GaInN light-emitting diodes with omnidirectional reflector using nanoporons SnO2 film

Enhancement of light extraction in a GaInN light-emitting diode(LED)employing an omni-directional reflector(ODR)consisting of GaN,SnO2 nanorod and an Ag layer was presented.The ODR comprises a transparent,quaxterwave layer of SnO2 nanorod claded by silver and serves as an ohmic contact to p-type were prepared by dip-coating technique.The average size of the spherical SnO2 particles obtained is 200 nm.The refractive index of the nanorod SnO2 film layer is 2.01.The GaInN LEDs with GaN/SnO2/Ag ODR show a lower forward voltage.This was attributed to the enhanced reflectivity of the ODR that employs the nanorod SnO2 film layer.Experimental results show that ODR-LEDs have lower optical losses and higher extraction efficiency as compared to conventional LEDs with Ni/Au contacts and conventional LEDs employing a distributed Bragg reflector(DBR).     

The beneficial effects of a p-type GaInN spacer layer on the efficiency of GaInN/GaN light-emitting diodes

Light-emitting diodes (LEDs) with a Mg-doped p-type Ga_1−xIn_xN (0 ≤ x ≤ 0.07) spacer layer located between an undoped GaN spacer layer and the electron blocking layer are investigated. The LEDs are found to have comparable peak efficiency but less efficiency droop when the crystal quality of the p-type Ga_1−xIn_xN spacer layer is well-controlled by lowering the growth temperature and by using a suitable In composition and Mg doping concentration. All LED samples with the p-type spacer layer show a smaller efficiency droop compared to a reference LED having an undoped GaN spacer. Among the sample sets investigated, an optical power enhancement of 12% at 111 A/cm² is obtained when inserting a 5 nm-thick p-type Ga_0.97In_0.03N spacer layer. The results support that carrier transport is the key factor in the efficiency droop observed in GaN-based LEDs.     

Omnidirectional reflector using nanoporous SiO2 as a low-refractive-index material

Triple-layer omnidirectional reflectors (ODRs) consisting of a semiconductor, a quarter-wavelength transparent dielectric layer, and a metal have high reflectivities for all angles of incidence. Internal ODRs (ambient material's refractive index n > 1.0) are demonstrated that incorporate nanoporous SiO2, a low-refractive-index material (n = 1.23), as well as dense SiO2 (n = 1.46). GaP and Ag serve as the semiconductor and the metal layer, respectively. Reflectivity measurements, including angular dependence, are presented. Calculated angle-integrated TE and TM reflectivities for ODRs employing nanoporous SiO2 are R(int)/TE = 99.9% and R(int)/TM = 98.9%, respectively, indicating the high potential of the ODRs for low-loss waveguide structures.     

Optical characteristics of light-emitting diodes with high transparent conductive film at low temperatures

In this paper, we report the synthesis and transmittance of a titanium–indium–tin oxide (TITO) film, fabricated through a low-temperature process. The TITO film was fabricated by incorporating a 2-nm-thick titanium barrier at the bottom of an ITO film. The transmittance characteristics of the TITO film were examined for light-emitting diodes (LEDs) of various wavelengths at different post-annealing temperatures. A saturated high transmittance was observed at a temperature of 550 °C, which is relatively low when compared to that in the case of a conventional ITO film. Photoluminescence studies demonstrated that a 450-nm-thick TITO film, fabricated at 550 °C, was highly effective in improving the performance of the LED, when compared to conventional ITO films. The X-ray diffraction peaks, scanning electron microscopy images, and transmittance electron microscopy images confirmed that titanium atoms could improve the crystallization of ITO. It was found that non-crystallization in ITO was effectively activated by the titanium barrier. Furthermore, the optical bandgap (3.77 eV for the conventional ITO film) was improved to 3.92 eV in the TITO film. An infrared LED fabricated with a TITO film displayed 70% higher light output power than that with a conventional ITO film. These results suggest that using a titanium barrier is essential to effectively improve inactive nucleation sites in ITO films grown at low temperatures.     

The enhancement of light-emitting efficiency using GaN-based multiple quantum well light-emitting diodes with nanopillar arrays

The quest for higher modulation speed and lower energy consumption has inevitably promoted the rapid development of semiconductor-based solid lighting devices in recent years. GaN-based light-emitting diodes （LEDs） have emerged as promising candidates for achieving high efficiency and high intensity, and have received increasing attention among many researchers in this field. In this paper, we use a self-assembled array-patterned mask to fabricate InGaN/GaN multi- quantum well （MQW） LEDs with the intention of enhancing the light-emitting efficiency. By utilizing inductively coupled plasma etching with a self-assembled Ni cluster as the mask, nanopillar arrays are formed on the surface of the InGaN/GaN MQWs. We then observe the structure of the nanopillars and find that the V-defects on the surface of the conventional structure and the negative effects of threading dislocation are effectively reduced. Simultaneously, we make a comparison of the photoluminescence （PL） spectrum between the conventional structure and the nanopillar arrays, achieved under an experimental set-up with an excitation wavelength of 325 mm. The analysis demonstrates that MQW-LEDs with nanopillar arrays achieve a PL intensity 2.7 times that of conventional LEDs. In response to the PL spectrum, some reasons are proposed for the enhancement in the light-emitting efficiency as follows： 1） the improvement in crystal quality, namely the reduction in V-defects; 2） the roughened surface effect on the expansion of the critical angle and the attenuated total reflection; and 3） the enhancement of the light-extraction efficiency due to forward scattering by surface plasmon polariton modes in Ni particles deposited above the p-type GaN layer at the top of the nanopillars.     

White light-emitting diodes using blue and yellow-orange-emitting phosphors

A blue-emitting phosphor, BaMgAl₁₀O₁₇:Eu²⁺ (BAM) and a yellow-orange phosphor, Ba²⁺-codoped Sr₃SiO₅:Eu²⁺ were prepared by the solid-state reaction. Excitation and emission spectra results showed that BAM and Ba²⁺-codoped Sr₃SiO₅:Eu²⁺ can be efficiently excited by near-ultraviolet (n-UV)-visible light from 250 to 440 nm. The effects of the doped-Eu²⁺ concentration in BAM and Ba²⁺-codoped Sr₃SiO₅:Eu²⁺ on the photoluminescence were investigated in detail. White light-emitting diodes (LED) was obtained by combining n-UV LED chip (GaN-based 380 nm emitting) with BaMgAl₁₀O₁₇:0.09Eu²⁺ and 0.1Ba²⁺-codoped Sr₃SiO₅: 0.2Eu²⁺ phosphors with the characteristic of color-rendering index of 86, CIE chromaticity coordinates (x,y) of (0.3216,0.3096), and color temperature T_c of 5700 K. As the current increases, the relative intensity simultaneously increases. The CIE chromaticity coordinates (x,y) of the white LED tends to decrease. The correlated color temperature T_c increases from 4100 to 7500 K and the color-rendering index R_a increases from 82 to 87 simultaneously.     

Sharply directed emission in microcavity organic light-emitting diodes with a cholesteric liquid crystal film

A flexible microcavity organic light-emitting diode (OLED) was fabricated and the emitting characteristics were examined. A pair of right- and left-handed cholesteric liquid crystal (CLC) films were attached to the microcavity OLED between aluminum (Al) and silver (Ag). Sharply directed spontaneous emission was observed from microcavity OLEDs, in which a typical luminescent material, 8-hydroxyquinoline aluminum (Alq₃), with a broad emission spectrum was used for emitting layer. The introduction of the CLC film improved both the emission bandwidth and directionality, preserving the turn-on voltage and maximum brightness.     

利用温变电容特性测量发光二极管结温的研究

结区的温度, 简称结温, 是发光二极管(LED) 的重要参数之一, 它对LED 器件的出光效率、光色、器件可靠性和寿命均有很大影响, 准确测量LED 器件的结温对制备LED 芯片、器件封装和应用有着重要的意义. 本文利用反向偏压下的LED的势垒电容随温度变化的特性, 提出了一种LED结温测量的新方法. 论文首先测量和分析了LED在室温下反向偏压时的电容-电压(C-V)曲线和不同反向偏压下的电容-温度(C-T)曲线, 结果表明, 在合适的偏压下, LED的电容随温度的增大而显著增加, 并呈现良好的线性关系. 在LED工作中监测其电容的变化, 并与C-T曲线进行对比, 实现了LED结温的测量, 其测量结果和传统的正向电压法的结果相对比, 两者符合较好. 最后, 利用上述方法测量了LED 在恒流和恒压条件下的结温的实时变化过程. 较传统的结温测量方法, 本方法的优点在于只须要一次定标测量, 且可实现LED在任意电压和电流下的结温测量.     

Efficiency-enhanced AlGaInP light-emitting diodes using transparent plasmonic silver nanowires

Silver nanowire(AgNW) networks have been demonstrated to exhibit superior transparent and conductive performance over that of indium-doped tin oxide(ITO) and have been proposed to replace ITO, which is currently widely used in optoelectronic devices despite the scarcity of indium on Earth. In this paper, the current spreading and enhanced transmittance induced by AgNWs, which are two important factors influencing the light output power, were analyzed. The enhanced transmittance was studied by finite-difference time-domain simulation and verified by cathodoluminescence measurements.The enhancement ratio of the light output power decreased as the Ga P layer thickness increased, with enhancement ratio values of 79%, 52%, and 15% for Ga P layer thicknesses of 0.5 μm, 1 μm, and 8 μm, respectively, when an AgNW network was included in Al Ga In P light-emitting diodes. This was because of the decreased current distribution tunability of the AgNW network with the increase of the Ga P layer thickness. The large enhancement of the light output power was caused by the AgNWs increasing carrier spread out of the electrode and the enhanced transmittance induced by the plasmonic AgNWs. Further decreasing the sheet resistance of AgNW networks could raise their light output power enhancement ratio.     

Red-enhanced white light-emitting diodes using external AlGaInP epilayers with various aperture ratios

A novel hybrid white light-emitting diode (LED) is proposed to enhance the red color and stability of the chromaticity under various operating currents using an AlGaInP photon-recycling (PR) epilayer. A blue/yellow white InGaN LED sample is covered with an external patterned AlGaInP epilayer. Under an optimum aperture ratio of 30%, the luminous efficiency of the PR-LED lamp can achieve a color coordinate of (x=0.340, y=0.295). When the injection current increases from 50 to 350 mA, the color temperature decreases from 4030 to 3700 K and the color rendering index increases from 81 to 86. These could be due to the fact that the AlGaInP epilayer was not attached onto the LED chip, alleviating the thermal effect on the color coordinate. The present white PR-LED samples have high potential for portable liquid-crystal-display backlight applications.     

Realization of high-performance blue organic light-emitting diodes using multi-emissive layers

High-performance blue organic light-emitting diodes （OLEDs） are developed. A concept of using multiple-emissive layer （EML） configuration is adopted. In this letter, bis（2-methyl-8-quinolinolate）-4- （phenylphenolato）A1 （BAlq） and 9,10-di（naphtha-2-yl）anthracene （ADN）, which serve n- and p-type EMLs, respectively, are used to evaluate and demonstrate the multi-EML concept for blue OLEDs. The thickness effect of individual EMLs and the number of EMLs, e.g., triple and quadruple EML components, on the power efficiency of blue OLEDs are systematically investigated. To illustrate the point, the total thickness of the emissive region in different blue OLEDs are kept contact at 30 nm for comparison. The power efficiency of blue OLEDs with a quadruple EML structure of BAlq/ADN/BAlq/ADN is about 40% higher than that of blue OLEDs having a single EML unit. The Commission Internationale deL＇eclairage color coordinates of multi-EML OLEDs have values that represent the average of blue emissions from individual EMLs of BAlq and ADN.     

Multi-stacked organic light-emitting diodes using zinc oxide nanoparticle interfacial layers

Stacked organic light-emitting diodes (SOLEDs) with 30-nm nanoparticle (NP) interfacial layers were investigated. Zinc oxide (ZnO) was used as an interfacial layer between two green polymer (GP) layers. SOLEDs with NP interfacial layers had higher device efficiency than did a single-unit device due to the high probability of exciton recombination that originated from the Auger electron-assisted energy up-conversion process. Although the current density and luminance of SOLEDs with ZnO NP interfacial layers were smaller than those of the reference device, the efficiency was doubled because of the big band alignment difference and the large band gap between GP and ZnO NP interfacial layers, which induced more radiative-exciton recombination.     

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).     

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.     

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     

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.     

纳米晶SnO2透明导电薄膜的研制

阐述了金属氧化物透明导电薄膜研究的发展情况及其应用前景。介绍了采用磁控溅射技术 ,使用混合气体Ar和O2 ,在衬底温度为 15 0～ 40 0℃的耐热玻璃基片上制备纳米晶SnO2 :Sb透明导电薄膜 ,通过测定X射线衍射谱 ,表明薄膜择优取向为 [110 ]和 [2 11]方向。测量了SnO2 :Sb薄膜的导电特性随衬底温度及氧分压的变化。光学特性测量结果表明薄膜具有较高的透过率。     

White phosphorescent organic light-emitting diodes using double emissive layer with three dopants for color stability

We fabricate white phosphorescent organic light-emitting diodes （PHOLEDs） with three dopants and double emissive layer （EML） to achieve color stability. The white PHOLEDs use FIrpic dopant for blue EML （B- EML）, and Ir（ppy）3：Ir（piq）3 dopants for green：red EML （GR-EML） with N,N＇-dicarbazolyl-3, 5-benzene （mCP） as host material. Thicknesses of B-EML and GR-EML are adjusted to form a narrow recombination zone at two EML＇s interface and charge trapping happens in EML according to wide highest occupied molecular orbital and/or lowest unoccupied molecular orbital energy band gap of mCP and smaller energy band gap of dopants. The total thickness of both EMLs is fixed at 30 nm in the device structure of ITO （150 nm）/MoO3 （2 nm）/N,N＇-diphenyl-N,N＇-bis（1-naphthyl-phenyl）-（1,1″-biphenyl）-4, 4＇-diamine （70 nm）/ meP：Firpic-8.0% （12 nm）/mCP：Ir（ppy）3-3.0%：Ir（piq）3-1.5% （18 nm）/2″,2＇,2＂＇-（1,3,5-benzinetriyl）-tris（1- phenyl-l-H-benzimidazole） （30 nm）/8-hydroxyquinolinolato-lithium （2 nm）/A1 （120 nm）. White PHOLED shows 18.25 cd/A of luminous efficiency and white color coordinates of （0.358 and 0.378） at 5000 cd/m2 and color stability with slight CIExy change of （0.028 and 0.002） as increasing luminance from 1000 to 5000 cd/m^2.     

All-in-one-type organic light-emitting diodes for color tuning using phosphor in glasses with Pb-free silicate powders

We demonstrate all-in-one-type organic light-emitting diodes (OLEDs) that are fabricated using a color converting plate as a substrate. The color converting plate is Pb-free phosphor-in-glass (PiG), which is prepared by mixing Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) and SiO₂–B₂O₃–RO (R = Ba, Zn) glass frit by sintering at 750 °C for 30 min. The maximum luminance, luminance efficiency, and power efficiency of blue OLEDs fabricated on commercial glass are measured as 10500 cd/m², 10.18 cd/A, and 2.95 lm/W, respectively. The Commission Internationale de l'Eclairge (CIE) coordinates of blue OLEDs is (0.167, 0.325). Our obtained results show that the luminance value decreased as the PiG thickness increased, and the glass to phosphor (GTP) ratio decreased. The OLED devices fabricated on the PiG substrate (GTP ratio = 9:1, thickness: 150 μm) showed a maximum luminance, luminance efficiency, and power efficiency of 7600 cd/m², 8.76 cd/A, and 2.85 lm/W, respectively. The CIE color coordinates changed to (0.286, 0.504) at 200 mA/cm². These results proved that color coordination can be easily adjusted by varying the GTP ratio and the thickness of the PiG.