Quantum-dot light-emitting diodes (QLEDs) promise high-performance and cost-effective electroluminescent devices. However, shelf stability of QLEDs, a must for practical applications, is currently overlooked. Here we reveal that the state-of-the-art QLEDs exhibit abnormal shelf-ageing behaviours, i.e., improvements in performance (positive ageing), followed by deterioration of performance (negative ageing). Mechanism studies show that the organic acids in the encapsulation acrylic resin induce in-situ reactions, which simultaneously improve electrical conductance and minimize interfacial exciton quenching at the positive-ageing stage. Progression of the in-situ reactions results in negative ageing. Inspired by these findings, we design an electron-transporting bi-layer structure, which delivers both improved electrical conductivity and suppressed interfacial exciton quenching. This design enables shelf-stable QLEDs with high operational performance, i.e., neglectable changes of external quantum efficiency (>20.0%) and ultra-long operational lifetime (T95: 5,500 hours at 1,000 cd m-2) after storage for 180 days. Our work paves the way towards the commercialization of QLEDs.
This article presents the use of flexible carbon substrates for the growth of III-nitride nanowire light emitters. A dense packing of gallium nitride nanowires were grown on a carbon paper substrate. The nanowires grew predominantly along the a-plane direction, normal to the local surface of the carbon paper. Strong photo- and electro-luminescence was observed from InGaN quantum well light emitting diode nanowires.
We demonstrate arbitrary helicity control of circularly polarized light (CPL) emitted at room temperature from the cleaved side-facet of a lateral-type spin-polarized light-emitting diode (spin-LED) with two ferromagnetic electrodes in an anti-parallel magnetization configuration. Driving alternate currents through the two electrodes results in polarization switching of CPL with frequencies up to 100 kHz. Furthermore, tuning the current density ratio in the two electrodes enables manipulation of the degree of circular polarization. These results demonstrate arbitrary electrical control of pola…
Light-emitting diodes (LEDs) are efficient light sources extensively applied in people's daily life nowadays, with profound effect on their psychological and physiological state subtly and deeply. The correlated color temperature (CCT) the eyes perceive is a key parameter, but the CCT tuning technology of LEDs remains in the development stage. In this study, an electrochromic (EC) material is employed in the tuning device, exhibiting a wide CCT tuning range from 3,200 K to 6,900 K, varying almost along the black-body locus. The driving voltage is lower than 2.4 V, with luminosity loss less tha…
Considerable progress has been made in organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency (EQE), among which the dipole orientation of OLED emitters has a remarkable effect. In most cases, EQE of the OLED emitter is theoretically predicted with only one orientation factor to match with corresponding experiments. Here, we develop a distribution theory with three independent parameters to fully describe the relationship between dipole orientations and power densities. Furthermore, we propose an optimal experiment configuration for measuring such distribution parame…
A GaN resonant cavity light emitting diode was built on a GaN-AlN distributed Bragg reflector grown on a silicon substrate. The electroluminescence output increased by 2.5 times for a GaN diode coupled to a properly designed resonant cavity. Theoretical calculations showed that this enhancement could increase up to four times for transmission through a sem-transparent metal contact design, up to eight times for a flip-chip design
We report on the demonstration of the first axial AlInN ultraviolet core-shell nanowire light-emitting diodes with highly stable emission in the UV wavelength range. During the epitaxial growth of AlInN layer, an AlInN shell is spontaneously formed, resulted in the reduced nonradiative recombination on nanowire surface. The AlInN nanowires exhibit high internal quantum efficiency of ~ 52% at room temperature for emission at 295nm. The peak emission wavelength can be varied from 290 nm to 355 nm by changing the growth condition. Moreover, significantly strong transverse magnetic (TM) polarized …
Perovskite nanoplatelets (NPls) hold great promise for light-emitting applications, having achieved high photoluminescence quantum efficiencies (PLQEs) approaching unity in the blue wavelength range, where other metal-halide perovskites have typically been ineffective. However, the external quantum efficiency (EQE) of blue-emitting NPl light-emitting diodes (LEDs) have only reached 0.12%, with typical values well below 0.1%. In this work, we show that the performance of NPl LEDs is primarily hindered by a poor electronic interface between the emitter and hole-injector. Through Kelvin Probe and…
To improve the light extraction efficiency of organic light emitting diodes (OLEDs), we developed a novel substrate, i.e., a metamaterial based flat Moire micro-lens array formed using double nanoimprint, termed Mlens-array, consisting of a hexagonal moiré pattern pillar array. By choosing a low refractive index dielectric material for the pillar array and a high refractive index dielectric material for the layer capped on top of the pillar array, we fabricated the Mlens-array behaving as a conventional convex optical micro-lens array. The Mlens-array was fabricated on a 4-inch wafer-size glas…
This paper reviews recent progress in the synthesis of near-infrared (NIR) lead chalcogenide (PbX; PbX=PbS, PbSe, PbTe) quantum dots (QDs) and their applications in NIR QDs based light emitting diodes (NIR-QLEDs). It summarizes the strategies of how to synthesize high efficiency PbX QDs and how to realize high performance PbX based NIR-QLEDs.
Unbalanced charge injection is deleterious for the performance of colloidal quantum dot (CQD) light emitting diodes (LEDs) as it deteriorates the quantum efficiency (QE), brightness and operational lifetime. CQD LEDs emitting in the infrared have previously achieved high quantum efficiencies but only when driven to emit in the low radiance regime. At higher radiance levels, required for practical applications, the efficiency decreased dramatically in view of the notorious efficiency droop. Here we report a novel methodology to regulate charge supply in multinary bandgap CQD composites that fac…
Silicon-based light sources including light-emitting diodes (LEDs) and laser diodes (LDs) for information transmission are urgently needed for developing monolithic integrated silicon photonics. Silicon doped by ion implantation with erbium ions (Er$^{3+}$) is considered a promising approach, but suffers from an extremely low quantum efficiency. Here we report an electrically pumped superlinear emission at 1.54 $μ$m from Er/O-doped silicon planar LEDs, which are produced by applying a new deep cooling process. Stimulated emission at room temperature is realized with a low threshold current of …
Utilization of triplet excitons, which generally emit poorly, is always fundamental to realize highly efficient organic light-emitting diodes (LEDs). While triplet harvest and energy transfer via electron exchange between triplet donor and acceptor are fully understood in doped organic phosphorescence and delayed fluorescence systems, the utilization and energy transfer of triplet excitons in quasi-two-dimensional (quasi-2D) perovskite are still ambiguous. Here, we use an orange-phosphorescence-emitting ultrathin organic layer to probe triplet behavior in the sky-blue-emitting quasi-2D perovsk…
A three-dimensional ordered and self-organized semiconductor system emitting highly-polarized light in the yellow-orange visible range (580-650 nm) is presented, comprising self-assembled in-plane AlInP wires vertically stacked in regularly-spaced columns. More than 200 wires per column without detectable defect formation could be stacked. Theoretical simulations and temperature-dependent photoluminescence provided a benchmark to engineer multilayered structures showing internal quantum efficiency at room temperature larger than comparable quantum wells emitting at similar wavelengths. Finally…
Quantum-dot light-emitting diodes (QD-LEDs) promise a new generation of efficient, low-cost, large-area, and flexible electroluminescent devices. However, the inferior performance of green and blue QD-LEDs is hindering the commercialization of QD-LEDs in display and solid-state lighting. Here, we demonstrate best-performing green and blue QD-LEDs with ~100% conversion of the injected charge carriers into emissive excitons. Key to this success is eliminating electron leakage at the organic/inorganic interface by using hole-transport polymers with low electron affinity and reduced energetic diso…
Singlet-polaron and triplet-polaron annihilation mechanisms are the most detrimental exciton quenching processes that lower the efficiency of organic light-emitting diodes (OLEDs) at high current densities, causing so-called efficiency roll-off in these devices. These exciton loss mechanisms are also the critical obstacles towards the realization of electrically pumped organic semiconductor lasers, which require very high current densities to reach threshold. Herein, under a relatively large external magnetic field, we demonstrate that the efficiency roll-off at high current densities in europ…
Despite widespread interest, ultrathin and highly flexible light-emitting devices that can be seamlessly integrated and used for flexible displays, wearables, and as bioimplants remain elusive. Organic light-emitting diodes (OLEDs) with $μ$m-scale thickness and exceptional flexibility have been demonstrated but show insufficient stability in air and moist environments due to a lack of suitable encapsulation barriers. Here, we demonstrate an efficient and stable OLED with a total thickness of $\approx$12 $μ$m that can be fully immersed in water or cell nutrient media for weeks without suffering…
The quantum efficiency of double hetero-junction light-emitting diodes (LEDs) can be significantly enhanced at low current density by tailoring the spatial profile of dopants to suppress Shockley-Read-Hall (SRH) recombination. To demonstrate this effect we model, design, grow, fabricate, and test a GaInAsP LED ($λ\approx$ 1330nm) with an unconventional dopant profile. Compared against that of our control design, which is a conventional $n^+$-$n$-$p^+$ double hetero-junction LED, the dopant profile near the $n$-$p^+$ hetero-structure of the new design displaces the built-in electric field in su…
III-Nitride light emitting diodes (LEDs) are widely used in a range of high efficiency lighting and display applications, which have enabled significant energy savings in the last decade. Despite the wide application of GaN LEDs, transport mechanisms across InGaN/GaN heterostructures in these devices are not well-explained. Fixed polarization sheet charges at InGaN/GaN interfaces lead to large interface dipole charges, which create large potential barriers to overcome. One-dimensional models for transport across such heterostructures predict turn-on voltages that are significantly higher than …
Artificial lighting is a widespread technology which consumes large amounts of energy. Triplet-triplet annihilation photochemical upconversion is a method of converting light to a higher frequency. Here, we show theoretically that photochemical upconversion can be applied to Watt-scale lighting, with performance closely approaching the 50% quantum yield upper limit. We describe the dynamic equilibrium of an efficient device consisting of an LED, an upconverting material, and an optical cavity from optical and thermal perspectives.
Single-photon emitting devices have been identified as an important building block for applications in quantum information and quantum communication. They allow to transduce and collect quantum information over a long distance via photons as so called flying qubits. In addition, substrates like silicon carbide provides an excellent material platform for electronic devices. In this work we combine these two features and show that one can drive single photon emitters within a silicon carbide p-i-n-diode. To achieve this, we specifically designed a lateral oriented diode. We find a variety of new…
To improve the power efficiency of light emitting diodes (OLEDs), we developed a novel OLED structure, termed Dielectric-Nanomesh OLED (DNM-OLED), fabricated by large-area nanoimprint lithography (NIL). A dielectric-nanomesh substrate with a subwavelength nanomesh grating patterned into glass releases the photons trapped in ITO and organic layers by guided modes into leaky modes. And the dielectric-nanomesh substrate modifies the entire OLED layers into a corrugated configuration, which further lowers the parasitic resistance and hence the operating voltage. For a red phosphorescent OLED in ou…
Light-emitting diodes based on perovskite nanocrystals (PNCs-LEDs) have gained great interest for next-generation display and lighting technologies prized for their color purity, high brightness and luminous efficiency approaching the intrinsic limit imposed by extraction of electroluminescence from the device structure. Although the time is ripe for the development of effective light outcoupling strategies to further boost the device performance, this technologically relevant aspect of PNC-LEDs is still without a definitive solution. Here, following theoretical guidelines and without the inte…
A silicon light source at communication wavelength is the bottleneck for developing monolithically integrated silicon photonics. Doping silicon with erbium ions was believed to be one of the most promising approaches but suffers from the aggregation of erbium ions that are efficient non-radiative centers, formed during the standard rapid thermal treatment. Here, we apply a deep cooling process following the high-temperature annealing to suppress the aggregation of erbium ions by flushing with Helium gas cooled in liquid nitrogen. The resultant light emitting efficiency is increased to a record…
Origin of Operating Voltage Increase in InGaN-based Light-emitting Diodes under High Injection: Phase Space Filling Effect on Forward Voltage Characteristics
In this work, performance and characteristics of AlGaN/AlGaN deep-ultraviolet light-emitting diodes (DUV LEDs) with varied number of quantum-well (QW) are investigated numerically. From our simulation, 1-QW structure give the best performance at low injection current. However, at higher injection current, 2-QWs structure give the largest power output due to its higher total radiative recombination rate and internal quantum efficiency (IQE) compared to other structures. The 2-QWs structure also has less serious efficiency droop at high current than 1-QW, which makes it an optimum structure for …
Lanthanide-based upconversion nanoparticles (UCNPs) generally require high power laser excitation. Here we report wide-field upconversion microscopy at single-nanoparticle sensitivity using incoherent excitation of a 970-nm light-emitting diode (LED). We show that due to its broad emission spectrum, LED excitation is about 3 times less effective for UCNPs and generates high background compared to laser illumination. To counter this, we use time-gated luminescence detection to eliminate the residual background from the LED source, so that individual UCNPs with high sensitizer (Yb3+) doping and …
Localized surface plasmons (LSPs) have played a significant role in improving the light emission efficiency of light emitting diodes (LEDs). In this report, polygonal nanoholes have been fabricated in the p-GaN layer of InGaN-based LEDs by using Ni nanoporous film as the etching mask, and then Au/Al metal nanoparticles are embedded in the nanoholes to form the LSP structure. The coupling between the LSP and the LED has been clearly observed. The results show that the light output of the LEDs has been increased by 46% at higher current injection condition, and together with a shift of the gain …
We report on the illustration of the first electron blocking layer (EBL) free AlInN nanowire light-emitting diodes (LEDs) operating in the deep ultraviolet (DUV) wavelength region (sub-250 nm). We have systematically analyzed the results using APSYS software and compared with simulated AlGaN nanowire DUV LEDs. From the simulation results, significant efficiency droop was observed in AlGaN based devices, attributed to the significant electron leakage. However, compared to AlGaN nanowire DUV LEDs at similar emission wavelength, the proposed single quantum well (SQW) AlInN based light-emitters of…
The low efficiency of metal halide perovskite light-emitting diodes (PeLEDs) with blue emission block their potential applications in large-area full-color displays and solid-state lighting. A delicate control over the entire electroluminescence process is indispensable to overcome the efficiency limitations of blue PeLEDs. Here, we demonstrate an efficient device architecture to synergistically reduce the energetic losses during electron-photon conversion and boost the extraction of trapped light in the device. An interfacial nucleation seeding scheme is proposed to control the crystallizatio…
Short wavelength ultraviolet (UV-C) light deactivates DNA of any germs, including multiresistive bacteria and viruses like COVID-19. Two-dimensional (2D) material-based UV-C light emitting diodes can potentially be integrated into arbitrary surfaces to allow for shadow-free surface disinfection. In this work, we perform a series of first-principles calculations to identify the core components of ultrathin LEDs based on hexagonal boron nitride (hBN). The electrons and holes are predicted to be confined in multiple quantum wells (MQWs) by combining hBN layers with different stacking orders. Vari…
Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent (EL) devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remains to be inefficient (external quantum efficiency, EQE <5%), due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (P…
The undoped BAlN electron-blocking layer (EBL) is investigated to replace the conventional AlGaN EBL in light-emitting diodes (LEDs). Numerical studies of the impact of variously doped EBLs on the output characteristics of LEDs demonstrate that the LED performance shows heavy dependence on the p-doping level in the case of the AlGaN EBL, while it shows less dependence on the p-doping level for the BAlN EBL. As a result, we propose an undoped BAlN EBL for LEDs to avoid the p-doping issues, which a major technical challenge in the AlGaN EBL. Without doping, the proposed BAlN EBL structure still …
Organic Light Emitting Diodes (OLED) have experienced vast attention in the scientific literature as the leading contender for the next generation of electronic lighting technology. However, while research continues to push the limit for OLED performance and efficiencies, minimal attention is paid to the processability and suitability for mass production. In this regard, Solution Processability (SP) and Thermally Activated Delayed Fluorescence (TADF) in OLEDs are two recent discoveries which have significantly improved the capacity for low-cost processing without compromise to their performanc…
Organic light-emitting diodes (OLEDs) are the leading self-emitting pixel technology in current and future small and large area displays. Once integrated with a certain layer architecture into the backplane layout, their emission colour and angular distribution is set by the optical properties of the layered system. In this paper, we demonstrate a pixel design that allows for actively controlled variation of the angular emission profile of the individual vertical pixel. For this, a tandem device is developed that comprises two units optimized for different angular emission pattern. We constrai…
The radiative recombination of injected charge carriers gives rise to electroluminescence (EL), a central process for light-emitting diode (LED) operation. It is often presumed in some emerging fields of optoelectronics, including perovskite and organic LEDs, that the minimum voltage required for light emission is the semiconductor bandgap divided by the elementary charge. Here we show for many classes of LEDs, including those based on metal halide perovskite, organic, chalcogenide quantum-dot and commercial III-V semiconductors, photon emission can be generally observed at record-low driving …
The electron and hole drift mobilities of organic semiconductor layers, localized tail state distributions, and bimolecular recombination constants in working polymer light-emitting diodes (PLEDs) are determined simultaneously using impedance spectroscopy (IS). The organic light-emitting layers of these PLEDs are composed of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). Electron and hole transit time effects are observed in the capacitance-frequency characteristics of the PLEDs and their drift mobilities are determined over wide temperature and electric field ranges. The drift mobilit…
Perovskite nanocrystal light-emitting diodes (LEDs) employing architecture comprising a ZnO nanoparticles electron-transport layer and a conjugated polymer hole-transport layer have been fabricated. The obtained LEDs demonstrate a maximum external-quantum-efficiency of 6.04%, luminance of 12998 Cd/m2 and stable electroluminescence at 519 nm. Importantly, such high efficiency and brightness have been achieved by employing solution processed transport layers, formamidinium lead bromide nanocrystals (CH(NH2)2PbBr3 NCs) synthesized at room-temperature and in air without the use of a Schlenk line, …
A two-dimensional (2D) ZnS photonic crystal was deposited on the surface of a one-dimensional (1D) III-nitride micro cavity light-emitting diode (LED), to intermix the light extraction features of both structures (1D+2D). The deposition of an ideal micro-cavity optical thickness of lambda/2 is impractical for III-nitride LEDs, and in realistic multi-mode devices a large fraction of the light is lost to internal refraction as guided light. Therefore, a 2D photonic crystal on the surface of the LED was used to diffract and thus redirect this guided light out of the semiconductor over several hun…
Spectroscopic Signature of Local Alloy Fluctuations in InGaN/GaN Multi-Quantum-Disk Light Emitting Diode Heterostructures and Its Impact on the Optical Performance
Blue light-emitting diodes (LEDs) consisting of a buried n+-p+ GaN tunnel junction, (In,Ga)N multiple quantum wells (MQWs) and a n+-GaN top layer are grown on single-crystal Ga-polar n+-GaN bulk wafers by plasma-assisted molecular beam epitaxy. The (In,Ga)N MQW active regions overgrown on the p+-GaN show chemically abrupt and sharp interfaces in a wide range of compositions and are seen to have high structural and optical properties as verified by X-ray diffraction and spatially resolved cathodoluminescence measurements. The processed LEDs reveal clear rectifying behavior with a low contact an…
Organic light-emitting diodes (OLEDs) have successfully entered the display market and continue to be attractive for many other applications. As state-of-the-art OLEDs can reach an internal quantum efficiency (IQE) of almost 100 %, light outcoupling remains one of the major screws left to be turned. The fact that no superior outcoupling structure has been found underlines that further investigations are needed to understand their prospect. In this paper, we use two-dimensional titanium dioxide (2D TiO$_2$) block arrays as a model of an internal light outcoupling structure and investigate the i…
The use of robust, inorganic charge-transport materials is always desired in quantum-dot light emitting diodes (QLEDs) because they are expected to allow higher stability and less cost than that of organic counterparts. Here we report an all-inorganic QLED with excellent efficiency by modifying the solution-processed NiO (s-NiO) surface with an ultrathin Al2O3 passivating layer. The localized electric field induced by nickel oxyhydroxide (NiOOH) is estimated to be ~ 70 MV/cm at a distance of 6 nm from the surface of s-NiO layer. Both transient resolution photoluminescence (TRPL) and X-Ray phot…
Carbene-metal-amide type photoemitters based on CF$_3$-substituted carbazolate ligands show sky-blue to deep-blue photoluminescence from charge-transfer excited states. They are suitable for incorporation into organic light-emitting diodes (OLEDs) by thermal vapour deposition techniques, either embedded within a high-triplet-energy host, or used host-free. We report high-efficiency OLEDs with emission ranging from yellow to blue (Commission Internationale de l'Éclairage (CIE) coordinates from [0.35, 0.53] to [0.17, 0.17]). The latter show a peak electroluminescence external quantum efficiency …
Spin-spin interactions in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are pivotal because radiative recombination is largely determined by triplet-to-singlet conversion, also called reverse intersystem crossing (RISC). To explore the underlying process, we apply a spin-resonance spectral hole-burning technique to probe electroluminescence. We find that the triplet exciplex states in OLEDs are highly spin-polarized and show that these states can be decoupled from the heterogeneous nuclear environment as a source of spin dephasing and can even b…
We present a comprehensive model to analyze, quantitatively, and predict the process of degradation of organic light-emitting diodes (OLEDs) considering all possible degradation mechanisms, i.e., polaron, exciton, exciton-polaron interactions, exciton-exciton interactions, and a newly proposed impurity effect. The loss of efficiency during degradation is presented as a function of quencher density, the density and generation mechanisms of which were extracted using a voltage rise model. The comprehensive model was applied to stable blue phosphorescent OLEDs (PhOLEDs), and the results showed th…
Excitable optoelectronic devices represent one of the key building blocks for implementation of artificial spiking neurons in neuromorphic (brain-inspired) photonic systems. This work introduces and experimentally investigates an opto-electro-optical (O/E/O) artificial neuron built with a resonant tunnelling diode (RTD) coupled to a photodetector as a receiver and a vertical cavity surface emitting laser as a the transmitter. We demonstrate a well defined excitability threshold, above which this neuron produces 100 ns optical spiking responses with characteristic neural-like refractory period.…
Radiative and non-radiative charge carrier recombination in thin-film diodes plays a key role in determining the efficiency of electronic devices made of next generation semiconductors such as organic, perovskite and nanocrystals. In this work, we show that lowering the bulk recombination does not necessarily result in enhanced performance metrics of electronic devices. From the perspective of charge carrier extraction and injection, the radiative limit of the open-circuit voltage of solar cells, noise current of photodetectors and lasing threshold of injection lasers cannot be improved if the…
