Impact of Reabsorption on the Emission Spectra and Recombination Dynamics of Hybrid Perovskite Single Crystals

High solid-state photoluminescence quantum yield of carbon-dot-derived molecular fluorophores for light-emitting devices

Several recent studies of carbon dots (CDs) synthesized by bottom-up methods under mild conditions have reported the presence of organic molecular fluorophores in CD dispersions. These fluorophores have a tendency to aggregate, and their properties strongly depend on whether they are present in the form of discrete molecules or aggregates. The aggregation becomes more prominent in the solid state, which motivates the study of the properties of the fluorophores associated with CDs in the solid state. Here, we report the solid-state characterization of N4,N11-dimethyldibenzo[a,h]phenazine-4,11-diamine (BPD) - a molecular fluorophore that forms CDs. Discrete BPD molecules show excitation-wavelength-independent photoluminescence (PL) emission in the green wavelength region at ∼520 nm. However, additional blue PL is also observed due to aggregation, making the PL emission significantly broad. For detailed studies, BPD is mixed in different solid matrices, and it is observed that the PL quantum yield (PLQY) of BPD films strongly depends on the concentration of BPD in the solid matrices. Increasing the concentration of BPD results in a considerable decrease in the PLQY. The PLQY of the films with an optimum concentration of BPD is 75.9% and 40.2% in polymethyl methacrylate and polystyrene, respectively. At higher concentrations, these PLQY values decrease to ∼11%. The significant decrease in the PLQY is ascribed to reabsorption and nonradiative exciton decay that is facilitated by BPD aggregation at higher concentrations. Finally, light-emitting devices (LEDs) were fabricated with almost pure white emission color, having CIE (International Commission on Illumination) coordinates of (0.35, 0.37) using BPD in the color-converting layer of blue-pumped LEDs. The device shows a luminous efficiency 3.8 lm W-1 and luminance of 43 331 cd m-2.

Can thick metal-halide perovskite single crystals have narrower optical bandgaps with near-infrared absorption?

Metal-halide perovskite (MHP) single crystals are emerging as potential competitors to their polycrystalline thin-film counterparts. These materials have shown the specific feature of extended absorbance towards the near-infrared (NIR) region, which promises further extension of their applications in the field of photovoltaics and photodetectors. This notable expansion of absorbance has been explained by the narrower effective optical bandgap of MHP single crystals promoted by their large thickness over several micrometres to millimetres. Herein, the attributes of the material's thickness and the measurement technique used to estimate these characteristics are discussed to elucidate the actual origins of the extended absorbance of MHP single crystals. Contrary to the general belief of the narrower bandgap of the MHP single crystals, we demonstrate that the extended NIR absorption in the MHP single crystals mainly originates from the combination of unique below-bandgap absorption of MHPs, the thickness of single crystals, and the technical limitation of the spectrophotometer, with the key attributes of (i) significantly large thickness of the MHP single crystals by suppressing the transmitted light and (ii) the detector's limited dynamic range. Combining the theoretical and experimental characterizations, we clarify the significant role of the large thickness together with the limited sensitivity of the detector in promoting the well-known red shift of the absorption onset of the MHP single crystals. The observations evidently show that in some special circumstances, the acquired absorption spectrum cannot reliably represent the optical bandgap of MHP materials. This highlights some misinterpretations in the estimation of the narrower optical bandgap of the MHP single crystals from conventional optical methods, while the optical bandgap is an inherent property independent of the thickness. The proposed broad applications of the MHP single crystals are dictated by their fascinating properties, and therefore, a deep insight into these features should be considered besides device applications, because much of their property-function relationships are still ambiguous and a subject of debate.

Rapid Exciton Transport and Structural Defects in Individual Porphyrinic Metal Organic Framework Microcrystals

To date, spectroscopic characterization of porphyrin-based metal organic frameworks (MOFs) has relied almost exclusively on ensemble techniques, which provide only structurally averaged insight into the functional properties of these promising photochemical platforms. This work employs time-resolved pump-probe microscopy to probe ultrafast dynamics in PCN-222 MOF single crystals. The simultaneous high spatial and temporal resolution of the technique enables the correlation of spectroscopic observables to both inter- and intracrystal structural heterogeneity. The pump-probe measurements show that significant differences in the excited state lifetime exist between individual PCN-222 crystals of an ensemble. On a single PCN-222 crystal, differences in excited state lifetime and photoluminescence quantum yield are found to correlate to microscale structural defects introduced at crystallization. Pump probe microscopy also enables the direct measurement of excited state transport. Imaging of exciton transport on individual MOF crystals reveals rapid, but subdiffusive exciton transport which slows on the 10s of ps time scale. Time-averaged exciton diffusion coefficients over the first 200 ps span a range of 0.27 to 1.0 cm2/s, indicating that excited states are rapidly transported through the porphyrin network of PCN-222 before being trapped. Together, these single-particle-resolved measurements provide important new insight into the role played by structural defects on the photochemical functionality of porphyrin-based MOFs.

Cathodoluminescence Nanoscopy: State of the Art and Beyond

Cathodoluminescence (CL) nanoscopy is proven to be a powerful tool to explore nanoscale optical properties, whereby free electron beams achieve a spatial resolution far beyond the diffraction limit of light. With developed methods for the control of electron beams and the collection of light, the dimension of information that CL can access has been expanded to include polarization, momentum, and time, holding promise to provide invaluable insights into the study of materials and optical near-field dynamics. With a focus on the burgeoning field of CL nanoscopy, this perspective outlines the recent advancements and applications of this technique, as illustrated by the salient experimental works. In addition, as an outlook for future research, several appealing directions that may bring about developments and discoveries are highlighted.

3D-Heterojunction Based on Embedded Perovskite Micro-Sized Single Crystals for Fast Photomultiplier Photodetectors with Broad/Narrowband Dual-Mode

A fast photomultiplier photodetector with a broad/narrowband dual mode is implemented using a new 3D heterostructure based on embedded perovskite micro-sized single crystals. Because the single-crystal size is smaller than the electrode size, the active layer can be divided into a perovskite microcrystalline part for charge transport and a polymer-embedded part for charge storage. This induces an additional radial interface in the 3D heterojunction structure, and allows a photogenerated built-in electric field in the radial direction, especially when the energy levels between the perovskite and embedding polymer are similar. This type of heterojunction has a small radial capacitance that can effectively reduce carrier quenching and accelerate the carrier response. By controlling the applied bias direction, up to 300-1000% external quantum efficiency (EQE) and microsecond response can be achieved not only in the wide range of ultraviolet to visible light from 320 to 550 nm, but also in the narrow-band response with a full width at half minimum (FWHM) of 20 nm. This shows great potential for applications in integrated multifunctional photodetectors.

Life on the Urbach Edge

The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley-Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

Large-area large-grain CsPbCl3perovskite films by confined re-growth for violet photodetectors

CsPbCl₃perovskite is an attractive semiconductor material with characteristics such as a wide bandgap, high chemical stability, and excellent optoelectronic properties, which broaden its application prospects for ultraviolet (UV) and violet photodetectors (PDs). However, large-area CsPbCl₃films with high coverage, large grains, and controllable thickness are still difficult to prepare by using the solution method due to the extremely low solubility of their precursors in conventional solvents. Herein, a water-assisted confined re-growth method is developed, and a CsPbCl₃microcrystalline film with an area of 3 cm × 3 cm is grown, the thickness of which is controllable within a range of several microns. The as-prepared thin film exhibits a flat and smooth surface, large grains, and enhanced photoluminescence. Furthermore, the fabricated violet PDs based on the prepared CsPbCl₃film show a high responsivity of 2.17 A W^-1, external quantum efficiency of 664%, on/off ratio of 2.58 × 10³, and good stability. This study provides a prospective solution for the growth of large-area, large-grain, and surface-smooth CsPbCl₃films for high-performance UV and violet PDs.

Role of solution concentration in formation kinetics of bromide perovskite thin films during spin-coating monitored by optical in situ metrology

Optoelectronic devices based on metal halide perovskites continue to show a improved performance, and solution-based coating techniques pave the way for large-area applications. However, not all parameters influencing the thin film formation process of metal halide perovskites are identified and entirely rationalised over their full compositional range, thus hampering optimised thin film fabrication. Furthermore, while the perovskite deposition via spin-coating and annealing is an easily accessible technique, more profound insights into the chemical formation process are still lacking. Varying the precursor solution concentration is commonly used to vary the resulting thin film thickness. This study shows that varying the precursor solution concentration also affects the thin film morphology and optoelectronic quality. Hence, we herein investigate the influence of the precursor solution concentration on the formation process of a pure bromide-based triple cation perovskite (Cs_0.05MA_0.10FA_0.85PbBr₃) by fiber-based optical in situ measurement. During the spin-coating process, in situ UV-vis and PL measurements reveal formation kinetics are strongly dependent on the concentration. Furthermore, we identify delayed nucleation and retarded growth kinetics for more concentrated precursor solutions. In addition, we quantify the shifting chemical equilibrium of colloidal pre-coordination in the precursor solution depending on concentration. Namely, colloids are pre-organised to a higher degree and higher-coordination lead–bromide complexes tend to form in more concentrated precursor solutions. Thus, the modified solution chemistry rationalises retarded perovskite formation kinetics and highlights the precursor concentration as an influential and optimisable parameter for solution-based thin film deposition.

The solution chemistry of the precursor ink defines the formation kinetics of metal halide perovskites. Thus, rationalizing the formation process from solution via the deposition paves the way for high-quality thin films for optoelectronic devices.

X-Ray-Induced Modification of the Photophysical Properties of MAPbBr3 Single Crystals

Methylammonium lead tribromide (MAPbBr₃) perovskite single crystals demonstrate to be excellent direct X-ray and gamma-ray detectors with outstanding sensitivity and low limit of detection. Despite this, thorough studies on the photophysical effects of exposure to high doses of ionizing radiation on this material are still lacking. In this work, we present our findings regarding the effects of controlled X-ray irradiation on the optoelectronic properties of MAPbBr₃ single crystals. Irradiation is carried out in air with an imaging X-ray tube, simulating real-life application in a medical facility. By means of surface photovoltage spectroscopy, we find that X-ray exposure quenches free excitons in the material and introduces new bound excitonic species. Despite this drastic effect, the crystals recover after 1 week of storage in dark and low humidity conditions. By means of X-ray photoelectron spectroscopy, we find that the origin of the new bound excitonic species is the formation of bromine vacancies, leading to local changes in the dielectric response of the material. The recovery effect is attributed to vacancy filling by atmospheric oxygen and water.

Resonant Degenerate Four-Wave Mixing at the Defect Energy Levels of 2D Organic-Inorganic Hybrid Perovskite Crystals

Two-dimensional organic-inorganic lead halide perovskites are generating great interest due to their optoelectronic characteristics such as high solar energy conversion efficiency and a tunable direct band gap in the visible regime. However, the presence of defect states within the two-dimensional crystal structure can affect these properties, resulting in changes to their band gap emission as well as the emergence of nonlinear optical phenomena. Here, we have investigated the effects of the presence of defect states on the nonlinear optical phenomena of the 2D hybrid perovskite (BA)₂(MA)₂Pb₃Br₁₀. When two pulses, one narrowband pump pulse centered at 800 nm and one supercontinuum pulse with bandwidth from 800-1100 nm, are incident on a perovskite flake, degenerate four-wave mixing (FWM) occurs, with peaks corresponding to the energy levels of the defect states present within the crystal. The longer carrier lifetime of the defect state, in comparison to that of virtual transitions that take place in nonresonant FWM processes, allows for a larger population of electrons to be excited by the second pump photon, resulting in increased FWM signal at the defect energy levels. The quenching of the two-photon luminescence as flake thickness increases is also observed and attributed to the increased presence of defects within the flake at larger thicknesses. This technique shows the potential of detecting defect energy levels in crystals using FWM for a variety of optoelectronic applications.

Using pulsed mode scanning electron microscopy for cathodoluminescence studies on hybrid perovskite films

The use of pulsed mode scanning electron microscopy cathodoluminescence (CL) for both hyperspectral mapping and time-resolved measurements is found to be useful for the study of hybrid perovskite films, a class of ionic semiconductors that have been shown to be beam sensitive. A range of acquisition parameters is analysed, including beam current and beam mode (either continuous or pulsed operation), and their effect on the CL emission is discussed. Under optimized acquisition conditions, using a pulsed electron beam, the heterogeneity of the emission properties of hybrid perovskite films can be resolved via the acquisition of CL hyperspectral maps. These optimized parameters also enable the acquisition of time-resolved CL of polycrystalline films, showing significantly shorter lived charge carriers dynamics compared to the photoluminescence analogue, hinting at additional electron beam-specimen interactions to be further investigated. This work represents a promising step to investigate hybrid perovskite semiconductors at the nanoscale with CL.

Local Energy Landscape Drives Long-Range Exciton Diffusion in Two-Dimensional Halide Perovskite Semiconductors

Halide perovskites are versatile semiconductors with applications including photovoltaics and light-emitting devices, having modular optoelectronic properties realizable through composition and dimensionality tuning. Layered Ruddlesden-Popper perovskites are particularly interesting due to their unique 2D character and charge carrier dynamics. However, long-range energy transport through exciton diffusion in these materials is not understood or realized. Here, local time-resolved luminescence mapping techniques are employed to visualize exciton transport in exfoliated flakes of the BA2MAn-1PbnI3n+1 perovskite family. Two distinct transport regimes are uncovered, depending on the temperature range. Above 100 K, diffusion is mediated by thermally activated hopping processes between localized states. At lower temperatures, a nonuniform energy landscape emerges in which transport is dominated by downhill energy transfer to lower-energy states, leading to long-range transport over hundreds of nanometers. Efficient, long-range, and switchable downhill transfer offers exciting possibilities for controlled directional long-range transport in these 2D materials for new applications.

Low temperature scintillation performance of a Br-doped CH3NH3PbCl3 single-crystalline perovskite

Time response and light yield are two of the most important features of a scintillation detector, and are mostly determined by the luminescence properties of the scintillator. Here we have investigated the radioluminescence (RL) characteristics of a single-crystalline hybrid lead halide perovskite at both room temperature and low temperature. A dual-channel single photon correlation (DCSPC) system with a vacuum chamber is employed for the measurement. A rise time faster than 100 ps and several times enhancement of the crystal scintillation performances at low temperature have been observed. These behaviors demonstrated that bulk solution-grown single crystals of hybrid lead halide perovskites (MAPbCl₃ and Br-doped MAPbBr_0.08Cl_2.92, where MA = CH₃NH₃) can serve as stable scintillating materials for pulsed gamma detectors. In addition, this work provides a pathway for perovskite application and also attracts attention to investigating low-temperature scintillators.

Time response and light yield are two of the most important features of a scintillation detector, and are mostly determined by the luminescence properties of the scintillator.

Halide perovskite nanocrystal arrays: Multiplexed synthesis and size-dependent emission

Halide perovskites have exceptional optoelectronic properties, but a poor understanding of the relationship between crystal dimensions, composition, and properties limits their use in integrated devices. We report a new multiplexed cantilever-free scanning probe method for synthesizing compositionally diverse and size-controlled halide perovskite nanocrystals spanning square centimeter areas. Single-particle photoluminescence studies reveal multiple independent emission modes due to defect-defined band edges with relative intensities that depend on crystal size at a fixed composition. Smaller particles, but ones with dimensions that exceed the quantum confinement regime, exhibit blue-shifted emission due to reabsorption of higher-energy modes. Six different halide perovskites have been synthesized, including a layered Ruddlesden-Popper phase, and the method has been used to prepare functional solar cells based on single nanocrystals. The ability to pattern arrays of multicolor light-emitting nanocrystals opens avenues toward the development of optoelectronic devices, including optical displays.

Direct Correlation of Charge Carrier Transport to Local Crystal Quality in Lead Halide Perovskites

Although solution processing methods provide an attractive route toward development of low-cost functional materials, these accessible fabrication approaches can engender high concentrations of microscopic structural defects that are detrimental to performance. In lead halide perovskites, structural disorder derived from solution processing has been implicated as an important determiner of photophysical properties. However, a direct correlation between the functional properties of these materials and the local crystal structure in which non-equilibrium states evolve has remained elusive, in part because structural heterogeneities occur on length scales that defy conventional characterization techniques. To address this knowledge gap, we have combined ultrafast pump-probe microscopy and electron backscattering diffraction to directly correlate charge carrier transport with the local diffraction pattern contrast, an indicator of crystal quality. Spatial correlation of these measurements strongly suggests that even on individual single crystal CsPbBr₃ domains, microscopic variability in the crystal quality profoundly impacts the efficiency of charge carrier transport.

Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH₃NH₃)PbI₃ methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures

Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. Naturally self-assembled inorganic-organic multiple quantum wells (IO-MQWs) show utility from room-temperature exciton emission features (binding energies ~200–250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one- and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced PL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/GW) and two-photon excitation cross-sections (η₂σ₂ ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA- and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the co-existence of both free and deep-level crumpled excitons with some traces of defect-induced trap state emission. We conclude that the two-photon absorption induced PL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks.

Large lattice distortions and size-dependent bandgap modulation in epitaxial halide perovskite nanowires

Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials. However, despite ongoing research from multiple perspectives, some fundamental questions regarding their optoelectronic properties remain controversial. One reason is the high-variance of data collected from, often unstable, polycrystalline thin films. Here we use ordered arrays of stable, single-crystal cesium lead bromide (CsPbBr₃) nanowires grown by surface-guided chemical vapor deposition to study fundamental properties of these semiconductors in a one-dimensional model system. Specifically, we uncover the origin of an unusually large size-dependent luminescence emission spectral blue-shift. Using multiple spatially resolved spectroscopy techniques, we establish that bandgap modulation causes the emission shift, and by correlation with state-of-the-art electron microscopy methods, we reveal its origin in substantial and uniform lattice rotations due to heteroepitaxial strain and lattice relaxation. Understanding strain and its effect on the optoelectronic properties of these dynamic materials, from the atomic scale up, is essential to evaluate their performance limits and fundamentals of charge carrier dynamics.

Visualizing Buried Local Carrier Diffusion in Halide Perovskite Crystals via Two-Photon Microscopy

Halide perovskites have shown great potential for light emission and photovoltaic applications due to their remarkable electronic properties. Although the device performances are promising, they are still limited by microscale heterogeneities in their photophysical properties. Here, we study the impact of these heterogeneities on the diffusion of charge carriers, which are processes crucial for efficient collection of charges in light-harvesting devices. A photoluminescence tomography technique is developed in a confocal microscope using one- and two-photon excitation to distinguish between local surface and bulk diffusion of charge carriers in methylammonium lead bromide single crystals. We observe a large dispersion of local diffusion coefficients with values between 0.3 and 2 cm²·s^-1 depending on the trap density and the morphological environment-a distribution that would be missed from analogous macroscopic or surface measurements. This work reveals a new framework to understand diffusion pathways, which are extremely sensitive to local properties and buried defects.

Crystal Orientation and Grain Size: Do They Determine Optoelectronic Properties of MAPbI3 Perovskite?

Growing large, oriented grains of perovskite often leads to efficient devices, but it is unclear if properties of the grains are responsible for the efficiency. Domains observed in SEM are commonly misidentified with crystallographic grains, but SEM images do not provide diffraction information. We study methylammoinium lead iodide (MAPbI3) films fabricated via flash infrared annealing (FIRA) and the conventional antisolvent (AS) method by measuring grain size and orientation using electron back-scattered diffraction (EBSD) and studying how these affect optoelectronic properties such as local photoluminescence (PL), charge carrier lifetimes, and mobilities. We observe a local enhancement and shift of the PL emission at different regions of the FIRA clusters, but we observe no effect of crystal orientation on the optoelectronic properties. Additionally, despite substantial differences in grain size between the two systems, we find similar optoelectronic properties. These findings show that optoelectronic quality is not necessarily related to the orientation and size of crystalline domains.

Spontaneous Self-Assembly of Cesium Lead Halide Perovskite Nanoplatelets into Cuboid Crystals with High Intensity Blue Emission

Colloidal all-inorganic perovskite nanocrystals have gained significant attention as a promising material for both fundamental and applied research due to their excellent emission properties. However, reported photoluminescence quantum yields (PL QYs) of blue-emitting perovskite nanocrystals are rather low, mostly due to the fact that the high energy excitons for such wide bandgap materials are easily captured by interband traps, and then decay nonradiatively. In this work, it is demonstrated how to tackle this issue, performing self-assembly of 2D perovskite nanoplatelets into larger size (≈50 nm × 50 nm × 20 nm) cuboid crystals. In these structures, 2D nanoplatelets being isolated from each other within the cuboidal scaffold by organic ligands constitute multiple quantum wells, where exciton localization on potential disorder sites helps them to bypass nonradiative channels present in other platelets. As a result, the cuboid crystals show an extremely high PL QY of 91% of the emission band centered at 480 nm. Moreover, using the same synthetic method, mixed-anion CsPb(Br/Cl)3 cuboid crystals with blue emission peaks ranging from 452 to 470 nm, and still high PL QYs in the range of 72-83% are produced.

Strong Cathodoluminescence and Fast Photoresponse from Embedded CH3NH3PbBr3 Nanoparticles Exhibiting High Ambient Stability

This study presents a comprehensive analysis of the strong cathodoluminescence (CL), photoluminescence (PL), and photoresponse characteristics of CH₃NH₃PbBr₃ nanoparticles (NPs) embedded in a mesoporous nanowire (NW) template. Our study revealed a direct correlation between the CL and PL emissions from the perovskite NPs (Per NPs), for the first time. Per NPs are fabricated by a simple spin-coating of a perovskite precursor on the surface of metal-assisted chemically etched mesoporous Si NW arrays. The Per NPs confined in the mesopores show blue-shifted and enhanced CL emission as compared to the bare perovskite film, while the PL intensity of Per NPs is dramatically high compared to that of their bulk counterpart. A systematic analysis of the CL/PL spectra reveals that the quantum confinement effect and ultralow defects in Per NPs are mainly responsible for the enhanced CL and PL emissions. Low-temperature PL and time-resolved PL analysis confirm the high exciton binding energy and radiative recombination in Per NPs. The room temperature PL quantum yield of the Per NP film on the NW template was found to be 40.5%, while that of Per film was 2.8%. The Per NPs show improved ambient air stability than the bare film due to the protection provided by the dense NW array, since a dense NW array can slow down the lateral diffusion of oxygen and water molecules in Per NPs. Interestingly, the Si NW/Per NP junction shows superior visible light photodetection and the prototype photodetector shows a high responsivity (0.223 A/W) with response speeds of 0.32 and 0.28 s of growth and decay in photocurrent, respectively, at 2 V applied bias, which is significantly better than the reported photodetectors based on CH₃NH₃PbBr₃ nanostructures. This work demonstrates a low-cost fabrication of CH₃NH₃PbBr₃ NPs on a novel porous NW template, which shows excellent photophysical and optoelectronic properties with superior ambient stability.

Quantitative optical assessment of photonic and electronic properties in halide perovskite

The development of high efficiency solar cells relies on the management of electronic and optical properties that need to be accurately measured. As the conversion efficiencies increase, there is a concomitant electronic and photonic contribution that affects the overall performances. Here we show an optical method to quantify several transport properties of semiconducting materials and the use of multidimensional imaging techniques allows decoupling and quantifying the electronic and photonic contributions. Example of application is shown on halide perovskite thin film for which a large range of transport properties is given in the literature. We therefore optically measure pure carrier diffusion properties and evidence the contribution of optical effects such as the photon recycling as well as the photon propagation where emitted light is laterally transported without being reabsorbed. This latter effect has to be considered to avoid overestimated transport properties such as carrier mobility, diffusion length or diffusion coefficient.

Several Orders of Magnitude Difference in Charge-Transfer Kinetics Induced by Localized Trapped Charges on Mixed-Halide Perovskites

Partial halide substitution in organolead halide perovskites MAPbX₃ (MA = CH₃NH₃⁺, X = Cl^-, Br^-, or I^-) leads to semiconductor heterostructures with precisely tuned band-gap energies, which facilitates efficient charge extraction or separation for high-performance solar cells and optoelectronic devices. In this study, partially iodide-substituted MAPbBr₃ perovskites were prepared through a halide-exchange reaction in the liquid phase, and in situ space- and time-resolved photoluminescence profiles were acquired by means of confocal microscopy. The rates of charge transfer from the bulk MAPbBr₃ to the surface MAPbBr_{3- x}I _x domains, which are widely distributed over a single crystal, were found to greatly depend on the excitation-power density. In particular, an abnormally slow charge-transfer process, lasting a few nanoseconds, was observed at higher excitation density. To explain the dependence of this rate on the excitation density, and its correlation with the charge-trapping rate in the bulk MAPbBr₃, we propose a plausible mechanism in which trap filling associated with surface-trapped holes induces band bending within the space charge region. This band bending modulates carrier dynamics near the surface, thereby leading to efficient charge extraction from the bulk. To validate the mechanism, the carrier dynamics was numerically simulated using a diffusion model that includes the effect of the localized electric field. Our findings provide significantly deeper insight into the carrier dynamics within heterostructured perovskites with nanoscale heterogeneities, and a robust route for manipulating the photogenerated charges in various types of perovskite devices.

Single Crystal Growth of Hybrid Lead Bromide Perovskites Using a Spin-Coating Method

Synthesis and studies of single crystals of hybrid perovskite are important for achieving a better understanding of the optoelectronic phenomena occurring in this material and for improving ongoing applications. Here, we report on the growth of micrometer-size single crystals of methylammonium lead bromide (MAPbBr3) using the spin coating deposition method on a quartz substrate. We studied the influence of the rotation speed and the use of three different additives N-cyclohexyl-2-pyrrolidone, dimethyl sulfoxide, and 4-tert-butylpyridine on the crystal size and shape. The introduction of an additive in the precursor solution is revealed to be very useful for obtaining crystals with well-defined geometries and for decreasing the amount of defects. In this way, high-quality single crystals that sustain optical resonating modes were obtained and characterized by transmittance and photoluminescence measurements.

Real-Time Observation of Exciton-Phonon Coupling Dynamics in Self-Assembled Hybrid Perovskite Quantum Wells

Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication, and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons, remains an open question. Here, we investigate two widely used materials, namely, butylammonium lead iodide (CH₃(CH₂)₃NH₃)₂PbI₄ and hexylammonium lead iodide (CH₃(CH₂)₅NH₃)₂PbI₄, both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton-phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm^-1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 and 137 cm^-1. Using the determined optical phonon energies, we analyzed photoluminescence broadening mechanisms. At low temperatures (<100 K), the broadening is due to acoustic phonon scattering, whereas at high temperatures, LO phonon-exciton coupling is the dominant mechanism. Our results help explain the broad photoluminescence line shape observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton-phonon coupling in these materials.

Photoluminescence from Radiative Surface States and Excitons in Methylammonium Lead Bromide Perovskites

In view of its band gap of 2.2 eV and its stability, methylammonium lead bromide (MAPbBr₃) is a possible candidate to serve as a light absorber in a subcell of a multijunction solar cell. Using complementary temperature-dependent time-resolved microwave conductance (TRMC) and photoluminescence (TRPL) measurements, we demonstrate that the exciton yield increases with lower temperature at the expense of the charge carrier generation yield. The low-energy emission at around 580 nm in the cubic phase and the second broad emission peak at 622 nm in the orthorhombic phase originate from radiative recombination of charges trapped in defects with mobile countercharges. We present a kinetic model describing both the decay in conductance as well as the slow ingrowth of the TRPL. Knowledge of defect states at the surface of various crystal phases is of interest to reach higher open-circuit voltages in MAPbBr₃-based cells.