The role of photon recycling in perovskite light-emitting diodes

Fabrication of red-emitting perovskite LEDs by stabilizing their octahedral structure

Light-emitting diodes (LEDs) based on metal halide perovskites (PeLEDs) with high colour quality and facile solution processing are promising candidates for full-colour and high-definition displays1-4. Despite the great success achieved in green PeLEDs with lead bromide perovskites5, it is still challenging to realize pure-red (620-650 nm) LEDs using iodine-based counterparts, as they are constrained by the low intrinsic bandgap6. Here we report efficient and colour-stable PeLEDs across the entire pure-red region, with a peak external quantum efficiency reaching 28.7% at 638 nm, enabled by incorporating a double-end anchored ligand molecule into pure-iodine perovskites. We demonstrate that a key function of the organic intercalating cation is to stabilize the lead iodine octahedron through coordination with exposed lead ions and enhanced hydrogen bonding with iodine. The molecule synergistically facilitates spectral modulation, promotes charge transfer between perovskite quantum wells and reduces iodine migration under electrical bias. We realize continuously tunable emission wavelengths for iodine-based perovskite films with suppressed energy loss due to the decrease in bond energy of lead iodine in ionic perovskites as the bandgap increases. Importantly, the resultant devices show outstanding spectral stability and a half-lifetime of more than 7,600 min at an initial luminance of 100 cd m-2.

Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7 cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

Quantum barriers engineering toward radiative and stable perovskite photovoltaic devices

Efficient photovoltaic devices must be efficient light emitters to reach the thermodynamic efficiency limit. Here, we present a promising prospect of perovskite photovoltaics as bright emitters by harnessing the significant benefits of photon recycling, which can be practically achieved by suppressing interfacial quenching. We have achieved radiative and stable perovskite photovoltaic devices by the design of a multiple quantum well structure with long (∼3 nm) organic spacers with oleylammonium molecules at perovskite top interfaces. Our L-site exchange process (L: barrier molecule cation) enables the formation of stable interfacial structures with moderate conductivity despite the thick barriers. Compared to popular short (∼1 nm) Ls, our approach results in enhanced radiation efficiency through the recursive process of photon recycling. This leads to the realization of radiative perovskite photovoltaics with both high photovoltaic efficiency (in-lab 26.0%, certified to 25.2%) and electroluminescence quantum efficiency (19.7 % at peak, 17.8% at 1-sun equivalent condition). Furthermore, the stable crystallinity of oleylammonium-based quantum wells enables our devices to maintain high efficiencies for over 1000 h of operation and >2 years of storage.

Valley-centre tandem perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) have emerged as a promising new light source for displays. The development roadmap for commercializing PeLEDs should include a tandem device structure, specifically by stacking a thin nanocrystal PeLED unit and an organic light-emitting diode unit, which can achieve a vivid and efficient tandem display; however, simply combining light-emitting diodes with different characteristics does not guarantee both narrowband emission and high efficiency, as it may cause a broadened electroluminescence spectra and a charge imbalance. Here, by conducting optical simulations of the hybrid tandem (h-tandem) PeLED, we have discovered a crucial optical microcavity structure known as the h-tandem valley, which enables the h-tandem PeLED to emit light with a narrow bandwidth. Specifically, the centre structure of the h-tandem valley (we call it valley-centre tandem) demonstrates near-perfect charge balance and optimal microcavity effects. As a result, the h-tandem PeLED achieves a high external quantum efficiency of 37.0% and high colour purity with a narrow full-width at half-maximum of 27.3 nm (versus 64.5 nm in organic light-emitting diodes) along with a fast on-off response. These findings offer a new strategy to overcome the limitations of nanocrystal-based PeLEDs, providing valuable optical and electrical guidelines for integrating different types of light-emitting device into practical display applications.

Unraveling the Luminescence Quenching Mechanism in Strong and Weak Quantum-Confined CsPbBr3 Triggered by Triarylamine-Based Hole Transport Layers

Luminescence quenching by hole transport layers (HTLs) is one of the major issues in developing efficient perovskite light-emitting diodes (PeLEDs), which is particularly prominent in blue-emitting devices. While a variety of material systems have been used as interfacial layers, the origin of such quenching and the type of interactions between perovskites and HTLs are still ambiguous. Here, we present a systematic investigation of the luminescence quenching of CsPbBr3 by a commonly employed hole transport polymer, poly[(9,9-dioctylfluorenyl-2,7diyl)-co-(4,4'-(N-(4-sec-butylphenyl) diphenylamine)] (TFB), in LEDs. Strong and weak quantum-confined CsPbBr3 (nanoplatelets (NPLs)/nanocrystals (NCs)) are rationally selected to study the quenching mechanism by considering the differences in their morphology, energy level alignments, and quantum confinement. The steady-state and time-resolved Stern-Volmer plots unravel the dominance of dynamic and static quenching at lower and higher concentrations of TFB, respectively, with a maximum quenching efficiency of 98%. The quenching rate in NCs is faster than that in NPLs owing to their longer PL lifetimes and weak quantum confinement. The ultrafast transient absorption results support these dynamics and rule out the involvement of Forster or Dexter energy transfer. Finally, the 1D 1H and 2D nuclear overhauser effect spectroscopy nuclear magnetic resonance (NOESY NMR) study confirms the exchange of native ligands at the NCs surface with TFB, leading to dark CsPbBr3-TFB ensemble formation accountable for luminescence quenching. This highlights the critical role of the triarylamine functional group on TFB (also the backbone of many HTLs) in the quenching process. These results shed light on the underlying reasons for the luminescence quenching in PeLEDs and will help to rationally choose the interfacial layers for developing efficient LEDs.

Energy transfer enhanced photoluminescence of 2D/3D CsPbBr3 hybrid assemblies

Energy transfer has been proven to be an effective method to optimize optoelectronic conversion efficiency by improving light absorption and mitigating nonradiative losses. We prepared 2D/3D CsPbBr3 hybrid assemblies at different reaction temperatures using the hot injection method and found that the photoluminescence quantum yields (PLQYs) of these hybrids were greatly enhanced from 53.4% to 72.57% compared with 3D nanocrystals (NCs). Femtosecond transient absorption measurements were used to study the PLQY enhancement mechanisms, and it was found that the hot carrier lifetime improved from 0.36 to 1.88 ps for 2D/3D CsPbBr3 hybrid assemblies owing to the energy transfer from 2D nanoplates to 3D NCs. The energy transfer benefits the excited carrier accumulation and prolonged hot carrier lifetime in 3D NCs in hybrid assemblies, as well as PLQY enhancement in materials.

Light Management of Metal Halide Scintillators for High-Resolution X-Ray Imaging

The ever-growing need to inspect matter with hyperfine structures requires a revolution in current scintillation detectors, and the innovation of scintillators is revived with luminescent metal halides entering the scene. Notably, for any scintillator, two fundamental issues arise: Which kind of material is suitable and in what form should the material exist? The answer to the former question involves the sequence of certain atoms into specific crystal structures that facilitate the conversion of X-ray into light, whereas the answer to the latter involves assembling these crystallites into particular material forms that can guide light propagation toward its corresponding pixel detector. Despite their equal importance, efforts are overwhelmingly devoted to improving the X-ray-to-light conversion, while the material-form-associated light propagation, which determines the optical signal collected for X-ray imaging, is largely overlooked. This perspective critically correlates the reported spatial resolution with the light-propagation behavior in each form of metal halides, combing the designing rules for their future development.

Hyperspectral mapping of nanoscale photophysics and degradation processes in hybrid perovskite at the single grain level

With solar cells reaching 26.1% certified efficiency, hybrid perovskites are now the most efficient thin film photovoltaic material. Though substantial effort has focussed on synthesis approaches and device architectures to further improve perovskite-based solar cells, more work is needed to correlate physical properties of the underlying film structure with device performance. Here, using cathodoluminescence microscopy coupled with unsupervised machine learning, we quantify how nanoscale heterogeneity globally builds up within a large morphological grain of hybrid perovskite when exposed to extrinsic stimuli such as charge accumulation from electron beams or milder environmental factors like humidity. The converged electron-beam excitation allows us to map PbI2 and the emergence of other intermediate phases with high spatial and energy resolution. In contrast with recent reports of hybrid perovskite cathodoluminescence, we observe no significant change in the PbI2 signatures, even after high-energy electron beam excitation. In fact, we can exploit the stable PbI2 signatures to quantitatively map how hybrid perovskites degrade. Moreover, we show how our methodology allows disentangling of the photophysics associated with photon recycling and band-edge emission with sub-micron resolution using a fundamental understanding of electron interactions in hybrid perovskites.

Perovskite Light-Emitting Diodes with an External Quantum Efficiency Exceeding 30

Perovskite light-emitting diodes (PeLEDs) are strong candidates for next-generation display and lighting technologies due to their high color purity and low-cost solution-processed fabrication. However, PeLEDs are not superior to commercial organic light-emitting diodes (OLEDs) in efficiency, as some key parameters affecting their efficiency, such as the charge carrier transport and light outcoupling efficiency, are usually overlooked and not well optimized. Here, ultrahigh-efficiency green PeLEDs are reported with quantum efficiencies surpassing a milestone of 30% by regulating the charge carrier transport and near-field light distribution to reduce electron leakage and achieve a high light outcoupling efficiency of 41.82%. Ni0.9 Mg0.1 Ox films are applied with a high refractive index and increased hole carrier mobility as the hole injection layer to balance the charge carrier injection and insert the polyethylene glycol layer between the hole transport layer and the perovskite emissive layer to block the electron leakage and reduce the photon loss. Therefore, with the modified structure, the state-of-the-art green PeLEDs achieve a world record external quantum efficiency of 30.84% (average =  29.05 ± 0.77%) at a luminance of 6514 cd m-2 . This study provides an interesting idea to construct super high-efficiency PeLEDs by balancing the electron-hole recombination and enhancing the light outcoupling.

Light management for perovskite light-emitting diodes

Perovskite light-emitting diodes (LEDs) have reached external quantum efficiencies of over 20% for various colours, showing great potential for display and lighting applications. Despite the internal quantum efficiencies of the best-performing devices already approaching unity, around 80% of the internally generated photons are trapped in the devices and lose energy through a variety of lossy channels. Significant opportunities for improving efficiency and maximizing photon extraction lie in the effective management of light. In this Review we analyse light management strategies based on the intrinsic optical properties of the perovskite materials and the extrinsic properties related to device structures. These approaches should allow the external quantum efficiencies of perovskite LEDs to substantially exceed the conventional limits of planar organic LED devices. By revisiting lessons learned from organic LEDs and perovskite solar cells, we highlight possible directions of future research towards perovskite LEDs with ultrahigh efficiencies.

Advances in the Application of Perovskite Materials

Nowadays, the soar of photovoltaic performance of perovskite solar cells has set off a fever in the study of metal halide perovskite materials. The excellent optoelectronic properties and defect tolerance feature allow metal halide perovskite to be employed in a wide variety of applications. This article provides a holistic review over the current progress and future prospects of metal halide perovskite materials in representative promising applications, including traditional optoelectronic devices (solar cells, light-emitting diodes, photodetectors, lasers), and cutting-edge technologies in terms of neuromorphic devices (artificial synapses and memristors) and pressure-induced emission. This review highlights the fundamentals, the current progress and the remaining challenges for each application, aiming to provide a comprehensive overview of the development status and a navigation of future research for metal halide perovskite materials and devices.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Energy and Charge Transfer Dynamics in Red-Emitting Hybrid Organo-Inorganic Mixed Halide Perovskite Nanocrystals

We report time-resolved spectral properties of highly stable and efficient red-emitting hybrid perovskite nanocrystals with the composition FA_0.5MA_0.5PbBr_0.5I_2.5 (FAMA PeNC) synthesized by using the hot-addition method. The PL spectrum of the FAMA PeNC shows a broad asymmetric band covering 580 to 760 nm with a peak at 690 nm which can be deconvoluted into two bands corresponding to the MA and FA domains. The interactions between the MA and FA domains are shown to affect the relaxation dynamics of the PeNCs from the subpicosecond to tens of nanoseconds scale. Time-correlated single-photon counting (TCSPC), femtosecond PL optical gating (FOG), and femtosecond transient absorption spectral (TAS) techniques were employed to study the intercrystal energy transfer (photon recycling) and intracrystal charge transfer processes between the MA and the FA domains of the crystals. These two processes are shown to increase the radiative lifetimes for the PLQYs exceeding 80%, which may play a key role in enhancing the performance of PeNC-based solar cells.

Reducing Nonradiative Losses in Perovskite LEDs through Atomic Layer Deposition of Al2O3 on the Hole-Injection Contact

Halide perovskite light-emitting diodes (PeLEDs) exhibit great potential for use in next-generation display technologies. However, scale-up will be challenging due to the requirement of very thin transport layers for high efficiencies, which often present spatial inhomogeneities from improper wetting and drying during solution processing. Here, we show how a thin Al₂O₃ layer grown by atomic layer deposition can be used to preferentially cover regions of imperfect hole transport layer deposition and form an intermixed composite with the organic transport layer, allowing hole conduction and injection to persist through the organic hole transporter. This has the dual effect of reducing nonradiative recombination at the heterojunction and improving carrier selectivity, which we infer to be due to the inhibition of direct contact between the indium tin oxide and perovskite layers. We observe an immediate improvement in electroluminescent external quantum efficiency in our p-i-n LEDs from an average of 9.8% to 13.5%, with a champion efficiency of 15.0%. The technique uses industrially available equipment and can readily be scaled up to larger areas and incorporated in other applications such as thin-film photovoltaic cells.

Simple Visualization of Universal Ferroelastic Domain Walls in Lead Halide Perovskites

Domain features and domain walls in lead halide perovskites (LHPs) have attracted broad interest due to their potential impact on optoelectronic properties of this unique class of solution-processable semiconductors. Using nonpolarized light and simple imaging configurations, ferroelastic twin domains and their switchings through multiple consecutive phase transitions are directly visualized. This direct optical contrast originates from finite optical reflections at the wall interface between two compositionally identical, orientationally different, optically anisotropic domains inside the material bulk. The findings show these domain walls serve as internal reflectors and steer energy transport inside halide perovskites optically. First-principles calculations show universal low domain-wall energies and modest energy barriers of domain switching, confirming their prevalent appearance, stable presence, and facile moving observed in the experiments. The generality of ferroelasticity in halide perovskites stems from their soft bonding characteristics. This work shows the feasibility of using LHP twin domain walls as optical guides of internal photoexcitations, capable of nonvolatile on-off switching and tunable positioning endowed by their universal ferroelasticity.

Efficient vertical charge transport in polycrystalline halide perovskites revealed by four-dimensional tracking of charge carriers

Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar cells. While lateral transient photoluminescence microscopies have been popularly used to characterize charge diffusion in perovskites, there exists a discrepancy between low diffusion coefficients measured and near-unity charge collection efficiencies achieved in practical solar cells. Here, we reveal hidden microscopic dynamics in halide perovskites through four-dimensional (directions x, y and z and time t) tracking of charge carriers by characterizing out-of-plane diffusion of charge carriers. By combining this approach with confocal microscopy, we discover a strong local heterogeneity of vertical charge diffusivities in a three-dimensional perovskite film, arising from the difference between intragrain and intergrain diffusion. We visualize that most charge carriers are efficiently transported through the direct intragrain pathways or via indirect detours through nearby areas with fast diffusion. The observed anisotropy and heterogeneity of charge carrier diffusion in perovskites rationalize their high performance as shown in real devices. Our work also foresees that further control of polycrystal growth will enable solar cells with micrometres-thick perovskites to achieve both long optical path length and efficient charge collection simultaneously.

Overcoming the Outcoupling Limit of Perovskite Light-Emitting Diodes with Artificially Formed Nanostructures

The external quantum efficiency (EQE) of state-of-the-art planar-structure perovskite light-emitting diodes (PeLEDs) is mainly limited by the outcoupling efficiency, which is around 20% and decreases significantly with the perovskite thickness. Here, an approach to artificially form textured perovskite films to boost the outcoupling limit of the PeLEDs is reported. By manipulating the dwell time of antisolvents, the perovskite phase precipitation mechanism, film-forming process, and surface texture can be finely controlled. The film surface roughness can be tuned from 15.3 to 241 nm, with haze increasing accordingly from 6% to >90% for films with an average thickness of 1.5 µm. The light outcoupling limit increases accordingly from 11.7% for the flat PeLEDs to 26.5% for the textured PeLEDs due to photon scattering at the interface. Consequently, the EQE is boosted significantly from around 10% to 20.5% with an extraordinarily thick emissive layer of 1.5 µm. This study provides a novel way of forming light-extraction nanostructures for perovskite optoelectronic devices.

Ultra-bright, efficient and stable perovskite light-emitting diodes

Metal halide perovskites are attracting a lot of attention as next-generation light-emitting materials owing to their excellent emission properties, with narrow band emission^1-4. However, perovskite light-emitting diodes (PeLEDs), irrespective of their material type (polycrystals or nanocrystals), have not realized high luminance, high efficiency and long lifetime simultaneously, as they are influenced by intrinsic limitations related to the trade-off of properties between charge transport and confinement in each type of perovskite material^5-8. Here, we report an ultra-bright, efficient and stable PeLED made of core/shell perovskite nanocrystals with a size of approximately 10 nm, obtained using a simple in situ reaction of benzylphosphonic acid (BPA) additive with three-dimensional (3D) polycrystalline perovskite films, without separate synthesis processes. During the reaction, large 3D crystals are split into nanocrystals and the BPA surrounds the nanocrystals, achieving strong carrier confinement. The BPA shell passivates the undercoordinated lead atoms by forming covalent bonds, and thereby greatly reduces the trap density while maintaining good charge-transport properties for the 3D perovskites. We demonstrate simultaneously efficient, bright and stable PeLEDs that have a maximum brightness of approximately 470,000 cd m^-2, maximum external quantum efficiency of 28.9% (average = 25.2 ± 1.6% over 40 devices), maximum current efficiency of 151 cd A^-1 and half-lifetime of 520 h at 1,000 cd m^-2 (estimated half-lifetime >30,000 h at 100 cd m^-2). Our work sheds light on the possibility that PeLEDs can be commercialized in the future display industry.

Ultrahigh-resolution full-color perovskite nanocrystal patterning for ultrathin skin-attachable displays

High-definition red/green/blue (RGB) pixels and deformable form factors are essential for the next-generation advanced displays. Here, we present ultrahigh-resolution full-color perovskite nanocrystal (PeNC) patterning for ultrathin wearable displays. Double-layer transfer printing of the PeNC and organic charge transport layers is developed, which prevents internal cracking of the PeNC film during the transfer printing process. This results in RGB pixelated PeNC patterns of 2550 pixels per inch (PPI) and monochromic patterns of 33,000 line pairs per inch with 100% transfer yield. The perovskite light-emitting diodes (PeLEDs) with transfer-printed active layers exhibit outstanding electroluminescence characteristics with remarkable external quantum efficiencies (15.3, 14.8, and 2.5% for red, green, and blue, respectively), which are high compared to the printed PeLEDs reported to date. Furthermore, double-layer transfer printing enables the fabrication of ultrathin multicolor PeLEDs that can operate on curvilinear surfaces, including human skin, under various mechanical deformations. These results highlight that PeLEDs are promising for high-definition full-color wearable displays.

Deciphering modes of long-range energy transfer in perovskite crystals using confocal excitation and wide-field fluorescence spectral imaging

Excitation energy migration beyond mesoscale is of contemporary interest for both solar photovoltaic and light-emissive devices, especially in context of organometal halide perovskites (OMHPs) which have been shown to have very long (charge carrier) diffusion lengths. While understanding the energy propagation pathways in OMHPs is crucial for further advancement of material design and improvement of opto-electronic features, the simultaneous existence of multiple processes like carrier diffusion, photon recycling, and photon transport makes it often complex to differentiate them. In this study, we unravel the diverse yet dominant excitation energy transfer mode(s) in crystalline MAPbBr3 micron-sized 1-D rods and plates by localized (confocal) laser excitation coupled with spectrally-resolved wide-field fluorescence imaging. While rarely used, this technique can efficiently probe excitation migration beyond the diffraction limit and can be realized by simple modification of existing epifluorescence microscopy setups. We find that in rods of length below ~2 microns, carrier diffusion dominates amongst the various energy transfer processes. However, the transient non-radiative defects severely inhibit the extent of carrier migration and also temporarily affect the radiative recombination dynamics of the photo-carriers. For MAPbBr3 plates of several tens of micrometers, we find that the photoluminescence (PL) spectral characteristics remain unaltered at short distances (< ~3 μm) whilst at a larger distance, the spectral profile is gradually red-shifted. This implies that carrier diffusion dominates over small distances, while photon recycling, i.e., repeated re-absorption and re-emission of photons propagates excitation energy transfer over extended length scales with assistance from wave-guided photon transport. Our findings can potentially be used for future studies on the characterization of energy transport mechanisms in semiconductor solids as well as for organic (molecular) self-assembled microstructures.

Hybrid modeling of perovskite light-emitting diodes with nanostructured emissive layers

Perovskite light-emitting diodes (PeLEDs) have attracted much attention due to their superior performance. When a bottleneck of energy conversion efficiency is achieved with materials engineering, nanostructure incorporation proves to be a feasible approach to further improve device efficiencies via light extraction enhancement. The finite-difference time-domain simulation is widely used for optical analysis of nanostructured optoelectronic devices, but reliable modeling of PeLEDs with nanostructured emissive layers remains unmet due to the difficulty of locating dipole light sources. Herein we established a hybrid process for modeling light emission behaviors of such nanostructured PeLEDs by calibrating light source distribution through electrical simulations. This hybrid modeling method serves as a universal tool for structure optimization of light-emitting diodes with nanostructured emissive layers.

Optical Visualization of Photoexcitation Diffusion in All-Inorganic Perovskite at High Temperature

All-inorganic halide perovskites are promising candidates for optoelectronic and photovoltaic devices because of their good thermal stability and remarkable optoelectronic properties. Among those properties, carrier transport properties are critical as they inherently dominate the device performance. The transport properties of perovskites have been widely studied at room and lower temperatures, but their high-temperature (i.e., tens of degrees above room temperature) characteristics are not fully understood. Here, the photoexcitation diffusion is optically visualized by transient photoluminescence microscopy (TPLM), through which the temperature-dependent transport characteristics from room temperature to 80 °C are studied in all-inorganic CsPbBr₃ single-crystalline microplates. We reveal the decreasing trend of diffusion coefficient and the almost unchanged trend of diffusion length when heating the sample to high temperature. The phonon scattering in combination with the variation of effective mass is proposed for the explanation of the temperature-dependent diffusion behavior.

Effect of electronic doping and traps on carrier dynamics in tin halide perovskites

Tin halide perovskites have recently emerged as promising materials for low band gap solar cells. Much effort has been invested on controlling the limiting factors responsible for poor device efficiencies, namely self-p-doping and tin oxidation. Both phenomena are related to the presence of defects; however, full understanding of their implications in the optoelectronic properties of the material is still missing. We provide a comprehensive picture of the competing radiative and non-radiative recombination processes in tin-based perovskite thin films to establish the interplay between doping and trapping by combining photoluminescence measurements with trapped-carrier dynamic simulations and first-principles calculations. We show that pristine Sn perovskites, i.e. sample processed with commercially available SnI₂ used as received, exhibit extremely high radiative efficiency due to electronic doping which boosts the radiative band-to-band recombination. Contrarily, thin films where Sn⁴⁺ species are intentionally introduced show drastically reduced radiative lifetime and efficiency due to a dominance of Auger recombination at all excitation densities when the material is highly doped. The introduction of SnF₂ reduces the doping and passivates Sn⁴⁺ trap states but conversely introduces additional non-radiative decay channels in the bulk that fundamentally limit the radiative efficiency. Overall, we provide a qualitative model that takes into account different types of traps present in tin-perovskite thin films and show how doping and defects can affect the optoelectronic properties.

Achieving Up-Conversion Amplified Spontaneous Emission through Spin Alignment between Coherent Light-Emitting Excitons in Perovskite Microstructures

Metal hybrid perovskites have presented interesting infrared-to-visible up-conversion light-emitting lasing properties through multi-photon absorption. Here, when the optical pumping switches between circular and linear polarization, up-conversion amplified spontaneous emission (ASE) intensity exhibits large and small amplitudes, respectively, leading to a positive up-conversion ΔASE in the CsPbBr3 perovskite microrods. This observed phenomenon demonstrates that the coherent interaction between coherent light-emitting excitons is indeed established at the up-conversion ASE regime in the CsPbBr3 perovskite microrods. In addition, the positive up-conversion ΔASE indicates the orbital magnetic dipoles between coherent light-emitting excitons are conserved during up-conversion ASE action. Essentially, the up-conversion ΔASE results provide evidence that shows up-conversion ASE can be realized by the orbit−orbit polarization interaction between light-emitting excitons. Moreover, up-conversion ASE proportionally increased as the pumping fluence increased, which shows that orbit–orbit polarization interaction can be gradually enhanced between coherent light-emitting excitons by increasing pumping density in the CsPbBr3 perovskite microrods. Substantially, our studies provide a fundamental understanding of the spin alignment between coherent light-emitting excitons towards developing spin-dependent nonlinear lasing actions in metal halide perovskites.

Anisotropic nanocrystal superlattices overcoming intrinsic light outcoupling efficiency limit in perovskite quantum dot light-emitting diodes

Quantum dot (QD) light-emitting diodes (LEDs) are emerging as one of the most promising candidates for next-generation displays. However, their intrinsic light outcoupling efficiency remains considerably lower than the organic counterpart, because it is not yet possible to control the transition-dipole-moment (TDM) orientation in QD solids at device level. Here, using the colloidal lead halide perovskite anisotropic nanocrystals (ANCs) as a model system, we report a directed self-assembly approach to form the anisotropic nanocrystal superlattices (ANSLs). Emission polarization in individual ANCs rescales the radiation from horizontal and vertical transition dipoles, effectively resulting in preferentially horizontal TDM orientation. Based on the emissive thin films comprised of ANSLs, we demonstrate an enhanced ratio of horizontal dipole up to 0.75, enhancing the theoretical light outcoupling efficiency of greater than 30%. Our optimized single-junction QD LEDs showed peak external quantum efficiency of up to 24.96%, comparable to state-of-the-art organic LEDs.

Controlling the transition-dipole-moment orientation in quantum dot solids at device level has not been achieved before. Here, the authors demonstrated intrinsic light out-coupling enhancement approach to boost the external quantum efficiency up to 25% by using the colloidal lead halide perovskite anisotropic nanocrystals.

Directional emission of white light via selective amplification of photon recycling and Bayesian optimization of multi-layer thin films

Over the last decades, light-emitting diodes (LED) have replaced common light bulbs in almost every application, from flashlights in smartphones to automotive headlights. Illuminating nightly streets requires LEDs to emit a light spectrum that is perceived as pure white by the human eye. The power associated with such a white light spectrum is not only distributed over the contributing wavelengths but also over the angles of vision. For many applications, the usable light rays are required to exit the LED in forward direction, namely under small angles to the perpendicular. In this work, we demonstrate that a specifically designed multi-layer thin film on top of a white LED increases the power of pure white light emitted in forward direction. Therefore, the deduced multi-objective optimization problem is reformulated via a real-valued physics-guided objective function that represents the hierarchical structure of our engineering problem. Variants of Bayesian optimization are employed to maximize this non-deterministic objective function based on ray tracing simulations. Eventually, the investigation of optical properties of suitable multi-layer thin films allowed to identify the mechanism behind the increased directionality of white light: angle and wavelength selective filtering causes the multi-layer thin film to play ping pong with rays of light.

Machine learning as a tool to engineer microstructures: Morphological prediction of tannin-based colloids using Bayesian surrogate models

ABSTRACT

Oxidized tannic acid (OTA) is a useful biomolecule with a strong tendency to form complexes with metals and proteins. In this study we open the possibility to further the application of OTA when assembled as supramolecular systems, which typically exhibit functions that correlate with shape and associated morphological features. We used machine learning (ML) to selectively engineer OTA into particles encompassing one-dimensional to three-dimensional constructs. We employed Bayesian regression to correlate colloidal suspension conditions (pH and pK _a) with the size and shape of the assembled colloidal particles. Fewer than 20 experiments were found to be sufficient to build surrogate model landscapes of OTA morphology in the experimental design space, which were chemically interpretable and endowed predictive power on data. We produced multiple property landscapes from the experimental data, helping us to infer solutions that would satisfy, simultaneously, multiple design objectives. The balance between data efficiency and the depth of information delivered by ML approaches testify to their potential to engineer particles, opening new prospects in the emerging field of particle morphogenesis, impacting bioactivity, adhesion, interfacial stabilization, and other functions inherent to OTA.

IMPACT STATEMENT

Tannic acid is a versatile bio-derived material employed in coatings, surface modifiers, and emulsion and growth stabilizers, which also imparts mild anti-viral health benefits. Our recent work on the crystallization of oxidized tannic acid (OTA) colloids opens the route toward further valuable applications, but here the functional properties tend to depend strongly on particle morphology. In this study, we eschew trial-and-error morphology exploration of OTA particles in favor of a data-driven approach. We digitalized the experimental observations and input them into a Gaussian process regression algorithm to generate morphology surrogate models. These help us to visualize particle morphology in the design space of material processing conditions, and thus determine how to selectively engineer one-dimensional or three-dimensional particles with targeted functionalities. We extend this approach to visualize other experimental outcomes, including particle yield and particle surface-to-volume ratio, which are useful for the design of products based on OTA particles. Our findings demonstrate the use of data-efficient surrogate models for general materials engineering purposes and facilitate the development of next-generation OTA-based applications.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1557/s43577-021-00183-4.

Microscale gratings patterned in perovskite films to promote the electron-optical conversion efficiency of perovskite light-emitting diodes

In this paper, we investigated an enhancement of luminance in a perovskite light-emitting diode (PeLED) by inserting periodic microscale gratings. Soft imprinting was employed to pattern the gratings onto the surface of perovskite film in the annealing process. It was found that the grain-refining effect deeply influenced the morphology and crystallinity of the perovskite film. The finite-element analysis was utilized to confirm the light extraction enhancement arising from the inserted microscale gratings. Additionally, the introduction of microscale gratings led to a 1.33 enhancement of the device's optoelectrical field intensity. Thus, the patterned PeLED obtained an enhancement of 2.96 and 2.1 of the luminance and external quantum efficiency, respectively.

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.

Distribution control enables efficient reduced-dimensional perovskite LEDs

Light-emitting diodes (LEDs) based on perovskite quantum dots have shown external quantum efficiencies (EQEs) of over 23% and narrowband emission, but suffer from limited operating stability¹. Reduced-dimensional perovskites (RDPs) consisting of quantum wells (QWs) separated by organic intercalating cations show high exciton binding energies and have the potential to increase the stability and the photoluminescence quantum yield^2,3. However, until now, RDP-based LEDs have exhibited lower EQEs and inferior colour purities^4-6. We posit that the presence of variably confined QWs may contribute to non-radiative recombination losses and broadened emission. Here we report bright RDPs with a more monodispersed QW thickness distribution, achieved through the use of a bifunctional molecular additive that simultaneously controls the RDP polydispersity while passivating the perovskite QW surfaces. We synthesize a fluorinated triphenylphosphine oxide additive that hydrogen bonds with the organic cations, controlling their diffusion during RDP film deposition and suppressing the formation of low-thickness QWs. The phosphine oxide moiety passivates the perovskite grain boundaries via coordination bonding with unsaturated sites, which suppresses defect formation. This results in compact, smooth and uniform RDP thin films with narrowband emission and high photoluminescence quantum yield. This enables LEDs with an EQE of 25.6% with an average of 22.1 ±1.2% over 40 devices, and an operating half-life of two hours at an initial luminance of 7,200 candela per metre squared, indicating tenfold-enhanced operating stability relative to the best-known perovskite LEDs with an EQE exceeding 20%^1,4-6.

Perovskite White Light Emitting Diodes: Progress, Challenges, and Opportunities

As global warming, energy shortages, and environment pollution have intensified, low-carbon and energy-saving lighting technology has attracted great attention worldwide. Light emitting diodes (LEDs) have been around for decades and are considered to be the most ideal lighting technology currently due to their high luminescence efficiency (LE) and long lifespan. Besides, along with the development of modern technology, lighting technologies with higher performance and more functions are desired. Perovskite based LEDs (PeLEDs) have recently emerged as ideal candidates for lighting technology owing to the extraordinary photoelectric properties of perovskite, such as high photoluminescence quantum yields (PLQYs), easy wavelength tuning, and low-cost synthesis. Herein, we open this review by introducing the background of white LEDs (WLEDs), including their light-emitting mechanism, typical characteristics, and key indicators in applications. Then, four main approaches to fabricate WLEDs are discussed and compared. After that, in accordance with the four categories, we focus on the recent progress of white PeLEDs (Pe-WLEDs), followed by the challenges and opportunities for Pe-WLEDs in practical application. Meanwhile, some pertinent countermeasures to their challenges are put forward. Finally, the development promise of Pe-WLEDs is explored.

Luminescence enhancement of lead halide perovskite light-emitting diodes with plasmonic metal nanostructures

Metal halide perovskites, as newly emerging light emitters, have been attracting considerable attention on luminescent materials and devices, due to their superior optoelectronic properties and potential practical applications. Recently, perovskite light-emitting diodes (PeLEDs) based on lead halide perovskites (LHPs) have been largely designed and intensively studied in laboratory platforms. However, to satisfy demand and promote their commercialization, it is crucial to improve the efficiency and stability of PeLEDs. Accordingly, the surface-plasmon (SP) effect provides a promising approach to enhance their luminescence, which is realized by incorporating plasmonic metal nanostructures (NSs) into PeLEDs. This review presents a comprehensive overview of the research status and prospect on LHP-based plasmonic PeLEDs together with the corresponding perovskite light-emission films (PeLEFs). Firstly, the recent development of the PeLEDs is briefly introduced. Secondly, the mechanisms and photophysics of the PeLEDs by SP manipulation are simply illustrated and analyzed. Then, the recent progress and achievements on the theoretical and experimental results of SP effect applications in the PeLEDs together with PeLEFs are presented in detail and systematically reviewed. Next, the current challenges and future directions of the PeLEDs are shown and discussed. Finally, a critical summary and outlook of the PeLEDs are summarized and proposed. Our results indicate that this new class of LHP-based plasmonic PeLEDs presents future research fields and demonstrates promising applications in lighting and displays, and further luminescence enhancement in exciton radiation processes and light extraction techniques are a hopeful route to obtain high-performance PeLEDs.

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.

Electrical Pumping of Perovskite Diodes: Toward Stimulated Emission

The success of metal halide perovskites in photovoltaic and light-emitting diodes (LEDs) motivates their application as a solid-state thin-film laser. Various perovskites have shown optically pumped stimulated emission of lasing and amplified spontaneous emission (ASE), yet the ultimate goal of electrically pumped stimulated emission has not been achieved. As an essential step toward this goal, here, a perovskite diode structure that simultaneously exhibits stable operation at high current density (≈1 kA cm-2 ) and optically excited ASE (with a threshold of 180 µJ cm-2 ) is reported. This diode structure achieves an electroluminescence quantum efficiency of 0.8% at 850 A cm-2 , which is estimated to be ≈3% of the charge carrier population required to reach ASE in the same device. It is shown that the formation of a large angle waveguide mode and the reduction of parasitic absorption losses are two major design principles for diodes to obtain a positive gain for stimulated emission. In addition to its prospect as a perovskite laser, a new application of electrically pumped ASE is proposed as an ideal perovskite LED architecture allowing 100% external radiation efficiency.

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.

Tackling light trapping in organic light-emitting diodes by complete elimination of waveguide modes

Conventional waveguide mode decoupling methods for organic light-emitting diodes (OLEDs) are typically not scalable and increase fabrication complexity/cost. Indium-tin-oxide-free transparent anode technologies showed efficiency improvement without affecting other device properties. However, previous works lack rigorous analysis to understand the efficiency improvement. Here, we introduced an ultrathin silver (Ag) film as transparent electrode and conducted systematic modal analysis of OLEDs and report that waveguide mode can be completely eliminated by designing an OLED structure that is below the cutoff thickness of waveguide modes. We also experimentally verified the waveguide mode removal in organic waveguides with the help of index-matching fluid and prism. The negative permittivity, extremely thin thickness (~5 nanometers), and highly conductive properties achieved by a uniform copper-seeded Ag film can suppress waveguide mode formation, enhancing external quantum efficiency without compromising any other characteristics of OLEDs, which paves the way for cost-effective high-efficiency OLEDs in current display industry.

Scrutinizing the stability and exploring the dependence of thermoelectric properties on band structure of 3d-3d metal-based double perovskites Ba2FeNiO6 and Ba2CoNiO6

Through the conventional DFT computation, we have designed new oxide double perovskites Ba2FeNiO6 and Ba2CoNiO6. The structural and thermodynamic stabilities are predicted by optimizing the crystal structure and evaluation of enthalpy of formation, respectively. Then by using the optimized lattice constant, we have explored the different physical properties. The GGA + mBJ electronic band-structure illustrates Ba2FeNiO6 is a half-metal with 100% spin polarization at the Fermi level. While Ba2CoNiO6 shows a ferromagnetic semiconducting nature. The change in the electronic structure when Fe is replaced by Co is explained with the help of the orbital diagram and exchange interaction. The eg-eg hybridization that happens via O-p states is strong because Fe-O-Ni and Co-O-Ni bond angles are strictly 180°. The narrow bandgaps in the semiconducting channels prompted us to analyze the applicability of these materials towards thermoelectric technology. Besides this, we have investigated the dependency of transport properties on electronic band structure. The semiconducting nature in Ba2CoNiO6 results in a significant ZT around 0.8 at room temperature makes it suitable for wasted-energy regeneration.

Reconciliation of dipole emission with detailed balance rates for the simulation of luminescence and photon recycling in perovskite solar cells

A theoretical description of light emission, propagation and re-absorption in semiconductor multilayer stacks is derived based on the transverse Green's function of the electromagnetic field in the presence of a complex dielectric. The canonical dipole emission model is parametrized in terms of the local optical material constants and the local quasi-Fermi level splitting using the detailed balance relation between local absorption and emission rates. The framework obtained in this way is shown to reproduce the generalized Kirchhoff relations between the luminescent emission from metal halide perovskite slabs under uniform excitation and the slab absorptance of light with arbitrary angle of incidence. Use of the proper local density of transverse photon states in the local emission rate includes cavity effects in the generalized Planck law for internal spontaneous emission, which are neglected in the conventional Van Roosbroeck-Shockley formalism and avoids spurious divergencies due to non-radiative energy transfer via longitudinal modes. Finally, a consistent treatment of re-absorption provides the local rate of secondary photogeneration required for the consideration of photon recycling in an opto-electronic device simulator that includes the effects of charge transport.

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.

High-performance quasi-2D perovskite light-emitting diodes: from materials to devices

Quasi-two-dimensional (quasi-2D) perovskites have attracted extraordinary attention due to their superior semiconducting properties and have emerged as one of the most promising materials for next-generation light-emitting diodes (LEDs). The outstanding optical properties originate from their structural characteristics. In particular, the inherent quantum-well structure endows them with a large exciton binding energy due to the strong dielectric- and quantum-confinement effects; the corresponding energy transfer among different n-value species thus results in high photoluminescence quantum yields (PLQYs), particularly at low excitation intensities. The review herein presents an overview of the inherent properties of quasi-2D perovskite materials, the corresponding energy transfer and spectral tunability methodologies for thin films, as well as their application in high-performance LEDs. We then summarize the challenges and potential research directions towards developing high-performance and stable quasi-2D PeLEDs. The review thus provides a systematic and timely summary for the community to deepen the understanding of quasi-2D perovskite materials and resulting LED devices.

Ligand-engineered bandgap stability in mixed-halide perovskite LEDs

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity^1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of commercial organic light-emitting diodes-have been demonstrated in both the infrared and the green emission channels^1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized^5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.

Computational Study of Dipole Radiation in Re-Absorbing Perovskite Semiconductors for Optoelectronics

Compared to organic emitters, perovskite materials generally have a small Stokes shift and correspondingly large re-absorption of dipole emission. Classical optical modelling methods ignoring re-absorption do not provide an adequate description of the observed light emission properties. Here, optical modelling methods and design rules for perovskite light-emitting diodes are presented. The transfer-matrix formalism is used to quantify the Poynting vectors generated by a dipole radiating inside a perovskite optoelectronic device. A strategy is presented to deal with non-radiative coupling to nearby emissive material that can otherwise lead to non-physical divergence in the calculation. Stability issues are also investigated regarding coherence of the light propagating in the substrate and the absence of a light absorber in the system. The benefit of the photon recycling effect is taken into account by recursive calculation of the dipole generation profile. The simulation results predict that a high external quantum efficiency of ≈40% is achievable in formamidinium lead triiodide-based perovskite light-emitting diodes, by optimization of microcavity, dipole orientation, and photon recycling effects. Contrary to conventional device structures currently reported, this work highlights the benefits of thick charge transport layers and thick perovskite with small Stokes shift.

Microscopic (Dis)order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites

Recent developments in the field of high efficiency perovskite solar cells are based on stabilization of the perovskite crystal structure of FAPbI₃ while preserving its excellent optoelectronic properties. Compositional engineering of, for example, MA or Br mixed into FAPbI₃ results in the desired effects, but detailed knowledge of local structural features, such as local (dis)order or cation interactions of formamidinium (FA) and methylammonium (MA), is still limited. This knowledge is, however, crucial for their further development. Here, we shed light on the microscopic distribution of MA and FA in mixed perovskites MA_1-x FA _x PbI₃ and MA_0.15FA_0.85PbI_2.55Br_0.45 by combining high-resolution double-quantum ¹H solid-state nuclear magnetic resonance (NMR) spectroscopy with state-of-the-art near-first-principles accuracy molecular dynamics (MD) simulations using machine-learning force-fields (MLFFs). We show that on a small local scale, partial MA and FA clustering takes place over the whole MA/FA compositional range. A reasonable driving force for the clustering might be an increase of the dynamical freedom of FA cations in FA-rich regions. While MA_0.15FA_0.85PbI_2.55Br_0.45 displays similar MA and FA ordering as the MA_1-x FA _x PbI₃ systems, the average cation-cation interaction strength increased significantly in this double mixed material, indicating a restriction of the space accessible to the cations or their partial immobilization upon Br^- incorporation. Our results shed light on the heterogeneities in cation composition of mixed halide perovskites, helping to exploit their full optoelectronic potential.

Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping

The outstanding optoelectronic performance of lead halide perovskites lies in their exceptional carrier diffusion properties. As the perovskite material dimensionality is reduced to exploit the quantum confinement effects, the disruption to the perovskite lattice, often with insulating organic ligands, raises new questions on the charge diffusion properties. Herein, we report direct imaging of >1 μm exciton diffusion lengths in CH₃NH₃PbBr₃ perovskite nanocrystal (PNC) films. Surprisingly, the resulting exciton mobilities in these PNC films can reach 10 ± 2 cm² V^-1 s^-1, which is counterintuitively several times higher than the carrier mobility in 3D perovskite films. We show that this ultralong exciton diffusion originates from both efficient inter-NC exciton hopping (via Förster energy transfer) and the photon recycling process with a smaller yet significant contribution. Importantly, our study not only sheds new light on the highly debated origins of the excellent exciton diffusion in PNC films but also highlights the potential of PNCs for optoelectronic applications.

Emerging Light-Emitting Materials for Photonic Integration

The arrival of the information explosion era is urging the development of large-bandwidth high-data-rate optical interconnection technology. Up to now, the biggest stumbling block in optical interconnections has been the lack of efficient light sources despite significant progress that has been made in germanium-on-silicon (Ge-on-Si) and III-V-on-silicon (III-V-on-Si) lasers. 2D materials and metal halide perovskites have attracted much attention in recent years, and exhibit distinctive advantages in the application of on-chip light emitters. Herein, this Progress Report reviews the recent progress made in light-emitting materials with a focus on new materials, i.e., 2D materials and metal halide perovskites. The report briefly introduces the current status of Ge-on-Si and III-V-on-Si lasers and discusses the advances of 2D and perovskite light-emitting materials for photonic integration, including their optical properties, preparation methods, as well as the light sources based on these materials. Finally, challenges and perspectives of these emerging materials on the way to the efficient light sources are discussed.