Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.153
Publication date: 18th July 2019
Mixing iodide and bromide in MAPb(I1-xBrx)3 perovskites is an effective strategy to obtain any desired bandgap intermediate to MAPbI3 (1.6 eV) and MAPbBr3 (2.3 eV). However, a major drawback of these mixed-halide perovskites is that under illumination (for x > 0.2), the halides segregate into iodide-rich and bromide-rich domains.[1] This makes the bandgap of mixed-halide perovskites unstable, which is detrimental for solar cell performance. That is, the iodide-rich lower bandgap domains act as a recombination centre for charges, which prevents them from being collected.
In this study, we investigate to what extent this halide segregation can be manipulated under pressure. Using pressure-dependent photoluminescence, we find that the pressure-dependent emission energy of the iodide-rich phase is positive (i.e. dEg/dP > 0). Notably, this is in contrast with pure MAPbI3 and MAPbBr3, where the bandgap shows a negative pressure dependence (i.e. dEg/dP < 0). Interestingly, this observation suggests that the composition of the iodide-rich phase depends on the pressure, indicating that the threshold for halide segregation shifts to higher x-values for increased pressure. In addition, we use pressure-dependent transient absorption spectroscopy to investigate the energetic landscape of the halide segregated perovskites and the rate at which charges funnel from the higher bandgap to the lower bandgap domains. Most importantly, our results suggest that contracting the unit cell of mixed-halide perovskites improves their stability against halide segregation for low x-values, paving the way toward rational design of stable perovskites with tunable bandgaps.
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