In Situ Exploration of the Structural Transition during Morphology‐ and Efficiency‐Conserving Halide Exchange on a Single Perovskite Nanocrystal

Abstract

Controlled fabrication of semiconductor nanostructures with unique physicochemical properties is vital for future technologies. In this study, transformation from red‐emitting metal halide perovskite CH3NH3PbI3 nanocrystals (NCs) to green‐emitting CH3NH3PbBr3 NCs was achieved without significant morphological changes and loss of photoluminescence (PL) efficiency via a controlled halide exchange reaction. In situ single‐particle PL imaging along with detailed structural and elemental characterizations revealed that sudden cooperative transitions between two light‐emitting states via intermediate dark states with >100 s durations during halide exchange originate from two distinct defect‐mediated reconstruction processes with different activation energies (0.072 and 0.40 eV), leading to an isokinetic temperature of ca. 314 K, across a solid‐state miscibility gap between the I‐ and Br‐rich phases inside a single NC.

Photo De‐Mixing in Dion‐Jacobson 2D Mixed Halide Perovskites

“…Apart from the nature of the organic cations, halide substitution or mixing provide an effective way to vary the material properties of both 2D and 3D halide perovskites. [ 10–12 ] However, 3D mixed halide perovskites (e.g., MAPb(I 0.5 Br 0.5 ) 3 ) have been shown to undergo segregation under illumination into Br‐rich and I‐rich compositions (photo de‐mixing). Such segregation process appears to be largely reversible as in the dark the de‐mixed phases re‐mix to form the original phase (dark re‐mixing).…”

“…[9] Apart from the nature of the organic cations, halide substitution or mixing provide an effective way to vary the material properties of both 2D and 3D halide perovskites. [10][11][12] However, 3D mixed halide perovskites (e.g., MAPb(I 0.5 Br 0.5 ) 3 ) have 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, the authors analyze the changes in the optical absorption of 2D Dion-Jacobson mixed halide perovskite thin films (encapsulated) based on (PDMA)Pb(I 0.5 Br 0.5 ) 4 (PDMA: 1,4-phenylenedimethanammonium spacer) exposed to a constant illumination.…”

Nanoscale Structural Heterogeneity and Efficient Intergrain Charge Diffusion in a Series of Mixed MA/FA Halide Perovskite Films

“…Fluorescence microscopy in combination with other techniques has served as a powerful tool to find nanoscale structure–function relationships in a variety of materials, including perovskite thin films, , and has provided insight into potential sources of nonradiative loss , and heterogeneity in perovskites. , Further studies using photoluminescence (PL) and electroluminescence (EL) imaging and spectroscopy have revealed the local photophysics of a variety of other perovskite materials, such as micro- and nanocrystals or quantum dots. − Effective utilization of these fluorescence techniques can give access to local energy transport, grain boundaries, and charge pathways to help rationally control these fundamental processes and improve the efficiency of solar cells, light-emitting diodes, or lasers. − …”

“…To address these important questions, we used wide-field fluorescence microscopy to better understand the interplay between local nonradiative recombination and structural heterogeneity in explaining the PL properties of mixed-cation hybrid perovskites. While in situ microscopic observation has brought valuable insight into the anion exchange mechanism in mixed-anion perovskites, , there have been no attempts so far to characterize on the nanoscale level the mixed-cation materials. We investigated the PL fluctuations and local PL spectra in a series of MA 1– x FA x PbI 3 polycrystalline films of ∼450 nm thickness with changing FA ratio from 0% ( x = 0) to 100% ( x = 1) using two different antisolvents, chlorobenzene (CB) and ethyl acetate (EA). , The films unexpectedly show PL blinking that is correlated over micrometer scale distances, as well as distributions of local PL spectra in the mixed films reflecting the local compositional heterogeneities.…”

Irreversibility in Anion Exchange Between Cesium Lead Bromide and Iodide Nanocrystals Imaged by Single-Particle Fluorescence

“…Single-particle optical measurements remove ensemble averaging to reveal differences in chemical and physical behavior among nanocrystals prepared within the same synthetic batch. ,,− We and others have used single-particle fluorescence microscopy to monitor chemical transformations in semiconductor NCs including cation exchange, anion exchange, and ion intercalation based on changes in the emission intensity and wavelength of individual NCs as they transform. − These measurements quantify heterogeneity in the reaction times for a population of NCs undergoing the same transformation. In every system studied so far, the switching times, which characterize the rate at which the PL intensity changes for individual NCs, are much shorter than the time it takes for the ensemble of NCs to transform.…”