Bandgap bowing in a zero-dimensional hybrid halide perovskite derivative: spin-orbit coupling versus lattice strain

We have considered a zero-dimensional hybrid halide perovskite derivative system, namely MA(3)(Sb1-xBix)(2)I-9, to study the bandgap dependence on metal substitution. Similar to tin-lead mixed halide perovskites (MASn(1-x)Pb(x)I(3)), the composition dependence of the optical bandgap in the MA(3)(Sb1-xBix)(2)I-9 solid-state alloys showed evidence of a quadratic (bow-like) behavior where an intermediate compound containing an equimolar contribution of antimony and bismuth, MA(3)(Sb0.5Bi0.5)(2)I-9 offered the narrowest bandgap of around 1.90 eV; this is markedly lower than the bandgap of the end members MA(3)Sb(2)I(9) (2.36 eV) and MA(3)Bi(2)I(9) (2.16 eV). In addition, we have observed the bowing in the transport gap of MA(3)(Sb1-xBix)(2)I-9 that has been derived from scanning tunneling spectroscopy and density of states spectra thereof. To explain the underlying mechanism, we speculate that an antagonism between spin orbit coupling and its competing component, namely lattice strain, may have led to this bow-like nature in both optical and transport gaps. The band-diagram of heterojunctions based on MA(3)(Sb1-xBix)(2)I-9 accordingly depended on the metal-composition; solar cell characteristics of the heterojunctions followed the change in the bandgap, morphology, and also the exciton binding energy.