Halide Segregation in Mixed-Halide Perovskites: Influence of A-Site Cations

Abstract

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br 0.5 I 0.5 ) 3 and FA 0.83 Cs 0.17 Pb(Br 0.4 I 0.6 ) 3 films. We report evidence for low-barrier ionic pathways in MAPb(Br 0.5 I 0.5 ) 3 , which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA 0.83 Cs 0.17 Pb(Br 0.4 I 0.6 ) 3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

“…The film was then exposed to the segregation-driving CW illumination and the measurements were repeated after stabilisation of the PL spectral changes, resulting in μ = 37.2 ± 0.6 cm 2 /(Vs) . Such similar μ values of charge carriers immediately after photoexcitation are consistent with halide segregation occurring only in a small portion of the film despite drastic changes in PL lineshape 2 , 9 , 17 , which results in the minimal impact of halide demixing on the charge transport properties of the majority phase. This finding agrees with our recent observations in a parallel study carried out on identical films, in which XRD diffraction showed good preservation of material crystallinity upon halide segregation 17 .…”

“…Such similar μ values of charge carriers immediately after photoexcitation are consistent with halide segregation occurring only in a small portion of the film despite drastic changes in PL lineshape 2 , 9 , 17 , which results in the minimal impact of halide demixing on the charge transport properties of the majority phase. This finding agrees with our recent observations in a parallel study carried out on identical films, in which XRD diffraction showed good preservation of material crystallinity upon halide segregation 17 . To understand the impact of heterogeneities on the transport properties, we tuned the OPTP photoexcitation wavelength to 720 nm.…”

Investigation of Opto-Electronic Properties and Stability of Mixed-Cation Mixed-Halide Perovskite Materials with Machine-Learning Implementation

“…Although the resulting increased bandgaps lower the optical absorption, the stability of bromide-based perovskites is significantly enhanced [56] and this is confirmed by thermal decomposition of MAPbX 3 perovskites [57]. Furthermore, the stability seems to be also linked to the A-site cation mixture as observed in the case of halide segregation [58]. Using binary mixtures of the aforementioned seven A-site cations and three halogens we assemble the perovskite structures of formula A x A' 1−x PbX y X' z X" 3−y−z .…”

Multication perovskite 2D/3D interfaces form via progressive dimensional reduction

“…Moreover, in such a sequence of reducing dimensionality, we would expect diffraction of larger RDPs to be weaker, since there would initially be few instances of adjacent and periodic high-n RDPs, but once they split into lower-n RDPs, these structures would be immediately adjacent to each other, thus promoting diffraction 21 . Considering the small percentage of bromine in the (MAPbBr 3 ) 0.05 (FAPbI 3 ) 0.95 films, we posit that the difference in the 2D/3D transformation pathways arises due to the greater stability of FAPbI 3 compared to MAPbI 3 , where the latter is known to exhibit poorer thermal stability due to loss of volatile species under thermal stress and is more easily converted to PbI 2 26,27 .…”