Nitride-based light-emitting diodes with Ni/ITO p-type ohmic contacts

The optical and electrical properties of Ni(5 nm)-Au(5 nm) and Ni(3.5 nm)-indium tin oxide (ITO) (60 nm) films were studied. It was found that the normalized transmittance of Ni/ITO film could reach 87% at 470 nm, which was much larger than that of the Ni-Au film. It was also found that the specific contact resistance was 5 /spl times/ 10/sup -4/ /spl Omega/ /spl middot/ cm/sup 2/ and 1 /spl times/ 10/sup -3/ /spl Omega/ /spl middot/ cm/sup 2/, respectively, for Ni-Au and Ni/ITO on p-GaN. Furthermore, it was found that the 20 mA output power of light-emitting diode (LED) with Ni-Au p-contact layer was 5.26 mW. In contrast, the output power could reach 6.59 mW for the LED with Ni/ITO p-contact layer.     

Nitride-based green light-emitting diodes with high temperature GaN barrier layers

High-quality InGaN-GaN multiquantum well (MQW) light-emitting diode (LED) structures were prepared by temperature ramping method during metalorganic chemical vapor deposition (MOCVD) growth. It was found that we could reduce the 20-mA forward voltage and increase the output intensity of the nitride-based green LEDs by increasing the growth temperature of GaN barrier layers from 700/spl deg/C to 950/spl deg/C. The 20-mA output power and maximum output power of the nitride-based green LEDs with high temperature GaN barrier layers was found to be 2.2 and 8.9 mW, respectively, which were more than 65% larger than those observed from conventional InGaN-GaN green LEDs. Such an observation could be attributed to the improved crystal quality of GaN barrier layers. The reliability of these LEDs was also found to be reasonably good.     

Nitride-based near-ultraviolet multiple-quantum well light-emitting diodes with AlGaN barrier layers

The In_0.05Ga_0.95N/GaN, In_0.05Ga_0.95N/Al_0.1Ga_0.9N, and In_0.05Ga_0.95N/Al_0.18Ga_0.82N multiple-quantum well (MQW) light-emitting diodes (LEDs) were prepared by metal-organic chemical-vapor deposition. (MOCVD). It was found that the 20-mA electroluminescence (EL) intensity of the InGaN/Al_0.1Ga_0.9N MQW LED was two times larger than that of the InGaN/GaN MQW LED. The larger maximum-output intensity and the fact that maximum-output intensity occurred at a larger injection current suggest that Al_0.1Ga_0.9N-barrier layers can provide a better carrier confinement and effectively reduce leakage current. In contrast, the EL intensity of the InGaN/Al_0.18Ga_0.82N MQW LED was smaller because of the relaxation that occurred in the MQW active region of the sample.     

Color temperature tunable white organic light-emitting diodes

In this work, we demonstrated color-tunable white organic light-emitting diodes by stacking upper orange transparent and lower blue bottom emission organic light-emitting diodes (OLEDs). By independently operating each OLED, it was possible to tune the color temperature in a range of 1500–10,000 K, which covers the full Planckian locus in the 1931 CIE space. In designing stable and efficient OLEDs, in addition to the electrical characteristics, the importance of internal microcavity was emphasized and implemented. In fabricating the upper transparent OLED, special attention was paid to the capping layer for enhancing the emission. Our results presented a general guideline that is practically useful in designing high-performance color-tunable OLEDs with transparent OLEDs.     

白光有机电致发光器件发光稳定性研究

基于红绿/蓝双发光层,制作了结构为ITO/MoO ₃(10nm)/NPB(40nm)/TCTA(10nm)/CBP:R-4B(2%):GIR1(14%,X nm)/mCP:Firpic(8%,Y nm/BCP(10nm)/Alq₃(40nm)/LiF(1nm)/Al( 100nm)的白色全磷光有机电致发光器件(OLED),通过 调节红绿发光层的厚度X与蓝光发光层的厚度Y,研究了不同发光层厚度器件发 光性能的影响。研究发现:当X 为23nm、Y为7nm时,器件的光效和色坐标都具有 很高的稳定性,在电压分别为5、 10和15V时,色坐标分别为(0.33,0.37)、(0.33,0. 37)和(0.34,0.38);在电压为 5V时,电流密度为0.674mA,亮度为158.7cd ,最大电流效率为26.87cd/A;利用电子阻 挡材料TCTA和空穴阻挡材料BCP能够显著提高载流子的复合效率。分析认为:发光层顺序 为红绿/蓝时,更有利于蓝光的出射,从而使白光的色坐标更稳定。     

Highly efficient solution-processed small-molecule white organic light-emitting diodes

Solution-processed small-molecule white organic light-emitting diodes (WOLEDs) were fabricated with a co-host of hole-transporter 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) and electron-transporter 2,7-Bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13). By doping 15 wt% FIrpic or F₃Irpic and 0.5 wt% Ir(MDQ)₂(acac) in to the TCTA/SPPO13 host, highly efficient white OLEDs have been achieved which exhibit nearly identical emission spectra at different luminance. The F₃Irpic and Ir(MDQ)₂(acac)-based WOLED shows maximum efficiencies of 40.9 cd/A, 36.7 lm/W and 16.9%, and even high efficiencies of 30.1 cd/A and 12.3% at the practical luminance of 1000 cd/m², which are among the highest efficiencies of the solution-processed small-molecule WOLEDs. These results demonstrate a convenient way to realize solution-processed WOLEDs with high efficiency and high spectral stability through full small-molecule materials system.     

Bright and efficient white stacked organic light-emitting diodes

High brightness and efficient white stacked organic light-emitting diodes have been fabricated by connecting individual blue and red emissive units with the anode–cathode layer (ACL) consisting of LiF (1 nm)/Ca (25 nm)/Ag (15 nm). Use 1,3-bis(carbazol-9-yl)benzene (mCP):bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FirPic) as the blue emitter and tris(8-hydroxy-quinolinato)aluminium (Alq₃):4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) as the red emitter, white light emission with CIE coordinates of (0.32, 0.38) was obtained at a driving voltage of 26 V with a luminance of 40,000 cd/m². By replacing the red fluorescent emitter with a phosphorescent one, the color coordinates were improved to (0.33, 0.31). The peak external quantum efficiency was enhanced from 5.3% (at 28.2 mA/cm²) to 10.5% (at 1.4 mA/cm²) as well.     

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.     

Fluorescent SiC and its application to white light-emitting diodes

Fluorescent-SiC(f-SiC),which contains donor and acceptor impurities with optimum concentrations, has high conversion efficiency from NUV to visible light caused by donor-acceptor-pair(DAP) recombination. This material can be used as a substrate for a near UV light-emitting diode(LED) stack,and leads to monolithic white LED device with suitable spectral property for general lighting applications.In this paper,we describe basic technologies of the white LED,such as optical properties of f-SiC substrate,and epitaxial growth of NUV stack on the f-SiC substrate.     

Efficient,color-stable flexible white top-emitting organic light-emitting diodes

Flexible white top-emitting organic light-emitting diodes (WTEOLEDs) with red and blue phosphorescent dual-emitting layers were fabricated onto polyethylene terephthalate (PET) substrates. By inserting a 2-nm thin tris(phenypyrazole)iridium between the red and the blue emitters as an electron/exciton blocking layer, significant improvements on luminous efficiency and color stability were observed, reaching 9.9 cd/A (3.74 lm/W) and a small chromaticity change of (0.019, 0.011) in a wide luminance range of 80–5160 cd/m². The origin on color stability was explored by analyzing the electroluminescent spectra, the time-resolved transient photoluminescence decay lifetimes of phosphors, and the tunneling phenomenon. In addition, mechanical bending lifetimes in WTEOLEDs with spin-coated     

Fluorescent SiC and its application to white light-emitting diodes

Fluorescent-SiC (f-SiC), which contains donor and acceptor impurities with optimum concentrations, has high conversion efficiency from NUV to visible light caused by donor-acceptor-pair (DAP) recombination. This material can be used as a substrate for a near UV light-emitting diode (LED) stack, and leads to monolithic white LED device with suitable spectral property for general lighting applications. In this paper, we describe basic technologies of the white LED, such as optical properties of f-SiC substrate, and epitaxial growth of NUV stack on the f-SiC substrate.     

氮化镓基白光发光二极管的快中子辐照效应

对注入量为 1×10¹⁴cm^-2 的快中子(1.2 MeV)对氮化镓(GaN)基白光发光二极管(LED)器件的辐照效应进行研究。通过测量和分析器件的电致发光谱(EL)、光功率-电流(L-I)和电流-电压(I-U)特性,发现器件辐照后光功率降低,而 EL 谱形状几乎没有变化,表明该注入量的中子辐照主要对器件中的蓝光 LED 芯片造成了损伤。进一步分析发现,中子辐照导致蓝光 LED 量子阱中产生大量非辐射复合中心,增加了漏电流并减小了量子阱中载流子密度,从而降低 LED 的输出光功率。由此,在原有 GaN 基蓝光 LED 等效电路模型的基础上,加入由中子辐照导致的影响因素,不仅有助于理解中子辐照对 LED 光功率的衰退影响机理,还为预测辐照后光功率的变化提供了可行性。     

All-small-molecule efficient white organic light-emitting diodes by multi-layer blade coating

Blue and white small-molecule organic light-emitting diodes are fabricated by multi-layer blade coating on hot plate at 80 °C with hot wind. Uniform multi-layer structures are made without dissolution due to rapid drying. Only small molecules originally developed for vacuum deposition are used. For hole transport layer of, 4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA), electron transport layer of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TBPI), emissive layer host of, 6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy), triplet emitters of bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic), and cathode of LiF/Al, the peak current efficiency for blue emission is 25.1 cd/A (10.8% and 9.3 lm/W). Orange emitter iridium(III)bis (4-(4-t-butylphenyl) thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01-TB) is added to obtain white emission with CIE coordinate of (0.39, 0.46) [1]. The current efficiency is 34.2 cd/A (11.6% and 12 lm/W) at maximum, 32.4 cd/A at 1000 cd/m², and 31 cd/A at 10,000 cd/m².     

Nitride-based laser diodes using thick n-AlGaN layers

We obtained 1 μm crack-free AlGaN layers up to an AlN molar fraction of 0.4 by growing directly on low-temperature-deposited buffer layers. The buffer layer is effective for growing AlGaN layers without the stress caused by the lattice mismatch. We also demonstrated nitride-based laser diodes with such a 1 μm crack-free n-AlGaN cladding layer/n-AlGaN contact layer/low-temperature-deposited buffer layer/sapphire structure, which showed a clear single spot in a far field pattern. The AlGaN-based structure can suppress optical leakage from the waveguide region to the underlying layer. The threshold current of the laser diode is about 230 mA, which is comparable to or better than that of our laser diodes with the conventional GaN-based structure.     

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.     

Enhanced carrier confinement in AlInGaN-InGaN quantum wells in near ultraviolet light-emitting diodes

To increase carrier confinement, the GaN barrier layer was substituted with an AlInGaN quaternary barrier layer which was lattice-matched to GaN in the GaN-InGaN multiple quantum wells (MQWs). Photoluminescence (PL) and high-resolution X-ray diffraction measurements showed that the AlInGaN barrier layer has a higher bandgap energy than the originally used GaN barrier layer. The PL intensity of the five periods of AlInGaN-InGaN MQWs was increased by three times compared to that of InGaN-GaN MQWs. The electroluminescence (EL) emission peak of AlInGaN-InGaN MQWs ultraviolet light-emitting diode (UV LED) was blue-shifted, compared to a GaN-InGaN MQWs UV LED and the integrated EL intensity of the AlInGaN-InGaN MQWs UV LED increased linearly up to 100 mA. These results indicated that the AlInGaN-InGaN MQWs UV LED has a stronger carrier confinement than a GaN-InGaN MQWs UV LED due to the larger barrier height of the AlInGaN barrier layer compared to a GaN barrier layer.     

Novel scattering and color converting substrates for simple-structured white organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have shown great success in the display applications recently. However, the applications of OLEDs in lighting are still limited due to their complex device structures. Here, we developed a novel phosphor doped glass substrate with both high scattering and excellent color conversion capability to greatly simplify the device structures of white organic light-emitting diodes (WOLEDs). A simple-structured WOLED comprising a blue OLED and the scattering fluorescent substrate was demonstrated to realize high quality white light for lighting applications. The WOLED exhibits a turn-on voltage of 2.7 V, a maximum power efficiency of 29.8 lm/W, an external quantum efficiency (EQE) of 14.2%, a color rendering index (CRI) of 86, and a correlated color-temperature (CCT) of 3900 K. The low turn-on voltage can be attributed to the single emissive layer structure used in the WOLED. The high power efficiency as well as the high EQE are due to both the high color conversion efficiency and the high scattering capability of the fluorescent substrate. In addition, the WOLED is favorable for high-quality solid-state lighting in our daily life due to its high color rendering ability along with an adequate CCT CC.     

基于混合配体Znq(DBM)的单色和白色OLED

以8-羟基喹啉(q)和1,3-二苯基-1,3-丙二酮定向合成了有机小分子配合物Znq(DBM),将其作为发光层制备了单色有机电致发光器件(OLED)。在结构为ITO/m-MTDATA(5nm)/NPB(40nm)/Znq(DBM)(60nm)/LiF(0.5nm)/Al(100nm)的器件中,启亮电压为5V,最大亮度达到4 575cd/m2。同时又在器件中引入间隔层BCP,研究其不同厚度对OLED性能的影响。在结构为ITO/m-MTDATA(5nm)/NPB(40nm)/BCP(x nm)/Znq(DBM)(60nm)/LiF(0.5nm)/Al(100nm)的器件中,当BCP层厚为0nm时,发光颜色为黄绿色;当BCP层厚为1nm时,发光颜色为白色,色坐标为(0.29,0.33),最大亮度为2 231cd/m2;当BCP层厚为5nm时,发光颜色为蓝色。根据器件结构和性能,讨论了其内部机理。     

Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure

In this paper, we report color stable phosphorescent white organic light-emitting diodes (OLEDs) based on a double emissive layer (EML) structure composed of blue and red/green phosphorescent units. Deep hole trapping situation of red and green dopants at the red/green EML could induce less voltage dependent white spectral characteristics by restricting the change of exciton generation zone. A wide band-gap host material, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), was used for achieving such deep-trap generation. Fabricated phosphorescent white OLED shows a slight color coordinate change of (?0.002, +0.002) from 1000 cd/m² to 5000 cd/m² with power efficiency of 38.7 lm/W and current efficiency of 46.4 cd/A at 1000 cd/m². In addition, negligible color changes were observed by delaying red dopant saturation time using optimum red dopant concentration.     

Color stable multilayer all-phosphor white organic light-emitting diodes with excellent color quality

The color stability of all-phosphor white organic light-emitting diodes (WOLEDs) is crucial and remains a challenge that must be overcome before the wide application of phosphor WOLEDs technology. Besides, color stable all-phosphor WOLEDs should also offer high color rendering index (CRI) and ideal correlated color temperature (CCT) simultaneously to make the technology competitive against other alternative technologies such as inorganic LEDs. In this work, we demonstrate a series of color stable all-phosphor WOLEDs with two emitters (blue and yellow), three emitters (blue, green/red, and yellow) and four emitters (blue, green, yellow and red) by introducing tris (phenylpyrazole) Iridium [Ir(ppz)₃] as interlayer. The results show that appropriate thickness of Ir(ppz)₃ interlayer not only can control exciton distribution in the emission zone, but also can improve the spectra stability. In particular, one efficient four-color device with double-interlayer yields fairly high CRI of 92 and 90, ideal CCT of 3703 K and 3962 K at illumination-relevant luminance of 100 cd m^–2 and 1000 cd m^–2, respectively, which is very appropriate to indoor lighting application. By further employing appropriate hosts to regulate the carrier injection, ultrahigh stable four-color devices with applicable CRI are finally achieved.