Non-doped-type white organic light-emitting diodes for lighting purpose

We demonstrate a non-doped white organic light-emitting diode (WOLED) in which the blue-, green- and red-emissions are generated from 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl, tris(8-hydroxyquinoline)aluminum (Alq) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyl-julolidyl 9-enyl)-4H-pyran (DCJTB), which is used as an ultrathin layer. The DCJTB ultrathin layer plays the chromaticity tuning role in optimizing the white spectral band by modulating the location of the DCJTB ultrathin layer in the green emissive Alq layer. The optimized WOLED gives the Commission Internationale de l’Eclairage-1931 xy coordinates of (0.319, 0.335), a color rendering index of 91.2 at 10 V, a maximum brightness of 21010 cd/m² at 12 V and a maximum current efficiency of 5.17 cd/A at 6.6 V. The electroluminescence mechanism of the white device is also discussed.     

高效率共轭聚合物主体绿光磷光发光二极管

因电致发光效率高和器件制备工艺简单,聚合物为主体的绿色磷光电致发光成为一个研究热点.共轭聚合物的三线态能级一般低于绿色磷光材料的三线态能级,易对磷光的发光引起猝灭导致低的发光效率,所以较少被用作绿色磷光材料的主体.通过增加聚乙烯基咔唑（PVK）作为空穴传输层,获得了高发光效率的共轭聚合物聚芴（PFO）作主体绿色磷光发射,甚至高于相同条件下以PVK为主体的绿色磷光发射.究其原因,PVK的电子阻挡作用使发光中心靠近PVK与PFO的界面,界面处PVK因为其高的三线态能级增强了绿色磷光的发光.当三-（2-苯基吡啶）-Ir（Ir（ppy）₃）掺杂浓度为2％时得到了最高的亮度效率24.8 cd/A,此时的电流密度为4.65 mA/cm²,功率效率为11 lm/W,最高亮度达到35054 cd/m²,色坐标是（0.39,0.56）. 关键词： 共轭聚合物 磷光 绿光发光     

基于复眼透镜的大面积均匀照明方案研究

为了实现大面积矩形均匀光斑,提出一种基于双排复眼透镜的设计方案。使用TracePro软件对该照明系统方案的效果进行建模仿真,分析了复眼透镜组的间距、后工作距离等参数对光斑面积和均匀性的影响。结果表明,复眼透镜间距越小、后工作距离越大,得到的光斑面积越大;而当间距控制在微透镜焦距附近时,光斑均匀性最佳。利用该设计方案,可在1 m的后工作距离得到尺寸350 mm200 mm的光斑,光斑中心280 mm160 mm范围内光照度均匀性＞0.85,该部分面积超过光斑整体面积的60%。该匀光方案有望在医用光疗、路灯照明等大面积均匀光照明领域得到广泛应用。     

Design of double freeform-surface lens for LED uniform illumination with minimum Fresnel losses

In this study, we demonstrated a double freeform-surface lens to realize uniform illumination and minimum Fresnel losses for light-emitting diode (LED). In the present design, the inner surface and outer surface of the freeform lens were designed simultaneously, thus the light path can be controlled more flexibly. The detailed calculation and design process were presented. Monte Carlo ray-tracing simulation and Fresnel losses calculation were conducted to validate the present freeform lens. The simulation and calculation results indicated that the present freeform lens could enhance the illumination uniformity greatly and realize minimum Fresnel losses as low as 7.67%.     

Highly efficient all phosphorescent white organic light-emitting diodes for solid state lighting applications

We report highly efficient all phosphorescent white organic light-emitting diodes (OLEDs) with an exciton-confinement structure. By stacking two emissive layers (EMLs) with different charge transporting properties, effective charges as well as exciton confinements were achieved. Accordingly, efficient blue OLEDs with a peak external quantum efficiency (EQE) over 22% and power efficacy (PE) over 50 lm/W were developed by using iridium(III) bis(4,6-(difluorophenyl) pyridinato-N,C2′)picolinate (FIrpic) as an electro-phosphorescent dopant. When the optimized orange and red EMLs were sandwiched between the stacked two blue EMLs, white OLEDs with an EQE and PE of 24.3% and 45.9 lm/W at a luminance of 1000 cd/m² were obtained without the use of any out-coupling techniques. In addition, these white OLEDs exhibit a color rendering index (CRI) value of 84 with high efficacy.     

High performance polymer light-emitting diodes doped with a novel phosphorescent iridium complex

High performance polymer light-emitting diodes (PLEDs) based on a phosphor of noble metal complex bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate) [(pbi)₂Ir(acac)] doped in poly(N-vinylcarbazole) (PVK) host with various concentration were demonstrated. The photoluminescence (PL) and electroluminescence (EL) spectra of the PLEDs exhibited an emission intensity decrease of PVK and a gradually enhanced feature of (pbi)₂Ir(acac) with increased doping concentration. The device with a 5 wt% (pbi)₂Ir(acac) doped PVK system showed a high power efficiency of 3.84 lm/W and a luminance of 26,006 cd/m². The results indicated that both energy transfer and charge trapping have a significant influence on the performance of PLEDs. The devices have a broadened EL spectrum of full-width at half-maximum (FWHM) more than 100 nm, which can be realized for WOLEDs.     

基于近焦点非球面透镜的LED均匀照明设计

依据几何光学和非成像光学理论,提出了一种基于大尺寸近焦点非球面透镜的大功率LED均匀光源设计方法。在该方法中,首先根据选定参数的LED通过几何光学理论初步设计非球面透镜参数,然后在ZEMAX软件中对非球面透镜参数进行基于评价函数的优化,得到焦距76.79 mm、直径为200 mm的非球面透镜。将非球面透镜导入TRACEPRO软件建模并进行光线追迹仿真,根据仿真结果获得最优透镜参数进行加工和下一步实验。实验结果表明：均匀照明系统可以在60 cm处实现发散角±8.53°的均匀照明,光斑均匀性达到95.82%。     

Coherence characteristics of light-emitting diodes

We report the measurement of coherence characteristics of light-emitting diodes (LEDs). Experiments were performed using red and green color LEDs directly illuminating the Young's double slit kept in the far-zone. Fourier transform fringe analysis technique was used for the measurement of the visibility of interference fringes from which the modulus of degree of spectral coherence was determined. Low degree of spectral coherence, typically 0.4 for red and 0.2 for green LED with double-slit separation of 400 μm was observed. A variable slit was then kept in front of the LEDs and the double slit was illuminated with the light coming out of the slit. Experiments were performed with various slit sizes and the visibility of the interference fringes was observed. It was found that visibility of the interference fringes changes drastically in presence of variable slit kept in front of LEDs and a high degree of spectral coherence, typically 0.85 for red and 0.8 for green LED with double-slit separation of 400 μm and rectangular slit opening of 500 μm was observed. The experimental results are compared with the theoretical counterparts. Coherence lengths of both the LEDs were also determined and it was obtained 5.8±2 and 24±4 μm for green and red LEDs, respectively.     

基于激子阻挡层的高效率绿光钙钛矿电致发光二极管

金属卤化物钙钛矿材料由于具有高的光致发光量子产率、高色纯度、带隙可调等杰出的光学性能,被作为发光材料广泛地用于制备钙钛矿电致发光二极管(perovskite light-emitting diodes,PeLEDs).虽然取得了较好的研究进展,但是其效率和稳定性还未达到商业化的要求,还需要进一步提高.为了提高PeLEDs的效率和稳定性,本文使用旋涂法,引入了一种具有宽带隙和较好空穴传输能力的有机小分子材料4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline](TAPC)作为激子阻挡层,获得了效率和寿命都得到提高的全无机PeLEDs.研究表明,PeLEDs效率和寿命得到提高的物理机制主要源于两方面:1)TAPC具有恰当的最高占有分子轨道能级,与PEDOT:PSS的最高占有分子轨道能级和CsPbBr3的价带边形成了阶梯式能级分布,有利于空穴注入和传输;同时TAPC具有较高的最低未占分子轨道能级,能够有效地阻止电子泄漏到阳极端,并能很好地将电子和激子限制在发光层内;2)TAPC层的引入可以避免钙钛矿发光层与强酸性的空穴注入材料Poly(3,4-ethylenedioxythiophene):poly(p-styrene sulfonate)(PEDOT:PSS)的直接接触,进而免除钙钛矿发光层由于与PEDOT:PSS的直接接触所导致的激子淬灭,从而提高了激子的发光辐射复合率.     

High power and high reliability GaN/InGaN flip-chip light-emitting diodes

High-power and high-reliability GaN/InGaN flip-chip light-emitting diodes (FCLEDs) have been demonstrated by employing a flip-chip design, and its fabrication process is developed. FCLED is composed of a LED die and a submount which is integrated with circuits to protect the LED from electrostatic discharge (ESD) damage. The LED die is flip-chip soldered to the submount, and light is extracted through the transparent sapphire substrate instead of an absorbing Ni/Au contact layer as in conventional GaN/InGaN LED epitaxial designs. The optical and electrical characteristics of the FCLED are presented. According to ESD IEC61000-4-2 standard (human body model), the FCLEDs tolerated at least 10\,kV ESD shock have ten times more capacity than conventional GaN/InGaN LEDs. It is shown that the light output from the FCLEDs at forward current 350mA with a forward voltage of 3.3\,V is 144.68\,mW, and 236.59\,mW at 1.0\,A of forward current. With employing an optimized contact scheme the FCLEDs can easily operate up to 1.0\,A without significant power degradation or failure. The life test of FCLEDs is performed at forward current of 200\,mA at room temperature. The degradation of the light output power is no more than 9\% after 1010.75\,h of life test, indicating the excellent reliability. FCLEDs can be used in practice where high power and high reliability are necessary, and allow designs with a reduced number of LEDs.     

High efficiency blue phosphorescent organic light-emitting diodes without electron transport layer

High efficiency blue phosphorescent organic light-emitting diodes were fabricated without an electron transport layer using a spirobifluorene based blue triplet host material. The simple blue PHOLEDs without the electron transport layer showed a high external quantum efficiency and current efficiency of 16.1% and 30.2 cd/A, respectively. The high device performances of the electron transport layer free blue PHOLEDs were comparable to those of blue PHOLEDs with the electron transport layer.     

Lensless digital holographic microscope with light-emitting diode illumination

We demonstrate the operation of a digital in-line microscope with LED illumination. We show with a practical example that, for typical setups, the limited temporal coherence and the spatial incoherence of the source do not affect the resolving power of the system. On the contrary, important advantages are obtained in terms of signal-to-noise ratio and alignment simplification.     

High brightness InGaN-based yellow light-emitting diodes with strain modulation layers grown on Si substrate

InGaN-based multiple quantum wells (MQWs) yellow light-emitting diodes (LEDs) were grown on Si substrate by metal organic vapor deposition. Blue MQWs were introduced as strain modulation layers for yellow MQWs. The LED chips emitted 72-mW yellow light with 566-nm dominant wavelength and 9.4 % external quantum efficiency (EQE) at 350 mA under room temperature, and it reached a peak EQE of 22.2 % at 0.7 mA. A comparison sample without strain modulation layers exhibited much weaker performance. The results reveal that long-wavelength emission of InGaN system is reliable if the strain of MQWs has been properly modulated.     

High power efficiency in single layer blue phosphorescent organic light-emitting diodes

High efficiency single layer blue phosphorescent organic light-emitting diodes (PHOLEDs) without any charge transport layer were developed. A mixed host of spirobifluorene based phosphine oxide (SPPO13) and 1, 1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) was used as the host in the emitting layer. A high maximum external quantum efficiency of 15.8% and a quantum efficiency of 8.6% at 1000 cd/m² were achieved in the single-layer blue PHOLEDs without any charge transport layer. The maximum power efficiency and power efficiency at 1000 cd/m² were 31.4 and 16.9 lm/W, respectively.     

Solution-processed white organic light-emitting diodes with mixed-host structures

Efficient white light-emitting diodes (WOLEDs) were fabricated with a solution-processed single emission layer composed of a molecular and polymeric material mixed-host (MH). The main host used was a blue-emitting molecular material of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) and the assisting host used was a hole-transport-type polymer of poly(9-vinylcarbazole) (PVK). By co-doping 4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl and 5,6,11,12-tetraphenylnaphacene into the MH, the performances of the fabricated devices made with different mixing ratio of host materials were investigated, and were to depend on the mixing ratios. Under the optimal PVK:DPVBi ratio (3:7), we achieved a maximum luminance of 14 110 cd/m² and a maximum current efficiency of 9.5 cd/A. These improvements were attributed to the MH structure, which effectively improved the thermal stability of spin-coated film and enhanced the hole-injection/transporting properties of WOLEDs.     

Blue InGaN light-emitting diodes with dip-shaped quantum wells

InGaN based light-emitting diodes (LEDs) with dip-shaped quantum wells and conventional rectangular quantum wells are numerically investigated by using the APSYS simulation software. It is found that the structure with dip-shaped quantum wells shows improved light output power, lower current leakage and less efficiency droop. Based on numerical simulation and analysis, these improvements on the electrical and the optical characteristics are attributed mainly to the alleviation of the electrostatic field in dip-shaped InGaN/GaN multiple quantum wells (MQWs).     

Status of GaN-based green light-emitting diodes

GaN-based blue light emitting diodes(LEDs) have undergone great development in recent years,but the improvement of green LEDs is still in progress.Currently,the external quantum efficiency(EQE) of GaN-based green LEDs is typically30%,which is much lower than that of top-level blue LEDs.The current challenge with regard to GaN-based green LEDs is to grow a high quality In GaN quantum well(QW) with low strain.Many techniques of improving efficiency are discussed,such as inserting Al GaN between the QW and the barrier,employing prestrained layers beneath the QW and growing semipolar QW.The recent progress of GaN-based green LEDs on Si substrate is also reported:high efficiency,high power green LEDs on Si substrate with 45.2% IQE at 35 A/cm2,and the relevant techniques are detailed.     

基于定抛物面调制的直下式匀光照明透镜设计

对于近场大角度的匀光照明系统,透镜的一般设计方法是采用内表面为球形的自由曲面透镜设计,计算的自由曲面透镜只需考虑一次折射。但是当角度达到一定范围时,在透镜的内表面容易发生全反射,导致目标面照度不均匀。为了在近场照明系统中得到更大角度的配光,提出一种以合适的抛物面作为自由曲面透镜的内表面的设计方法,内抛物面先对光源的能量进行一次扩散,再计算自由曲面面型,通过模拟仿真,实现了在距离光源15 mm处形成半径40 mm的光斑,均匀度达到95%,能量利用率达到97.8%,相对于传统的直下式透镜,提高了照明的均匀度,同时减少了箱体的厚度。     

Photoinduced improvement of the properties of light-emitting diodes

Results from studies of the effect of the action of optical radiation on the characteristics of light-emitting diodes (LEDs) produced using the binary heterostructure Ga_xAl_1−xAs (λ=0.88 μm) are presented. High sensitivity of the LED to the following parameters of the optical radiation is shown: flux density, quantum energy, and exposure dose. The action of optical radiation in the form of a band with a maximum at 255 nm on the LED heterostructures lowers the leakage current into the bulk, decreases the loss identified as surface leakage current by about an order of magnitude, increases the radiated power by 50–100% in the current region up to 10⁻³ A, and increases the overall light output of the diodes. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 109–114, August, 1997.     

Ultra-violet light-emitting diodes with quasi acceptor-free AlGaN polarization doping

The development and application of nitride-based light-emitting diodes (LEDs) is handicapped by the low hole conductivity of Mg-doped layers. Mg-doping becomes increasingly difficult with higher Al-content of the p-AlGaN layers as required for ultra-violet (UV) light emission. Polarization-induced hole doping of graded AlGaN was recently demonstrated as an alternative doping method. Using advanced numerical device simulation, this paper investigates the impact of polarization-doping on the internal device physics of UV-LEDs and compares the conventional Ga-face growth to the novel N-face growth direction. Various LED design options are explored to maximize the internal quantum efficiency.