  Halide double perovskites have gained significant attention, owing to their
composition of low-toxicity elements, stability in air and long charge-carrier
lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide
bandgaps, which limit photo conversion efficiencies. The bandgap can be reduced
through hallowing with Sb3+, but Sb-rich alloys are difficult to synthesise due
to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We
develop a solution-based route to synthesis phase-pure Cs2Ag(SbxBi1-x)Br6 thin
films, with the mixing parameter x continuous varying over the entire
composition range. We reveal that the mixed alloys (x between 0.5 and 0.9)
demonstrate smaller bandgaps (as low as 2.08 eV) than the pure Sb- (2.18 eV)
and Bi-based (2.25 eV) compounds, with strong deviation from Vegard's law.
Through in-depth computations, we propose that bandgap lowering arises from the
Type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch
between the Bi and Sb s and p atomic orbitals, coupled with their non-linear
mixing, results in the alloys adopting a smaller bandgap than the pure
compounds. Our work demonstrates an approach to achieve bandgap reduction and
highlights that bandgap bowing may be found in other double perovskite alloys
by pairing together materials forming a Type II band alignment.
