Atomistic Origins of Enhanced Band Gap, Miscibility, and Oxidation Resistance in α-CsPb_1–xSn_xI₃ Mixed Perovskite

Abstract

The advance made in perovskite-based solar cell technology demands the search for materials with better properties, namely, high stability in operational conditions and suitable electronic structure parameters. In this work, we provide a detailed study for cubic CsPb1–x Sn x I3 alloys. We employed a theoretical model that combines quasiparticle effects via the density functional theory (DFT)-1/2 method and spin–orbit corrections with a rigorous statistical disordered description of the alloy. As the main result, a reliable quantitative expression for the variation of energy gap with the composition that can be directly compared with experiments is given. A particular situation is verified for x = 0.80, where the alloy is predicted to have a band gap of 0.984, 0.033 eV lower than the gap of CsSnI3. Additionally, the model encompasses the nuances necessary to understand the properties’ behavior in this complex system. We verified that “more mixed” configurations are energetically favored, leading to ordering in very small temperatures that evolve to a very stable alloy in the usual growth conditions. Also, based on the thermodynamic results, an antioxidant mechanism for this alloy is proposed. The bowing mechanism is explained in terms of band character and spin–orbit interaction. Finally, the conclusions support CsPb1–x Sn x I3 alloys as a very good potential candidate for photovoltaic applications.

Optoelectronic Properties of Tin–Lead Halide Perovskites

“…Because changes in the metal halide bond angle in these perovskites alter the bandgap, this structural relaxation contributes significantly to bandgap bowing, as has been identified both from first-principles calculations and through experiments. ,, The magnitude of bowing varies somewhat with the choice of A cation and halide anion, which mediate structural relaxation effects through microstrain in the crystal structure . Chemical effects, wherein the atomic orbitals of tin and lead that respectively form the valence band and conduction band edges are mismatched in energy, also have a significant influence on bandgap bowing. , The contribution of spin–orbit coupling to bandgap bowing, previously disputed, can also be understood in relation to chemical effects as spin–orbit coupling enhances the influence of lead on the conduction band minimum so that the mismatch in energy between lead and tin orbitals becomes significant …”

“…While bandgap bowing is prominent in tin–lead halide perovskites, yielding experimental values of b for a range of A cation compositions of between 0.5 and 0.9 at room temperature, ,,, it is almost absent for metal halide perovskites upon halide substitution. , Understanding the origins of bandgap bowing in tin–lead perovskites is therefore helpful to the realization of lowest achievable bandgaps. The emerging literature consensus points toward bandgap bowing in mixed tin–lead perovskites arising mostly from a combination of structural relaxation effects and chemical effects mediated by spin–orbit coupling. ,,− Structural relaxation accommodates the random placement of differently sized lead and tin ions throughout the lattice by bond bending, as a result of which the structure varies locally and is not well described by an average value . Because changes in the metal halide bond angle in these perovskites alter the bandgap, this structural relaxation contributes significantly to bandgap bowing, as has been identified both from first-principles calculations and through experiments. ,, The magnitude of bowing varies somewhat with the choice of A cation and halide anion, which mediate structural relaxation effects through microstrain in the crystal structure .…”

“…57,63 The contribution of spin−orbit coupling to bandgap bowing, 61 previously disputed, 63 can also be understood in relation to chemical effects as spin−orbit coupling enhances the influence of lead on the conduction band minimum so that the mismatch in energy between lead and tin orbitals becomes significant. 64 In addition to the local variations in structure which contribute to bandgap bowing, the metal ratio in mixed tin− lead perovskites affects overall crystal structure. The smaller size of tin cations compared to lead cations results in decreasing lattice parameter values as tin content increases, as has been observed via shifts in X-ray diffraction peak position with tin content.…”

Tunable Band Gap and Rhombohedral Distortion in Lead-Free CsSn1–xGexI3 Mixed Perovskites

“…This result explains why the present studied system does not present the same strong nonlinearity earlier observed in Pb–Sn mixed perovskites. In these materials, the CBM and VBM bands are dominated by different metals, causing intermediate compositions to have narrower gaps than the pure Sn and Pb components. , …”

“…In these materials, the CBM and VBM bands are dominated by different metals, causing intermediate compositions to have narrower gaps than the pure Sn and Pb components. 31,32 IV. CONCLUSIONS In summary, we report a detailed and rigorous analysis of a perovskite of high technological interest, including spin−orbit and quasiparticle corrections as well as the disorder and thermodynamics.…”