Maximum Internal Quantum Efficiency Research Articles

Developing an efficient garnet-type near-infrared phosphor Na1.5Gd1.5Sc2Ge3O12:Cr3+ with relatively long-wavelength emission.

Developing efficient near-infrared (NIR) phosphor with an emission peak wavelength over 850 nm is crucial to realize multi-functional spectroscopy applications, while this remains a significant challenge. Herein, a garnet-type Na1.5Gd1.5Sc2Ge3O12:Cr3+ phosphor with an emission peak located at 882 nm was created from NaδGdδCa3-2δSc2Ge3O12:Cr3+ (0 ≤ δ ≤ 1.5) solid solutions basing on the effect of cationic disorder. The maximum internal quantum efficiency (IQE) of Na1.5Gd1.5Sc2-xGe3O12:xCr3+ reached 84.6% when x = 0.01. The maximum external quantum efficiency (EQE) can be optimized to 20.6%. Using this material, a prototype NIR phosphor-converted light-emitting diodes (pc-LEDs) device was fabricated, which demonstrates an excellent prospect in spectroscopy applications.

Efficient and droop-free AlGaN-based UV-C LED using the inverted linearly graded active region and engineered hole source layer

The poor efficiency of the aluminum gallium nitride (AlGaN)-based ultraviolet (UV)-C light emitting diode (LED) remains as the major hindrance towards its commercialization. Thus, to enhance the internal quantum efficiency (IQE) and light output power (LOP) and to mitigate the efficiency droop of AlGaN-based UV-C LED, this simulation work proposes and examines a new engineering strategy, i.e., inverted linearly graded quantum wells along with quantum barriers over a series of samples A, B and C. To further improve the characteristics of AlGaN-based UV-C LED, an engineered linearly graded p-AlGaN layer has been proposed in the device structure. This increases the electron and hole concentrations by ∼2-fold and ∼8-fold, respectively. The 1.2-fold reduction in hole effective potential barrier height improves the hole injection. Also, the 1.5-fold increase in electron effective potential barrier height limits electron spill-off from the active region. The proposed structural-engineering reduces quantum confined stark effect (QCSE) confirmed by the reduction of quantum well slopes. Reducing carrier spill-off and QCSE increases average radiative recombination rate by ∼8-fold. The maximum IQE is improved by ∼44 % and the efficiency droop is also reduced to a promising level of ∼2 % in case of the sample under consideration. Finally, the LOP increases 10-fold, promisingly. Thus, the proposed structure is supposed to be highly potential for improving the UV-C LED performance. The electroluminescence spectra show that despite the gradual differences introduced in the structure, all the samples emit at ∼276 nm with similar full width at half maximum providing a reference for the comparison.

Advantages of AlGaN Tunnel Junction in N-Polar 284 nm Ultraviolet-B Light Emitting Diode

Smart, low cost and environmentally safe aluminum gallium nitride (AlGaN)-based ultraviolet-B light-emitting diodes (UV-B LEDs) are promising in real-world applications including medical as well as agricultural sciences. Higher efficiency droops, low hole injection efficiency, and high operating voltage are the key problems that AlGaN-based UV-B LEDs are facing. In this work, a smart and clean AlGaN-based UV-B LED at 284 nm emission wavelength has been studied. Here an approach is presented to electrically operate the quantum tunnelling probability by exploiting the transported carriers at the interface of p-AlGaN/n-AlGaN/n++-AlGaN tunnel junction (TJ) with moderate Si and Mg-doping levels and optimized thickness with the help of simulation study. The simulation results show that the Augur recombination rate is successfully suppressed and quite a high radiative recombination rate is achieved in the 284 nm N-polar AlGaN-based TJ UV-B LEDs, which is attributed to the improved hole injection toward the MQWs when compared to C-LED (conventional-LED). It is found that C-LED has a maximum IQE (internal quantum efficiency) of 40% under 200 A cm−2 injection current with an efficiency drop of 15%, while the TJ-LED has a maximum IQE of 93% with an efficiency droop of 0%. In addition, TJ-based AlGaN LED emitted power has been improved by 6 times compared to the C-LED structure. The emitted powers of TJ-LED increase linearly under varying current densities, whereas in the case of C-LED, the emitted power changes nonlinearly under varying current densities. This is attributed to the lower Augur recombination rate in the MQWs of N-AlGaN-based TJ UV-B LED. The operating voltages were reduced from 5.2 V to 4.1 V under 200 mA operation, which is attributed to the thickness and doping optimization in TJ and better selection of relatively lower Al-content in the contact layer. N-polar AlGaN-based TJ is explored for UV-B LEDs and the demonstrated work opens the door to epitaxial growth of high-performance UV emitters in MOCVD and MBE for a plethora of biomedical applications.

Temperature‐ and Excitation Power Density‐Resolved Photoluminescence of AlGaN‐Based Multiple Quantum Wells Emitting in the Spectral Range of 220–260 nm

Internal quantum efficiency (IQE) of AlGaN‐based multiple quantum wells (MQWs) on face‐to‐face‐annealed sputter‐deposited AlN templates is examined by photoluminescence spectroscopy. The excitation power density dependence of IQE is evaluated as a function of temperature under the selective excitation of the quantum wells. The temperature dependences of the maximum IQE and the corresponding excitation power density (EPD) are analyzed based on the rate equation models for carriers and excitons. The decrease of the maximum IQE and increase of the corresponding EPD are mainly due to the increase in the nonradiative recombination rate via nonradiative recombination centers. Furthermore, the nonradiative recombination rate for the MQW with an emission wavelength around 220 nm is activated at a lower temperature than the other samples, which is expected to lead to the lower IQE of the MQW with an emission wavelength around 220 nm.

Non-heavy doped pnp-AlGaN tunnel junction for an efficient deep-ultraviolet light emitting diode with low conduction voltage.

While traditional tunnel junction (TJ) light-emitting diodes (LEDs) can enhance current diffusion and increase hole injection efficiency, their reliance on highly doped AlGaN layers to improve hole tunneling efficiency results in a higher conduction voltage, adversely impacting LED device performance. This paper proposes a non-heavy doped pnp-AlGaN TJ deep ultraviolet (DUV) LED with a low conduction voltage. By inserting the TJ near the active region, between the electron blocking layer and the hole supply layer, the need for heavily doped AlGaN is circumvented. Furthermore, the LED leverages the polarization charge in the pnp-AlGaN TJ layer to decrease the electric field strength, enhancing hole tunneling effects and reducing conduction voltage. The non-heavy doped pnp-AlGaN TJ LED effectively enhances carrier concentration in the quantum well, achieving a more uniform distribution of electrons and holes, thus improving radiative recombination efficiency. Consequently, at an injection current of 120 A/cm2, compared to the traditional structure LED (without TJ), the proposed LED exhibits a 190.7% increase in optical power, a 142.8% increase in maximum internal quantum efficiency (IQE) to 0.85, and a modest efficiency droop of only 5.8%, with a conduction voltage of just 4.1V. These findings offer valuable insights to address the challenges of high heavy doped TJ and elevated conduction voltage in high-performance TJ DUV LEDs.

Constructing highly efficient and ultra-stable (against Temperature, Humidity and Sunlight) luminescent solar concentrators by using dendritic CsPbBr3@SiO2 particles

Conventional perovskite quantum dots (PQDs) suffer long-term stability, further limiting practical application of luminescent solar concentrators (LSCs). In this work, novel dendritic CsPbBr3@SiO2 particles with bright solid-state green light emission and 46 % of photoluminescence quantum yield (PLQY) have first been synthesized via one-step solid-phase calcination method. The dendritic CsPbBr3@SiO2 particles have then been used as fluorescent material for construction of LSCs along with using organosilicon polymers with better compatibility as waveguide materials. Single-layer thin film LSCs and overall curing LSCs are easily prepared by the casting and curing method, exhibiting high external optical efficiency (ηopt) of 7.88 % and 10.92 %, respectively. The maximum internal quantum efficiency (ηint) of thin film LSCs and overall curing LSCs reach 45.09 % and 44.07 %, respectively, while their maximum external quantum efficiency (ηext) of 29.42 % and 34.07 % are obtained, respectively. Importantly, both types of LSCs show excellent stability, maintaining above 92 % of the initial ηopt under high-relative-humidity (RH = 90 %), high-temperature (60 °C) and sunlight exposure conditions for two weeks or room temperature environment for twenty weeks. Moreover, both types of LSCs at optimal conditions have good transparency (>40 % over 515 nm of light wavelength). Otherwise, both types of LSCs have higher ηopt than that of the previous related work.

Thermally-stable AlN-based green and red composite ceramic phosphor wheel for LD lighting with high CRI

Phosphor conversion materials with excellent thermal stability and broad-spectrum is critical to obtaining laser lighting devices with high luminous flux and CRI. In this paper, high thermally stable green AlN-Lu3Al5O12:Ce (ASL) and red AlN–CaAlSiN3:Eu (ASR) composite phosphor ceramics were prepared by embedding phosphor into AlN ceramic matrix. With mass ratio of phosphor increasing, luminescence intensity and quantum efficiency gradually increased, and the maximum internal quantum efficiency reached 62.8 % and 78.9 % for ASL and ASR, respectively. Under laser exciting in reflection mode, ASL-20 and ASR-50 got the max luminous emittance of 2827.5 lm/mm2 and 268.7 lm/mm2 with a saturation laser power density of 60.4 W/mm2 and 16.7 W/mm2, respectively. The diffusion coefficient of Ca, Al, Si between AlN and CASN grains was 2–4 magnitude higher than that of Lu, O, N between AlN and LuAG grains, confirming facile solid solubility of AlN and CASN. Finally, luminescence performances of green-red color wheel fabricated by ASL and ASR were evaluated. When the area ratio of green to red was 2:1, the color wheel yielded the biggest CRI of 82.6. This work presents a facile strategy for fabricating high thermally stable AlN-based composite phosphor ceramic and its application in high-quality laser lighting and display devices.

Performance Improvement of AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes with Multigradient Electron Blocking Layer and Triangular Last Quantum Barrier

The performance improvements of AlGaN‐based deep ultraviolet light‐emitting diodes (DUV‐LEDs) with multigradient electron blocking layer (EBL) and triangular last quantum barrier (LQB) are investigated. The results show that the maximum internal quantum efficiency (IQE) is improved by 44.9% and light output power (LOP) is improved by 58.1% at 200 A cm−2, exhibiting a tremendous improvement compared to the conventional structure. These improvements are mainly attributed to the fact that both multigradient EBL and triangular LQB can generate negative polarization charges in the graded composition layer, which leads to a significant increase in hole concentration compared to the conventional EBL and LQB structures. Meanwhile, the multigradient EBL can transform the conventional triangular barrier into an arc‐shaped barrier, increasing the electron potential barrier height and decreasing the hole potential barrier height. This unique design suppresses electron leakage and enhances hole injection, leading to a significant enhancement of the IQE and alleviation of the efficiency droop.

Vigorous high-efficiency YAGG:Ce phosphor-in-glass film for next-generation laser lighting applications

It is an important challenge to select suitable color converters and apply them to laser diodes (LDs) lighting field. Sapphire plates are used as substrates for phosphor-in-glass films (PiFs) due to their ultrahigh thermal conductivity (∼ 30 W m−1 K−1). Herein, Y3Al3.5Ga1.5O12:Ce3+ (YAGG:Ce) phosphor mixing silicate glass powder is successfully sintered on sapphire plates to fabricate YAGG:Ce phosphor-in-glass film (PiF). The microstructure appearance and photoelectric characteristics of the efficient green color converter are studied in detail. In addition, it is worth noting that the maximum internal quantum efficiency of the prepared PiF is 92%, which is close to that of the original phosphor (98%). Meanwhile, the saturation threshold of YAGG:Ce PiF reaches 38.96 W/mm2 with luminous efficiency (LE) of 230.85 lm/W under the excitation of 460 nm blue LD. Notably, PiF has ultra-high power density while still maintaining such high LE. In order to highlight the application of this color converter in laser lighting, we assemble laser flashlights and fireflies lights. The test of the laser flashlight shows excellent performances such as high brightness, strong focusing effect and wide irradiation range. All indications manifest that YAGG:Ce PiF is an excellent green color converter.

Structural, Optical, and Electrical Characterization of 643 nm Red InGaN Multiquantum Wells Grown on Strain‐Relaxed InGaN Templates

Red‐emitting (≈643 nm) InGaN multiquantum well active device layers and micro‐LEDs are grown by metal organic chemical vapor deposition (MOCVD) on relaxed InGaN templates, the latter created via thermal decomposition of an InGaN underlayer, and examined via power‐ and temperature‐dependent photoluminescence and electrical measurements. Maximum internal quantum efficiencies are determined to be 7.5% at an excitation power density of 13 W cm−2, radiative recombination occurs through monomolecular recombination, and the fabricated micro‐LEDs do not show any efficiency degradation with decreasing size. Peak on‐wafer external quantum efficiency (EQE) of a 5 × 5 μm2device is 0.44%, demonstrating that thermally decomposed InGaN “strain‐relaxing” underlayers may be useful for long wavelength micro‐LED applications.

Direct Determination of the Internal Quantum Efficiency of Light‐Emitting Diodes

The maximum internal quantum efficiency (IQE) of light‐emitting diodes (LEDs) can be obtained using the so‐called ABC model. Since the ABC model is based on the experimentally not directly accessible carrier density, the light over current data is usually fitted as a function of the light output to yield the maximum IQE. This article shows a method to directly obtain the IQE from experimental data without fitting, which also reflects changes of the A, B, or C parameters over the current range.

Proposing the n+-AlGaN tunnel junction foranefficient deep-ultraviolet light-emitting diodeat254 nm emission.

Toxic and low-pressure deep-ultraviolet (DUV) mercury lamps have been used widely for applications of surface disinfection and water sterilization. The exposure of pathogens to 254nm DUV radiations has been proven to be an effective and environmentally safe way to inactivate germs as well as viruses in short time. To replace toxic mercury DUV lamps, an n +-A l G a N tunnel junction (TJ)-based DUV light-emitting diode (LED) at 254nm emission has been investigated. The studied conventional LED device has maximum internal quantum efficiency (IQE) of 50% with an efficiency droop of 18% at 200A/c m 2. In contrast, the calculated results show that a maximum IQE of 82% with a 3% efficiency droop under a relatively higher injection current was estimated by employing a 5nm thin n +-A l G a N TJ with a 0.70 aluminum molar fraction. In addition, the TJ LED emitted power has been improved significantly by 2.5 times compared with a conventional LED structure. Such an efficient n +-A l G a N TJ-based DUV LED at 254nm emission might open a new way, to the best of our knowledge, for the development of safe and efficient germicidal irradiation sources.

Understanding Microscopic Properties of Light‐Emitting Diodes from Macroscopic Characterization: Ideality Factor, S‐parameter, and Internal Quantum Efficiency

Herein, how the macroscopic characterizations can be utilized to extract information on the defect level and crystal quality of the epitaxial layers of the light‐emitting diodes (LEDs) is presented. After review of the current–voltage (I–V) and light output power–current (L–I) characteristics, actual examples are utilized to show how different defect levels in the devices are reflected in macroscopic characteristics including the internal quantum efficiency (IQE). We show that the ideality factor from the I–V and the S‐parameter from the L–I can serve as useful guides to denote the dominance of the radiative recombination in the active region of the device. The minimum ideality factor is proposed as a possible figure of merit for the defect level of the epitaxial layers of the device and shown to be correlated with the maximum IQE value.

Compositionally graded AlGaN hole source layer for deep-ultraviolet nanowire light-emitting diode without electron blocking layer

The electron blocking layer (EBL) plays a vital role in blocking the electron overflow from an active region in the AlGaN-based deep-ultraviolet light-emitting diode (DUV-LED). Besides the blocking of electron overflow, EBL reduces hole injection toward the active region. In this work, we proposed a DUV nanowire (NW) LED structure without EBL by replacing it with a compositionally continuous graded hole source layer (HSL). Our proposed graded HSL without EBL provides a better electron blocking effect and enhanced hole injection efficiency. As a result, optical power is improved by 48% and series resistance is reduced by 50% with 4.8 V threshold voltage. Moreover, graded HSL without EBL offer reduced electric field within the active region, which leads to a significant increment in radiative recombination rate and enhancement of spontaneous emission by 34% at 254 nm wavelength, as a result, 52% maximum internal quantum efficiency with 24% efficiency drop is reported.

All-inorganic high efficiency LuAG:Ce3+ converter based on phosphor-in-glass for laser diode lighting

High-power, high-brightness laser lighting technology put forward new requirements for the service stability of color conversion materials. Phosphor-in-glass materials (PiGs) have emerged with their unique advantages of withstanding high power excitation density. High luminous efficiency (LE) and high internal quantum efficiency (IQE) LuAG:Ce PiG is still an urgent demand for high power laser lighting conversion materials. In this paper, LuAG:Ce PiGs was successfully prepared by careful design of glass matrix composition. The maximum internal quantum efficiency of LuAG:Ce PiG is 93.8%. The conversion efficiency (CE) of LuAG:Ce PiG from blue to green light is increased to 56.3%. The luminous efficiency (LE) even reaches 225 lm W−1, which is the best performance of LuAG:Ce in LD lighting so far. These results show that this work has taken a big step in improving the performance of LuAG:Ce Green converter and will greatly promote the development of LD lighting.

High Internal Quantum Efficiency AlGaN Epilayer Grown by Molecular Beam Epitaxy on Si Substrate

We report the molecular beam epitaxy growth and characterization of aluminum gallium nitride (AlGaN) epilayer on an Si substrate. The AlGaN epilayer was grown on an AlN buffer layer through a coalescence process on a nanowire template. The AlGaN epilayer possessed a relatively smooth surface. The room-temperature internal quantum efficiency (IQE) was investigated using power-dependent photoluminescence experiments and theoretical analysis. A maximum IQE of around 50% is derived, with an estimated carrier density of 3 × 1018 cm−3. This IQE is significantly improved compared to the previously reported bulk AlGaN epilayers grown on sapphire and/or AlN-on-sapphire templates. The efficiency droop mechanism is also discussed, which could presumably be attributed to the carrier delocalization.

Sr3LiTaO6:xMn4+ red-emitting phosphors for indoor plant growth lighting: High thermal stability and quantum efficiency

A series of Sr3LiTaO6:xMn4+ red-emitting phosphors were synthesized by high temperature solid-state method for the first time. The samples were characterized using XRD, SEM, PLE, PL, UV–vis spectra, and internal quantum efficiency (IQE). The Sr3LiTaO6:0.2%Mn4+ phosphor exhibited a strong emission peak at 695 nm with a maximum IQE of 61.46%, which is suitable for use in indoor plant application. The intense excitation band of the phosphor located at the near ultraviolet region, which can match well with the emission of commercial ultraviolet LED chip. Moreover, the CIE chromaticity coordinate of Sr3LiTaO6:0.2%Mn4+ phosphor is (0.730, 0.270), which falls in deep red range with a color purity of 98.7%. The Sr3LiTaO6:0.2%Mn4+ phosphor also showed a good thermal stability with an emission intensity decrease of only 23.3% at 423 K from that at 289 K (I423K/I289K=76.7%). Finally, a LED device was fabricated for light supplement on wheat, showing an obvious improvement on the content of chlorophyll. It shows that the optimized phosphor may have a good potential for the application in indoor plant growth lighting LEDs.

Hole Transport in Ultraviolet Light-Emitting Diode via p-Type Injection Channel

In order to enable lateral hole injection, a unique structure with a p-type AlGaN injection channel is proposed to replace the conventional deep-ultraviolet light-emitting diode (DUV LED) structure. This type of modification results in the probability of obtaining a 1.65-fold higher light output power when the channel band gap is between the bandgap of quantum-well (QW) and p-AlGaN, which is demonstrated at the current density of 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In comparison, the maximum internal quantum efficiency is increased by 23.43% relative to the traditional AlGaN DUV LED structure with a 10.27% reduced efficiency droop at 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The effective injection of holes would provide greater confinement of electrons. The ultimate effect would result in superior radiative recombination of the proposed DUV LED structure inside the QW.

Multifunctional derivatives of pyrimidine-5-carbonitrile and differently substituted carbazoles for doping-free sky-blue OLEDs and luminescent sensors of oxygen

IntroductionEvolution of organic light-emitting diodes (OLEDs) reached the point, which allows to obtain maximum internal quantum efficiency of 100% partly using heavy-metal-free emitters exhibiting thermally activated delayed fluorescence (TADF). Such emitters are also predictively perfect candidates for new generation of optical sensors since triplet harvesting can be sensitive to different analytes (at least to oxygen). Although many organic TADF emitters have been reported so far as OLED emitters, the investigation of materials suitable for both OLEDs and optical sensors remains extremely rare. ObjectivesAiming to achieve high photoluminescence quantum yields in solid-state and triplet harvesting abilities of organic semiconductors with efficient bipolar charge transport required for application in both blue OLEDs and optical sensors, symmetrical donor–acceptor-donor organic emitters containing pyrimidine-5-carbonitrile electron-withdrawing scaffold and carbazole, tert-butylcarbazole and methoxy carbazole donor moieties were designed, synthesized and investigated as the main objectives of this study. MethodsNew compounds were tested by many experimental methods including optical and photoelectron spectroscopy, time of flight technique, electrochemistry and thermal analyses. ResultsDemonstrating advantages of the molecular design, the synthesized emitters exhibited sky-blue efficient TADF with reverse intersystem crossing rates exceeding 106 s−1, aggregation-induced emission enhancement with photoluminescence quantum yields in solid state exceeding 50%, hole and electron transporting properties with charge mobilities exceeding 10-4 cm2/V·s, glass-forming properties with glass transition temperatures reaching 177 °C. Sky-blue OLEDs with non-doped light-emitting layers of the synthesized emitter showed maximum external efficiency of 12.8% while the doped device with the same emitter exhibited maximum external efficiency of 14%. The synthesized emitters were also used as oxygen probes for optical sensors with oxygen sensitivity estimated by the Stern-Volmer constant of 3.24·10-5 ppm−1. ConclusionThe developed bipolar TADF emitters with pyrimidine-5-carbonitrile and carbazole moieties showed effective applicability in both blue OLEDs and optical sensors.

Improved Performance of GaN-Based Ultraviolet LEDs With Electron Blocking Layers Composed of Double-Peak p-Type Al x Ga1−x N/GaN Superlattice Layers

Past studies have demonstrated the positive impact of step-graded $p$ -type Al x Ga1− x N/GaN superlattice (SL) electron blocking layer (EBL) structures on the efficiency performance of ultraviolet (UV) GaN-based light-emitting diodes (LEDs). However, the optimal Al-grading structure of these SL EBLs remains unclear owing to a lack of systematic investigation. The present work addresses this issue by applying EBL composed of alternating half-peak, single-peak, and double-peak $p$ -type Al x Ga1− x N/GaN SL structures with varying values of $x$ ranging from 0.05 to 0.15 in 0.05 step increments. Simulation analysis is employed to obtain the internal quantum efficiency (IQE), energy band diagrams, polarization compensation factor, hole concentration, and electron concentration of GaN-based UV LEDs with three different SL-EBL structures. The results obtained at an injection current of 200 mA demonstrate that UV LEDs with double-peak SL-EBL structures provide the maximum IQE, which is ~38% greater than that of devices employing the conventional EBL in simulation experiment. This SL-EBL is demonstrated to improve the hole injection and electron overflow performance of GaN-based UV LEDs owing to the polarization charge and lattice mismatch at the p- Al x Ga1− x N/GaN interfaces. The reduced Al composition on the p- GaN side reduces the potential barrier of hole injection. Moreover, the predicted increase in the IQE of GaN-based UV LEDs with the optimal SL-EBL is verified experimentally.