Operation-induced degradation of GaP light-emitting diodes

After GaP red light-emitting diodes (LED's) had been degraded by the forward-biased operation, changes of the light output, the minority carrier lifetime, the photocurrent, and the photocapacitance were measured. There was a linear decreasing relation between the light output and the carrier lifetime. A center which was related to the degradation of the light output was observed by means of photocurrent and photocapacitance measurements. This center's energy level is about 0.8 eV from either the valence or conduction band. This center was observed to increase in concentration with degradation. However, the contribution of this center to the minority carrier lifetime is not established since the capture cross sections of the center are unknown and is not known whether the center occurs on the n-side, or the p-side, or on both sides. This center does not appear to be an oxygen donor.     

The degradation of GaAlAs red light-emitting diodes under continuous and low-speed pulse operations

Long-term accelerated degradation tests on GaAlAs red light-emitting diodes were performed under continuous and low-speed pulse operation, and the differences in the degradation and lifetime were clarified. The major factor causing the degradation was suggested to be the decrease in the radiative recombination probability due to defect generation.     

Temperature-dependent minority-carrier lifetime measurements of red AlGaAs light emitting diodes

Electroluminescent decay and internal quantum efficiency measurements are made as a function of temperature on two double heterostructure AIGaAs light emitting diodes (LEDs) that emit in the visible (red) portion of the spectrum. The electroluminescent lifetimes increase by more than a factor of ten and the internal quantum efficiency falls by a factor of three as the temperature is raised from 90 to 400K. By analyzing the data with a model that accounts for the transfer with increasing temperature of the minority-carrier electrons from the direct-gap to the indirect-gap minima in the p-type active layer of these near-crossover LEDs, values for the radiative and nonradiative lifetimes as a function of temperature are obtained. A fit to the radiative-lifetime data results in an estimate of 1.3 × 10⁻¹⁰ cm³s⁻¹ for the room-temperature radiative recombination coefficient of Al_0.39Ga_0.61As, which is very similar to values reported for GaAs. The nonradiative lifetimes are found to be nearly independent of temperature from 220 to 400K and provide upper limits of 940 and 1250 cms⁻¹ for the interface recombination velocities of the two samples. These values are roughly an order of magnitude lower than any previously reported values for high-Al-content (x > 0.3) Al_xGa_1−xAs heterostructures.     

Enhancement of red organic light-emitting diodes via cascade energy transfer

We have demonstrated that efficient red electroluminescence is obtained via cascade energy transfer from Alq to fluorescent dye Coumarin(C545) and then from C545 to 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). The cell structure was indium tin oxide (ITO)/ N,N′-bis-(1-naphenyl)-N,N′-biphenyl-1,1′-bipheny1-4-4′-diamine (NPB)/ tris (8-hydroxyquinoline) aluminum (Alq): C545: DCJTB/Alq/LiF/Al. An additional dopant, C545, was used to assist the energy transfer from Alq to the red dopant. Compared with the devices where the emitting layer is only composed of Alq and DCJTB, the emission efficiency and color purity were improved. We attribute these improvements to the assistant dopant C545 which leads to the more efficient energy transfer from Alq to DCJTB. The co-doping system is a promising method for red organic light-emitting diodes.     

Hole injection polymer effect on degradation of organic light-emitting diodes

Polythienothiophene:poly(perfluoroethylene-perfluoroethersulfonic acid) (PTT:PFFSA) has been used to enhance hole injection into small molecule OLEDs. Compared to devices with polyethylene dioxythiophene polystyrene sulfonate (PEDOT:PSS) as the hole injection layer (HIL), the OLED using PTT:PFFSA as a HIL gives enhanced efficiency and a slower luminance decay as well as a slower rise in operating voltage. Further studies of capacitance–voltage characteristics reveal that positive trapped charges accumulate in the hole transporting layer during operation. These results thus highlight the significance of hole injection layer to OLED operational stability.     

Migration of small molecules during the degradation of organic light-emitting diodes

Degradation process of organic light-emitting diodes (OLEDs) is examined directly by X-ray photoelectron spectroscopy with in situ high-energy C₆₀⁺ and low-energy Ar⁺ co-sputtering. It is proven that this analytical technique clearly indicates the elemental depth profile of as-prepared OLED device (Al/LiF/2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)/4,4′-bis(carbazol-9-yl)biphenyl:bis(3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) iridium (III) (CBP:FIrpic)/poly(ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS)/ITO). Devices operated for different durations are subjected to this profiling technique for studying the change of elemental distribution. In addition to some of the accepted degradation mechanisms, it is observed that small TPBi molecules migrate towards the ITO anode under a direct driving voltage while retaining its original structure. It is also observed that oxygen diffused into the device through the Al cathode and along the Al–organic interface. Molecules with high stereo-hindrance may have less degradation due to the electron migration.     

Estimation of the degradation of InGaN/AlGaN blue light-emitting diodes

The long-term accelerated degradation of GaN blue light-emitting diodes under current stress was investigated. From the degradation pattern of optical output with respect to time, the dependence of the degradation on current stress and an equation for estimation of the half-life of the diode were obtained. The major factor causing the degradation is the decrease in the radiative recombination probability due to defect level generation.     

High-radiance light-emitting diodes

The properties of AlxGa1-xAs heterojunction incoherent edge emitters are described. Single (SH) and double-heterojunction (DH) diodes emitting at about 8200 Å were studied. The highest CW radiance measured perpendicular to the emitting facet was 95 W/cm².sr at 4200 A/cm²for a stripe-geometry SH device. This high radiance level was found to be consistent with an operating life of many thousands of hours. The near- and far-field patterns of the diodes are presented as well as the spectral characteristics and the radiance as a function of drive current, both pulsed and dc.     

Light degradation test and design of thermal performance for high-power light-emitting diodes

Light-emitting diodes (LEDs), which are generally used for indicator lights, have been continuously developed for the past 50 years. With an urgent need for energy conservation and pollution reduction, the trend of replacing traditional incandescent or fluorescent lamps with high-power LEDs is growing more and more popular. Consequently, LEDs have attracted the attention of many industries that incorporate LED researches and designs into their products. However, a low electro-optical conversion efficiency of LEDs can induce a high percentage of input power that transforms into redundant heat. This leads to an increase in the junction temperature, which indicates that thermal management is an important issue in high-power LEDs. In this research, a light degradation test is implemented and a chip-in-substrate-type LED packaging structure is proposed. A finite element (FE) model of the chip-in-substrate-type structure with an effective methodology, which is validated through the forward voltage method, is established. With regard to the design concepts of LED packaging, various parameters of chip-in-substrate-type structure are investigated.     

Changes in electrical characteristics associated with degradation of InGaN blue light-emitting diodes

Because of the high concentration of threading dislocations, the reverse current-voltage (I–V) characteristics for either homo- or heterojunctions made on GaN-based materials grown on sapphire often show a strong electric field dependence (called a soft breakdown characteristic), which can be described by a power law I=Vⁿ, with n between 4 to 5. We find a significant increase of reverse currents associated with the early degradation of emission in InGaN blue single-quantum-well light-emitting diodes (LEDs) subjected to aging tests (injected current of 70 mA over a total time of about 300 h). The formation of dislocations might be due to the relaxation of strain in the thin InGaN active layer during the aging tests.     

CMOS-compatible organic light-emitting diodes

We report a new method for the integration of light-emitting devices on a silicon substrate. As an active layer, we use unsubstituted PPV or PPV-based organic macromolecules with a p⁺-silicon anode and a cathode made from aluminum or titanium. The polymer is deposited by spin-coating the precursor, followed by a thermal conversion step to form the macromolecules. All process steps, including the possibility of dry etching of the active layer and the upper electrode, are typical for MOS technology. Spectrum analysis, current-voltage, and intensity measurements have been carried out for device characterization. These organic light-emitting diodes allow the monolithic integration of microelectronic circuits and light-emitting devices on one silicon chip applying only typical MOS process steps     

InGaN microring light-emitting diodes

The fabrication and performance of an InGaN light-emitting diode (LED) array based on a microring device geometry is reported. This design has been adopted in order to increase the surface area for light extraction and to minimize losses due to internal reflections and reabsorption. Electrical characteristics of these devices are similar to those of a conventional large-area LED, while the directed light extraction proves to be superior. In fact, these devices are found to be more efficient when operated at higher currents. This may be attributed to improved heat sinking due to the large surface area to volume ratio. The potential applications of these devices are also discussed.     

Improved degradation stability of blue-green II-VI light-emitting diodes with excluded nitrogen-doped ZnSe-based layers

Degradation characteristics of p-i-n BeZnSe/Zn(Be)CdSe light-emitting diodes were investigated. Undoped short-period superlattices, which provide efficient hole transport from the p ⁺-BeTe:N near-contact region (hole injector) into the active region, were used instead of the p-doped BeZnSe:N emitter. It is demonstrated that this makes it possible to considerably lengthen the operating life of the light-emitting diodes at highest direct current densities (～4.5 kA/cm²) at room temperature.     

Response times of light-emitting diodes

Response times of light emission from a GaAs0.6P0.4electroluminescent diode have been measured for stepwise applications of voltage. Turn-on times decrease either with an increase of the steady-state level of current or with superposition of a dc bias voltage. The main features of the experiment are in reasonable agreement with those of the calculated curves based on an equivalent circuit with a depletion-layer capacitance and a voltage-dependent diode conductance.     

Optical fluctuations of light-emitting diodes

Intensity fluctuations of GaAs light-emitting diodes (LED) have been measured at 25°C ambient temperature in the frequency range of 25 Hz to 20 kHz. A dependence of optical fluctuations at low frequencies on diode current and also on the direction of emitted radiation has been found.     

1,8-Naphthalimide-based hybrids for efficient red thermally activated delayed fluorescence organic light-emitting diodes

The connection of an electron deficient 1,8-naphthalimide acceptor moiety with a phenoxazine or phenothiazine donor moiety afforded 6-(10H-phenoxazin-10-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (PXZ-NAI) and 6-(10H-phenothiazin-10-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (PTZ-NAI) as two novel red thermally activated delayed fluorescence (TADF) emitters. The two emitters exhibited distinct TADF characteristics with small energy gaps between the lowest singlet and triplet excited states, which originated from the well-separated highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The optimized TADF organic light-emitting diodes based on PXZ-NAI and PTZ-NAI offered maximum external quantum efficiencies (EQEs) of 13.0% and 11.4% with maximum power efficiencies of 14.8 and 9.8 lm/W, respectively. Both devices emitted red electroluminescence spectra with peaks at 624 and 632 nm, and Commission Internationale de L'Eclairage coordinates of (0.610, 0.388) and (0.630, 0.368), respectively. These findings demonstrate that the combination of 1,8-naphthalimide with phenoxazine or phenothiazine could be an effective pathway for developing red TADF emitters.     

Stability of polymer light-emitting diodes

In this paper, we report on the lifetime of polymer LEDs fabricated at Philips Research. For single-layer LEDS, we find that the operational lifetime in nitrogen gas is limited by the stability of the indium-tin-oxide (ITO) anode. By using a polymeric capping layer for the ITO, we obtain more stable devices. In air, the lifetime is limited by black spot formation. Small pinholes in the cathode layer are the origins of the black spots. Water or oxygen may diffuse through these pinholes and react with the cathode, causing degradation. By encapsulating the devices we can prevent black spot formation. Our present 8 cm² devices have lifetimes of many thousands of hours at daylight visibility under ambient conditions.     

Thermal resistance of light-emitting diodes

A detailed analysis of the heat flow in a light-emitting diode is carried out in the present paper. In the thermal model of a light-emitting diode, the heat flow from the active region throughout the area between it and the top contact, the nonuniform heat flux density distribution in the active region due to the current-spreading effect as well as the temperature dependence of the thermal conductivity of the semiconductor material, are taken into account. The solutions of the thermal conduction equation for a light-emitting diode are obtained for both the steady-state condition and the transient-state condition. The heat-spreading in the heat-sink is analyzed. The effect of LED construction parameters on its thermal resistance is illustrated in numerous figures.     

高效叠层OLED白光器件进展

叠层有机发光二极管（Organic Light-Emitting Diode,OLED）白光器件具备低功耗、高亮度、高色域等性能优势。然而,由于效率、寿命及驱动电压等性能仍有较大改进空间,叠层结构的材料及电学结构仍需进一步优化。本文重点介绍叠层OLED白光器件的最新研究进展,总结了三类电荷产生层（Charge Generation Layer,CGL）在工程化应用中存在的问题以及其非破坏性检测方法;综述高效叠层OLED白光器件的“全磷光体系”、“并行通道激子收集”及“混合磷光热活性型延迟荧光（Thermally Activated Delayed Fluorescence,TADF）体系”最新研究成果,对器件寿命问题进行总结,探讨分析“分级掺杂”、“四色混合TADF体系”等从结构方面提出优化方案,并针对不同发光材料体系中的CGL材料及结构综述叠层OLED白光器件实现较低工作电压的技术方法,最后对叠层OLED白光器件的材料和结构提出改进建议。