Perovskite Emission Research Articles

Role of Exciton Diffusion in the Efficiency of Mn Dopant Emission in Two-Dimensional Perovskites

Role of Exciton Diffusion in the Efficiency of Mn Dopant Emission in Two-Dimensional Perovskites

Managing Edge States in Reduced-Dimensional Perovskites for Highly Efficient Deep-Blue LEDs.

Pure-halide reduced-dimensional perovskites, featuring large exciton binding energy and tunable bandgap, show great potential for high-efficiency deep-blue perovskite light-emitting diodes (PeLEDs). However, their efficiency, particularly in the low n-value phase domain ("n" represents the number of octahedral sheets), lags behind analogous perovskite emitters. Herein, it is demonstrated that the vibration of edge-dangling octahedra in the low n-value phase activates notorious exciton-phonon (EP) coupling, thereby deteriorating efficiency. To address this issue, an approach is reported to manage edge-state lattices by introducing tris(4-fluorophenyl) phosphine (TFP) ligands. Attributed to the large steric hindrance of TFP ligands and their strong binding affinity for edge-dangling octahedra, the edged-octahedral tilting reconstruction can effectively suppress lattice vibration and inhibit EP coupling. This strategy yields deep-blue emitting film with a spectral linewidth of 21nm and a photoluminescence quantum yield of 85% at low excitation densities. The resulting PeLEDs achieve deep-blue emission at 469nm, with a maximum luminance of 2,428cdm-2 and a maximum external quantum efficiency of 10.4%, marking them among the most efficient deep-blue PeLEDs reported. The strategy also showcases universality for higher n-value reduced-dimensional perovskites. It is believed that the observation, along with the edge-state management strategy, lays the groundwork for further advancements in reduced-dimensional perovskite optoelectronic devices.

Exciton Dissociation and Long‐Lived Delayed Components in High‐Efficiency Quasi‐Two‐Dimensional Green Perovskite Light‐Emitting Diodes

AbstractQuasi‐two‐dimensional (quasi‐2D) perovskites, consisting of multi‐quantum wells (MQWs) separated by organic intercalating cations, exhibit high luminescence efficiency while the photophysical processes involved remain partially obscure due to the uncertainty of the MQWs structure. Herein, a synergetic dual‐additive strategy is adopted to prepare quasi‐2D perovskite films, where 18‐crown‐6 and tris(4‐fluorophenyl)phosphine oxide are utilized to suppress the formation of low‐dimensional perovskites, diminish defect density, and enhance photoluminescence quantum yield to 93.7%. Notably, a long‐lived delayed component, spanning hundreds of microseconds, is identified for the first time in perovskite emitters, which correlates with exciton dissociation and recombination, and the differences in temperature‐dependent radiative lifetime of the delayed components match with the luminescence properties. Finally, benefiting from lower trap density and more efficient energy‐funneling, green PeLEDs treated with dual additives have achieved a high external quantum efficiency of 28.9% and a commendable operating half‐lifetime of 17.8 h at an initial brightness of 500 cd m−2. The observation of the long‐lived delayed components, coupled with insights into exciton dissociation, provides a profound and comprehensive understanding of the fundamental carrier behavior in perovskite emitters and establishes a direct correlation between the properties of perovskite emitters and their photophysical processes.

Lead-Free Perovskite Light-Emitting Diodes.

Metal halide perovskites have been identified as a promising class of materials for light-emitting applications. The development of lead-based perovskite light-emitting diodes (PeLEDs) has led to substantial improvements, with external quantum efficiencies (EQEs) now surpassing 30% and operational lifetimes comparable to those of organic LEDs (OLEDs). However, the concern over the potential toxicity of lead has motivated a search for alternative materials that are both eco-friendly and possess excellent optoelectronic properties, with lead-free perovskites emerging as a strong contender. In this review, the properties of various lead-free perovskite emitters are analyzed, with a particular emphasis on the more well-reported tin-based variants. Recent progress in enhancing device efficiencies through refined crystallization processes and the optimization of device configurations is also discussed. Additionally, the remaining challenges are examined, and propose strategies that may lead to stable device operation. Looking forward, the potential future developments for lead-free PeLEDs are considered, including the extension of spectral range, the adoption of more eco-friendly deposition techniques, and the exploration of alternative materials.

The origin of broadband blue emission in zero-dimensional organic lead iodine perovskites: A first-principles study.

Broadband blue emission in zero-dimensional perovskites has received considerable attention, which is very important for the realization of stable blue-light emitters; however, the underlying formation mechanism remains unclear. Based on first-principles calculations, we have systematically studied the self-trapped excitons (STEs) behavior and luminescence properties in 0D-(DMA)4PbI6 perovskite. Our calculations show that there is a significant difference between the intrinsic STE luminescence mechanism (∼2.51eV) and experimental observations (∼2.70eV). In contrast, we found that the iodine vacancy (VI) is energetically accessible and exhibits a shallow charge transition level at ∼2.69eV (0/+1) above the valence band maximum, which provides the initial local well for the STEs formation. Moreover, the low electronic dimension synergistic Jahn-Teller distortion facilitates the formation of extrinsic excitons self-trapping. Further excited state electronic structure analysis and configuration coordinate diagram calculations confirmed that the broadband blue emission in 0D-(DMA)4PbI6 is the origin of VI-induced extrinsic STEs instead of intrinsic STEs. Therefore, our simulation results rationalize the experimental phenomena and provide important insights into the formation mechanism of STEs in low-dimensional perovskite systems.

Color-Tunable Lead Halide Perovskite Single-Mode Chiral Microlasers with Exceptionally High glum.

Chiral microlasers hold great promise for optoelectronics from integrated photonic devices to high-density quantum information processing. Despite significant progress in lead-halide perovskite emitters, chiral lasing with high dissymmetry factors (glum) has not yet been realized. Here, we demonstrate chiral single-mode microlasers with exceptional stability and tunable emission across the visible range by combining CsPbClxBr3-x perovskite microrods (MRs) with a cholesteric liquid crystal (CLC) layer. The MRs lase via a whispering gallery mode (WGM) microcavity and confer chirality through the encapsulated CLC layer, thus exhibiting circularly polarized lasing with dissymmetry factors reaching 1.62. Importantly, we demonstrate wavelength-tunable high dissymmetry chiral lasers in a broad spectral range by tuning the halide composition and using CLC layers with the desired photonic bandgap (PBG). This facile approach to generate chiral lasing not only is applicable to semiconductor nano- and microcrystals but also paves the way for potential integration into nanoscale photonic devices.

Efficient Polycrystalline Single-Cation Perovskite Light-Emitting Diodes by Simultaneous Intracrystal and Interfacial Defect Passivation.

Polycrystalline perovskite light-emitting diodes (PeLEDs) have shown great promise with high efficiency and easy processability. However, PeLEDs using single-cation polycrystalline perovskite emitters have demonstrated low efficiency due to defects within the grains and at the interfaces between the perovskite layer and the charge injection contact. Thus, simultaneous defect engineering of perovskites to suppress exciton loss within the grains and at the interfaces is crucial for achieving high efficiency in PeLEDs. Here, 1,8-octanedithiol which is a strong nucleophile, is used to increase the luminescence efficiency of a single-cation perovskite by suppressing non-radiative recombination within the grains of their polycrystalline emitter film as well as at their interface with an anode. The dithiol additive performs a multifunctional role in defect passivation, spatial confinement of excitons, and prevention of exciton quenching at the interface between the perovskite layer and the underlying hole-injection layer. Photoluminescence studies demonstrate that incorporating the dithiol additive significantly enhances the charge carrier dynamics in perovskites, resulting in an external quantum efficiency (EQE) of up to 23.46% even in a simplified PeLED that does not use a hole-injection layer. This represents the highest level of EQE achieved among devices utilizing polycrystalline single-cation perovskites.

Phonon Directionality Impacts Electron-Phonon Coupling and Polarization of the Band-Edge Emission in Two-Dimensional Metal Halide Perovskites.

Two-dimensional metal halide perovskites are highly versatile for light-driven applications due to their exceptional variety in material composition, which can be exploited for the tunability of mechanical and optoelectronic properties. The band-edge emission is defined by the structure and composition of both organic and inorganic layers, and electron-phonon coupling plays a crucial role in the recombination dynamics. However, the nature of the electron-phonon coupling and what kind of phonons are involved are still under debate. Here we investigate the emission, reflectance, and phonon response from single two-dimensional lead iodide microcrystals with angle-resolved polarized spectroscopy. We find an intricate dependence of the emission polarization with the vibrational directionality in the materials, which reveals that several bands of low-frequency phonons with nonorthogonal directionality contribute to the band-edge emission. Such complex electron-phonon coupling requires adequate models to predict the thermal broadening of the emission and provides opportunities to design polarization properties.

Metal Halide Perovskites Blue Emitters and Light‐Emitting Diodes: A Review on Morphological and Molecular Dimensions

AbstractMetal halide perovskites have recently garnered significant attention as a fundamental emitter due to their high carrier mobility, strong light absorption, high monochromaticity, and tunable emission bandgap. However, the brightness, efficiency, and stability issues have limited their potential applications in blue region. To break through these limitations, researchers have investigated different dimensions of perovskites to seek an excellent emitter. To date, the quantum confinement and coordination octahedron monomer arrangement perovskites still lack of systematic cognition. In this review, the molecular and morphological dimensions of blue perovskite emitters are summarized. The differences and relationships between these dimensions in terms of their fabrication and properties are also explored. Furthermore, recent advances in blue light enhancement strategies to address the challenges in various morphological and molecular structures are highlighted. Finally, the main challenges and future directions are discussed for the development of perovskite blue LEDs.

Ultra‐Stable and Bright Pure‐Red Perovskite Nanocrystals for Backlit Displays

AbstractMetal halide perovskite nanocrystals (NCs) have emerged as an alternative to conventional phosphors for wide color gamut displays. However, the issues of poor stability and low emission efficiency of pure‐red CsPbBrxI3‐x NCs set an obstacle to their practical application. Herein, the synergistic effect of Mn doping and mesoporous SiO2 sealing is reported through in situ formation of Mn‐doped NCs into mesoporous SiO2 nanospheres (MnNCs@SiO2) to structurally and spatially stabilize the perovskite NCs for bright and stable pure‐red emitter. The large bonding energy of Mn‐halogen effectively suppresses the halide vacancy defects, prompting the highest photoluminescence quantum yield (PLQY) up to 84% (634 nm) among the pure‐red composite analogs. More importantly, the nanocrystal structure stability is fundamentally improved by the enhanced formation energy with Mn doping, together with the spatial isolation by SiO2 nanospheres. The MnNCs@SiO2 films exhibit impressive stabilities against high‐temperature heating, thermal cycle test, UV irradiation, water, and acid/alkali erosion, representing one of the most stable pure‐red perovskite emitters. By integrating fabricated all‐perovskite CsPbX3 single color conversion film into the liquid crystal display (LCD) modules, a wide color gamut of 128% of NTSC is achieved, enabling an excellent color rendition for high‐saturation object colors toward practical applications.

Monoammonium Modified Dion-Jacobson Quasi-2D Perovskite for High Efficiency Pure-Blue Light Emitting Diodes.

Quasi-2D perovskites exhibit impressive optoelectronic properties and hold significant promise for future light-emitting devices. However, the efficiency of perovskite light-emitting diodes (PeLEDs) is seriously limited by defect-induced nonradiative recombination and imbalanced charge injection. Here, the defect states are passivated and charge injection balance is effectively improved by introducing the additive cyclohexanemethylammonium (CHMA) to bromide-based Dion-Jacobson (D-J) structure quasi-2D perovskite emission layer. CHMA participates in the crystallization of perovskite, leading to high quality film composed of compact and well-contacted grains with enhanced hole transportation and less defects. As a result, the corresponding PeLEDs exhibit stable pure blue emission at 466nm with a maximum external quantum efficiency (EQE) of 9.22%. According to current knowledge, this represents the highest EQE reported for pure-blue PeLEDs based on quasi-2D bromide perovskite thin films. These findings underscore the potential of quasi-2D perovskites for advanced light-emitting devices and pave the way for further advancements in PeLEDs.

The origins of dual-peak emission and anomalous exciton decay in 2D Sn-based perovskites.

Two-dimensional (2D) Sn-based perovskites exhibit significant potential in diverse optoelectronic applications, such as on-chip lasers and photodetectors. Yet, the underlying mechanism behind the frequently observed dual-peak emission in 2D Sn-based perovskites remains a subject of intense debate, and there is a lack of research on the carrier dynamics in these materials. In this study, we investigate these issues in a representative 2D Sn-based perovskite, namely, PEA2SnI4, through temperature-, excitation intensity-, angle-, and time-dependent photoluminescence studies. The results indicate that the high- and low-energy peaks originate from in-face and out-of-face dipole transitions, respectively. In addition, we observe an anomalous increase in the non-radiative recombination rate as temperature decreases. After ruling out enhanced electron-phonon coupling and Auger recombination as potential causes of the anomalous carrier dynamics, we propose that the significantly increased exciton binding energy (Eb) plays a decisive role. The increased Eb arises from enhanced electronic localization, a consequence of weakened lattice distortion at low temperatures, as confirmed by first-principles calculations and temperature-dependent x-ray diffraction measurements. These findings offer valuable insights into the electronic processes in the unique 2D Sn-based perovskites.

2D hybrid organic-inorganic perovskite displaying narrow-band violet-blue photoluminescence

Hybrid organic-inorganic perovskites represent a drastically expanding class of solid-state semiconducting materials for optoelectronics and photovoltaics. Thanks to the easy chemical tunability, this class of materials can offer emitters of very wide spectral range, however, violet-blue emitting hybrid perovskites still remain underrepresented. In this paper we report on a novel two-dimensional hybrid organic-inorganic perovskite (ethylammonium)2CdBr4. Crystal structure of the reported compound is composed of infinite inorganic 2D layers that are created by [CdBr6]4- octahedra connected in corner-by-corner manner. These layers are separated by organic ethyammonium cations which create hydrogen bonds with the inorganic part. Optical absorption measurements revealed the presence of a large band gap of 4.25 eV. Importantly, this material displays narrow photoluminescence band at 388 nm (full width at half maximum is 32 nm). Chromaticity coordinates of the perovskite emission are close to NTSC blue standard. Band structure and DOS calculations show that the states close to Fermi level predominantly originate from cadmium and bromide. Thus, here we offer a facile way towards violet-blue luminophore containing simple and widely available organic cation ethylammonium.

Magnetic Assembly of Magnetite/Perovskite Hybrid Nanorods for Circularly Polarized Luminescence

AbstractMagnetic fields are uniquely valuable for creating colloidal nanostructured materials, not only providing a means for controlled synthesis but also guiding their self‐assembly into distinct superstructures. In this study, a magnetothermal process for synthesizing hybrid nanostructures comprising ferrimagnetic magnetite nanorods coated with fluorescent perovskite nanocrystals is reported and their magnetic assembly into superstructures capable of emitting linear and circularly polarized light are demonstrated. Under UV excitation, the superstructures assembled in a liner magnetic field produce linear polarized luminescence, and those assembled in a chiral magnetic field exhibit strong circularly polarized luminescence (CPL) with a glum value up to 0.44 (±0.004). The CPL is believed to originate from the dipolar interaction between neighboring perovskite nanocrystals attached to the chiral assemblies and the chiral‐selective absorption of the perovskite emission by the magnetite phase. The magnetic synthesis and assembly approaches and the resulting distinctive chiral superstructures are anticipated to open up new avenues for designing diverse functional chiroptical devices.

Stretchable Metal Halide Perovskite Color Conversion Layers with Block Copolymer Dispersant

AbstractStretchable luminescent nanocomposites with metal halide perovskite (MHP) emitters are in high demand as deformable color‐conversion layers (CCLs) for vivid‐colored form‐factor free displays. However, the immiscibility of the MHP crystals with an elastomeric polymer matrix induces particle aggregation with resultant luminescence quenching, hindering the development of MHP‐based CCLs. Herein, a simple method is proposed to produce highly luminescent MHP nanoparticles (NPs) and stably disperse them in an elastomeric poly(dimethylsiloxane) (PDMS) polymer matrix with block copolymer (BCP) dispersant. The nanocomposites are developed through sequential steps of synthesis of MHP‐NP dispersion solution with BCP and blending with PDMS followed by thermal curing. It is revealed that Lewis basic poly(2‐vinylpyridine)‐block‐poly(dimethylsiloxane) (P2VP‐b‐PDMS) BCP added in the synthesis step promotes crystallization of the MHPs as nanocrystals with an average diameter of 5.9 ± 1.2 nm, whereas MHPs with hundreds of nm are produced without BCP. The resultant MHP‐NPs with P2VP‐b‐PDMS exhibit excellent photophysical properties, such as narrow band emission, high photoluminescence (PL) quantum yield, and long PL lifetime. The MHP‐NPs are stably dispersed in the PDMS matrix by P2VP‐b‐PDMS without particle aggregation, leading to highly deformable and stretchable nanocomposites with stable PL emission upon repetitive stretching, and excellent environmental stability.

Defects in Halide Perovskites: Does It Help to Switch from 3D to 2D?

Two-dimensional (2D) organic-inorganic hybrid iodide perovskites have been put forward in recent years as stable alternatives to their three-dimensional (3D) counterparts. Using first-principles calculations, we demonstrate that equilibrium concentrations of point defects in the 2D perovskites PEA2PbI4, BA2PbI4, and PEA2SnI4 (PEA, phenethylammonium; BA, butylammonium) are much lower than in comparable 3D perovskites. Bonding disruptions by defects are more destructive in 2D than in 3D networks, making defect formation energetically more costly. The stability of 2D Sn iodide perovskites can be further enhanced by alloying with Pb. Should, however, point defects emerge in sizable concentrations as a result of nonequilibrium growth conditions, for instance, then those defects likely hamper the optoelectronic performance of the 2D perovskites, as they introduce deep traps. We suggest that trap levels are responsible for the broad sub-bandgap emission in 2D perovskites observed in experiments.

Alkoxysilane‐Treated SnO2 Interlayer for Energy Band Alignment of SnO2 Electron Injection Layer in Inverted Perovskite Light‐Emitting Diodes

AbstractEfficient inverted perovskite light‐emitting diodes (PeLEDs) are demonstrated by the introduction of tetraethyl orthosilicate (TEOS)‐incorporated tin oxide (SnO2) interlayer between the SnO2 electron injection layer and the perovskite emission layer. The TEOS incorporation into the SnO2 solution spontaneously converts it to a SiO2–SnO2 composite colloidal solution with a wide band gap, thermal stability, transparency, and chemical stability toward perovskite. The TEOS‐incorporated SnO2 interlayer effectively restricts the charge transfer from perovskite into SnO2 and promotes electron injection from SnO2 into perovskite due to the shift toward favorable energy band alignment. In addition, the TEOS‐treated interlayer balances the electron injection rate and the hole injection rate, thereby facilitating radiative recombination of the charge carriers injected into perovskite. As a result, the inverted PeLEDs exhibit significantly improved performance of 33 996 cd m−2 luminance, 9.99% of external quantum efficiency, and 44.83 cd A−1 of current efficiency.

Interfacial Coulomb-enhanced charge injection for efficient perovskite light-emitting diodes

Unbalanced electron-hole injection in perovskite light-emitting diodes (PeLEDs) has always been a major stumbling block limiting device performance. Here we develop a heterointerface Coulomb enhancement strategy to efficiently promote electron injection between perovskite emitter and transport layer and thereby balance the electron-hole recombination equilibrium through manipulating defect-rich positive-charge centers induced Coulomb interactions in LiF interlayer. Through multiscale polarization response characterizations combined with capacitance-voltage and electrochemical impedance spectroscopy, it is confirmed that diffusing holes trapped in LiF layer leads to the increased interfacial polarization. Moreover, the polarization response reversal and high-frequency-dependent dissipation sensitivity further corroborate the LiF-induced interfacial Coulomb enhancement mechanism. The universality of the concept is also successfully verified in devices with electron-only, hole-only, and PN structures. Consequently, the modified PeLEDs show a lower leakage current, along with a significant performance improvement by 1.9 times in external quantum efficiency (18.8%) and 5.7 times in luminance compared with control samples. This study provides a unique perspective to promote carrier transport balance by taking advantage of physical effects within interlayer.

High-brightness thermally evaporated perovskite light-emitting diodes via dual-interface engineering

Thermal evaporation emerges as a promising method for the scale-up fabrication of perovskite light-emitting diodes (PeLEDs) due to its superior repeatability and compatibility with the existing display industry compared to conventional solution process. However, the brightness of current high-efficiency thermally evaporated PeLEDs is generally constrained to a range of thousands of cd/m2. Herein, we propose a dual-interface engineering strategy to modulate the charge injection properties and passivate surface defects in the co-evaporated Cs–Pb–Br perovskite emitter. Our findings demonstrate that introducing 4,4′-Cyclohexylidenebis [N, N-bis(4-methylphenyl) benzenamine] additive to the bottom hole transport layer poly (9-vinylcarbazole) not only facilitates hole injection but also increases the conductivity. Meanwhile, the deposition of functionalized phenylethylammonium bromide salts with a specific number of fluorine atoms passivates the top surface defects and enhances the charge transport simultaneously. Consequently, the optimal PeLEDs achieve a maximum luminance of 75,012 cd/m2, marking one of the brightest thermally evaporated PeLEDs reported to date. The proposed strategy holds significant potential to guide the preparation of high-performance thermally evaporated PeLEDs for high-radiance display applications.

Phonon‐Bottleneck Enhanced Exciton Emission in 2D Perovskites

AbstractLayered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin‐orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher‐energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low‐temperature magneto‐optical measurements with sophisticated many‐particle theory, the origin of the bright exciton emission in perovskites is elucidated by tracking the thermalization of dark and bright excitons under a magnetic field. The unexpectedly high emission is clearly attributed to a pronounced phonon‐bottleneck effect, considerably slowing down the relaxation toward the energetically lowest dark states. It is demonstrated that this bottleneck can be tuned by manipulating the bright‐dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications.