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Reduced Barrier for Ion Migration in Mixed-Halide Perovskites.

ABSTRACT: Halide alloying in metal halide perovskites is a useful tool for optoelectronic applications requiring a specific bandgap. However, mixed-halide perovskites show ion migration in the perovskite layer, leading to phase segregation and reducing the long-term stability of the devices. Here, we study the ion migration process in methylammonium-based mixed-halide perovskites with varying ratios of bromide to iodide. We find that the mixed-halide perovskites show two separate halide migration processes, in contrast to pure-phase perovskites, which show only a unique halide migration component. Compared to pure-halide perovskites, these processes have lower activation energies, facilitating ion migration in mixed versus pure-phase perovskites, and have a higher density of mobile ions. Under illumination, we find that the concentration of mobile halide ions is further increased and notice the emergence of a migration process involving methylammonium cations. Quantifying the ion migration processes in mixed-halide perovskites shines light on the key parameters allowing the design of bandgap-tunable perovskite solar cells with long-term stability.

SUBMITTER: McGovern L

PROVIDER: S-EPMC8715422 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Hysteretic Ion Migration and Remanent Field in Metal Halide Perovskites

Project description:Abstract The gap in understanding how underlying chemical dynamics impact the functionality of metal halide perovskites (MHPs) leads to the controversy about the origin of many phenomena associated with ion migration in MHPs. In particular, the debate regarding the impact of ion migration on current–voltage (I–V) hysteresis of MHPs devices has lasted for many years, where the difficulty lies in directly uncovering the chemical dynamics, as well as identifying and separating the impact of specific ions. In this work, using a newly developed time?resolved time?of?flight secondary ion mass spectrometry CH3NH3+ and I? migrations in CH3NH3PbI3 are directly observed, revealing hysteretic CH3NH3+ and I? migrations. Additionally, hysteretic CH3NH3+ migration is illumination?dependent. Correlating these results with the I–V characterization, this work uncovers that CH3NH3+ redistribution can induce a remanent field leading to a spontaneous current in the device. It unveils that the CH3NH3+ migration is responsible for the illumination?associated I–V hysteresis in MHPs. Hysteretic ion migration has not been uncovered and the contribution of any ions (e.g., CH3NH3+) has not been specified before. Such insightful and detailed information has up to now been missing, which is critical to improving MHPs photovoltaic performance and developing MHPs?based memristors and synaptic devices. Ion migration plays critical roles in the functionalities of metal halide perovskites. Herein, using newly developed time?resolved time?of?flight secondary ion mass spectrometry, hysteretic CH3NH3+ and I? migrations in CH3NH3PbI3 are observed, where CH3NH3+ migration hysteresis is illumination?dependent. The ion redistribution in CH3NH3PbI3 can lead to a permanent spontaneous current after the application of electric bias.

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

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| S-EPMC7888268 | biostudies-literature

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| S-EPMC6403211 | biostudies-literature

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Project description:Solar cells based on organic-inorganic halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current-voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first-principles calculations based on density functional theory, we study the properties of an important defect in hybrid perovskites-interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization-enhanced defect migration following the Bourgoin-Corbett mechanism. Our analysis highlights the structural flexibility of organic-inorganic perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells.

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Project description:Mixed-halide lead perovskites (MHPs) are promising materials for photovoltaics and optoelectronics due to their highly tunable band gaps. However, they phase segregate under continuous illumination or an electric field, the mechanism of which is still under debate. Herein we systematically measure the phase segregation behavior of polymer-encapsulated CH3NH3Pb(BrxI1-x)3 MHPs as a function of excitation intensity and the nominal halide ratio by in situ photoluminescence microspectroscopy and observe surprising phase dynamics at the beginning of the illumination. The initial phase segregation to I-rich and Br-rich phases is observed followed by the formation of a new mixed-halide phase within several seconds that has not been reported before. We propose that the photothermal effect is amplified at the small-size I-rich domains, which significantly changes the local phase segregation in the otherwise uniform film within milliseconds after illumination.

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