High-performance Light-emitting Diodes Research Articles

Control of n-Phase Distribution in Quasi Two-Dimensional Perovskite for Efficient Blue Light-Emitting Diodes.

Pure-bromide quasi-2D perovskite (PBQ-2DP) promises high-performance light-emitting diodes (LEDs), while a challenge remains on control over its n-phase distribution for bright true-blue emission. Present work addresses the challenge through exploring the passivation molecule of amino acid with reinforced binding energy, which generates narrow n-phase distribution preferentially at n = 3 with true blue emission at 478 nm. Consequently, a peak external quantum efficiency of 5.52% and a record brightness of 512 cd m-2 are achieved on the PBQ-2DP-based true blue PeLED, these both values located among the top in the records of similar devices. We further reveal that the electron-phonon coupling results in the red-shifted emission in the PBQ-2DP film, suggesting that the view of n-phase distribution dominated true-blue emission in PBQ-2DP needs to be revisited, pointing out a guideline of electron-phonon coupling suppression to relieve the strait of realizing true blue or even deep blue emission in the PBQ-2DP film.

Radiative Recombination Processes in Halide Perovskites Observed by Light Emission Voltage Modulated Spectroscopy.

The kinetics of light emission in halide perovskite light-emitting diodes (LEDs) and solar cells is composed of a radiative recombination of voltage-injected carriers mediated by additional steps such as carrier trapping, redistribution of injected carriers, and photon recycling that affect the observed luminescence decays. These processes are investigated in high-performance halide perovskite LEDs, with external quantum efficiency (EQE) and luminance values higher than 20% and 80000 Cd m-2 , by measuring the frequency-resolved emitted light with respect to modulated voltage through a new methodology termed light emission voltage modulated spectroscopy (LEVS). The spectra are shown to provide detailed information on at least three different characteristic times. Essentially, new information is obtained with respect to the electrical method of impedance spectroscopy (IS), and overall, LEVS shows promise to capture internal kinetics that are difficult to be discerned by other techniques.

Versatile Use of 1,12-Diaminododecane as an Efficient Charge Balancer for High-Performance Quantum-Dot Light-Emitting Diodes

Balancing the charge injection rates in the emissive layer (EML) is a key factor in achieving high-performance quantum-dot light-emitting diodes (QD-LEDs). This has mostly been pursued by controlling the injection rate of one type of charge carrier (typically electrons). Simultaneous engineering of both types of charge carriers, however, would be more efficient in achieving the desired charge balance within a device. Herein, the versatile use of 1,12-diaminododecane (DAD) ligands is proposed to achieve balanced charge injection rates into the QD EML, thereby enhancing QD-LED performance. By utilizing the dipole moment and short chain lengths, DAD ligands function as efficient electron retarders and hole boosters. Specifically, DAD treatment on the surface of zinc oxide generates an electronic barrier to QDs. Meanwhile, DAD treatment on the surface of the QDs reduces the potential difference between the organic hole transport layers and QDs. Consequently, the QD-LEDs with the adoption of DAD at the two different positions exhibit balanced charge injection rates into the QDs, enhancing device efficiency and operational stability. The present study proposes a simple yet efficient chemical method that controls both electron and hole injection properties to achieve high-performance QD-LEDs.

Deciphering the Role of Water in Promoting the Optoelectronic Performance of Surface-Engineered Lead Halide Perovskite Nanocrystals.

Lead halide perovskites are promising candidates for high-performance light-emitting diodes (LEDs); however, their applicability is limited by their structural instability toward moisture. Although a deliberate addition of water to the precursor solution has recently been shown to improve the crystallinity and optical properties of perovskites, the corresponding thin films still do not exhibit a near-unity quantum yield. Herein, we report that the direct addition of a minute amount of water to post-treated formamidinium lead bromide (FAPbBr3) nanocrystals (NCs) substantially enhances the stability while achieving a 95% photoluminescence quantum yield in a NC thin film. We unveil the mechanism of how moisture assists in the formation of an additional NH4Br component. Alongside, we demonstrate the crucial role of moisture in assisting localized etching of the perovskite crystal, facilitating the partial incorporation of NH4+, which is key for improved performance under ambient conditions. Finally, as a proof-of-concept, the application of post-treated and water-treated perovskites is tested in LEDs, with the latter exhibiting a superior performance, offering opportunities toward commercial application in moisture-stable optoelectronics.

Complementary ligand decorated cesium lead bromide nanoplatelets for high performance light-emitting diodes

Two dimensional cesium lead bromide (CsPbBr3) nanoplatelets (NPLs) have gained numerous attention due to its tunable thickness, large exciton binding energy and excellent optical properties which can be applied in various photoelectric device. However, synthesizing blue emitting NPLs with long-term stability and high photoluminescence quantum yields (PLQYs) was still difficult. Here, we demonstrate a strategy based on surface ligand engineering, using didodecyldimethylammonium bromide and sodium dodecylbenzene sulfonate as surface ligands to impart higher PLQYs to perovskite materials while improving the stability of CsPbBr3 NPLs. These ligands strongly interact with CsPbBr3 NPLs, compared with untreated materials, effectively eliminating defects on the surface of NPLs and significantly improving the structural stability of perovskite materials. High-quality CsPbBr3 NPLs with high color purity (narrow peaks located at ∼470 nm) and PLQYs up to 90 % were prepared by this method. In addition, the stability of treated NPLs under some harsh environments (including long-term storage, high temperature and 365 nm UV light irradiation) has been significantly improved. White light-emitting diodes were finally fabricated using these highly bright and stable CsPbBr3 NPLs.

Hyperbranched Conjugated Polymer with Multiple Charge Transfer Enables High-Efficiency Nondoped Red Electroluminescence with Low Driving Voltage

Conjugated polymers featuring thermally activated delayed fluorescence (TADF) attract tremendous attention in both academic and industry communities due to their easy solution processing for fabricating large-area and low-cost high-performance polymer light-emitting diodes (PLEDs). However, current nondoped solution-processed PLEDs frequently encounter significant efficiency roll-offs and unreasonably high operating voltages at high brightness, especially for red-emitting polymers. Herein, we design hyperbranched conjugated polymers (HCPs) with D-A-D type TADF characteristics for high-performance red-emitting PLEDs. Multiple intramolecular charge transfer (ICT) channels induced by quasi-equivalent donors of the TADF core strongly boost the reverse intersystem crossing (RISC) process and singlet excitons radiative transition. Coupling with the efficient energy transfer process generated by structure advantages of HCPs, the strongly electron-withdrawing oxygen atoms located on the TADF cores further accelerate hole transportation from the host chains to the TADF cores. Under a rational regulation of the TADF core ratio, the related nondoped red-emitting device performs an outstanding performance with an EQEmax of 8.39% and exhibits no roll-off while the luminance is less than 100 cd/m2 and only 3.3% decrease at 500 cd/m2. Simultaneously, the EQE can maintain 7.4% under 1000 cd/m2. Furthermore, the corresponding nondoped device exhibits a low turn-on voltage of around 2.5 V and achieves a luminance of 500 cd/m2 at 3.5 V and even 1000 cd/m2 at 3.9 V. To our knowledge, this is the best performance among all nondoped red PLEDs with high brightness obtained at low operating voltage.

Does interfacial exciton quenching exist in high-performance quantum dot light-emitting diodes?

In quantum dot light-emitting diodes (QLEDs), even seemingly with interfacial exciton quenching between quantum dots (QDs) and the electron transport layer (ETL) limiting the device efficiency, the internal quantum efficiency of such QLEDs approaches 100%. Therefore, it is a puzzle that QLEDs exhibit high performance although they suffer from interfacial exciton quenching. In this work, we solve this puzzle by identifying the cause of the interfacial exciton quenching. By analyzing the optical characteristics of pristine and encapsulated QD-ETL films, the interfacial exciton quenching in the pristine QD-ETL film is attributed to O2-induced charge transfer. We further investigate the charge transfer mechanism and its effect on the performance of QLEDs. Finally, we show the photodegradation of the pristine QD-ETL film under UV irradiation. Our work bridges interfacial exciton quenching and high performance in hybrid QLEDs and highlights the significance of encapsulation in QLEDs.

Incorporation of a Polyfluorinated Acrylate Additive for High-Performance Quasi-2D Perovskite Light-Emitting Diodes

Incorporation of a Polyfluorinated Acrylate Additive for High-Performance Quasi-2D Perovskite Light-Emitting Diodes

Review on the promising roles of alkali metals toward highly efficient perovskite light-emitting diodes

Alkali metals have been used to obtain high-performance perovskite light-emitting diodes. In this review, we present an overview of the advances in alkali metal ions applied in PeLEDs. Challenges and the outlook are presented for their future applications.

High-performance perovskite light-emitting diodes based on grain boundary passivation: progress, challenges and perspectives

Metal halide perovskites are emerging as promising candidates for next-generation display and lighting technologies.

Performance Enhancement of Perovskite Quantum Dot Light-Emitting Diodes via Management of Hole Injection.

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is widely used in optoelectronic devices due to its excellent hole current conductivity and suitable work function. However, imbalanced carrier injection in the PEDOT:PSS layer impedes obtaining high-performance perovskite light-emitting diodes (PeLEDs). In this work, a novel poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,40-(N-(p-butylphenyl))diphenylamine)] (TFB) is applied as the hole transport layers (HTLs) to facilitate the hole injection with cascade-like energy alignment between PEDOT:PSS and methylammonium lead tribromide (MAPbBr3) film. Our results indicate that the introduced TFB layer did not affect the surface morphology or lead to any additional surface defects of the perovskite film. Consequently, the optimal PeLEDs with TFB HTLs show a maximum current efficiency and external quantum efficiency (EQE) of 21.26 cd A-1 and 6.68%, respectively. Such EQE is 2.5 times higher than that of the control devices without TFB layers. This work provides a facile and robust route to optimize the device structure and improve the performance of PeLEDs.

Zwitterions Narrow Distribution of Perovskite Quantum Wells for Blue Light-Emitting Diodes with Efficiency Exceeding 15.

While quasi-two-dimensional (quasi-2D) perovskites have emerged as promising semiconductors for light-emitting diodes (LEDs), the broad-width distribution of quantum wells hinders their efficient energy transfer and electroluminescence performance in blue emission. In particular, the underlying mechanism is closely related to the crystallization kinetics and has yet to be understood. Here for the first time, the influence of bifunctional zwitterions with different coordination affinity on the crystallization kinetics of quasi-2D perovskites is systematically investigated. The zwitterions can coordinate with Pb2+ and also act as co-spacer organic species in quasi-2D perovskites, which collectively inhibit the aggregation of colloidal precursors and shorten the distance of quantum wells. Consequently, restricted nucleation of high-n phases and promoted growth of low-n phases are achieved with moderately coordinated zwitterions, leading to the final film with a more concentrated n distribution and improved energy transfer efficiency. It thus enables high-efficiency blue LEDs with a recorded external quantum efficiency of 15.6% at 490nm, and the operation stability has also been prolonged to 55.3min. These results provide useful directions for tuning the crystallization kinetics of quasi-2D perovskites, which is expected to lead to high-performance perovskite LEDs.

In-Composition Graded Quantum Barriers for Polarization Manipulation in InGaN-Based Yellow Light-Emitting Diodes

Highly efficient indium gallium nitride (InGaN)-based yellow light-emitting diodes (LEDs) with low efficiency droop have always been pursued for next-generation displays and lighting products. In this work, we report an InGaN quantum barrier (QB) with linear-increase In-composition along [0001] direction for InGaN-based yellow LEDs. With the In-composition in QBs systematically engineered, three QB structures including linear-increase QB (LIQB), linear-decrease QB (LDQB) and commonly used flat QB (FQB) were investigated by simulation. The results show that the LIQB not only yields enhanced electron confinement, but also contributes to suppressed polarization field. Consequently, the yellow LED incorporated with LIQBs demonstrates improved radiative recombination rates and the efficiency droop is alleviated. Under a current density of 100 A/cm2, the efficiency droop ratios of LEDs with FQBs, LDQBs and LIQBs are 58.7%, 62.2% and 51.5%, respectively. When current density varies from 1 A/cm2 to 60 A/cm2, the blueshift values of peak emission wavelength for LEDs with FQBs, LDQBs and LIQBs are 14.4 nm, 16.5 nm and 13.0 nm, respectively. This work is believed to provide a feasible solution for high-performance InGaN-based LEDs in long-wavelength spectral region.

Mixed halide perovskite nanocrystals with surface engineering based on pseudohalide passivation and Short-Chain ligand exchange for High-Performance blue Light-Emitting diodes

Perovskite nanocrystals (PeNCs) have attracted considerable interest in light-emitting diodes (LEDs) owing to their unique optoelectronic properties. Although significant progress has been made in green and red perovskite LEDs (PeLEDs) with PeNCs, the performance of blue PeLEDs is still limited. The main obstacles for blue PeLEDs based on PeNCs are their Cl vacancy-induced low quantum efficiency in mixed halide (Br/Cl) systems and high electrical resistance from long organic capping ligands. In this study, to overcome these issues and achieve high-performance blue PeLEDs, a surface engineering (SE) strategy based on pseudohalide defect passivation and short-chain ligand exchange was introduced. By passivating Cl vacancies with SCN– and exchanging long-chain oleic acid and oleylamine ligands with short octylphosphonic acid and 3,3-diphenylpropyl amine, respectively, the optoelectronic properties of the PeNCs were significantly enhanced. The blue PeLEDs based on PeNCs, using SE, exhibited a maximum luminance of 1874 cd/m2 and maximum external quantum efficiency of 4.9 % at an electroluminescence wavelength of 467 nm, which satisfies the blue primary color requirement (467 nm) in Rec. 2020 color space.

One-step large-scale fabricating aggregation-induced emission carbon dots with strong solid-state fluorescence emission

Carbon dots (CDs) possessing favorable stability and optical properties have attracted extensive attention for their potential application in solid-state light-emitting devices. However, due to the stacking of π–π conjugated systems, solid-state CDs often suffer aggregation-caused quenching effect, resulting in fluorescence quenching. There are few reports on solid-state fluorescent CDs fabricated without using other solid matrices. In this work, we report a facile approach for one-step synthesis of yellow-green solid-state CDs without using matrix. The synthesized CDs show an independent excitation and emission in various organic solvents. Importantly, the prepared CDs show solvatochromism and aggregation-induced emission redshift effect. By simply drying the colloidal solution of CDs, yellow-green solid-state fluorescent CD powders with a photoluminescence quantum yield of 25.5% can be obtained. High-performance white light-emitting diodes with Commission Internationale de L'Eclairage coordinates at (0.35, 0.37) are prepared by employing the yellow-green solid-state CD powders. The corresponding color temperature and color rendering index are 4691 K and 91.5 under ultraviolet excitation of 365 nm, respectively. The facile synthesis method of solid-state fluorescent CDs with unique optical features offers promising future for CDs-based optoelectronic devices.

Defect-selective-etched porous GaN as a buffer layer for high efficiency InGaN/GaN light-emitting diodes.

Substrate-induced biaxial compressive stress and threading dislocations (TDs) have been recognized to severely impair the performance, stability, and reliability of InGaN/GaN light-emitting diodes (LEDs) for quite some time. In this study, a defect-selective-etched (DSE) porous GaN layer is fabricated employing electro-chemical etching and applied as a buffer layer for the development of InGaN/GaN LEDs with high quantum efficiency. Based on the analysis of photoluminescence and micro-Raman spectra, it has been revealed that the overgrown GaN epilayer on the DSE porous GaN has a relatively low TDs and relaxation of compressive stress in comparison to the conventional GaN epilayer. The remarkable improvement in the internal quantum efficiency of the InGaN/GaN LEDs is directly attributable to the strong radiative recombination in InGaN/GaN multi-quantum-wells caused by stress relaxation and TDs annihilation. Our findings indicate that the use of DSE porous GaN as a buffer layer may be a viable approach for producing crystalline GaN epilayers and high-performance LEDs.

Fabrication of ZnO dual electron transport layer via atomic layer deposition for highly stable and efficient CsPbBr3 perovskite nanocrystals light-emitting diodes

Electron transport layers (ETLs) are important components of high-performance all-inorganic perovskite nanocrystals light-emitting diodes (PNCs-LED). Herein, atomic layer deposition (ALD) of inorganic ZnO layer is combined to the organic 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) to form dual ETLs to enhance both the efficiency and stability of PNCs-LED simultaneously. Optimization of ZnO thickness suggested that 10 cycles ALD yields the best performance of the devices. The external quantum efficiency of the device reaches to 7.21% with a low turn-on voltage (2.4 V). Impressively, the dual ETL PNCs-LED realizes maximum T 50 lifetime of 761 h at the initial luminance of 100 nit, which is one of the top lifetimes among PNCs-LEDs up to now. The improved performance of dual ETL PNCs-LED is mainly due to the improved charge transport balance with favorable energy level matching. These findings present a promising strategy to modify the function layer via ALD to achieve both highly efficient and stable PNCs-LED.

Stabilizing dynamic surface of highly luminescent perovskite quantum dots for light-emitting diodes

Lead halide perovskite quantum dot (QD) shows great potential for application in light-emitting diodes (LEDs) due to its high photoluminescence quantum yield (PLQY), high exciton binding energy, tunable emission spectra and solution processibility. However, the dynamic binding of conventional organic acid/amine ligands to the QD surface significantly deteriorates its optoelectronic properties and stability, to a large extent affecting the electroluminescence performance of QD-based LEDs. Herein, the zwitterionic molecule, betaine (BET) that could firmly anchor on the QD surface, is introduced to stabilize the dynamic surface and simultaneously passivate the surface defects of QDs. Systematically experimental studies and theoretical calculations reveal that BET-treatment could remove the dynamically bonded OA/OAm ligands and substantially diminish the surface defects of QDs, leading to a boosted PLQY of up to 92 % with improved stability of BET-QDs. Consequently, the electroluminescence performance of green-emitting QD-LEDs was largely improved that the external quantum efficiency was increased to 10.8 %, along with a decreased turn-on voltage of 3.2 V. This work provides new insights into the design criterion of surface ligands for modulating the surface chemistry of perovskite QDs to realize high-performance LEDs or other optoelectronic devices.

Enhanced luminescence properties and device applications of Tb3+-doped Cs4PbBr6 perovskite quantum dot glass

Cs4PbBr6 perovskite quantum dots (QDs) have unique optoelectronic properties and are expected to become a new generation of luminescent materials. However, poor stability, low photoluminescence quantum yield (PLQY), and poor understanding as to the origin of photoluminescence behavior limit its further application. In this study, a series of Tb3+-doped Cs4PbBr6 perovskite QD glasses with excellent stability were obtained through the optimization of Tb3+ doping concentration and in-situ crystallization temperature. Density functional theory calculations and experimental characterization showed that an appropriate amount of lattice-incorporated Tb3+ ions can reduce structural defects in QDs, improve the PLQY, and reduce the QD heavy-metal requirements. Notably, the maximum PLQY value reached 47%, which is near to the Cs4PbBr6 perovskite crystal. Furthermore, a high-performance white light-emitting diode (WLED) device was prepared. The device featured a color rendering index of 80 and luminous efficiency of 41 lm W−1. Finally, a QD glass with double emission peaks was prepared by controlling the in-situ crystallization temperature (550 °C). The temperature sensitivity of the QD glass was then studied using the fluorescence intensity ratio method. The maximum relative temperature sensitivity (Sr) reached 2.03% K−1, which is higher than the previously reported value. Thus, the method proposed in this study can greatly improve the photoluminescence properties of Cs4PbBr6 QD glass and expand its applications in WLED and visual temperature sensing.

High-Performance Perovskite Light-Emitting Diodes Enabled by Passivating Defect and Constructing Dual Energy-Transfer Pathway through Functional Perovskite Nanocrystals.

Quasi-2D perovskites have emerged as a promising luminescent material for perovskite light-emitting diodes (Pe-LEDs). However, efficiency and stability are still obstacles to practical application due to numerous defects and inefficient energy transfer of perovskite films. Herein, functional phenethylammonium bromine-modified CsPbBr3 nanocrystals (PEA-CsPbBr3 NCs) are first introduced as multifunctional additive to simultaneously improve abovementioned problems. PEA-CsPbBr3 NCs not only serve as heteronuclear seeds and trigger growth, thus greatly reducing leakage current, but also deliver Cs+ and Br- to passivate the intrinsic defects inside film. More importantly, the PEA-CsPbBr3 construct a new carrier-transfer pathway from the small-n phase of the quasi-2D perovskite to the PEA-CsPbBr3 , which not only accelerates the energy-transfer process but also promotes radiation recombination of carriers due to stronger quantum confinement effect. Afterward, the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/poly[9,9-dioctylfluoreneco-N-[4-(3-methylpropyl)]diphenylamine]:black phosphorus quantum dot double hole-transport layer is successfully constructed to enhance its carrier-injection and charge-transport abilities. Consequently, a champion external quantum efficiency of 25.32% and maximal brightness of 128 842cd m-2 are achieved, which is the record efficiency of the quasi-2D Pe-LED with pure green emission at 530nm. Moreover, an impressive 174min lifetime is obtained at T50 , which is about five times longer than the control device.