Ion Migrations in Lead Halide Perovskite Single Crystals with Different Halide Components
Xin WangYuwei LiYubing XuYuzhu PanYao WuGuanwen LiQianqian HuangQi ZhangQing LiXiaobing ZhangJing ChenWei Lei
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Abstract:
The ion migrations in lead halide perovskite (LHP)‐based devices are complex processes due to the generation of charges and migration multipaths. The migrating ions are generated mainly from element defects and dislocation defects, and transport along paths of element vacancies and grain boundaries. In this regard, it is of interest to investigate the different influences of the two defect types and paths. Herein, the current–voltage hysteresis and X‐ray photoelectron spectra of almost‐dislocation‐free LHP single crystals (LHPSCs) with different halide components are investigated. The ions cannot migrate in iodine‐based LHPSCs. A decrease in halide vacancy content in a bromine–chlorine‐based perovskite single crystal can effectively limit the ion migration. This study provides useful insights to understand the different dominant factors determining the ion migration in LHPs.Keywords:
Hysteresis
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The transient current analysis was conducted in order to reveal the mechanism of hysteresis phenomena in organolead halide perovskite solar cells. The change of the hysteresis in OPV-type device of CH3NH3PbI3 was analyzed depending on scan condition. The change of time constant depending on the scan condition was observed. Our results suggest that the elimination of the hysteresis for slow scan speed cannot be explained by simple first-order relaxation model such as charging into capacitance. The model should include the parameters which depend on the scan speed and history of the bias scan.
Hysteresis
Transient (computer programming)
Time constant
Horizontal scan rate
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Abstract Potassium bromide treated with bromine and chlorine vapor respectively, was used as catalyst for the dehydrochlorination of t‐butyl chloride. The results indicated a clear correlation between the presence of color centers (V‐centers) in the halides and their catalytic activity. The activation energies of the catalytic dehydrochlorination were measured.
Potassium bromide
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Halogen electrodes, according to the authors, are the most convenient electrodes for thermodynamic studies of reactions involving halogens in molten alkali metal halides when the emf method is used. The authors measured the potential differences between the chlorine and bromine electrode and between the chlorine and iodine electrode in cells described here. From the results of measurements at different temperatures, the authors generalized interpolation relations between the potentials of the bromine and iodine electrode in molten alkalimetal bromides and iodides and their mixtures relative to the chlorine electrode in chlorides of the same metals, on one hand, and the temperature and cation's crystal radius on the other hand.
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Glass electrode
Metal halides
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Migration of halogen vacancies is one of the primary sources of phase segregation and material degradation in lead-halide perovskites. Here we use first principles density functional theory to compare migration energy barriers and paths of bromine vacancies in the bulk and at a (001) surface of cubic CsPbBr$_3$. Our calculations indicate that surfaces might facilitate bromine vacancy migration in these perovskites, due to their soft structure that allows for bond lengths variations larger than in the bulk. We calculate the migration energy for axial-to-axial bromine vacancy migration at the surface to be only half of the value in the bulk. Furthermore, we study the effect of modifying the surface with four different alkali halide monolayers, finding an increase of the migration barrier to almost the bulk value for the NaCl-passivated system. Migration barriers are found to be correlated to the lattice mismatch between the CsPbBr$_3$ surface and the alkali halide monolayer. Our calculations suggest that surfaces might play a significant role in mediating vacancy migration in halide perovskites, a result with relevance for perovskite nanocrystals with large surface-to-volume ratios. Moreover, we propose viable ways for suppressing this undesirable process through passivation with alkali halide salts.
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Metal-ion doping into halide perovskite materials is a promising approach to enhance long-term film stability and, thus, improve solar cell performance. Moreover, the idea of doping metals into halide perovskites can also be driven by applications in heterogeneous catalysis. Here, we investigate the incorporation of the catalytically active transition metals Pt and Pd into mixed-halide perovskite materials. We find that Pt aggregates in large clusters in iodine- and bromine-rich perovskite film compositions, whereas Pd incorporation depends on the underlying bromine to iodine ratio. By using optical and structural characterization methods, we observe that phase segregation in the perovskite layer is suppressed upon doping of either of these metals into the material. This approach further shows the potential to obtain catalytically active halide perovskite films in which the perovskite can serve as a host matrix.
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A hybrid method is proposed to synthesize methylammonium lead iodine, bromine and chlorine MAPbI 3 , MAPbBr 2 I and MAPbCl 2 I Perovskite by mixing two-step methods. The descriptions of perovskite samples included structural, morphological and optical properties. The intensity and orientation in X-ray diffraction (XRD) patterns appear clearly and squeaky. FESEM images of the perovskite films appeared as a rough surface due to iodine and more crystalline size using bromine and chlorine. Optical properties shifted towards low wavelengths (blue shift) and the value changed from 830 nm to 420 nm when the lead halide is changed from iodine to bromine and then to chlorine.
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