Electroluminescence Peak Research Articles

Optically invariant InGaN nanowire light-emitting diodes on flexible substrates under mechanical manipulation

The integration of GaN-based light-emitting diodes (LEDs) onto flexible platforms provides opportunities for conformal lighting, wearable electronics, and bendable displays. While this technology may enhance the functionality of the light source, the development of flexible GaN LEDs suffers from performance degradation, when mechanical bending is applied during operation. A unique approach to eliminate the degradation employs dot-in-wire structures, using cylindrical light-emitting heterostructures that protrude above the flexible platform, separating the active light-emitting region from the bending substrate. Here, we demonstrate the optical enhancement of nanowire light emitters by changing the geometric orientation within a 1 × 1 mm2 array of nanowires on a flexible platform through bending of the substrate platform. The flexible structures were achieved by transferring GaN nanowire LEDs from sapphire substrates onto flexible polyethylene terephthalate (PET) using a “paste-and-cut” integration process. The I–V characteristics of the nanowire LEDs showed negligible change after integration onto the PET, with a turn-on voltage of 2.5 V and a forward current of 400 μA at 4 V. A significant advantage for the nanowire devices on PET was demonstrated by tilting the LEDs through substrate bending that increased the electroluminescence (EL) intensity, while the I–V characteristics and the EL peak position remained constant. Through finite-element analysis and three-dimensional finite-difference time-domain modeling, it was determined that tilting the protruding devices changed the effective distance between the structures, enhancing their electromagnetic coupling to increase light output without affecting the electrical properties or peak emission wavelength of the LEDs.

InAlN-based LEDs emitting in the near-UV region

Fully functional InAlN-based ultraviolet LEDs emitting at 340–350 nm were demonstrated for the first time; detailed electrical and optical characterization is presented and discussed. Results from the measurements at pulsed conditions are in agreement with the attribution of the dominant electroluminescence peak to near-band-edge emission. The composition of the AlGaN barriers was chosen to give the same internal polarization field as that of the InAlN wells. A simulation study of this polarization-matched heterostructure shows a significant increase in the electron-hole overlap integral if compared with a standard AlGaN/AlGaN active region having the same level of carrier confinement. Limitations and problems of these preliminary devices are also presented, and possible future work aimed at increasing their efficiency is discussed.

Engineering Color-Stable Blue Light-Emitting Diodes with Lead Halide Perovskite Nanocrystals.

Nanocrystalline lead halide perovskites are promising as emissive layers for light-emitting diodes due to their bright, tunable emission with very narrow linewidths. Blue perovskite light-emitting diodes, in the wavelength range useful for display applications (460-470 nm), could be made with CsPb(Br/Cl)3 nanocrystals (NCs). However, mixed halide perovskites suffer from color instability, foremost, due to the segregation of halide ions. In this study, we address this issue with several measures. First, we show that thinner CsPb(Br/Cl)3 NC layers are less prone to color instability. Additionally, inefficient hole injection due to the deep-lying valence band of CsPb(Br/Cl)3 NCs detrimentally affects the device performance, and we mitigate this problem by stepwise hole injection using two hole-transporting materials. Next, we employ NCs capped with zwitterionic ligands that allow for a more thorough washing of the NC solutions. Furthermore, our new device layout explores the use of polystyrene in the emitting layer to limit the current leakage. Undertaking these steps, we show light-emitting diodes with a stable electroluminescence peak wavelength of 463 nm over the lifetime of the device and a peak external quantum efficiency of over 1%. The results prove that perovskite NCs are a viable contender in the development of blue-emissive, active pixel displays.

Large Area, High Performance and Stable Perovskite Light Emitting Diodes

Large area, high performance and stable CsPbBr3 perovskite light emitting diodes (PeLEDs) were demonstrated by readily commercializable spray-coating method. The PeLEDs with controlled crystal grain size exhibited maximum electroluminescence peak position of ~534 nm, emission spectrum of ~18 nm of full width at half maximum, maximum luminance of 20388 cd m-2, maximum current efficiency of 51.0 cd A-1, and maximum external quantum efficiency of 11.19%. The spray-coated PeLED exhibited ~3 fold longer average half life-time (> 22000 h) at 100 cd m-2 than the spin-coated one (> 7000 h) because it has lower trap density and more uniform perovskite film than the spin-coated one and consequently it reduces the temperature elevation of device. The large area glass substrate-based rigid PeLED and polymer substrate-based flexible PeLED were also successfully demonstrated. In addition, the PeLEDs with controlled crystal grain size could generate electric power in a daytime like solar cells, which gained power conversion efficiency of 0.43-1.03% at 1 sun condition (AM1.5G 100 mW/cm2).

Direct observation of state-filling at hybrid tin oxide/organic interfaces

Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. Here, we combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer [ladder type poly-(para-phenylene)], we find a one-to-one correspondence between the blueshift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which we unambiguously assign to state filling. Our data are resembled best by a model considering the combination of an exponential density of states with a doped semiconductor.

Two thermally stable and AIE active 1,8-naphthalimide derivatives with red efficient thermally activated delayed fluorescence

Red thermally activated delayed fluorescence (TADF) emitters with aggregation-induced emission (AIE) properties have been developed, based on strong electron-with-drawing 1,8-naphthalimide (NI) acceptor (A). With strong electron-rich 10-H-phenothiazine (Pz) or 9,9-dimethyl-9,10-dihydroacridine (DMAC) groups as electron donors (D), 2,6-dimethylphenyl (DM) as a π-bridge, these highly twist D-π-A architecture NI derivatives showed typical AIE feature and excellent solid-state TADF emission with high photoluminescence quantum yield (PLQY) up to 55%. Moreover, the NI derivatives exhibited excellent thermal morphological stability in their solid amorphous states, providing a stable EML for practical applications. Finally, a high external quantum efficiency (EQE) of 7.13% from Pz-DMNI, was achieved in the red OLEDs device with electroluminescence (EL) peak of 635 nm.

Electroluminescence Dynamics in Perovskite Solar Cells Reveals Giant Overshoot Effect.

High performance of both photovoltaic and electroluminescent devices requires low nonradiative recombination losses. In perovskites, such loses strongly depend on the carrier traps related to the mobile ions and vacancies, causing I- V hysteresis of solar cells and influencing the performance of other optoelectronic devices, such as photodetectors and LEDs. To address the dynamics of the mobile ions, here we investigate electroluminescence time evolution in perovskite solar cells under constant and pulsed voltage conditions. We propose a model, accounting for the spatial ion accumulation and explaining the complex electroluminescence dynamics both on fast (microseconds) and slow (seconds) time scales. We demonstrate the appearance of a high-intensity short electroluminescence peak (overshoot pulse) immediately after termination of the electrical pulse. The generation of a giant overshoot pulse suggests a simple way to achieve high pulsed luminescence intensity with a low current density, which opens new prospects toward optical gain and implementation of electrically pumped lasers.

Direct Evidence of Ion-Migration-Induced Degradation of Ultrabright Perovskite Light-Emitting Diodes.

Low operational lifetime is a critical issue in perovskite light-emitting diodes. The forward-bias currents for light emission accelerate device degradation, which needs to be identified and understood to be able to improve the device stability. Here, we systematically analyze the degradation mechanism of perovskite light-emitting diodes (PeLEDs) fabricated with a sequential deposition method that produce a compact and pinhole-free perovskite film. The device exhibits an efficient green electroluminescence (peak wavelength at 533 nm and full width at half-maximum of 22 nm) with a maximum luminance of more than 67 000 cd/m2. The lifetime, however, is quite short; under the constant current bias for an initial luminance of 1000 cd/m2, the decay time to reach half of the initial luminance is approximately 13 min. Dark spots are created and enlarged as a result of perovskite film deterioration and ion migration. By investigating morphological changes in the perovskite films and the amount of ion accumulation under the Al electrode for the unoperated, T50 (luminance decay to 50% of the initial value), and T10 (luminance decay to 10% of the initial value) devices, we propose a degradation mechanism for PeLEDs. The ion migration from the perovskite layer experienced electrochemical interactions with the Al electrode, causing device degradation.

Investigation of sintering temperature and Ce3+ concentration in YAG:Ce phosphor powder prepared by microwave combustion for white-light-emitting diode luminance applications

Abstract Cerium-doped transparent Y3Al5O12 (YAG:Ce3+) is the most widely used phosphor for converting blue to white light in light-emitting diodes (LEDs). Here, YAG:Ce3+ powders were synthesised through microwave-assisted combustion by using organic fuel urea. In YAG system applications, the powder structure and properties play key roles in the performance of terms of white LEDs (WLEDs). Accordingly, this study aimed to determine the effects of thermal treatment and Ce3+ concentration on the structure, morphology, chemical composition and luminescence properties of YAG powders. A direct conversion from the monoclinic and hexagonal phases to the cubic one was achieved at >850 °C. This conversion was accompanied by increased particle size in the range of 50–100 nm and decreased lattice strain. However, powder sintering at high temperature was required to incorporate Ce3+ into the YAG matrix and eliminate residual impurity phases. The highest electroluminescence (EL) emission was achieved for the sintered powders at 1050 °C with low dopant concentration (2.0 mol%), offering daylight WLED with a correlated colour temperature of 6834 K and corresponding colour-rendering index of 68.2. Finally, a slightly decreased intensity and red-shift in the EL peak position were observed. These findings were attributed to the presence of a separated CeO2 cubic phase (Ce4+) and concentration quenching for YAG ceramics with >2.0 mol% cerium content.

Tunable Deep-Red Electroluminescence From Flexible Quasi-2D Perovskites Light-Emitting Diodes

In this letter, we synthesize and characterize a new series of organic/inorganic hybrid quasi-2D perovskites PEA 2(FA)n–1PbnI3n+1 ( $n = 2, 3, 5$ ). The quantum well thickness of perovskites could be adjusted by regulating the crystal structure, which resulted in a tunable bandgap and emission spectra. The deep-red emissions from the flexible light-emitting diodes have been successfully achieved. The optimal device presents a maximum luminance of 212 cd/m2, a maximum current efficiency of 0.44 cd/A, and a maximum external quantum efficiency of 0.148% with an electroluminescent spectrum peak of 720 nm and the corresponding CIE coordinates of (0.691, 0.291).

Synthesis and Characteristics of Organic Red-Emissive Materials Based on Phenanthro[9,10-d]imidazole.

Red emission is one of the three primary colors that are essential for the realization of full-color displays and solid-state lightings. A high solid-state efficiency is a crucial factor for the applications in organic light-emitting diodes (OLEDs). In this work, two new donor-acceptor-donor type phenanthro[9,10-d]imidazole (PIM)-based derivatives, (2Z,2'Z)-2,2'-(1,4-phenylene)bis(3-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)acrylonitrile) (PIDSB) and 2,3-bis(4'-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1'-biphenyl]-4-yl)fumaronitrile (PIDPh), are designed and synthesized. Both of them possess high thermal stabilities. PIDPh shows typical characteristics of aggregation-induced emission enhancement, while PIDSB displays an aggregation-caused quenching effect. They both exhibit significant red-shifted emissions compared with PIM owing to intramolecular charge transfer. In the film state, the emission peaks of PIDSB and PIDPh are located at 538 nm and 605 nm with high photoluminescent quantum yields of 63.82 % and 41.26 %, respectively. The non-doped OLED using PIDPh as the active layer shows the maximum external quantum efficiency of 2.06 % with a very low efficiency roll-off, and exhibits the electroluminescent peak at 640 nm with a Commission Internationale de l'Éclairage coordinate of (0.617,0.396), meeting well the criteria of red OLEDs.

Electroluminescence and photo-response of inorganic halide perovskite bi-functional diodes

Abstract In this work, we report our efforts to develop a novel inorganic halide perovskite-based bi-functional light-emitting and photo-detecting diode. The bi-functional diode is capable of emitting a uniform green light, with a peak wavelength of 520 nm, at a forward bias of >2 V, achieving a high luminance of >103 cd/m2 at 7 V. It becomes an efficient photodetector when the bi-functional diode is operated at a reverse bias, exhibiting sensitivity over a broadband wavelength range from ultraviolet to visible light. The bi-functional diode possesses very fast transient electroluminescence (EL) and photo-response characteristics, e.g. with a short EL rising time of ~6 μS and a photo-response time of ~150 μS. In addition, the bi-functional diode also is sensitive to 520 nm, the wavelength of its peak EL emission. The ability of the bi-functional diodes for application in high speed visible light communication was analyzed and demonstrated using two identical bi-functional diodes, one performed as the signal generator and the other acted as a signal receiver. The dual functions of light emission and light detection capability, enabled by bi-functional diodes, are very attractive for different applications in under water communication and visible light telecommunications.

Stark effect of hybrid charge transfer states at planar ZnO/organic interfaces

We investigate the bias-dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photo-doping allow us to confirm that this bias-induced spectral shifts relate to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.

Efficiency Improvements in AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes with Graded Superlattice Last Quantum Barrier and Without Electron Blocking Layer

The characteristics of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) with electroluminescence peak wavelength of emission about 289 nm have been investigated by optimizing the last quantum barrier (LQB) in the active region. The results demonstrate that the internal quantum efficiency and radiative recombination rate of DUV-LEDs with a graded superlattice last quantum barrier (GSL LQB) and without an electron blocking layer (EBL) are higher than for other structures under current of 180 mA. Also, the electron and hole leakage currents are reduced for the GSL LQB structure. This structure contributes to effective electron confinement and hole injection owing to increased overlap of electron and hole wavefunctions resulting from low electrostatic fields in the active region. As a result, the optical output power of the structure with a GSL LQB and without an EBL is increased by 1.62 times, and the spontaneous emission intensity by 1.56 times, compared with the conventional structure.

Inverted Device Architecture for Enhanced Performance of Flexible Silicon Quantum Dot Light-Emitting Diode.

Here we report for the first time highly flexible quantum dot light-emitting diodes (QLEDs), in which a layer of red-emitting colloidal silicon quantum dots (SiQDs) works as the optically active component, by replacing a rigid glass substrate with a thin sheet of polyethylene terephthalate (PET). The enhanced optical performance for electroluminescence (EL) at room temperature in air is achieved by taking advantage of the inverted device structure. Our QLEDs do not exhibit parasitic EL emissions from the neighboring compositional layers or surface states of QDs over a wide range of driving voltages and do not exhibit a shift in the EL peak position as the operational voltage increases. Compared to the previous Si-QLEDs with a conventional device structure, our QLED has a longer device operational lifetime and a long-lived EQE value. The currently obtained brightness (∼5000 cd/m2), the 3.1% external quantum efficiency (EQE), and a turn-on voltage as low as 3.5 V are sufficiently high to encourage further developments of Si-QLEDs.

Determining External Quantum Efficiency From Energy Conversion Efficiency for Light-Emitting Diodes

We propose a convenient method for estimating external quantum efficiency (EQE) of light-emitting diode directly from the energy conversion efficiency, based on two facts. One lies in that the EQE approximately reaches its maximum when the power source provides the amount of energy that equals the electroluminescence peak energy ( ${E}_{\textsf {ELp}}$ ) to every electron. For the other, an electron carrying higher electrical energy than ${E}_{\textsf {ELp}}$ will release its excessive portion to lattice vibration during radiative recombination, thus emitting a photon with the energy around ${\text E}_{\textsf {ELp}}$ . In comparison with traditional methods, our approach maintains a decent accuracy of mostly 0.4%.

Theoretical study of electroluminescence from device based on silicon nanocrystals

In this work, we have proposed a theoretical model allowing the calculation of electroluminescence (EL) from a light-emitting device, taken from the literature, composed of a silicon nitride film (containing silicon nanocrystals (Si-ncs)) between an indium thin oxide (ITO) layer and an aluminum (Al) anode.The EL model suggests that following the application of an electric field, carriers are injected by the Fowler-Nordheim process. The carriers transporting process to the cathode is modeled by the Hopping conduction and causing the generation of other carriers inside Si-ncs by ionic impact. The EL is created by the recombination of the carriers inside Si-ncs.Results exhibit that selective size distribution centered at a Si-nc radius of less than 1 nm allows blue emission for thin active film. The optimal carrier injection is obtained for bias voltages lower than 27 V that allowed the potential barrier diminution.A comparison between the experimental and the theoretical results indicated that the present model is able to satisfactorily explain the highest observed EL peak.

Nanoscale Electronic Conditioning for Improvement of Nanowire Light-Emitting-Diode Efficiency.

Commercial III-Nitride LEDs and lasers spanning visible and ultraviolet wavelengths are based on epitaxial films. Alternatively, nanowire-based III-Nitride optoelectronics offer the advantage of strain compliance and high crystalline quality growth on a variety of inexpensive substrates. However, nanowire LEDs exhibit an inherent property distribution, resulting in uneven current spreading through macroscopic devices that consist of millions of individual nanowire diodes connected in parallel. Despite being electrically connected, only a small fraction of nanowires, sometimes <1%, contribute to the electroluminescence (EL). Here, we show that a population of electrical shorts exists in the devices, consisting of a subset of low-resistance nanowires that pass a large portion of the total current in the ensemble devices. Burn-in electronic conditioning is performed by applying a short-term overload voltage; the nanoshorts experience very high current density, sufficient to render them open circuits, thereby forcing a new current path through more nanowire LEDs in an ensemble device. Current-voltage measurements of individual nanowires are acquired using conductive atomic force microscopy to observe the removal of nanoshorts using burn-in. In macroscopic devices, this results in a 33× increase in peak EL and reduced leakage current. Burn-in conditioning of nanowire ensembles therefore provides a straightforward method to mitigate nonuniformities inherent to nanowire devices.

Boosting the Efficiency of Near‐Infrared Fluorescent OLEDs with an Electroluminescent Peak of Nearly 800 nm by Sensitizer‐Based Cascade Energy Transfer

AbstractA sensitization‐based cascade energy transfer channel is proposed to boost the electroluminescent performances of the solution‐processed near‐infrared organic light‐emitting devices (OLEDs) featuring an electroluminescent peak of 786 nm from a new fluorescent emitter of N4,N4,N9,N9‐tetra‐p‐tolylnaphtho[2,3‐c][1,2,5]thiadiazole‐4,9‐diamine (NZ2mDPA) with unique aggregation‐induced emission (AIE) property. The optimized device is composed of 4,4′‐N,N‐dicarbazole‐biphenyl (CBP) as the host, bis(2‐phenyl‐1,3‐benzothiozolato‐N,C2′)iridium (Ir(bt)2(acac)) as the sensitizer, and NZ2mDPA as the emitter, where the cascade energy transfer can occur via two steps realizing unexpected triplet–singlet energy transfer by the Förster mechanism. The first step features efficient triplet harvesting from CBP to Ir(bt)2(acac), and then the second step involves in resonant energy transfer from the phosphorescent sensitizer to the near‐infrared AIE emitter of NZ2mDPA, which finally endows two channels of harvesting singlet and triplet excitons. The unique scheme achieves not only more efficient Förster energy transfer but also the higher utilization efficiency of triplet excitons. As a result, the near‐infrared OLEDs can realize a factor of 2.7 enhancement of external quantum efficiency by employing the phosphor‐sensitized AIE lumogen compared with the commonly used binary host–guest system.

Ultraviolet light-emitting diodes and photodiodes grown by plasma-assisted molecular beam epitaxy

We demonstrate a Schottky ultraviolet photodiode (UV-PD) and a UV light-emitting diode (UV-LED) based on AlGaN heterostructures grown by plasma-assisted molecular beam epitaxy. The spectral responsivity of the Schottky UV-PD is 3 mA/W at 271 nm at zero bias and decreases by two orders of magnitude for the spectral range of longer wavelengths ( > 300 nm). The sub-monolayer digital alloying technique was used for growing the AlGaN quantum wells in UV-LEDs emitting a single electroluminescence peak at 273 nm.