Bowing of transport gap in hybrid halide perovskite alloys (CH3NH3Sn1-xPbxI3): Which band is responsible?

Apart from being a unique material for efficient solar cells, hybrid halide perovskites possess more mysteries than ever. An anomalous bandgap behavior in CH3NH3Sn1-xPbxI3 alloys has been reported recently [Hao et al., J. Am. Chem. Soc. 136, 8094 (2014)], in which the composition-dependent optical bandgap follows nonmonotonic and nonlinear characteristics instead of a linear trend or Vegard's law; the bandgap of the intermediate compounds was lower than that of the end members. In this article, we study composition-dependent conduction and valence band energies through scanning tunneling spectroscopy to deliberate on the role of the two bands in the bandgap bowing phenomenon and the underlying mechanism. We observe a nonlinear behavior of the two bands with respect to the alloy composition, leading to an anomalous behavior in the transport gap as well. We confirm that two competing events, namely, a spin-orbit coupling parameter appearing due to inclusion of a high-Z material and structural distortion affecting molecular orbitals responsible for the formation of the valence and the conduction bands, result in bandgap bowing in CH3NH3Sn1-xPbxI3 alloys.