Band-gap engineering in methylammonium bismuth bromide perovskites for less-toxic perovskite solar cells

Abstract

Bismuth-based perovskites could be a viable and efficient alternative to lead-based perovskite materials. To widen possible applications, it is necessary to be able to alter the band gap of the absorber material (i.e. for water splitting or multijunction cells). In this work we show the possibility of colour-management in methylammonium bismuth halide perovskites. When synthesised using DMSO, methylammonium bismuth bromide is characterised and identified as possessing a zero-dimensional perovskite type structure with a formula of MA₃BiBr₆ DMSO solvate, and a band gap of 2.51 eV. An increasing mole fraction of iodide ions into methylammonium bismuth bromide is shown to produce a single-phase with a decreasing band gap, with an associated colour change from yellow (2.51 eV) for methylammonium bismuth bromide to red (1.88 eV) for methylammonium bismuth iodide. The methylammonium bismuth iodide structure is tolerant to high degrees of bromide substitution, up to 40%, before significant effects in band gap or morphology are observed. Halide substitution was also found to aid thin-film stability over 10 weeks in air. The insights obtained from this study could provide additional stability at a reduced cost to lead-free bismuth halide perovskite materials.