Bimodal Bandgaps in Mixed Cesium Methylammonium Lead Bromide Perovskite Single Crystals

Alloying inorganic cations such as Cs+ or Rb+ into hybrid lead halide perovskites has been shown to provide long-term material stability critical to the implementation in solar cells, but little is known about how cation alloying affects the electronic properties. Here we study single crystals of mixed-cation lead bromide perovskite, Cs-x(CH3NH3)(1-x) PbBr3, in a range of cation mixing ratios, x = 0.05-0.3. In contrast to the continuous bandgap tunability known for mixed-anion alloys, we find that the bandgaps of Cs-x(CH3NH3)(1-x) PbBr3 single crystals adopt a bimodal distribution, with bandgaps similar to those of pure CH3NH3PbBr3 for x <= 0.13 and pure CsPbBr 3 for x > 0.13. Single-crystal X-ray diffraction reveals a structural origin of this abrupt change in bandgap: with increasing Cs concentration, there is a similar to 3% lattice shrinkage and appearance of twin splitting at x >= 0.13. These findings are manifestations of the structural complexity and phase instability of lead halide perovskites.