Vapor-Phase Epitaxial Growth of Aligned Nanowire Networks of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I)

With the intense interest in inorganic cesium lead halide perovskites and their nanostructures for optoelectronic applications, high-quality crystalline nanomaterials with controllable morphologies and growth directions are desirable. Here, we report a vapor-phase epitaxial growth of horizontal single-crystal CsPbX₃ (X = Cl, Br, I) nanowires (NWs) and microwires (MWs) with controlled crystallographic orientations on the (001) plane of phlogopite and muscovite mica. Moreover, single NWs, Y-shaped branches, interconnected NW or MW networks with 6-fold symmetry, and, eventually, highly dense epitaxial network of CsPbBr₃ with nearly continuous coverage were controllably obtained by varying the growth time. Detailed structural study revealed that the CsPbBr₃ wires grow along the [001] directions and have the (100) facets exposed. The incommensurate heteroepitaxial lattice match between the CsPbBr₃ and mica crystal structures and the growth mechanism of these horizontal wires due to asymmetric lattice mismatch were proposed. Furthermore, the photoluminescence waveguiding and good performance from the photodetector device fabricated with these CsPbBr₃ networks demonstrated that these well-connected CsPbBr₃ NWs could serve as straightforward platforms for fundamental studies and optoelectronic applications.

CsPb(IxBr1-x)3 solar cells

Owing to its nice performance, low cost, and simple solution-processing, organic-inorganic hybrid perovskite solar cell (PSC) becomes a promising candidate for next-generation high-efficiency solar cells. The power conversion efficiency (PCE) has boosted from 3.8% to 25.2% over the past ten years. Despite the rapid progress in PCE, the device stability is a key issue that impedes the commercialization of PSCs. Recently, all-inorganic cesium lead halide perovskites have attracted much attention due to their better stability compared with their organic-inorganic counterpart. In this progress report, we summarize the properties of CsPb(I_xBr_1-x)₃ and their applications in solar cells. The current challenges and corresponding solutions are discussed. Finally, we share our perspectives on CsPb(I_xBr_1-x)₃ solar cells and outline possible directions to further improve the device performance.

Visualization and Studies of Ion-Diffusion Kinetics in Cesium Lead Bromide Perovskite Nanowires

The facile chemical transformation of metal halide perovskites via ion exchange has been attributed to their "soft" crystal lattices that enable fast ion migration. Kinetic studies of such processes could provide mechanistic insights on the ion migration dynamics. Herein, by using aligned single-crystal nanowires of cesium lead bromide (CsPbBr₃) perovskite on epitaxial substrates as platforms, we visualize and investigate the cation or anion interdiffusion kinetics via spatially resolved photoluminescence measurement on heterostructures fabricated by stacking CsPbCl₃, MAPbI₃, or MAPbBr₃ microplates on top of CsPbBr₃ nanowires. Time-dependent confocal photoluminescence microscopy and energy-dispersive X-ray spectroscopy showed the solid-state anion interdiffusion readily occurs to result in halide concentration gradients along CsPbBr_{3-3 x}Cl_{3 x} ( x = 0-1) nanowires. Quantitative analysis of such composition profiles using Fick's law allowed us, for the first time, to extract interdiffusion coefficients of the chloride-bromide couple and an activation energy of 0.44 ± 0.02 eV for ion diffusion from temperature-dependent studies. In contrast, iodide-bromide interdiffusion is limited, likely due to the complex phase behaviors of mixed alloys of CsPb(Br,I)₃. In contrast to the relatively mobile anions, A-site cation interdiffusion across the MAPbBr₃/CsPbBr₃ junctions was barely observed at room temperature. Our results present a general method to investigate the kinetics of the solid-state ion migration, and the gained insights on ion diffusion can provide guidelines for rationally designing perovskite heterostructures that could lead to new properties for fundamental studies and technological applications.

Au Exchange or Au Deposition: Dual Reaction Pathways in Au-CsPbBr3 Heterostructure Nanoparticles

We have designed a facile synthetic strategy for the selective deposition of Au metal on all-inorganic CsPbBr₃ perovskite nanocrystals that includes the addition of PbBr₂ salt along with AuBr₃ salt. PbBr₂ is necessary because the addition of Au³⁺ to solutions of CsPbBr₃ nanocrystals otherwise results in the exchange of Au³⁺ ions from solution with Pb²⁺ cations within the nanocrystal lattice to produce Cs₂Au^IAu^IIIBr₆ nanocrystals with a tetragonal crystal structure and a band gap of about 1.6 eV, in addition to Au metal deposition. Including excess Pb²⁺ ions in solution prevents the exchange reaction. Au metal deposits on the surface of the nanocrystals to produce the Au-CsPbBr₃ heterostructure nanoparticles with an Au particle diameter determined by the Au³⁺ ion concentration. Fluorescence quenching caused by Au deposition monotonically increases with deposition size, but the fluorescence quantum yield (QY) is significantly greater than if any cation exchange has occurred. An optimized synthesis can produce Au-CsPbBr₃ nanoparticles with 70% QY and no evidence of cation exchange.

Probing Linewidths and Biexciton Quantum Yields of Single Cesium Lead Halide Nanocrystals in Solution

Cesium lead halide (CsPbX₃, X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide composition and size. The spectroscopy of single PNCs enhances our understanding of the effect of confinement on excitations in PNCs in the absence of obfuscating ensemble averaging and can also inform synthetic efforts. However, single PNC studies have been hampered by poor PNC photostability under confocal excitation, precluding interrogation of all but the most stable PNCs, and leading to a lack of understanding of PNCs in the regime of high confinement. Here, we report the first comprehensive spectroscopic investigation of single PNC properties using solution-phase photon-correlation methods, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. With minimally perturbative solution-phase photon-correlation Fourier spectroscopy (s-PCFS), we establish that the ensemble emission linewidth of PNCs of all sizes and compositions is predominantly determined by the intrinsic single PNC linewidth (homogeneous broadening). The single PNC linewidth, in turn, dramatically increases with increasing confinement, consistent with what has been found for II-VI semiconductor nanocrystals. With solution-phase photon antibunching measurements, we survey the biexciton-to-exciton quantum yield ratio (BX/X QY) in the absence of user-selection bias or photodegradation. Remarkably, the BX/X QY ratio depends both on the PNC size and halide composition, with values between ∼2% for highly confined bromide PNCs and ∼50% for intermediately confined iodide PNCs. Our results suggest a wide range of underlying Auger rates, likely due to transitory charge carrier separation in PNCs with relaxed confinement.