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Register a new userThermodynamic stabilization of mixed-halide perovskites against phase segregation
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Mixing iodide and bromide in halide perovskite semiconductors is an effective strategy to tune their bandgap, therefore mixed-halide perovskites hold great promise for color-tunable LEDs and tandem solar cells. However, the bandgap of mixed-halide perovskites is unstable under (sun-)light, since the halides segregate into domains of different bandgaps. Using pressure-dependent ultrafast transient absorption spectroscopy, we show that high external pressure increases the range of thermodynamically stable halide mixing ratios. Chemical pressure, by inserting a smaller cation, has the same effect, which means that any iodide-to-bromide ratio can be thermodynamically stabilized by tuning the crystal volume and compressibility. We interpret this stabilization by an alteration of the Helmholtz free energy via the largely overlooked PdeltaV term.
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The bandgap tunability of mixed-halide perovskites makes them promising candidates for light emitting diodes and tandem solar cells. However, illuminating mixed-halide perovskites results in the formation of segregated phases enriched in a single-halide. This segregation occurs through ion migration, which is also observed in single-halide compositions, and whose control is thus essential to enhance the lifetime and stability. Using pressure-dependent transient absorption spectroscopy, we find that the formation rates of both iodide- and bromide-rich phases in MAPb(BrxI1-x)3 reduce by two orders of magnitude on increasing the pressure to 0.3 GPa. We explain this reduction from a compression-induced increase of the activation energy for halide migration, which is supported by first-principle calculations. A similar mechanism occurs when the unit cell volume is reduced by incorporating a smaller cation. These findings reveal that stability with respect to halide segregation can be achieved either physically through compressive stress or chemically through compositional engineering.
Two-dimensional (2D) halide perovskites feature a versatile structure, which not only enables the fine-tuning of their optoelectronic properties but also makes them appealing as model systems to investigate the fundamental properties of hybrid perovskites. In this study, we analyzed the changes in the optical absorption of 2D Dion-Jacobson mixed halide perovskite thin films (encapsulated) based on (PDMA)Pb(I0.5Br0.5)4 (PDMA: 1,4-phenylenedimethanammonium spacer) exposed to a constant illumination. We demonstrate that these 2D mixed-halide perovskites undergo photo de-mixing with direct transformation from the pristine phase to the de-mixed phases. Almost complete re-mixing of these phases occurs when the sample is left in the dark, showing that the process is reversible in terms of optical properties. On the other hand, exposure to light appears to induce structural changes in the thin film that are not reversible in the dark. We have further investigated temperature-dependent absorption measurements under light to extract the photo de-mixed compositions and to map the photo-miscibility-gap. This work thereby reveals that photo de-mixing occurs in Dion-Jacobson two-dimensional hybrid perovskites and provides strategies to address the role of light in the thermodynamic properties of these materials.
Organic-inorganic hybrid perovskites are considered to be most promising photovoltaic materials. Highest efficiencies of perovskite solar cells have been achieved by using appropriate cation and anion mixtures. Mixed perovskite solar cells also show an improved stability. For both performance as well as stability, experimental information on electronic and ionic charge carriers is key, an information that so far has only been provided for methylammonium lead iodide; there we also found that light can enhance not only electronic but also ionic conductivities by more than one order of magnitude. We also proposed a mechanism for this surprising photo-ionic effect and explained its impact on photo-decomposition. Here we quantitatively deconvolute ionic and electronic transport properties for the practically relevant substitutions and mixtures. Specifically, we investigate various cation and anion substitutions (Cs; FA; Br) with a special eye on their photo-ionic effect. The results are not only of importance for light-induced degradation but also for light-induced demixing. As far as the photo-ionic effect is concerned, we find that the choice of the halide is of crucial importance, while the cationic substitutions are less relevant. The huge ionic conductivity enhancement found for iodide perovskites, is weakened by bromide substitution and eventually becomes insignificant for the pure bromide. Based on these experimental results, we provide a rationale for the experimentally observed photo-demixing.
Inorganic halide perovskites have emerged as a promising platform in a wide range of applications from solar energy harvesting to computing, and light emission. The recent advent of epitaxial thin film growth of halide perovskites has made it possible to investigate low-dimensional quantum electronic devices based on this class of materials. This study leverages advances in vapor-phase epitaxy of halide perovskites to perform low-temperature magnetotransport measurements on single-domain cesium tin iodide (CsSnI$_3$) epitaxial thin films. The low-field magnetoresistance carries signatures of coherent quantum interference effects and spin-orbit coupling. These weak anti-localization measurements reveal a micron-scale low-temperature phase coherence length for charge carriers in this system. The results indicate that epitaxial halide perovskite heterostructures are a promising platform for investigating long coherent quantum electronic effects and potential applications in spintronics and spin-orbitronics.
The paper presents the results of measurements of XPS valence band spectra of SiO2/MAPbI3 hybrid perovskites subjected to irradiation with visible light and annealing at an exposure of 0-1000 hours. It is found from XPS survey spectra that in both cases (irradiation and annealing) a decrease in the I:Pb ratio is observed with aging time, which unambiguously indicates PbI2 phase separation as a photo and thermal product of degradation. The comparison of the XPS valence band spectra of irradiated and annealed perovskites with density functional theory calculations of the MAPbI3 and PbI2 compounds have shown a systematic decrease in the contribution of I 5p-states and allowed us to determine the threshold for degradation, which is 500 hours for light irradiation and 200 hours for annealing.
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