPs with high stability for solar cells application.