Quantification of re-absorption and re-emission processes to determine photon recycling efficiency in perovskite single crystals

Ultra-Long Carrier Lifetime of Spiral Perovskite Nanowires Realized through Cooperative Strategy of Selective Etching and Passivation

Spiral inorganic perovskite nanowires (NWs) possess unique morphologies and properties that allow them highly attractive for applications in optoelectronic and catalytic fields. In popular solution-based synthesis methodology, however, challenges persist in simultaneously achieving precise and facile control over morphological twisting and fantastic carrier lifetimes. Here, a cooperative strategy of concurrently employing selective etching and ligand engineering is applied to facilitate the formation of spiral CsPbBr3 perovskite NWs with an ultralong carrier lifetime of ≈2 µs. Specifically, a novel amine of 1-(p-tolyl)ethanamine is introduced to functionalize as both a selective etchant and the source of forming an effective ligand to passivate the exposed facets, favoring the structural twisting and the enhancement of carrier lifetimes. The twisting behaviors are dependent on the etch ratios, which are essentially associated with the densities of grain boundaries and dislocations in the NWs. The ultralong carrier lifetime and long-term stability of the spiral NWs open up new possibilities for all-inorganic perovskites in optoelectronic and photocatalytic fields, while the cooperative synthesis strategy paves the way for exploring complex spiral structures with tunable morphology and functionality.

Photoexcited Carrier Dynamics in Iodine-Doped CH3NH3PbBr3 Single Crystals

Iodine-doped bromide perovskite single crystals (IBPSCs) have important applications in optoelectronic fields, such as in solar cells. Currently, much research has aimed to study the phase separation phenomenon and device performance improvements in IBPSCs. However, important intrinsic photoexcited carrier dynamics are often overlooked in IBPSCs. Here, we explored the photoexcited carrier dynamics in typical iodine-doped MAPbBr3 single crystals using the excitation intensity-dependent steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) technique. We found that the trap state density changes with an increase in the amount of doped iodine. Further, we noticed that there is an influence of carrier diffusion on the photoexcited carrier dynamics, and then, we evaluated the carrier diffusion coefficients and recombination constants via numerical simulations of the PL kinetics. Consequently, we found that the electron shallow trap-related carrier behaviors substantially impacted the PL kinetics. Our results greatly facilitate a deeper understanding of the fundamental characteristics of mixed halide perovskite material.

Photophysics and its application in photon upconversion

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interaction, where the energy of the emitted photons is higher than that of the incident photons. PL upconversion has promising applications in optoelectronic devices, displays, photovoltaics, imaging, diagnosis and treatment. In this review, we summarize the mechanism of PL upconversion and ultrafast PL physical processes. In particular, we highlight the advances in laser cooling, biological imaging, volumetric displays and photonics.

Additive-Enhanced Crystallization of Inorganic Perovskite Single Crystals for High-Sensitivity X-Ray Detection

Inorganic cesium lead halide perovskite single crystals are particularly intriguing to ionizing radiation detection by virtue of their material stability and high attenuation coefficients. However, the growth of high-quality inorganic perovskite single crystals remains challenging, mainly due to the limited solubility. In this work, an additive-enhanced crystallization method is proposed for cesium lead perovskites. The additive can remarkably increase the solubility of cesium bromide in dimethyl sulfoxide (DMSO) forming a balanced stoichiometric precursor solution, which prevents the formation of impurity phases. In addition, the additives would react with DMSO generating glyoxylic acid (GLA) via nucleophilic substitution and Kornblum oxidation reactions. The GLA can form stable PbBr₂ -DMSO-GLA complexes, which enables better crystallinity, uniformity and much longer carrier lifetimes for the grown single crystals. The X-ray detectors using the additive-enhanced crystals exhibit an ultra-high sensitivity of 3.0 × 10⁴  µC Gy_air ^-1 cm^-2 which is more than two orders of magnitude higher than that for the control devices.

Colored Daytime Radiative Cooling Textiles Supported by Semiconductor Quantum Dots

Radiative cooling, a zero-energy, eco-friendly cooling technology, has attracted tremendous attention recently for its potential of fighting global warming and climate changes. Radiative cooling fabrics with diffused solar reflections typically have reduced light pollution and can be mass-produced with currently available techniques. However, the monotonous white color has hindered its further applications and no colored radiative cooling textiles are available yet. In this work, we electrospun PMMA textiles containing CsPbBr_xI_3-x quantum dots as the colorant to achieve colored radiative cooling textiles. A theoretical model to predict the 3D color volume and cooling threshold was proposed for this system. As indicated by the model, a sufficiently high quantum yield (>0.9) will guarantee a wide color gamut and strong cooling ability. In the real experiments, all of the fabricated textiles show excellent color agreement with the theory. The green fabric containing CsPbBr₃ quantum dots achieved a subambient temperature of ∼4.0 °C under direct sunlight with an average solar power density of 850 W/m². The reddish fabric containing CsPbBrI₂ quantum dots also managed to cool 1.5 °C compared to the ambient temperature. The fabric containing CsPbI₃ quantum dots failed to achieve subambient cooling with a slightly increased temperature. Nevertheless, all of the fabricated colored fabrics outperformed the regular woven polyester fabric when placed on a human hand. We believed that the proposed colored textiles may widen the range of applications for radiative cooling fabrics and have the potential to become the next-generation colored fabrics with stronger cooling ability.

Charge Carrier Recombination Dynamics in MAPb(BrxCl1-x)3 Single Crystals

Understanding carrier recombination processes in MAPb(Br_xCl_1-x)₃ crystals is essential for their photoelectrical applications. In this work, carrier recombination dynamics in MAPb(Br_xCl_1-x)₃ single crystals were studied by steady-state photoluminescence (PL), time-resolved photoluminescence (TRPL), and time-resolved microwave photoconductivity (TRMC). By comparing TRPL and TRMC, we find TRPL of MAPb(Br_xCl_1-x)₃ (x < 0.98) single crystals is dominated by a hole trapping process while the long-lived component of TRMC is dominated by an electron trapping process. We also find both electron and hole trapping rates of MAPb(Br_xCl_1-x)₃ (x < 0.98) crystals decrease with an increase in Br content. A temperature-dependent PL study shows there are shallow trap states besides the deep level trap states in the MAPb(Br_0.82Cl_0.18)₃ crystal. The activation energy for holes in shallow trap states detrapped into the valence band is ∼0.1 eV, while the activation energy for free holes to be trapped into deep trap states is ∼0.4 eV. This work provides insight into carrier recombination processes in MAPb(Br_xCl_1-x)₃ single crystals.

Unveiling a Counterintuitive Intermode Interplay in a Prototype Plasmonic Nanosystem

We demonstrate a counterintuitive intermode interplay in the plasmonic system of gold nanorods, i.e., energy transfer (EnT) from the lower-energy longitudinal (L) mode to the higher-energy transverse (T) mode. The opening of this EnT(L→T) channel is enabled by an energy upconversion process with the L mode, in which the solvent environment plays a critical role. Switching from a strong thermal-conductivity solvent (i.e., water) to a much weaker one (i.e., chloroform) brings on prolongation of plasmonic hot-electron lifetime and enhancement of phonon emission, thereby increasing the probability of L-mode energy upconversion assisted by self-absorption of phonon emission. The pertinent justification and further manipulation of EnT(L→T) are provided by control experiments mainly from ultrafast spectroscopy. Besides, a subtle intermode dynamic screening effect in this unary plasmonic system is also addressed. This work refreshes our knowledge about the elusive intermode interplay in plasmonic systems and offers implementable strategies to harness hot electrons toward plasmon-mediated applications.

Direct Observation of Size-Dependent Phase Transition in Methylammonium Lead Bromide Perovskite Microcrystals and Nanocrystals

Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr₃) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr₃ microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.

Intrinsic Carrier Diffusion in Perovskite Thin Films Uncovered by Transient Reflectance Spectroscopy

Carrier diffusion and surface recombination are key processes influencing the performance of conventional semiconductor devices. However, the interplay of photon recycling together with these processes in halide perovskites obfuscates our understanding. Herein, we discern these inherent processes in a thin FAPbBr₃ perovskite single crystal (PSC) utilizing a unique transient reflectance technique that allows accurate diffusion modeling with clear boundary conditions. Temperature-dependent measurements reveal the coexistence of shallow and deep traps at the surface. The inverse quadratic dependence of temperature on carrier mobility μ suggests an underlying scattering mechanism arising from the anharmonic deformation of the PbBr₆ cage. Our findings ascertain the fundamental limits of the intrinsic surface recombination velocity (S) and carrier diffusion coefficient (D) in PSC samples. Importantly, these insights will help resolve the ongoing debate and clarify the ambiguity surrounding the contributions of photon recycling and carrier diffusion in perovskite optoelectronics.

Two-photon absorption in halide perovskites and their applications

Active research on halide perovskites has given us a deep understanding of this family of materials and their potential for applications in advanced optoelectronic devices. One of the prominent outcomes is the use of perovskite materials for nonlinear optical applications. Two-photon absorption in perovskites, in particular their nanostructures, has been extensively studied and shows huge promise for many applications. However, we are still far from a thorough understanding of two-photon absorption in halide perovskites from a micro to macro perspective. Here we summarize different techniques for studying the two-photon absorption in nonlinear optical materials. We discuss the in-depth photophysics in two-photon absorption in halide perovskites. A comprehensive summary about the factors which influence two-photon absorption provides the direction to improve the two-photon absorption properties of halide perovskites. A summary of the recent applications of two-photon absorption in halide perovskites provides inspirations for engineers to utilize halide perovskites in two-photon absorption device development. This review will help readers to have a comprehensive and in-depth understanding of the research field of two-photon absorption of halide perovskites from microscopic mechanisms to applications. The article can serve as a manual and give inspiration for future researchers.

Perovskite-Gallium Nitride Tandem Light-Emitting Diodes with Improved Luminance and Color Tunability

Tandem structures with different subpixels are promising for perovskite-based multicolor electroluminescence (EL) devices in ultra-high-resolution full-color displays; however, realizing excellent luminance- and color-independent tunability considering the low brightness and stability of blue perovskite light-emitting diodes (PeLEDs) remains a challenge. Herein, a bright and stable blue gallium nitride (GaN) LED is utilized for vertical integration with a green MAPbBr₃ PeLED, successfully achieving a Pe-GaN tandem LED with independently tunable luminance and color. The electronic and photonic co-excitation (EPCE) effect is found to suppress the radiative recombination and current injection of PeLEDs, leading to degraded luminance and current efficiency under direct current modulation. Accordingly, the pulse-width modulation is introduced to the tandem device with a negligible EPCE effect, and the average hybrid current efficiency is significantly improved by 139.5%, finally achieving a record tunable luminance (average tuning range of 16631 cd m^-2 at an arbitrary color from blue to green) for perovskite-based multi-color LEDs. The reported excellent independent tunability can be the starting point for perovskite-based multicolor EL devices, enabling the combination with matured semiconductor technologies to facilitate their commercialization in advanced display applications with ultra-high resolution.

Novel Cu(I)-5-nitropyridine-2-thiol Cluster with NIR Emission: Structural and Photophysical Characterization

A novel Cu(I) cluster compound has been synthesized by reacting CuI with the 2,2'-dithiobis(5-nitropyridine) ligand under solvothermal conditions. During the reaction, the original ligand breaks into the 5-nitropyridine-2-thiolate moiety, which acts as the coordinating ligand with both N- and S-sites, leading to a distorted octahedral Cu₆S₆ cluster. The structure has been determined by single-crystal X-ray diffraction and FT-IR analysis, and the photophysical properties have been determined in the solid state by means of steady-state and time-resolved optical techniques. The cluster presents a near-infrared emission showing an unusual temperature dependence: when passing from 77 to 298 K, a blue-shift of the emission band is observed, associated with a decrease in its intensity. Time-dependent-density functional theory calculations suggest that the observed behavior can be ascribed to a complex interplay of excited states, basically in the triplet manifold.

High-Luminance Microsized CH3NH3PbBr3 Single-Crystal-Based Light-Emitting Diodes via a Facile Liquid-Insulator Bridging Route

Micro-/nanosized organic-inorganic hybrid perovskite single crystals (SCs) with appropriate thickness and high crystallinity are promising candidates for high-performance electroluminescent (EL) devices. However, their small lateral size poses a great challenge for efficient device construction and performance optimization, causing perovskite SC-based light-emitting diodes (PSC-LEDs) to demonstrate poor EL performance. Here, we develop a facile liquid-insulator bridging (LIB) strategy to fabricate high-luminance PSC-LEDs based on single-crystalline CH₃NH₃PbBr₃ microflakes. By introducing a blade-coated poly(methyl methacrylate) (PMMA) insulating layer to effectively overcome the problems of leakage current and possible short circuits between electrodes, we achieve the reliable fabrication of PSC-LEDs. The LIB method also allows us to systematically boost the device performance through crystal growth regulation and device architecture optimization. Consequently, we realize the best CH₃NH₃PbBr₃ microflake-based PSC-LED with an ultrahigh luminance of 136100 cd m^-2 and a half-lifetime of 88.2 min at an initial luminance of ∼1100 cd m^-2, which is among the highest for organic-inorganic hybrid perovskite LEDs reported to date. Moreover, we observe the strong polarized edge emission of the microflake-based PSC-LEDs with a high degree of polarization up to 0.69. Our work offers a viable approach for the development of high-performance perovskite SC-based EL devices.

Transient quantum beatings of trions in hybrid organic tri-iodine perovskite single crystal

Utilizing the spin degree of freedom of photoexcitations in hybrid organic inorganic perovskites for quantum information science applications has been recently proposed and explored. However, it is still unclear whether the stable photoexcitations in these compounds correspond to excitons, free/trapped electron-hole pairs, or charged exciton complexes such as trions. Here we investigate quantum beating oscillations in the picosecond time-resolved circularly polarized photoinduced reflection of single crystal methyl-ammonium tri-iodine perovskite (MAPbI₃) measured at cryogenic temperatures. We observe two quantum beating oscillations (fast and slow) whose frequencies increase linearly with B with slopes that depend on the crystal orientation with respect to the applied magnetic field. We assign the quantum beatings to positive and negative trions whose Landé g-factors are determined by those of the electron and hole, respectively, or by the carriers left behind after trion recombination. These are \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${g}_{[001]}^{e}$$\end{document}g[001]e = 2.52 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${g}_{[1\bar{1}0]}^{e}\,$$\end{document}g[11¯0]e= 2.63 for electrons, whereas \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\big|{g}_{[001]}^{h}\big|\,$$\end{document}g[001]h= 0.28 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\big|{g}_{[1\bar{1}0]}^{h}\big|\,$$\end{document}g[11¯0]h= 0.57 for holes. The obtained g-values are in excellent agreement with an 8-band K.P calculation for orthorhombic MAPbI₃. Using the technique of resonant spin amplification of the quantum beatings we measure a relatively long spin coherence time of ~ 11 (6) nanoseconds for electrons (holes) at 4 K.

Understanding photo-physics giving rise to quantum beating oscillations in hybrid organic-inorganic perovskites aids their applications in spintronics and quantum information science. Here, authors demonstrate that quantum beatings observed in single crystal perovskite at cryogenic temperatures are originating from positive and negative trions.

Strain Engineering: A Pathway for Tunable Functionalities of Perovskite Metal Oxide Films

Perovskite offers a framework that boasts various functionalities and physical properties of interest such as ferroelectricity, magnetic orderings, multiferroicity, superconductivity, semiconductor, and optoelectronic properties owing to their rich compositional diversity. These properties are also uniquely tied to their crystal distortion which is directly affected by lattice strain. Therefore, many important properties of perovskite can be further tuned through strain engineering which can be accomplished by chemical doping or simply element substitution, interface engineering in epitaxial thin films, and special architectures such as nanocomposites. In this review, we focus on and highlight the structure–property relationships of perovskite metal oxide films and elucidate the principles to manipulate the functionalities through different modalities of strain engineering approaches.

Spatially resolved fluorescence of caesium lead halide perovskite supercrystals reveals quasi-atomic behavior of nanocrystals

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled cesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modeling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.

By utilizing spatially resolved fluorescence (-lifetime) measurements and high precision X-ray nanodiffraction, the authors correlate the influence of structural misalignment and fluorescence (-lifetime) properties of all-inorganic CsPbX₃ (X^– = Br^–, Cl^–) perovskite superlattices.

Effects of photon recycling and scattering in high-performance perovskite solar cells

Efficient external radiation is essential for solar cells to achieve high power conversion efficiency (PCE). The classical limit of 1/2n² (n, refractive index) for electroluminescence quantum efficiency (ELQE) has recently been approached by perovskite solar cells (PSCs). Photon recycling (PR) and light scattering can provide an opportunity to surpass this limit. We investigate the role of PR and scattering in practical device operation using a radiative PSC with an ELQE (13.7% at 1 sun) that significantly surpasses the classical limit (7.4%). We experimentally analyze the contributions of PR and scattering to this strong radiation. A novel optical model reveals an increase of 39 mV in the voltage of our PSC. This analysis can provide design principles for future PSCs to approach the Shockley-Queisser efficiency limit.

Photon Recycling in CsPbBr3 All-Inorganic Perovskite Nanocrystals

Photon recycling, the iterative process of re-absorption and re-emission of photons in an absorbing medium, can play an important role in the power-conversion efficiency of photovoltaic cells. To date, several studies have proposed that this process may occur in bulk or thin films of inorganic lead-halide perovskites, but conclusive proof of the occurrence and magnitude of this effect is missing. Here, we provide clear evidence and quantitative estimation of photon recycling in CsPbBr₃ nanocrystal suspensions by combining measurements of steady-state and time-resolved photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. The steady-state PL shows clear spectral modifications including red shifts and quantum yield decrease, while the time-resolved measurements show prolonged PL decay and rise times. These effects grow as the nanocrystal concentration and distance traveled through the suspension increase. Monte Carlo simulations of photons diffusing through the medium and exhibiting absorption and re-emission account quantitatively for the observed trends and show that up to five re-emission cycles are involved. We thus identify 4 quantifiable measures, PL red shift, PL QY, PL decay time, and PL rise time that together all point toward repeated, energy-directed radiative transfer between nanocrystals. These results highlight the importance of photon recycling for both optical properties and photovoltaic applications of inorganic perovskite nanocrystals.

Overcoming Outcoupling Limit in Perovskite Light-Emitting Diodes with Enhanced Photon Recycling

Photon recycling (PR), reabsorption and reemission of photons, can randomize the propagation direction of photons trapped in the waveguide mode and potentially increase the outcoupling efficiency of perovskite light-emitting diodes (PeLEDs). However, the contribution of PR in PeLEDs has not been experimentally quantified in real device structures. Here, we show that, with the PR effect, the external quantum efficiency (EQE) of PeLEDs remains above 15% with extraordinary thick perovskite layers up to 2200 nm, which is much higher than the outcoupling efficiency (4.3%) of the thick emissive layer device with an emission zone near the TPBi layer without PR. We designed monolithic device structures to experimentally quantify the PR contribution under device working conditions and reveal that the PR can contribute 2.4%-40.4% of the total emission in PeLEDs depending on film thickness. This work provides an important way of manipulation and quantification of PR contribution in perovskite optoelectronic devices.

Photon Recycling in Semiconductor Thin Films and Devices

Photon recycling (PR) plays an important role in the study of semiconductor materials and impacts the properties of their optoelectronic applications. However, PR has not been investigated comprehensively and it has not been demonstrated experimentally in many different kinds of semiconductor materials and devices. In this review paper, first, the authors introduce the background of PR and describe how this phenomenon was originally identified in semiconductors. Then, the theory and modelling of PR is reviewed and some of the important parameters that are used to quantify PR are highlighted. Next, a variety of the methods used to achieve and characterize PR in materials and devices are discussed. Examples of how the performance parameters of different types of optoelectronic devices are affected by PR are described. Finally, a summary of the roles of PR in semiconductor materials and devices and an outlook on how PR can be used to solve existing problems and challenges in the field of optoelectronics are provided. From this review, it is apparent that PR can have a positive impact on optoelectronic device performance, and that further in-depth theoretical and experimental studies are needed to rigorously demonstrate the advantages and importance of PR.

Photon-recycling effect in perovskites for photovoltaic applications: a Monte Carlo study

Photon recycling has been shown to play an important role in the optoelectronic properties and device performance of perovskite solar cells recently. However, there lacks an analytical method to accurately predict the dynamics of charge carriers and photons and the device performance with photon recycling due to the complexity of multiple electron-photon conversion processes involved in photon recycling. We propose a model based on the Monte Carlo simulation method that combines charge carrier diffusion and photon radiation transport to analyze the effects of photon recycling on electron-photon dynamics and device performance of perovskite solar cells. We show that the carrier lifetime can be significantly boosted by photon recycling in the radiative limit, which yields a 37 meV increase in the open-circuit voltage for a 500 nm thick perovskite solar cell. Our results provide insights for the working mechanisms of perovskite solar cells, and the new model can be further applied to other types of solar cells with photon recycling.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

Impact of Vertical Inhomogeneity on the Charge Extraction in Perovskite Solar Cells: A Study by Depth-Dependent Photoluminescence

In perovskite solar cells (PSCs), the vertical inhomogeneities which include uneven grains, voids, and grain boundaries are closely linked to the underlying charge transport layer which controls the nucleation and grain growth in the perovskite film. Herein, the vertical inhomogeneity of perovskite films in the device structure is analyzed by depth-dependent photoluminescence (PL) achieved with different excitation wavelengths. An analytical representation between vertical inhomogeneity and depth-dependent PL, parametrized with a factor, b, is introduced to understand the relation between inhomogeneity and charge recombination. Lower values of b correlate to lower vertical inhomogeneity and hence reduced recombination. The analytical representation is validated in two sets of devices that show remarkable differences in perovskite film morphology, device based on mesoporous TiO₂ and planar SnO₂. By exploring the morphological properties and the PL emission from different depths across the device structures, we show that the lower vertical inhomogeneity leads to more efficient charge carrier extraction in planar SnO₂-based devices. Moreover, the SnO₂-based devices exhibit lower Urbach energy, which concurs with the slow transient photovoltage decay, suggesting less defects and recombination losses. This work provides a broader understanding of the impact of vertical inhomogeneity on the charge extraction efficiency and presents a methodology to study quantitatively the inhomogeneity of perovskite films in device structures.

Crystallization of CsPbBr3 single crystals in water for X-ray detection

Metal halide perovskites have fascinated the research community over the past decade, and demonstrated unprecedented success in optoelectronics. In particular, perovskite single crystals have emerged as promising candidates for ionization radiation detection, due to the excellent opto-electronic properties. However, most of the reported crystals are grown in organic solvents and require high temperature. In this work, we develop a low-temperature crystallization strategy to grow CsPbBr₃ perovskite single crystals in water. Then, we carefully investigate the structure and optoelectronic properties of the crystals obtained, and compare them with CsPbBr₃ crystals grown in dimethyl sulfoxide. Interestingly, the water grown crystals exhibit a distinct crystal habit, superior charge transport properties and better stability in air. We also fabricate X-ray detectors based on the CsPbBr₃ crystals, and systematically characterize their device performance. The crystals grown in water demonstrate great potential for X-ray imaging with enhanced performance metrics.

Perovskite single crystals are commonly grown in organic solvents, which require relatively high temperature condition. Here, the authors develop a low-temperature crystallisation strategy to grow CsPbBr₃ single crystals in water with improved charge transport properties and stability.

High-Quality All-Inorganic Perovskite CsPbBr3 Microsheet Crystals as Low-Loss Subwavelength Exciton-Polariton Waveguides

Nanostructured all-inorganic metal halide perovskites have attracted considerable attention due to their outstanding photonic and optoelectronic properties. Particularly, they can exhibit room-temperature exciton-polaritons (EPs) capable of confining electromagnetic fields down to the subwavelength scale, enabling efficient light harvesting and guiding. However, a real-space nanoimaging study of the EPs in perovskite crystals is still absent. Additionally, few studies focused on the ambient-pressure and reliable fabrication of large-area CsPbBr₃ microsheets. Here, CsPbBr₃ orthorhombic microsheet single crystals were successfully synthesized under ambient pressure. Their EPs were examined using a real-space nanoimaging technique, which reveal EP waveguide modes spanning the visible to near-infrared spectral region. The EPs exhibit a sufficient long propagation length of over 16 μm and a very low propagation loss of less than 0.072 dB·μm^-1. These results demonstrate the potential applications of CsPbBr₃ microsheets as subwavelength waveguides in integrated optics.

Phonon, thermal, and thermo-optical properties of halide perovskites

Metal halide perovskites are semiconductors with many fascinating characteristics and their widespread use in optoelectronic devices has been expected. High-quality thin films and single crystals can be fabricated by simple chemical solution processes and their fundamental electrical, optical, and thermal properties can be changed significantly by compositional substitution, in particular halogen ions. In this perspective, we provide an overview of phonon and thermal properties of metal halide perovskites, which play a decisive role in determining device performance. After a brief introduction to fundamental material properties, longitudinal-optical phonons and unusual thermal properties of metal halide perovskites are discussed. Remarkably, they possess very low thermal conductivities and very large thermal expansion coefficients despite their crystalline nature. In line with these discussions, we present optical properties governed by the strong electron-phonon interactions and the unusual thermal properties. By showing their unique thermo-optic responses and novel application examples, we highlight some aspects of the unusual thermal properties.

Static Rashba Effect by Surface Reconstruction and Photon Recycling in the Dynamic Indirect Gap of APbBr3 (A = Cs, CH3NH3) Single Crystals

Recently, halide perovskites have gained significant attention from the perspective of efficient spintronics owing to the Rashba effect. This effect occurs as a consequence of strong spin-orbit coupling under a noncentrosymmetric environment, which can be dynamic and/or static. However, there exist intense debates on the origin of broken inversion symmetry since the halide perovskites typically crystallize into a centrosymmetric structure. In order to clarify the issue, we examine both dynamic and static effects in the all-inorganic CsPbBr₃ and organic-inorganic CH₃NH₃PbBr₃ (MAPbBr₃) perovskite single crystals by employing temperature- and polarization-dependent photoluminescence excitation spectroscopy. The perovskite single crystals manifest the dynamic effect by photon recycling in the indirect Rashba gap, causing dual peaks in the photoluminescence. However, the effect vanishes in CsPbBr₃ at low temperatures (<50 K) accompanied by a striking color change of the crystal, arising presumably from lower degrees of freedom for inversion symmetry breaking associated with the thermal motion of the spherical Cs cation compared with the polar MA cation in MAPbBr₃. We also show that the static Rashba effect occurs only in MAPbBr₃ below 90 K, presumably due to surface reconstruction via MA-cation ordering, which likely extends across a few layers from the crystal surface to the interior. We further demonstrate that this static Rashba effect can be completely suppressed upon surface treatment with polymethyl methacrylate (PMMA) coating. We believe that our results provide a rationale for the Rashba effects in halide perovskites.

Energy transport and light propagation mechanisms in organic single crystals

Unambiguous information about spatiotemporal exciton dynamics in three-dimensional nanometer- to micrometer-sized organic structures is difficult to obtain experimentally. Exciton dynamics can be modified by annihilation processes, and different light propagation mechanisms can take place, such as active waveguiding and photon recycling. Since these various processes and mechanisms can lead to similar spectroscopic and microscopic signatures on comparable time scales, their discrimination is highly demanding. Here, we study individual organic single crystals grown from thiophene-based oligomers. We use time-resolved detection-beam scanning microscopy to excite a local singlet exciton population and monitor the subsequent broadening of the photoluminescence (PL) signal in space and on pico- to nanosecond time scales. Combined with Monte Carlo simulations, we were able to exclude photon recycling for our system, whereas leakage radiation upon active waveguiding leads to an apparent PL broadening of about 20% compared to the initial excitation profile. Exciton-exciton annihilation becomes important at high excitation fluence and apparently accelerates the exciton dynamics leading to apparently increased diffusion lengths. At low excitation fluences, the spatiotemporal PL broadening results from singlet exciton diffusion with diffusion lengths of up to 210 nm. Surprisingly, even in structurally highly ordered single crystals, the transport dynamics is subdiffusive and shows variations between different crystals, which we relate to varying degrees of static and dynamic electronic disorders.

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.

The role of photon recycling in perovskite light-emitting diodes

Perovskite light-emitting diodes have recently broken the 20% barrier for external quantum efficiency. These values cannot be explained with classical models for optical outcoupling. Here, we analyse the role of photon recycling (PR) in assisting light extraction from perovskite light-emitting diodes. Spatially-resolved photoluminescence and electroluminescence measurements combined with optical modelling show that repetitive re-absorption and re-emission of photons trapped in substrate and waveguide modes significantly enhance light extraction when the radiation efficiency is sufficiently high. In this manner, PR can contribute more than 70% to the overall emission, in agreement with recently-reported high efficiencies. While an outcoupling efficiency of 100% is theoretically possible with PR, parasitic absorption losses due to absorption from the electrodes are shown to limit practical efficiencies in current device architectures. To overcome the present limits, we propose a future configuration with a reduced injection electrode area to drive the efficiency toward 100%.

Perovskite light-emitting diodes have shown unexpected high external quantum efficiency of 20%, breaking the ray-optics limit. Here Cho et al. reveal that photon recycling is responsible for the enhancement and propose photonic structures to further improve the device efficiency.

Rubidium Doping to Enhance Carrier Transport in CsPbBr3 Single Crystals for High-Performance X-Ray Detection

The direct band gap CsPbBr₃ perovskite is regarded as a promising alternative for low-cost and high-performance X-ray radiation detectors. Despite the fact that CsPbBr₃ nanocrystals have been shown to be good scintillators in the indirect conversion mode, the direct X-ray conversion with CsPbBr₃ single crystals is expected to yield higher spatial resolution. Here, rubidium (Rb) doping is demonstrated to be an efficient approach to improve carrier transport and X-ray detection performance in the direct-conversion X-ray detectors based on Cs_(1-x)Rb_xPbBr₃ single crystals. Electrical properties' characterizations as combined with X-ray photoelectron spectroscopy (XPS) measurements have revealed that Rb doping in Cs_(1-x)Rb_xPbBr₃ single crystals can enhance the atomic interaction and orbital coupling between Pb and Br atoms, leading to an enhancement of carrier transport and X-ray detection performance. X-ray detectors based on a small amount (0.037%) of Rb-doped Cs_(1-x)Rb_xPbBr₃ single crystals exhibited a high X-ray sensitivity of 8097 μC Gy_air^-1 cm^-2. This work offers a feasible strategy to improve the X-ray detection performance by chemical doping in all-inorganic perovskite X-ray detectors.

In-plane stimulated emission of polycrystalline CH3NH3PbBr3 perovskite thin films

Hybrid organic–inorganic lead halide perovskites have been investigated extensively within the last decades, for its great potential in efficient solar cells and as an ideal light source. Among the studies on stimulated emission (SE), the emission is either out-of-plane for polycrystalline films or in-plane with randomly aligned single microcrystals and nanowires. In this work, we revealed in-plane propagation of SE from bromine-based perovskite polycrystalline thin films (CH₃NH₃PbBr₃, or MAPbBr₃). The output from in-plane SE is an order higher than the out-of-plane emission. It is proposed that large crystalline flakes in the films lead to the in-plane lasing phenomena. The output coupling can be found at grain boundaries, intergrain gaps, and artificial structures. Simulative results support the experimental phenomenon that large crystalline grains are profitable for in-plane propagation and over 90% photons can be sufficiently outcoupled when the gap is larger than a micron. Considering the fabrication and handling convenience, we propose that the MAPbBr₃ thin films can be easily integrated for in-plane applications as the light source for photonic chips etc.

MAPbBr₃ perovskite thin film contains large crystal flakes, which support the in-plane stimulated emission and its propagation within these polycrystalline films. The emission scatters at the natural or artificial edge of the film.

Extrinsic photoluminescence properties of individual micro-particle of Cs4PbBr6 perovskite with "defect" structure

Optical performance of the lead halide perovskites with zero-dimension (0D) structure has been in a hot debate for optoelectronic applications. Here, Cs₄PbBr₆ hexagonal micro-particles with a remarkable green emission are first fabricated via a low-temperature solution-process employed ethanol as solvent. Our results underline that the existence of bromine vacancies and the introduction of hydroxyl induce a narrowed band gap with the formation of a defect level, which contributes to the extrinsic photoluminescence (PL) properties synergistically. Thanks to the high exciton binding energy and the unique morphology with a regular geometric structure of the as-obtained micro-particles, two-photon pumped amplified spontaneous emission (ASE) and single mode lasing from an individual Cs₄PbBr₆ particle are realized. Our results not only provide an insight into the origin of optical emission from Cs₄PbBr₆, but also demonstrate that the versatile Cs₄PbBr₆ offers a new opportunity for novel nonlinear photonics applications as an up-conversion laser.

Vapor-Phase Incommensurate Heteroepitaxy of Oriented Single-Crystal CsPbBr3 on GaN: Toward Integrated Optoelectronic Applications

Integrating metallic halide perovskites with established modern semiconductor technology is significant for promoting the development of application-level optoelectronic devices. To realize such devices, exploring the growth dynamics and interfacial carrier dynamics of perovskites deposited on the core materials of semiconductor technology is essential. Herein, we report the incommensurate heteroepitaxy of highly oriented single-crystal cesium lead bromide (CsPbBr₃) on c-wurtzite GaN/sapphire substrates with atomically smooth surface and uniform rectangular shape by chemical vapor deposition. The CsPbBr₃ microplatelet crystal exhibits green-colored lasing under room temperature and has a structural stability comparable with that grown on van der Waals mica substrates. Time-resolved photoluminescence spectroscopy studies show that the type-II CsPbBr₃-GaN heterojunction effectively enhances the separation and extraction of free carriers inside CsPbBr₃. These findings provide insights into the fabrication and application-level integrated optoelectronic devices of CsPbBr₃ perovskites.

Homogeneous Freestanding Luminescent Perovskite Organogel with Superior Water Stability

Metal-halide perovskites have become appealing materials for optoelectronic devices. While the fast advancing stretchable/wearable devices require stability, flexibility and scalability, current perovskites suffer from ambient-environmental instability and incompatible mechanical properties. Recently perovskite-polymer composites have shown improved in-air stability with the protection of polymers. However, their stability remains unsatisfactory in water or high-humidity environment. These methods also suffer from limited processability with low yield (2D film or beads) and high fabrication cost (high temperature, air/moisture-free conditions), thereby limiting their device integration and broader applications. Herein, by combining facile photo-polymerization with room-temperature in-situ perovskite reprecipitation at low energy cost, a one-step scalable method is developed to produce freestanding highly-stable luminescent organogels, within which CH₃ NH₃ PbBr₃ nanoparticles are homogeneously distributed. The perovskite-organogels present a record-high stability at different pH and temperatures, maintaining their high quantum yields for > 110 days immersing in water. This paradigm is universally applicable to broad choices of polymers, hence casting these emerging luminescent materials to a wide range of mechanical properties tunable from rigid to elastic. With intrinsically ultra-stretchable photoluminescent organogels, flexible phosphorous layers were demonstrated with > 950% elongation. Rigid perovskite gels, on the other hand, permitted the deployment of 3D-printing technology to fabricate arbitrary 2D/3D luminescent architectures.

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.

The photoluminescence mechanism of CsPb2Br5 microplates revealed by spatially resolved single particle spectroscopy

CsPb2Br5 is a new member of the all-inorganic lead halide perovskite family with unique structures and optoelectronic properties for various applications. As an indirect band gap semiconductor, the photoluminescence (PL) mechanism of CsPb2Br5 is still under debate. To resolve this issue, CsPb2Br5 microplates with strong green PL have been prepared by a hot-injection method. Characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis and ultraviolet-visible (UV-vis) absorption spectroscopy indicates the existence of a small amount of embedded CsPbBr3 phase. The removal of the embedded CsPbBr3 phase by treatment with water containing ethanol solvent resulted in complete PL quenching, suggesting the origin of PL due to the embedded CsPbBr3 phase. Spatially resolved PL and time-resolved PL mapping have been further employed to directly visualize the spatial distribution of different emission centers. Our single particle spectroscopic studies indicated the existence of three different emission centers with different PL lifetimes: two types of embedded CsPbBr3 phases (clumped and randomly distributed CsPbBr3 nanocrystals) and intrinsic defects of CsPb2Br5. The embedded CsPbBr3 phases with fast and intermediate PL lifetimes are the primary contributors to PL of the CsPb2Br5 microplates while their intrinsic defects with slow PL lifetimes make only a minor contribution. These studies have unambiguously clarified the PL mechanisms of the CsPb2Br5 microplates and provided the direct mapping of different emission centers, which resolve the contradictory explanation and debate about the PL mechanism of the CsPb2Br5 microplates.

Investigation into the Photoluminescence Red Shift in Cesium Lead Bromide Nanocrystal Superlattices

The formation of cesium lead bromide (CsPbBr₃) nanocrystal superlattices (NC SLs) is accompanied by a red shift in the NC photoluminescence (PL). The values of the PL red shift reported in the literature range from none to ∼100 meV without unifying explanation of the differences. Using a combination of confocal PL microcopy and steady-state optical spectroscopies we found that an overall PL red shift of ∼96 meV measured from a macroscopic sample of CsPbBr₃ NC SLs has several contributions: ∼ 10-15 meV from a red shift in isolated and clean SLs, ∼ 30 meV from SLs with impurities of bulklike CsPbBr₃ crystals on their surface, and up to 50 meV or more of the red shift coming from a photon propagation effect, specifically self-absorption. In addition, a self-assembly technique for growing micron-sized NC SLs on the surface of perfluorodecalin, an inert perfluorinated liquid and an antisolvent for NCs, is described.

Photoluminescence enhancement in wide spectral range excitation in CsPbBr3 nanocrystal/Ag nanostructure via surface plasmon coupling

This work demonstrates the surface plasmon (SP)-exciton coupling effect on the photoluminescence (PL) enhancement in CsPbBr₃ nanocrystal (NC) at PMMA/Ag nanostructure (NS) in wide spectral range excitation. The spectra dependent time resolved PL measurement reveals that the emission photons are from the recombination of localized excitons and the PL enhancement can be attributed to the near-field effect, which is also supported by evidence that the enhancements are nearly the same in the whole excitation wavelength from 200 nm to 900 nm. The non-spectral dependence of the enhancement factor suggests that there is the same dynamic process of hot electrons in CsPbBr₃ NC in multiphoton excitation. The hot electrons will relax into localized exciton states, and the electric field generated by SPs will enhance the radiative recombination of excitons. This work will have benefits for revealing dynamics of hot electron relaxing and interactions in multi-photon absorption, as well as the inner mechanism of SP coupling effects.

Indirect tail states formation by thermal-induced polar fluctuations in halide perovskites

Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr₆ octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.

The weak effects induced by lattice disorder on the optoelectronic properties of halide perovskites still remain elusive. Here Wu et al. confirm the indirect transition tail states in perovskite crystals which explain their low photoluminescence quantum yield, dual emission peaks and difficulties in realizing lasing.

Temperature and excitation wavelength-dependent photoluminescence of CH3NH3PbBr3 crystal

We have investigated temperature and excitation wavelength-dependent luminescent properties of CH₃NH₃PbBr₃ (MAPbBr₃) perovskite crystal using steady-state and time-resolved photoluminescence (PL) spectroscopy. Optical spectroscopic results indicate that the PL intensity, peak wavelength, and full width at half maximum are jumped due to the phase transition from orthorhombic to tetragonal and cubic. As temperature increases, the peaks of above-bandgap and intrinsic PL have blueshift and redshift, respectively. The above-bandgap PL emerges under one-photon laser excitation and vanishes under the excitation of near-infrared femtosecond laser due to reabsorption. The initial redshift of PL peak and lifetime change at various wavelengths reveals the existence of a trap-assisted excitonic state. It is found that upconversion PL in all phases is composed of a photoinduced thermal-assisted and an intrinsic excitonic state, which produces the observation of biexponential dynamics. Our results can facilitate the understanding of the thermal effect of exciton recombination in hybrid perovskite crystal.

Photon-induced carrier recombination in the nonlayered-structured hybrid organic-inorganic perovskite nano-sheets

The hybrid organic-inorganic perovskites (HOIPs) have attracted much attention recently due to their preeminent efficiency in solar cells. According to the difference on the crystalline structure, the HOIPs could be classified into layered and non-layered perovskites. Very recently, it has been realized that the non-layered HOIPs with common-vertex structure possess even better opto-electrical performance. Yet the carrier recombination mechanism in perovskite remains not very clear, and a clear understanding of this mechanism is essential to pinpoint the working mechanism of photovoltaic and electroluminescent materials. Here we report the optical studies on the hybrid perovskite crystalline nano-sheet of CH₃NH₃PbBr₃ with common-vertex structure. It is shown that the non-layered perovskite crystalline nanosheets possess the exciton binding energy about two orders of magnitude smaller than that of the layered perovskite and the colloidal nanoplates, which is beneficial for the designing of the high-efficiency photovoltaic devices. By measuring the temperature-dependent photoluminescence (PL) spectra, the excitation-power-variant PL spectra, and the time-resolved PL spectra, we identify that both the free-carrier and the localized exciton recombination channels may coexist in the crystallites. Further, for the thin crystallite (∼60 nm), the free-carrier recombination channel dominates; whereas when the thickness increases beyond 200 nm, the localized exciton recombination channel plays the major role. We suggest these results are helpful to improve further the photovoltaic and electroluminescent performances of perovskite devices.

High Resolution Mapping of Two-Photon Excited Photocurrent in Perovskite Microplate Photodetector

We fabricate photodetectors based on solution-processed single CH₃NH₃PbBr₃ microcrystals (MCs) and map the two-photon absorption (TPA) excited photocurrent (PC) with spatial resolution of 1 μm. We find that the charge carrier transport length in the MCs depends on the applied electric field, and increases from 5.7 μm for 0.02 V bias (dominated by carrier diffusion) to 23.2 μm for 2 V bias (dominated by carrier drift). Furthermore, PC shows strong spatial variations. Combining the PC mapping results with time-resolved photoluminescence microscopy, we demonstrate that the spatial distribution of PC mainly originates from the inhomogeneous distribution of trap-states across perovskite MCs. This suggests that there is still large margin for improvement of perovskite single crystal devices by better controlling of the traps.

High-efficiency energy transfer in perovskite heterostructures

Here, we report the energy transfer in (PEA)₂PbI₄/MAPbBr₃ perovskite heterostructures. Under two-photon excitation, the photoluminescence (PL) emission of the (PEA)₂PbI₄ flake is nearly completely quenched, while that of the MAPbBr₃ microplate is greatly increased (6.5 folds higher) in the heterostructure. The opposite variation character of the PL emissions is attributed to the radiative energy transfer from the (PEA)₂PbI₄ flake to the MAPbBr₃ microplate. The radiative energy transfer occurs on an ultrafast timescale with a high efficiency (~100%). In addition, a strongly thickness- and wavelength-dependent interlayer interaction is observed under one-photon excitation. This work advocates great promise for revealing the interlayer interaction of perovskite heterostructures and developing high-performance optoelectronic devices.

Photonics and Optoelectronics of 2D Metal-Halide Perovskites

In the growing list of 2D semiconductors as potential successors to silicon in future devices, metal-halide perovskites have recently joined the family. Unlike other conversional 2D covalent semiconductors such as graphene, transition metal dichalcogenides, black phosphorus, etc., 2D perovskites are ionic materials, affording many distinct properties of their own, including high photoluminescence quantum efficiency, balanced large exciton binding energy and oscillator strength, and long carrier diffusion length. These unique properties make 2D perovskites potential candidates for optoelectronic and photonic devices such as solar cells, light-emitting diodes, photodetectors, nanolasers, waveguides, modulators, and so on, which represent a relatively new but exciting and rapidly expanding area of research. In this Review, the recent advances in emerging 2D metal-halide perovskites and their applications in the fields of optoelectronics and photonics are summarized and insights into the future direction of these fields are offered.