Short period superlattice Il-VI blue light emitting diodes

The authors report, for the first time, high colour purity short period superlattice (SPSL) II-VI blue light emitting diodes (LEDs) grown by molecular beam epitaxy. The room temperature electroluminescence spectrum showed pure blue emission at 460 nm with a 13 nm full width at half maximum (FWHM). The external quantum efficiency is ~0.1%. Operating at 14 A cm^-2, the half-intensity lifetime was >13 h at room temperature     

Abnormal blue shift of InGaN micro-size light emitting diodes

Blue InGaN micro-size light emitting diodes (LEDs) with diameters from 3 to 70 μm have been fabricated. An ion implantation technique and a 12 μm electro-ridge were used to simplify the fabrication processes. The 3–70 μm LEDs exhibiting a large emission of photon blue shift (40–240 meV) were observed in electro-luminescence (EL) spectra. The dependence of the blue shift on size is studied. The characteristics of the micro-size LEDs are also investigated numerically with the use of an advanced physical model of semiconductor devices (APSYS). The experimental measurements and simulation results are in close agreement for maximum blue shift. Base on the simulation, the difference between blue shift caused by band filling effect and red shift caused by lateral carrier confinement are approximately the same. Hence, the maximum blue shift increases when the size of micro-size LED increases because of decreased red shift resulted from thermal effect.     

Efficient blue emitting organic light emitting diodes based on fluorescent solution processable cyclic phosphazenes

Solution processable blue fluorescent dendrimers based on cyclic phosphazene (CP) cores incorporating amino-pyrene moieties have been prepared and used as emissive layers in organic light emitting diodes (OLEDs). These dendrimers have high glass transition temperatures, are monodisperse, have high purity via common chromatographic techniques, and form defect-free amorphous films via spin/dip coating. The solution processable blue light emitting OLEDs reach current efficiencies of 3.9 cd/A at brightness levels near 1000 cd/m². Depending on the molecular bridge used to attach the fluorescent dendron to the inorganic core, the emission wavelength changes from 470 to 545 nm, corresponding to blue and green light respectively. Via dilution experiments we show that this shift in emission wavelength is likely associated with molecular stacking of the amino-pyrene units.     

Efficient non-doped blue light emitting diodes based on novel carbazole-substituted anthracene derivatives

In this study, we synthesized three anthracene derivatives featuring carbazole moieties as side groups - 2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz⁹PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl}anthracene (tCz⁹PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz⁹Ph₂Ant) - for use in blue organic light emitting devices (OLEDs). The anthracene derivatives presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited strong blue emissions in solution, with high quantum efficiencies (91%). We studied the electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL provided superior EL performance than did those lacking the HTL. The highest brightness (6821 cd/m²) was that for the tCz⁹PhAnt-based device; the greatest current efficiency (2.1 cd/A) was that for the tCz⁹Ph₂Ant-based device. The devices based on these carbazole-substituted anthracene derivatives also exhibited high color purity.     

A new degradation phenomenon in blue light emitting silicon carbide diodes

In 6H-SiC blue emitting diodes prepared by sawing epitaxial wafers the development of greenish striations in the luminescing layer is observed together with a decrease in external quantum efficiency. It is proposed that this degradation is due to the development of numerous stacking faults leading to an intermediate (cubic?) state of lower band gap than the 6H modification.     

Influence of light absorption on the metallic nanotextured reflectors of GaN-based light emitting diodes

Metallic nanotextured reflectors have been widely used in light emitting diodes (LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however, only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference time-domain (FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon (SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.     

Single crystal growth of SiC substrate material for blue light emitting diodes

The fabrication of 6H-SiC ingot single crystals with up to 20-mm diam and 24-mm length is described. Crystal growth was realized with a modified Lely method using a suitable seed crystal. The growth temperature was 2200°C to obtain preferential growth of the 6H-modification with 2.9-eV bandgap useful for blue light emitting silicon carbide diodes. It is the first time that 6H-SiC ingot crystals yielding substrates for the industrial production of devices based on this compound were obtained. With controlled Al doping of the crystal it was possible to fabricate blue light diodes with the highest quantum efficiency reported so far.     

染料掺杂剂的位置对有机电致发光器件性能的影响

利用3种有机小分子荧光染料dimethylquinacridone(DMQA),4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran(DCJTB)和5,6,11,12-tetraphenylnaphthacene(Rubrene)作为掺杂剂,研究了其掺杂位置对于有机电致发光器件(OLED)性能的影响。利用Fo¨rster能量传递(ET)理论和直接载流子俘获(DCT)理论,结合器件的电致发光(EL)光谱,讨论了这3种材料作为掺杂染料的类型。实验结果表明,小分子荧光染料DMQA、DCJTB和Rubrene同时具有ET型和DCT型掺杂剂的双重特性;另外,掺杂剂的位置对器件的EL光谱有较大影响,并讨论了EL光谱差异的形成原因。     

High-brightness blue organic light emitting diodes with different types of guest-host systems

We demonstrate high-brightness blue organic light emitting diodes (OLEDs) using two types of guest-host systems. A series of blue OLEDs were fabricated using three organic emitters of dibenz anthracene (perylene), di(4-fluorophenyl) amino-di (styryl) biphenyl (DSB) and 4,4''-bis[2-(9-ethyl-3-carbazolyl)vinyl]biphenyl (BCzVBi) doped into two hosting materials of 4,4''-bis(9-carbazolyl) biphenyl (CBP) and 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) as blue emitting layers, respectively. We achieve three kinds of devices with colors of deep-blue, pure-blue and sky-blue with the Commission Internationale de L''Eclairage (CIE) coordinates of (0.16, 0.10), (0.15, 0.15) and (0.17, 0.24), respectively, by employing PBD as host material. In addition, we present a microcavity device using the PBD guest-host system and achieve high-purity blue devices with narrowed spectrum.     

发光二极管表面结构对出光特性的影响

用传输矩阵法模拟计算了AlGaInP发光二极管(LED)不同表面结构的光学特性,用等离子体增强化学气相沉积(PECVD)或磁控溅射掺铟氧化锡(ITO)设备,在带有DBR结构的外延衬底上制备出具有不同表面层结构的LED.通过实验结果对比表明,表面生长λ/4n SiON加λ2n ITO增透膜结构复合增透膜的LED,器件光学性能提高最佳,在20 mA注入电流下,光强和光通量分别达到141.7 mcd和0.4733 lm.比同样结构的无增透膜LED轴向光强和光通量分别提高138%和91%.     

High-brightness AlGaInP light emitting diodes

First commercially introduced in 1990, AlGaInP light emitting diodes (LEDs) currently are the highest (luminous) efficiency visible solid-state emitters produced to date in the red through yellow spectral regime. The attainment of this high-efficiency performance is a result of the development of advanced metalorganic chemical vapor deposition crystal growth techniques, which have facilitated the high-quality growth of this quaternary alloy as well as the implementation of complex device designs. Furthermore, the highest efficiency family of AlGaInP devices (based upon a transparent-substrate platform and commercially introduced in 1994) have been realized as result of the development and implementation of direct compound semiconductor wafer bonding technology. As a result, the luminous efficiency of AlGaInP LEDs exceeds or rivals that of unfiltered incandescent lamps and other conventional lighting sources. Further improvements in these techniques (and the realization of efficient, high-power LEDs) are expected to make AlGaInP LEDs even more competitive with conventional lamp technology, thus enhancing the position of LED's in many applications as a preferred lighting source     

Materials for light emitting diodes

III–V semiconductor display diodes fall into three material categories: direct, indirect, and diode-phosphor combinations. The light generation mechanisms are well understood in each case. On the basis of the luminous efficacy achieved in the laboratory, the merit of the various materials is in the order: GaP: N (green), GaP: Zn, O (red), GaInP (yellow), GaAsP (red), GaAlAs (red) and GaAs-phosphor combinations (red, green, blue). A more comprehensive assessment of quality must take into account factors such as the economic feasibility of the methods of synthesis, the applicability to monolithic arrays, and the suitability of spectra (hue, contrast, saturation, etc.). The degree of importance of these matters and the limitations of the various materials are less well-defined at present. New developments which may have an impact on the future of display diodes include the use of Al_xGa_1-xP in place of GaP to shift spectra to shorter wavelengths, the possible development of very bright direct alloys of InP with either GaP or A1P, and the doping of GaAsP with N to extend its range of colors.     

Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes

We investigated the electrical and structural qualities of Mg-doped p-type GaN layers grown under different growth conditions by metalorganic chemical vapor deposition (MOCVD). Lower 300 K free-hole concentrations and rough surfaces were observed by reducing the growth temperature from 1,040°C to 930°C. The hole concentration, mobility, and electrical resistivity were improved slightly for Mg-doped GaN layers grown at 930°C with a lower growth rate, and also an improved surface morphology was observed. In_0.25Ga_0.75N/GaN multiple-quantum-well light emitting diodes (LEDs) with p-GaN layers grown under different conditions were also studied. It was found from photoluminescence studies that the optical and structural properties of the multiple quantum wells in the LED structure were improved by reducing the growth temperature of the p-layer due to a reduced detrimental thermal annealing effect of the active region during the GaN:Mg p-layer growth. No significant difference in the photoluminescence intensity depending on the growth time of the p-GaN layer was observed. However, it was also found that the electroluminescence (EL) intensity was higher for LEDs having p-GaN layers with a lower growth rate. Further improvement of the p-GaN layer crystalline and structural quality may be required for the optimization of the EL properties of long-wavelength (∼540 nm) green LEDs.     

A recent advances of blue perovskite light emitting diodes for next generation displays

The halide perovskite blue light emitting diodes (PeLEDs) attracted many researchers because of its fascinating opto-electrical properties.This review introduces the recent progress of blue PeLEDs which focuses on emissive layers and interlay-ers.The emissive layer covers three types of perovskite structures:perovskite nanocrystals (PeNCs),2-dimensional (2D) and quasi-2D perovskites,and bulk (3D) perovskites.We will discuss about the remaining challenges of blue PeLEDs,such as lim-ited performances,device instability issues,which should be solved for blue PeLEDs to realize next generation displays.     

Spectroscopic and electrical characteristics of highly efficient tetraphenylsilane-carbazole organic compound as host material for blue organic light emitting diodes

Thermal, electrical and spectroscopic properties have been studied for bis(3,5-di(9H-carbazol-9-yl) phenyl)diphenylsilane (SimCP2) which has exhibited high external quantum efficiency of 17.7% and power efficiency of 24.2 lm/W when it is used as host material for iridium bis(4,6-difluorophenypyridinato)picolate (FIrpic) blue emitter. They are compared with 1,3-bis (9-carbazolyl) benzene (mCP) and 3,5-bis (9-carbazolyl) tetraphenylsilane (SimCP) which have been also used as host for blue emitters. SimCP2 exhibits a highest glass transition temperature (148 °C) and is morphologically more stable. The electron and hole mobilities are higher (4.8 × 10⁻⁴ and 2.7 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively, at electric field of 9 × 10⁴ V cm⁻¹) than those of mCP and SimCP. The zero-phonon S₁ emission band is observed at 344 nm, while the T₁ emission band at 412 nm, i.e., this material preserves the characteristics of wide band-gap of 3.56 eV and high T₁ triplet energy of 3.01 eV. From the intensity ratio of the T₁ emission to the S₁ emission, it is suggested that the intersystem crossing rate is smaller for SimCP2 than for mCP and SimCP. From these results, we clarify the reasons why SimCP2 is superior to mCP and SimCP as the host material for blue phosphorescence emitter in organic light emitting diodes.     

InGaN／AlGaN双异质结构蓝光LED的电学和光学性质

研究了ＩｎＧａＮ／ＡｌＧａＮ双异质结构（ＤＨ）蓝光发光二极管（ＬＥＤ）的电学和光学性质，实验表明，器件正向偏压下的Ｉ－Ｖ特性偏离了ｐｎ结二极管的肖克莱模型的结果，并且载流子的主要输运机制与载流子隧穿有关。通过对电致发光（ＥＬ）谱的测量，得到位于２．８ｅＶ的发射峰和位于３．２ｅＶ弱发射峰，随着电流增大而均出现蓝移。对大脉冲电流下ＬＥＤ的特性的退化作了研究。     

Modification effect of hole injection layer on efficiency performance of wet-processed blue organic light emitting diodes

We examined the performance of solution-processed organic light emitting diodes (OLEDs) by modifying the hole injection layer (HIL), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS). Atomic force microscopy (AFM) showed morphological changes with surface roughness (R_RMS) of 1.47, 1.73, and 1.37 nm for pristine PEDOT: PSS, PEDOT: PSS modified with a 40 v% deionized water and with a 30 v% acetone, respectively. The surface hydrophobicity of the acetone modified PEDOT:PSS HIL layer was decreased by 34% as comparing with the water modified counterpart. Electrical conductivity was increased to two orders of magnitude for the water and acetone modified PEDOT:PSS as compared to pristine. We observed a low refractive index and high transmittance for the modified HILs. We fabricated and explored electroluminescent properties of bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) based sky blue device by utilizing HIL with and without modification. The changes in electrical conductivity, surface roughness, refractive index, and transmittance of the modified HILs strongly influenced the performance of devices. By utilizing a 30% acetone modified HIL, the power efficiency was increased from 14.2 to 24.2 lm/W, an increment of 70% and the EQE from 8.5 to 13.1% at 100 cd/m², an increment of 54%. The maximum luminance also increased from 11,780 to 18,190 cd/m². The findings revealed herein would be helpful in designing and fabricating high efficiency solution processed OLEDs.     

Correlations of impedance-voltage characteristics and carrier mobility in organic light emitting diodes

The correlation of accumulation charges at the interfaces of organic layers and carrier mobility in organic light emitting devices (OLEDs) were investigated through the impedance versus voltage (Z-V) characteristics of devices. The properties of devices with various combinations of cathode structures, HTLs and ETLs were investigated to understand the impedance transition in Z-V characteristics of OLEDs. It was observed that there is an extra impedance transition before devices turn on when the hole mobility in the HTL is much greater than the electron mobility in the ETL in the devices, which makes the Z-V characteristics a potential tool to compare the electron mobility in ETL and hole mobility in HTL.     

Degradation of AlGaN-based ultraviolet light emitting diodes

Aging the LEDs by driving at high current, results in the decrease of optical power proportional to the reciprocal square route of stress time. With aging time, change in the current–voltage characteristics indicates decrease of the current at low voltage below the light emission threshold, decrease of the forward voltage drop at high currents and usually no change in the series resistance. No change in the peak wavelength and half bandwidth were found with aging. Low frequency noise measured at low and high currents either did not depend on aging time or decreased. No correlation between noise, the device power, and the rate of the power degradation were found. These results are in strong contrast to previous studies of longer wavelength GaN-based LEDs. The possible degradation mechanism is the diffusion of the Al atoms out from the p-type cladding layer and lowering of the cladding layer potential barrier as a result.     

高亮度LED调光技术

高亮度发光二极管(HB LED)在各种领域应用普及.并要求HB LED具备有调光功能.在现有的几种调光技术中,从简单的可变电阻负载到复杂的脉冲宽度调制(PWM)开关,每一种方法均有其利弊.PWM调光的效率最高,电流控制也最精准.论述了HB LED在调光时的特性,介绍了LM3405评估板及其功能.包括其布局、原理图和元件清单.