Degradation of light emitting diodes: a proposed methodology

Due to their long lifetime and high efficacy, light emitting diodes have the potential to revolutionize the illumination industry. However, self heat and high environmental temperature which will lead to increased junction temperature and degradation due to electrical overstress can shorten the life of the light emitting diode. In this research, a methodology to investigate the degradation of the LED emitter has been proposed. The epoxy lens of the emitter can be modelled using simplified Eyring methods whereas an equation has been proposed for describing the degradation of the LED emitters.     

Exciton–Polaron‐Induced Aggregation of Wide‐Bandgap Materials and its Implication on the Electroluminescence Stability of Phosphorescent Organic Light‐Emitting Devices

The degradation mechanisms of phosphorescent organic light‐emitting devices (PhOLEDs) are studied. The results show that PhOLED degradation is closely linked to interactions between excitons and positive polarons in the host material of the emitter layer (EML), which lead to its aggregation near the EML/electron transport layer (ETL) interface. This exciton–polaron‐induced aggregation (EPIA) is associated with the emergence of new emission bands at longer wavelengths in the electroluminescence spectra of these materials, which can be detected after prolonged device operation. Such EPIA processes are found to occur in a variety of wide‐bandgap materials commonly used as hosts in PhOLEDs and are correlated with device degradation. Quite notably, the extent of EPIA appears to correlate with the material's bandgap rather than with the glass‐transition temperature. The findings uncover a new degradation mechanism, caused by polaron‐exciton interactions, that appears to be behind the lower stability of OLEDs utilizing wide‐bandgap materials in general. The same degradation mechanism can be expected to be present in other organic optoelectronic devices.     

Chemical degradation mechanisms of highly efficient blue phosphorescent emitters used for organic light emitting diodes

The stability and the degradation processes of two highly efficient blue-emitting phosphorescent materials, iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) and bis(2-(4,6-difluorophenyl) pyridyl-N,C2′)iridium(III)picolinate (FIrpic), which are commonly used as emitters in organic light emitting diodes (OLEDs), are investigated. Using single layers devices, the optical response and the half-lifetime behavior of the materials are investigated. Layers of FIr6 exposed to UV-light show the formation of a red emitting degradation product. We analyze the chemical reactions of the materials using laser desorption/ionization time-of-flight mass spectrometry. Several products related to the chemical dissociation of the FIr6 molecule as well as charge complex formation between the emitter and the emitter dissociation products are detected. FIr6 and FIrpic are also compared by lifetime studies on commonly used OLED structures. We show that single layers and OLEDs based on FIrpic exhibit higher stability than those based on FIr6. An explanation for this behavior can be found by considering the chemical structure of the molecules.     

Temperature dependence of electroluminescence degradation in organic light emitting devices without and with a copper phthalocyanine buffer layer

Temperature dependence of electroluminescence degradation was investigated in two types of organic light emitting devices (OLEDs) based on tris(8-hydroxyquinoline) aluminum (AlQ₃) emitter molecule, one without and another with copper phthalocyanine (CuPc) buffer layer at the hole-injecting contact interface. Electroluminescence degradation in time was measured for devices operated at 22 and 70 °C. Results unexpectedly showed that devices without the CuPc buffer layer demonstrated negligible change in half-life when operated at 22 or 70 °C, while devices with the CuPc layer showed the expected decrease in half-life when the temperature was increased. The results are explained within the framework of recently proposed OLED degradation mechanism, which identifies AlQ₃ cations as unstable, leading to device degradation.     

Engineering of Mixed Host for High External Quantum Efficiency above 25% in Green Thermally Activated Delayed Fluorescence Device

Highly efficient thermally activated delayed fluorescence (TADF) devices are developed by engineering mixed host materials in the emitting layer. Mixed hosts with deep highest occupied molecular orbital and high singlet energy without any exciplex formation are ideal as the host material for the TADF organic light‐emitting diodes. A high external quantum efficiency of 28.6% is achieved in the green TADF organic light‐emitting diodes using a mixed host of 1,3‐bis(N‐carbazolyl)benzene:1,3,5‐tri[(3‐pyridyl)‐phen‐3‐yl]benzene and green emitting (4s,6s)‐2,4,5,6‐tetra(9H‐carbazol‐9‐yl)isophthalonitrile TADF emitter.     

The influence of emitter material on silicon nitride passivation-induced degradation in InP-based HBTs

The influence of emitter material on silicon-nitride (SiN) passivation-induced degradation in InP-based heterojunction bipolar transistors (HBTs) has been studied. It has been found that, compared to InP, InAlAs has a much higher resistance to NH/sub 3/-related plasma-induced damage. InP-based HBTs using InAlAs as the emitter can effectively suppress the degradation of device performance caused by dielectric passivation giving least deterioration on the device characteristics compared to the previously reported results concerning the passivation quality using different passivation schemes. Short-term high temperature and high current electrical stress tests indicates that the SiN-passivated devices using InAlAs as the emitter may have better stability than those with InP emitter. Our results suggest that engineering of emitter layer structures could be an alternative approach to suppress passivation-induced degradation in InP-based HBTs.     

Applications of Inverted Pyramids Corroded by TMAH in PERL Silicon LED

In this paper,a novel and reliable structure of the side passivated emitter and the rear locally-diffused(PERL)silicon light emitting diodes(LEDs)is proposed.The inverted pyramids surface,the important interface in this structure,is given according to the experiment.The results show that the inverted pyramids surface has a low refection about 8%,in the anisotropic etching 70 ℃,5% TMAH concentration,corrosion time of 90 min or 30 min.Low refection means high light emitting rate.Most of the structure and manu...     

Highly Efficient Greenish‐Yellow Phosphorescent Organic Light‐Emitting Diodes Based on Interzone Exciton Transfer

Phosphorescent organic light emitting diodes (PHOLEDs) have undergone tremendous growth over the past two decades. Indeed, they are already prevalent in the form of mobile displays, and are expected to be used in large‐area flat panels recently. To become a viable technology for next generation solid‐state light source however, PHOLEDs face the challenge of achieving concurrently a high color rendering index (CRI) and a high efficiency at high luminance. To improve the CRI of a standard three color white PHOLED, one can use a greenish‐yellow emitter to replace the green emitter such that the gap in emission wavelength between standard green and red emitters is eliminated. However, there are relatively few studies on greenish‐yellow emitters for PHOLEDs, and as a result, the performance of greenish‐yellow PHOLEDs is significantly inferior to those emitting in the three primary colors, which are driven strongly by the display industry. Herein, a newly synthesized greenish‐yellow emitter is synthesized and a novel device concept is introduced featuring interzone exciton transfer to considerably enhance the device efficiency. In particular, high external quantum efficiencies (current efficiencies) of 21.5% (77.4 cd/A) and 20.2% (72.8 cd/A) at a luminance of 1000 cd/m² and 5000 cd/m², respectively, have been achieved. These efficiencies are the highest reported to date for greenish‐yellow emitting PHOLEDs. A model for this unique design is also proposed. This design could potentially be applied to enhance the efficiency of even longer wavelength yellow and red emitters, thereby paving the way for a new avenue of tandem white PHOLEDs for solid‐state lighting.     

White Organic Light‐Emitting Diodes Based on Quench‐Resistant Fluorescent Organophosphorus Dopants

The control of the doping ratio of a blue‐emitting matrix by an orange emitter with high accuracy still remains very challenging in the development of reproducible white organic light‐emitting diodes (WOLEDs). In this work, the development of an organophosphorus dopant that presents a high doping rate in order to reach white emission is reported. The increase of the doping rate has a small impact on the CIE co‐ordinates and on the EQE. These results are very appealing towards the development of “easy‐to‐make” WOLEDS.     

Highly Efficient Non‐Doped Blue‐Light‐Emitting Diodes Based on an Anthrancene Derivative End‐Capped with Tetraphenylethylene Groups

A novel blue‐emitting material, 2‐tert‐butyl‐9,10‐bis[4‐(1,2,2‐triphenylvinyl)phenyl]anthracene ( TPVAn ), which contains an anthracene core and two tetraphenylethylene end‐capped groups, has been synthesized and characterized. Owing to the presence of its sterically congested terminal groups, TPVAn possesses a high glass transition temperature (155 °C) and is morphologically stable. Organic light‐emitting diodes (OLEDs) utilizing TPVAn as the emitter exhibit bright saturated‐blue emissions (Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of x = 0.14 and y = 0.12) with efficiencies as high as 5.3 % (5.3 cd A^–1)—the best performance of non‐doped deep blue‐emitting OLEDs reported to date. In addition, TPVAn doped with an orange fluorophore served as an authentic host for the construction of a white‐light‐emitting device that displayed promising electroluminescent characteristics: the maximum external quantum efficiency reached 4.9 % (13.1 cd A^–1) with CIE coordinates located at (0.33, 0.39).     

Monothiatruxene‐Based,Solution‐Processed Green,Sky‐Blue,and Deep‐Blue Organic Light‐Emitting Diodes with Efficiencies Beyond 5% Limit

The development of blue materials with good efficiency, even at high brightness, with excellent color purity, simple processing, and high thermal stability assuring adequate device lifetime is an important remaining challenge for organic light‐emitting didoes (OLEDs) in displays and lightning applications. Furthermore, these various features are typically mutually exclusive in practice. Herein, four novel green and blue light‐emitting materials based on a monothiatruxene core are reported together with their photophysical and thermal properties, and performance in solution‐processed OLEDs. The materials show excellent thermal properties with high glass transition temperatures ranging from 171 to 336 °C and decomposition temperatures from 352 to 442 °C. High external quantum efficiencies of 3.7% for a deep‐blue emitter with CIE color co‐ordinates (0.16, 0.09) and 7% for green emitter with color co‐ordinates (0.22, 0.40) are achieved at 100 cd m^?2. The efficiencies observed are exceptionally high for fluorescent materials with photoluminescence quantum yields of 24% and 62%, respectively. The performance at higher brightness is very good with only 38% and 17% efficiency roll‐offs at 1000 cd m^?2. The results indicate that utilization of this unique molecular design is promising for efficient deep‐blue highly stable and soluble light‐emitting materials.     

Minimizing Optical Energy Losses for Long-Lifetime Perovskite Light-Emitting Diodes

Regardless of the rapid advance on perovskite light-emitting diodes (PeLEDs), the lack of long-term operational stability hinders the practicality of this technology. Particularly, thermal management is indispensable to control the Joule heating induced by charge transport and parasitic re-absorption of internally confined photons. Herein, a synergetic device architecture is proposed for minimizing the optical energy losses in PeLEDs toward high efficiency and long lifetime. By adopting a carefully modified perovskite emitter in combination with an improved light outcoupling structure, red PeLEDs emitting at 666 nm achieve a peak external quantum efficiency of 21.2% and an operational half-lifetime of 4806.7 h for an initial luminance of 100 cd m^-2. The enhanced light extraction from trapped modes can efficiently reduce the driving current and suppress optical energy losses in PeLEDs, which in turn ameliorate the heat-induced device degradation during operation. This work paves the way toward high-performance PeLEDs for display and lighting applications in the future.     

Chemical degradation processes of highly stable red phosphorescent organic light emitting diodes

We investigate the chemical degradation processes of highly stable red organic light emitting diodes (OLEDs) based on the triplet emitter tris(1-phenylisoquinoline)iridium(III) by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The analysis of LDI-TOF spectra, collected on OLEDs driven at different current densities, shows a direct correlation between the lifetime of the devices and the formation of the three different reaction products: a BPhen dimer, an adduct of BPhen dimer with cesium, and the complex [BAlq₂ + Al(Me-q)₂]⁺ as well. Additionally it was possible to identify another degradation product, whose chemical structure is related to the α-NPD molecule as well to the fluorine of the used p-dopant. This product is only observable in devices aged at very high current densities.     

GaAs---GaAlAs heterojunction transistor for high frequency operation

A bipolar transistor structure is proposed having application for either high frequency operation or integration with certain types of light emitting devices. The structure involves liquid phase epitaxially grown layers of GaAs for the collector and base regions, and of Ga_1−xAl_xAs for the heterojunction emitter. The high frequency potential of this device results primarily from the high electron mobility in GaAs and the ability to heavily dope the base region with slowly diffusing acceptors. The Ga_1−xAl_xAs emitter region provides a favorable injection efficiency and, because it is etched preferentially relative to GaAs, access to the base layer for making contact. Transistor action with d.c. common emitter current gains of 25 have been thus for observed. Calculations of the high speed capability of this transistor are presented.     

New methodology for the assessment of the thermal resistance of laser diodes and light emitting diodes

In this paper, we propose a new method to evaluate the thermal resistance of laser diodes and light emitting diodes based on the analysis of common emitter characteristics (emission spectrum, power-current and voltage-current characteristics) measured in CW condition. This method has been used to assess the thermal resistance of commercial GaAlAs laser and light emitting diodes emitting at 780 nm and 872 nm respectively. Finally, the method is compared with classic threshold current method carried out on the laser diodes.     

Exciton management by co-doping of blue triplet emitter as a lifetime improving method of blue thermally activated delayed fluorescent devices

Co-doping of a blue phosphorescent emitter in a thermally activated delayed fluorescent (TADF) emitter based emitting layer was developed as an approach to extend the lifetime of blue TADF devices by managing excitons and polarons in the emitting layer. The blue phosphorescent emitter was doped at a very low doping concentration below 1 wt% to suppress triplet-triplet and triplet-polaron quenching effect in the TADF emitting layer. The doping of the blue phosphorescent emitter led to great extension of the lifetime of the TADF devices by hole trapping effect of the blue triplet emitter which widened exciton formation zone in the TADF emitting layer. More than twice extension of the operational lifetime of the device was demonstrated by the co-doping approach irrespective of the doping concentration of the TADF emitter in the emitting layer.     

Correlation of emission and electrical properties of gallium arsenide transistor structures

Transistor structures were fabricated in si-doped solution, grown GaAs. The p-n junction used as anl emitter was formed during the growth of the epitaxial layer employing amphoteric Si doping. The collector junction was made by a planar sulphur diffusion. The behavior of the emitted light and the collector current as a function of the emitter base current are observed. The degradation in quantum efficiency and transistor alpha is noted to be due to the same diode current component. The degradation of alpha due to emitter crowding at high current levels is directly exhibited.     

Efficiency Enhancement of Organic Light‐Emitting Diodes Incorporating a Highly Oriented Thermally Activated Delayed Fluorescence Emitter

An organic light‐emitting diode (OLED) with the blue emitter CC2TA showing thermally activated delayed fluorescence (TADF) is presented exhibiting an external quantum efficiency () of 11% ± 1%, which clearly exceeds the classical limit for fluorescent OLEDs. The analysis of the emission layer by angular dependent photoluminescence (PL) measurements shows a very high degree of 92% horizontally oriented transition dipole moments. Excited states lifetime measurements of the prompt fluorescent component under PL excitation yield a radiative quantum efficiency of 55% of the emitting species. Thus, the radiative exciton fraction has to be significantly higher than 25% due to TADF. Performing a simulation based efficiency analysis for the OLED under investigation allows for a quantification of individual contributions to the efficiency increase originating from horizontal emitter orientation and TADF. Remarkably, the strong horizontal emitter orientation leads to a light‐outcoupling efficiency of more than 30%.     

Achieving Above 60% External Quantum Efficiency in Organic Light‐Emitting Devices Using ITO‐Free Low‐Index Transparent Electrode and Emitters with Preferential Horizontal Emitting Dipoles

Comprehensive theoretical and experimental studies are reported on organic light‐emitting devices (OLEDs) adopting either the conventional high‐index indium tin oxide (ITO) electrode or the low‐index conducting polymer electrode, either isotropic emitters or emitters having preferentially horizontal emitting dipoles, and different layer structures. Intriguingly, with the use of low‐index electrode in the device, in addition to the known suppression of waveguided modes, the surface plasmon modes can also be effectively suppressed with larger emitter‐to‐metal distances yet with better immunity to accompanied increase of the competing waveguided modes (induced by thicker organic layers) as in the ITO device. As a result, overall coupling efficiencies of OLED internal radiation into substrates can be significantly enhanced over those with ITO electrodes. Through effective extraction of radiation within substrates, green phosphorescent OLEDs adopting both the low‐index ITO‐free electrode and the preferentially horizontal dipole emitter (with a horizontal dipole ratio of 76%) achieve a high external quantum efficiency (EQE) of up to ≈64%. The simulation also predicts that very high EQEs of ≥80% are possible with highly horizontal dipole emitters for all red/green/blue/white OLEDs, clearly revealing the potential of combining low‐index transparent electrodes and horizontal dipole emitters for high‐efficiency OLEDs.     

High efficiency fluorescent white organic light-emitting diodes having a yellow fluorescent emitter sensitized by a blue thermally activated delayed fluorescent emitter

Fluorescent white organic light-emitting diodes having a blue thermally activated delayed fluorescent emitter and a yellow fluorescent emitter was developed by co-doping the blue and yellow emitters in a single emitting layer. The blue delayed fluorescent device showed high quantum efficiency of 22.6% at a very high doping concentration of 50% and the white devices exhibited a high quantum efficiency of 15.5% even though a fluorescent yellow emitter was doped in the blue thermally activated delayed fluorescent emitting layer. Minimized charge trapping and Dexter energy transfer by low yellow doping concentration of 0.05% as well as efficient Förster energy transfer could develop the high efficiency fluorescent white organic light-emitting diodes.