Electroluminescence Intensity Research Articles

Influence of well position on the electroluminescence characteristics of InGaN/GaN single quantum well red light-emitting diodes

In this paper, the influence of the position of InGaN layer well on the electroluminescence (EL) properties of red InGaN/GaN single quantum well (SQW) light-emitting diodes (LEDs) is investigated numerically. It is found that as the InGaN well shifts from the P-region to the N-region, the LED's EL intensity decreases, and the spectrum red-shifts. By analyzing the EL spectra, energy band structures and carrier distribution, it is considered that, when the well position moves away from the P-region, the injection barrier width of holes increases, while that of electrons decreases. Thus, the injection efficiency of electrons and holes is enhanced and weakened, respectively. Compared with electrons, the injection efficiency of holes with larger effective mass can be significantly reduced by a thicker barrier. As a result, the hole concentration in InGaN QW is reduced more severely, leading to a reduction in the total amount of carriers. The decreased concentration of injected carriers may weaken the carrier screening effect to the polarization electrical field, thereby the polarization-induced quantum confined-Stark effect is enhanced. As a consequence, for the SQW red LED whose InGaN well is far away from the P-region, the EL intensity of the SQW red LED is reduced and the peak wavelength becomes longer.

Carbon Nitride with Rationally Designed π-Conjugated Structure for Bright Blue-Violet Light-Emitting Diodes.

Carbon nitride consisting of the broken π-conjugated structure (bc-CN) is designed as the emitting layer in a blue-violet light emitting diode (LED). The bc-CN is prepared by a metal-oxide (MgO) template-assisted method, in which the low reaction temperature and nano MgO jointly control the degree of polymerization to form cyano groups and broken π-conjugation in the bc-CN nanosheets (bc-CN NS) which emit intense blue-violet photoluminescence at 412nm. The broken π-conjugated heptazine-ring structure in the bc-CN NS mitigates non-radiation energy loss and promotes the d*-LP transition. As a result, a high quantum efficiency of 73.1% is achieved. The excellent dispersing ability of the bc-CN NS enables solution-based fabrication of the light emitting diode (LED). The LED exhibits intense electroluminescence of 236cd m-2 at 412nm with an external quantum efficiency of 0.46%. The broken π-conjugation modulates the optical properties of the polymerized carbon nitride semiconductor giving rise to intense blue-violet electroluminescence, which is very desirable for printable and wide-color-gamut display devices.

Observation of distributed feedback lasing in silicon nanocrystals under electrical pumping

Electrically pumped distributed feedback (DFB) lasing in silicon nanocrystals (SiNCs) has been observed. The DFB SiNC laser was made based on a SiNC light-emitting diode (LED) by incorporating a second-order DFB grating. The light emission covered the visible regime. Optical gains in the order of 102 cm−1 were found in the light emitting regime. The DFB grating was so designed and fabricated that lasing wavelength in the red regime was selected. Signals emitting from the surface of the DFB SiNC laser were recorded. The laser was pumped by a pulsed direct-current power supply at room temperature. Four typical criteria of lasing were identified, i.e., 1) threshold behavior of the electroluminescence (EL) intensity versus pumping power density, 2) evident spectral narrowing of the EL spectrum, 3) well-recognized degree of polarization, and 4) small spread of light emitting angle. The latter three were for the pumping power density beyond the threshold one.

On the accurate characterization of quantum-dot light-emitting diodes for display applications

Quantum dot light-emitting diodes (QLEDs) are a class of high-performance solution-processed electroluminescent (EL) devices highly attractive for next-generation display applications. Despite the encouraging advances in the mechanism investigation, material chemistry, and device engineering of QLEDs, the lack of standard protocols for the characterization of QLEDs may cause inaccurate measurements of device parameters and invalid comparison of different devices. Here, we report a comprehensive study on the characterizations of QLEDs using various methods. We show that the emission non-uniformity across the active area, non-Lambertian angular distributions of EL intensity, and discrepancies in the adopted spectral luminous efficiency functions could introduce significant errors in the device efficiency. Larger errors in the operational-lifetime measurements may arise from the inaccurate determination of the initial luminance and inconsistent methods for analyzing the luminance-decay curves. Finally, we suggest a set of recommended practices and a checklist for device characterizations, aiming to help the researchers in the QLED field to achieve accurate and reliable measurements.

Highly efficient and thermally stable red emitting CaGdMgNbO6:Mn4+ double perovskites for indoor plant growth LEDs

Nowadays, more and more attention has been paid to artificial lighting sources for plant growth. In this paper, transition metal Mn4+ activated CaGdMgNbO6 (CGMNO:Mn4+) red phosphor was prepared high temperature solid state method. The crystal structure and cell parameters of novel CGMNO host were analyzed. The experimental results indicated that CGMNO:Mn4+ phosphor exhibit an emission band (701 nm), which belongs to the 2Eg→4A2g transition of Mn4+. The concentration quenching occurs when Mn4+ doping concentration is greater than 0.5 mol%. The variable temperature emission spectra proved that the emission intensity of CGMNO:0.005Mn4+ at 423 K maintained 75.2 % of that at 298 K. The CIE color coordinates and IQE of CGMNO:0.005Mn4+ are (0.7136, 0.2863) and 49.45 %, respectively. Finally, a LED device was packaged with CGMNO:0.005Mn4+ phosphor and a high-power chip (440 nm), the LED device shows stable electroluminescence intensity under different currents driving. All the results indicate that phosphors are suitable for lighting contemporary plant growth.

Improving the photovoltaic properties of GaAs/GaAsBi pin diodes by inserting a compositionally graded layer at the hetero-interface

We investigated the effect of inserting a compositionally graded layer at the GaAsBi/GaAs interface on the photovoltaic and light-emission properties of GaAs/GaAsBi pin diodes. When the pin diode is operated as a solar cell, inserting a graded layer improves the open-circuit bandgap–voltage offset (W oc) to 0.51 V. This is comparable to or better than other materials—such as GaInNAs(Sb), which has a bandgap of 1.0 eV—that are expected to be used in multijunction solar cells. In contrast, W oc can be as large as 0.71 V for a pin diode without a graded layer. When it is operated as a light-emitting diode, inserting a graded layer in such a diode suppresses nonradiative recombination by a factor of 1/50 based on its electroluminescence intensity. Inserting a graded layer also makes it possible to avoid deterioration of the peculiar hetero-interface where the transition from the non-metallic nature of GaAs to the metallic nature of GaAsBi occurs. Moreover, the graded layer is effective in avoiding a pile-up of oxygen at the interface at low temperatures when growth is interrupted just before growing the GaAsBi layer. Thus, inserting a graded layer is the key to improving the performance of minority-carrier devices containing GaAsBi.

Enhancing the Performance of Perovskite Light-Emitting Diodes by Humidity Treatment.

While humidity treatment has been employed for enhancing the performance of perovskite solar cells and light-emitting diodes (LEDs), only very limited success has been achieved in quasi-two-dimensional (2D) perovskite LEDs (PeLEDs). Here, for the first time, we demonstrate more than one order of magnitude enhancement of the external quantum efficiency (EQE) and electroluminescence (EL) intensity in blue CsPb(Cl/Br)3 PeLEDs with an organic cation of 2,2-(ethylenedioxy)bis(ethylammonium) (EDBE). Upon humidity treatment, the crystallinity of the three-dimensional (3D) perovskite phase in the EDBE-based perovskite is improved, contributing to an enhancement of photoluminescence quantum yield (PLQY). This work suggests that elaborately modulating the molecular structure of large cations under humidity treatment can serve as an effective strategy to improve the performance of quasi-2D PeLEDs.

Improved transient electroluminescence technique based on time-correlated single-photon counting technology to evaluate organic mobility

The transient electroluminescence (EL) technique is widely used to evaluate the carrier mobility in the field of organic light emitting diodes. The traditional analog detection strategy using oscilloscopes is generally limited since the background noise causes an underestimation of the mobility value. In this paper, we utilize time-correlated single-photon counting (TCSPC) to probe the transient EL for mobility calculation. The measurements on tris(8-hydroxyquinoline) aluminum (Alq3) show that the electron mobilities obtained using the TCSPC technique are slightly higher than those obtained from the analog method at all the investigated voltages. Moreover, the TCSPC mobilities demonstrate weaker dependence on the root of electrical field compared to the oscilloscope mobilities. These improvements are attributed to the unique principle of TCSPC, which quantifies the EL intensity by counting the number of single-photon pulses, improving its single-photon sensitivity and eliminating the negative impacts of electrical noise. These advantages make TCSPC a powerful technique in the characterization of time-resolved electroluminescence.Graphical

One-pot hydrothermal synthesis of fluorescent carbon quantum dots with tunable emission color for application in electroluminescence detection of dopamine

Existing carbon quantum dot (CQD) synthesis approaches for the detection of dopamine (DA) require additional processes for separating quantum dots, which makes them complex and decreases the yield. Therefore, a facile CQD synthesis approach without complicated processing steps or reaction conditions is required. In this study, CQDs were synthesized via an environment-friendly one-step hydrothermal approach using glucose and potassium phosphate as the starting materials. The fluorescent colors emitted by the synthesized CQDs were dependent on the amount of potassium phosphate used. The mechanism of CQDs emitting different colors of fluorescence was theoretically speculated by computational simulation. The electronic and optical properties of CQDs models with different sizes are studied by density functional theory (DFT). An electroluminescence (ECL) sensing platform based on the fluorescent CQDs was developed for the detection of DA. Under optimal conditions, the ECL intensity of the CQDs decreased linearly when the DA concentration was controlled within the range of 10−8–10−4 M. The results showed that the ECL sensing platform can be successfully applied for the determination of DA content within a certain concentration range, with a detection limit of 62 nM.

Green electroluminescence of Al2O3 film

Silicon-based Al2O3 film electroluminescent devices were fabricated by using vacuum electron beam vapor deposition technique. The thin films were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and UV–visible grating spectrometer. The electroluminescence spectrum of the device shows two emission peaks in the visible region, the main peak in the green region (503 nm) and the sub-peak in the red region (680 nm). When a forward bias voltage of about 10V is applied, the luminescence of the device is clearly visible to the naked eye in a dark room. And when the forward bias voltage increases to 30V, the luminescence intensity reaches the maximum. In addition, the experiment found that when the annealing temperature of the Al2O3 film is higher than 600 °C, the electroluminescent intensity of the Al2O3 film device gradually deteriorates, until the annealing temperature is higher than 800 °C, the electroluminescence of Al2O3 film devices disappears completely. This work provides a basis for the application of Al2O3 thin film in the field of electroluminescence.

Influence of Charge Injection on Dynamic Characteristics of Space Charge in Polymer Thin Films Under AC Stress

At present, most studies focused on the formation and accumulation of space charge under DC stress, relatively few studies focused on dynamic characteristics of space charge under AC stress. However, space charge dynamics under AC stress is of significance because the majorities of polymer insulation systems are subjected to AC voltage. In order to study the effect of charge injection mechanism from the electrodes on dynamic characteristics of space charge under AC stress, bipolar charge transport model are used to simulate the current density, electroluminescence intensity, space charge densities in polyethylene with different charge injection mechanisms. It is found that the simulated electroluminescence intensity and space charge densities under AC stress assuming the exponential functional approximation injection is more reasonable than those assuming the Schottky injection.

Silver-doped nickel oxide as an efficient hole-transport layer in polymer light-emitting diodes.

In this study, silver-doped nickel oxide (NiO:Ag) was successfully synthesized by a sol-gel method and spin-coated on indium titanium oxide (ITO) as a hole-transport layer for polymer light-emitting diodes (PLED). After the calcination of the NiO:Ag/ITO substrate at 300°C for 1 h, stable conductive regions and the mean work-function on the NiO:Ag/ITO surface reached 89.43% and 5.53 eV, respectively, which were greater than those on a conventional poly [3,4-ethylenedioxythiophene] polystyrene sulfonate (PEDOT:PSS)/ITO surface. When NiO:Ag (300°C)/ITO was used as an anode window substrate for PLEDs, the enhancement factor for the average current efficiency in the current-density range of 20-50 mA/cm2 and electroluminescence intensity at an applied bias of 8.0V were 4.60 and 2.55 times, respectively, in comparison with those of PLED based on a conventional PEDOT:PSS/ITO anode. HIGHLIGHTS: NiO:Ag is synthesized by a sol-gel method and spin-coated on ITO as a HTL for PLED. NiO:Ag/ITO calcined at 300°C for 1h has the best microscopic electrical properties. The performance for proposed PLED is much better than that for typical PLED.

A Waveguide-Integrated Two-Dimensional Light-Emitting Diode Based on p-Type WSe2/n-Type CdS Nanoribbon Heterojunction.

Transition metal dichalcogenides (TMDs) have emerged as two-dimensional (2D) building blocks to construct nanoscale light sources. To date, a wide array of TMD-based light-emitting devices (LEDs) have been successfully demonstrated. Yet, their atomically thin and planar nature entails an additional waveguide/microcavity for effective optical routing/confinement. In this sense, integration of TMDs with electronically active photonic nanostructures to form a functional heterojunction is of crucial importance for 2D optoelectronic chips with reduced footprint and higher integration capacity. Here, we report a room-temperature waveguide-integrated light-emitting device based on a p-type monolayer (ML) tungsten diselenide (WSe2) and n-type cadmium sulfide (CdS) nanoribbon (NR) heterojunction diode. The hybrid LED exhibited clear rectification under forward biasing, giving pronounced electroluminescence (EL) at 1.65 eV from exciton resonances in ML WSe2. The integrated EL intensity against the driving current shows a superlinear profile at a high current level, implying a facilitated carrier injection via intervalley scattering. By leveraging CdS NR waveguides, the WSe2 EL can be efficiently coupled and further routed for potential optical interconnect functionalities. Our results manifest the waveguided LEDs as a dual-role module for TMD-based optoelectronic circuitries.

AlGaN multiple quantum well deep‐ultraviolet micro‐light‐emitting diodes for high color conversion efficiency quantum dots display

AbstractDisplays based on inorganic micro‐light‐emitting diodes (micro‐LEDs) are highly anticipated for next‐generation technologies. AlGaN‐based deep‐ultraviolet (deep‐UV) micro‐LED as the excitation source for quantum dots display with high efficiency was reported in this paper. To achieve optimized electro‐optical performance, deep‐UV micro‐LEDs with different electrodes were fabricated and analyzed in sizes from 200 × 200 to 10 × 10 μm2. At the same forward bias, the devices with Ti/Al‐based electrodes achieved 10 times injection current and three times electroluminescence intensity than those with Cr/Al‐based electrodes. The blueshift phenomenon of deep‐UV light was observed from 292 nm at 2 A/cm2 to 287 nm at 200 A/cm2 with the increasing current density. By the excitation of deep‐UV micro‐LED, quantum dot film achieved high light conversion efficiency and optimized color rendering, as the converted color emission peak was separate from the pumping source. The high energy of deep‐UV photons and the narrow emission bandwidth of QDs resulted in prominent color purity. The forward voltage and electroluminescence intensity uniformity of a 250 × 250 micro‐LED array with each pixel size of 30 × 30 μm2 were further discussed. The optical microscope images of green QD film pumped by a deep‐UV micro‐LED demonstrated its competitive application in the color‐converted display.

Enhancing the Performance of Quasi-2D Perovskite Light-Emitting Diodes Using Natural Cyclic Molecules with Distinct Phase Regulation Behaviors

In this study, two natural small molecules, α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD), are used as additives to improve the performance of quasi-2D PEA2Csn-1PbnBr3n+1 (n = 3, herein) PeLEDs. Both of them are shown to efficiently passivate the quasi-2D perovskite films to afford improved film quality and morphology, but they exhibit distinct phase regulation behaviors possibly due to their different pore sizes. It reveals that α-CD effectively suppresses the formation of the low-n phases (n ≤ 2), while β-CD better regulates the phase with a medium-n value (n = 3). Because of effectively suppressing the formation of low-n phases, the CD-assisted quasi-2D perovskite films possess facilitated exciton energy transfer and reduced nonradiative recombination. Consequently, the optimized α-CD-derived PeLED shows the highest luminance (Lmax) of 37,825 cd/m2 with an external quantum efficiency (EQE) of 3.81%, while the β-CD-derived PeLED delivers a lower Lmax of 24,793 cd/m2 with an EQE of 3.09%. Compared to the pristine device, Lmax is enhanced by 6.3 and 3.8 times for α-CD- and β-CD-based PeLEDs, respectively, and EQE is enhanced by ∼4.8 times for both devices; besides, both CD-assisted devices also exhibit improved color purity and a lower bias dependency of electroluminescent intensity.

Low‐Energy UV Ultrafast Laser Controlled Lift‐Off for High‐Quality Flexible GaN‐Based Device

AbstractA one‐step laser lift‐off (LLO) for patterned gallium nitride (GaN) film and GaN‐based light‐emitting diode (LED) device is achieved using 355 nm picosecond laser irradiation in this research. The laser fluence required for separation is 0.09–0.13 J cm−2, which is much lower than that for the currently reported LLO methods. The separated GaN film is intact with only 0.04 GPa of residual stress. The ultra‐smooth separated surface with root mean square roughness of only 5.2 nm is attributed to the interconnection of microcrack‐free flat cavities formed by the combination of high photon energy‐induced intrinsic absorption and subsequent plasma generation. The flat cavity with a depth‐to‐width ratio of 1:4000 limits the delamination region to a few nanometers at the GaN/sapphire interface. GaN‐based LED is transferred with perfect electroluminescence (EL) by the strategy. The stable EL spectral peak positions and intensity independent of the bending state prove that the presented low‐energy ultrafast LLO technique ensured the flexibility of the separated LED device without affecting the performance. This research provides a promising strategy to achieve the LLO of GaN devices with low energy consumption, high controllability, and high efficiency, which is significant for the industrial fabrication of flexible GaN‐based electronics.

Investigation of methods for texturing light-emitting diodes based on AlGaAs/GaAs heterostructures

Investigations of methods for texturing the light-emitting surface of IR light-emitting diodes (LEDs) (wavelength 850 nm) based on AlGaAs/GaAs heterostructures with Bragg reflectors have been carried out. Developed were methods of liquid and plasma-chemical etching of solid solution for creating peaks (pyramids) of different form, 0.2-1.5 μm height. Estimation of the effect of texturing methods and also configuration of peaks on the light-emitting diode electroluminescence intensity has been performed. The increase of the electroluminescence intensity by 25% has been achieved. Keywords: light-emitting diode, texturing, etching methods, electroluminescence.

Two-state lasing in injection microdisks with InAs/InGaAs quantum dots

Spectral dependencies of the electroluminescence intensity of a microdisk laser with a diameter of 31 μm with active region based on InAs/InGaAs quantum dots, operating in the continuous-wave regime, are investigated in a wide range of injection currents. Simultaneous lasing through the ground and excited states of quantum dots under intense excitation is demonstrated in injection microdisk laser for the first time. At low pumping powers lasing occurs via ground states of quantum dots only. Keywords: microlaser, quantum dots, two-state lasing, ground state, excited state.

Двухуровневая лазерная генерация в инжекционных микродисках на основе квантовых точек InAs/InGaAs

Spectral dependencies of the electroluminescence intensity of microdisk laser with a diameter of 31 µm with active region based on InAs/InGaAs quantum dots, operating in a continuous-wave regime, are investigated in a wide range of injection currents. Simultaneous lasing through the ground and excited states of quantum dots under intense excitation is demonstrated in injection microdisk laser for the first time. At low pumping powers lasing occurs via ground states of quantum dots only.

Steady-state and transient optoelectronic characteristics of styrene-and quinoline-based derivative

Styrene and quinoline groups are commonly incorporated into the organic fluorescent materials for organic light-emitting diodes (OLEDs). In this work, a type of small molecule derived from styrene and quinoline, with a chemical structure of 2,2'-(2,5-dimethoxy-1,4-phylenedivinylene)bis-8- acetoxyquinoline (MPV-AQ), is employed as the emitter and electron transporting material in the OLEDs, and its optoelectronic characteristics such as charge-carrier injection, transporting and recombination are investigated by the steady-state and transient technologies. It is found that the electron injection from the cathode into the MPV-AQ layer shows the Fowler-Nordheim (FN) tunneling characteristic in the N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB)/MPV-AQ bilayer OLED, which is different from the Richardson-Schottky (RS) thermionic emission in the electron-only device based on the MPV-AQ single-layer. The difference in electron injection is attributed to the bend of energy bands of MPV-AQ in the NPB/MPV-AQ device, which can be caused by the charge accumulation at the NPB/MPV-AQ interface. The accumulated charges should mainly be the holes on the side of NPB layer because the electron mobility of MPV-AQ is much lower than the hole mobility of NPB. Owing to the bending of lowest unoccupied molecular orbital (LUMO) of MPV-AQ, the tunneling distance for electrons is significantly reduced, which is favorable for the FN tunneling. The barrier height for electron injection is calculated to be 0.23 eV by fitting the current-voltage curve of the NPB/MPV-AQ bilayer OLED. And the electron mobility of MPV-AQ is determined by the delay time of transient electroluminescence (EL) and shows field-dependence with the value on the order of 10<sup>–6</sup> cm<sup>2</sup>/(V·s). In addition, the electron-hole recombination coefficient is obtained from the long time component of the temporal decay of the EL intensity, and the coefficient is found to decrease with the applied voltage increasing, which is consistent with the efficiency roll-off in this bilayer OLED. This study may provide a foundation for understanding the electronic processes of carrier injection, transport and recombination in the OLEDs, which is helpful in improving the device performance.