Tandem organic light-emitting diodes with an effective charge-generation connection structure

The tandem organic light-emitting diodes (OLEDs) with an effective charge-generation connection structure of Mg-doped tris(8-hydroxyquinoline) aluminum (Alq₃)/Molybdenum oxide (MoO₃)-doped 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) were presented. At a current density of 50 mA/cm², the current efficiency of the tandem OLED with two standard NPB/Alq₃ emitting units is 4.2 cd/A, which is 1.7 times greater than that of the single EL device. The tandem OLED with the similar connection structure of Mg-doped PTCDA/ MoO₃-doped PTCDA was also fabricated and the influences of the different connection units on the current efficiency of the tandem OLED were discussed as well.     

Tandem organic light-emitting diode with molybdenum tri-oxide thin film interconnector layer

A 10-nm thickness molybdenum tri-oxide (MoO₃) thin film was used as the interconnector layer in tandem organic light-emitting devices (OLEDs). The tandem OLEDs with two identical emissive units consisting of N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB)/tris(8-hydroxyquinoline) aluminum (Alq₃) exhibited current efficiency-current density characteristics superior to the conventional single-unit devices. At 20 mA/cm², the current efficiency of the tandem OLEDs using the interconnector layers of MoO₃ thin film was about 4.0 cd/A, which is about twice of that of the corresponding conventional single-unit device (1.8 cd/A). The tandem OLED showed a higher power efficiency than the conventional single-unit device for luminance over 1200 cd/m². The experimental results demonstrated that a MoO₃ thin film with a proper thickness can be used as an effective interconnector layer in tandem OLEDs. Such an interconnector layer can be easily fabricated by simple thermal evaporation, greatly simplifying the device processing and fabrication processes required by previously reported interconnector layers. A possible explanation was proposed for the carrier generation of the MoO₃ interconnector layer.     

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.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

High-performance organic red-light-emitting device based on DCJTB and a new host material

An efficient red-light-emitting device using a new host material (DPF) and a red dopant (DCJTB) with a configuration of ITO/NPB (50 nm)/DCJTB:DPF (2%, 10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Mg:Ag has been fabricated and investigated. The red OLED yields a brightness of 9270 cd/m² at 10 V, a maximum current efficiency of 4.2 cd/A and a maximum power efficiency of 3.9 lm/W. Using DPF as host material, the performance is much better than that of a prototypical Alq₃-based device, which has a maximum efficiency of 1.9 cd/A and 0.6 lm/W. The performance is even comparable with red OLEDs using an assist dopant or a cohost emitter system. Results of this work indicate that DPF is a promising host material for red OLEDs with high efficiency and simple device structure.     

利用p型掺杂技术制备的高效有机电致发光器件

利用氧化钼(MoO_x)作为p型掺杂剂,以掺杂层4,4'-bis(carbazol-9-yl)biphenyl(CBP):MoO_x作为空穴注入层,制备了一种结构为ITO/MoO_x/CBP:MoO_x/CBP/CBP:tris(2-phenylpyridine)iridium(III)(Ir(ppy)₃)/4,7-diphenyl-1,10-phenanthroline(Bphen)/LiF/Al的有机电致发光器件.器件中CBP同时作为空穴注入层、空穴传输层以及发光层母体材料,这种结构具有结构简单同时能有效降低空穴注入势垒等优点.研究发现,随着空穴注入层厚度的增加,器件的电流密度增加,表明p型掺杂层的引入能够有效增强空穴的注入;通过优化器件空穴注入层与空穴传输层厚度,器件性能有所提高,最大电流效率为29.8 cd/A,可以认为合理的优化空穴注入层和空穴传输层的厚度,使载流子在发光层中的分布更加平衡是提高器件发光效率的主要原因.值得指出的是,从电流效率最大值到亮度为 20 000 cd/m²时,优化后器件的效率衰减仅为17.7%,而常规器件的效率衰减则为62.1%,优化后器件效率衰减现象得到了明显的改善.分析认为优化后的器件中未掺杂的CBP有助于展宽激子形成区宽度,进而减弱了三线态-三线态湮灭、三线态-极化子淬灭现象,激子形成区的展宽是改善效率衰减的主要原因.     

Color tunable high efficiency microcavity organic light-emitting diodes

Color tunable microcavity organic light-emitting diodes (OLEDs) with structure of distributed Bragg reflectors (DBR)/indium-tin-oxide (ITO)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/tris(8-hydroxyquinoline) aluminum (Alq₃)/LiF/Al were fabricated. Orange red and green light emissions with full width at half maximum (FWHM) of less than 20 nm were obtained through simply changing the thickness of NPB layer. Furthermore, due to the effective modification of the spontaneous emission within microcavity, the brightness and electroluminescent (EL) efficiency of the microcavity OLEDs were significantly enhanced. The maximum brightness and current efficiency, respectively, reached 31000 cd/m² at a current density of 480.0 mA/cm² and 8.3 cd/A at a current density of 110.0 mA/cm² for green devices, and 9700 cd/m² at a current density of 180.0 mA/cm² and 6.6 cd/A at a current density of 36.4 mA/cm² for red devices, which are over 1.5 times higher than those of noncavity OLEDs.      

High performance yellow light-emitting organic electrophosphorescent devices based on Re(I) complex

Yellow organic light-emitting devices (OLEDs) with (2,3-diphenylpyrazino [2,3-f] [1, 10] phenanthroline) Re(CO)₃Br (Re-Dppp) doped CBP as emissive layer were studied. It is found experimentally that Re-Dppp based yellow OLEDs possess high electroluminescent efficiency and brightness: a maximum brightness of 6249 cd/m² at 15 V and a highest current efficiency of 14.2 cd/A with a brightness of 180 cd/m² occurring at 1.3 mA/cm² are achieved, which indicate that Re(I) complexes have a potential application in commercialized PMOLEDs. PACS 85.60.Jb; 33.50.Dq; 33.50.-j; 42.79.Kr; 85.30.De     

基于新型电荷生成层的P-i-N结构白色有机叠层电致发光器件

采用2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline(BCP): 5 wt.% cesium carbonate(Cs₂CO₃)和N, N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine(NPB): 20 wt.% molybdenum oxide(MoO₃)分别作为器件的电子注入层和空穴注入层,研究了N型和P-i-N结构有机电致发光器件的载流子传输特性.载流子传输层中BCP: Cs₂CO₃和NPB:MoO₃的引入有效增强了载流子注入能力,从而降低了器件的驱动电压.基于新型电荷生成层BCP: 5 wt.% Cs₂CO₃/ NPB: 20 wt.% MoO₃制备了色稳定、高效率P-i-N结构有机叠层器件.与单元器件相比,引入新电荷生成层有机叠层器件的最大电流效率增大了2.5倍,表明该电荷生成层可以有效地将电子和空穴分别注入到相邻发光单元中.采用该电荷生成层制备了P-i-N结构白色有机叠层器件,器件的上下发光单元分别为橙光和蓝光发射.当发光亮度从500增加到5 000 cd/m²时,器件的色坐标稳定在(0.33, 0.29)附近,接近白光等能点.利用单色发光单元堆叠制备白色有机叠层器件的方法为实现色稳定、高效率的白色有机电致发光器件提供了一种有效的途径.     

Improvement of performance of tetraphenylporphyrin-based red organic light emitting diodes using WO3 and C60 buffer layers

One of the porphyrin derivatives, meso-tetraphenylporphyrin (TPP), has been synthesized and examined as an emitter material (EM) for efficient fluorescent red organic light-emitting diodes (OLEDs). By inserting a tungsten oxide (WO₃) layer into the interface of anode (ITO) and hole transport layer N,N′-Di-[(1-napthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (NPB) and by using fullerene (C₆₀) in contact with a LiF/Al cathode, the performance of devices was markedly improved. The current density–voltage–luminance (J–V–L) characterizations of the samples show that red OLEDs with both WO₃ and C₆₀ as buffer layers have a lower driving voltage and higher luminance compared with the devices without buffer layers. The red OLED with the configuration ITO/WO₃ (3 nm)/NPB (50 nm)/TPP (60 nm)/BPhen (30 nm)/C₆₀ (5 nm)/LiF (0.8 nm)/Al (100 nm) achieved the high luminance of 6359 cd/m² at the low driving voltage of 8 V. At a current density of 20 mA/cm², a pure red emission with CIE coordinates of (0.65; 0.35) is observed for this device. Moreover, a power efficiency of 2.07 lm/W and a current efficiency of 5.17 cd/A at 20 mA/cm² were obtained for the fabricated devices. The study of the energy level diagram of the devices revealed that the improvement in performance of the devices with buffer layers could be attributed to lowering of carrier-injecting barrier and more balanced charge injection and transport properties.     

Interfacial electronic structure of molybdenum oxide on the fullerene layer,a potential hole-injecting layer in inverted top-emitting organic light-emitting diodes

The interfacial electronic structures of molybdenum oxide (MoO_x) deposited on fullerene (C₆₀) which could be used as a hole-injecting layer in inverted top-emitting organic light-emitting diodes (TE-OLEDs) were investigated by photoemission spectroscopy. The hole-injecting barrier height (Φ_B^h) at each interface investigated by an ultraviolet photoemission spectroscopy was reduced to from 1.4 to 0.1 eV as the thickness of MoO_x (Θ_MoOx) was increased from 0.1 to 5.0 nm on C₆₀. In these interface system, the sign of vacuum-level shift, highest occupied molecular orbital (HOMO)-level shift, and core-level shifts were all positive indicating that the interface mechanism is attributed to the work-function differences due to a band bending at these interfaces. Moreover, the near-edge X-ray absorption fine structure spectra at carbon K-edge did not show any structural modification as well as any chemical reaction at the MoO_x-on-C₆₀ interfaces when Θ_MoOx was changed on C₆₀. From these results, the inverted TE-OLED with C₆₀ (5.0 nm)/MoO_x (5.0 nm) showed the power efficiency of 1.7 lm/W at a luminance of about 1000 cd/m² and the maximum luminance of about 76.000 cd/m² at the bias voltage of 11.0 V. It exhibited the highest performance among the inverted TE-OLEDs fabricated as a function of MoO_x thickness from 0 to 5.0 nm.     

Synthesis, Photo- and Electro-Luminescence of 3-Benzoxazol-2-yl-Coumarin Derivatives

Two coumarin derivatives containing electron-transporting benzoxazolyl moiety, 7-(diethylamino)-3-(benzoxazol-2-yl)coumarin (DABOC) and 3-(benzoxazol-2-yl)benzo[5,6]coumarin (BOBC), were synthesized and characterized. The photoluminescence and electroluminescence of the compounds were investigated detailedly. The compounds exhibited strong blue-green emissions in both solution and solid states, but the devices with DABOC as the emitting layer exhibited orange emission and maximum luminous efficiency of 2.8 cd/A and maximum luminance of 8,800 cd/m², and the devices with BOBC displayed orange-white emission and maximum luminous efficiency of 0.13 cd/A and maximum luminance of 540 cd/m².     

High-efficiency organic light-emitting diodes (OLEDs) with a mixed layer structure

Improved performance of organic light-emitting diodes (OLEDs) as obtained by a mixed layer was investigated. The OLEDs with a mixed layer which were composed of N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB), tris-(8-hydroxyquinolato) aluminum (Alq₃) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) showed the highest brightness and efficiency, which reached 19048 cd/m² at 17 V and 4.3 cd/A at 10 mA/cm², respectively. The turn-on voltage of the device is 2.6 V. Its Commission Internationale del’Eclairage (CIE) coordinate is (0.497, 0.456) at 17 V, and the CIE coordinates of the device are largely insensitive to the driving voltages, which depicts stabilized yellow color.     

White organic light-emitting diodes based on benzothiazole derivative

This paper describes the white organic light-emitting diodes (WOLEDs) made from a benzothiazole derivative, N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylnaphthalen-1-amine (BPNA). The bright yellowish-white emission was obtained from a non-doped triple-layer device: ITO/NPB (40 nm)/BPNA (50 nm)/Alq₃ (40 nm)/LiF/Al. The Commission Internationale de L’Eclairage (CIE) coordinates of the device were (0.24, 0.36) at 10 V. The maximum brightness of the device was 9225 cd/m² at 14.4 V. A current efficiency of 3.08 cd/A, a power efficiency of 1.21 lm/W and an external quantum efficiency of 1.18% at a driving current density of 20 mA/cm² were achieved. WOLED with a DCJTB-doped structure of ITO/TcTa/BPNA/BPNA: DCJTB (0.5%)/BPNA/BCP/Alq₃/LiF/Al was fabricated in comparison with the non-doped device. The device emitted bright white light with the CIE coordinates of (0.33, 0.29) at 10 V and a maximum luminance of 7723 cd/m² at 14.8 V.     

Bright flexible organic light-emitting devices on copy paper substrates

We have developed flexible top emission organic light-emitting devices (OLEDs) using copy paper substrates. Poly-2-chloro-p-xylylene and SiO₂ buffer layers were coated on the copy paper for protecting the water absorption and improving the barrier and morphological characteristics. Sputtered Ni and thermally evaporated Ca/Ag films were used as anode and semitransparent cathode, respectively. The electrical conduction, luminance, and efficiency characteristics of the copy paper-based flexible OLEDs were investigated. The SiO₂ buffer layer was critically important in obtaining the high performance devices. The device exhibited a high luminance of 2200 cd/m² at a driving voltage of 13 V.     

Efficiency control of organic light-emitting diode for high contrast ratio performance in active matrix display applications

We demonstrate the possibility of controlling organic light-emitting diodes (OLEDs) efficiency with an optimized hole-delay layer (HDL) in order to achieve very high contrast ratio needed for active matrix display applications. No triplet exciton confinement structure and a molybdenum trioxide (MoO₃) HDL at ITO/hole transport layer interface were used for a careful placing and adjusting of the exciton recombination zone. An optimized thickness of 50 nm thick MoO₃ allows to cut off the efficiency at the black level of active matrix driving mode while it gets greater at higher luminance at high current density region over 20 mA/cm² compared to conventional device. Our suggested efficiency control way is believed to be very useful to future active matrix OLED displays.     

Screen-printed white OLED based on polystyrene as a host polymer

We demonstrate the use of screen printing in the fabrication of single-layer organic-light-emitting devices (OLEDs). The organic layer is a single-layer of polystyrene, in which we incorporate rubrene for orange emission and α-NPD, DPVBi for blue emission. An appropriate mixing of the two colors produced white emission by incomplete Förster energy transfer. We showed the role of each constituent, α-NPD, DPVBi and rubrene in the emission characteristics of OLEDs. The turn-on voltage of screen-printed white OLEDs was about 10 V with maximum brightness and luminous efficiency up to 1300 cd/m² and 9 cd/A, respectively.     

Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L2₁结构Co₂FeSi高有序合金之间的Co₅₀Fe_50-xSi_x合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似(GGA+U)方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co₂FeSi合金的分子磁矩和居里温度分别达到5.92μ_B和777 ℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co₂FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内. 关键词： 磁性 Heusler合金 结构相变     

Enhanced power efficiency for white OLED with MoO3 as hole injection layer and optimized charge balance

White OLEDs with a different hole injection layer (MoO₃ or m-MTDATA), and a different electron transport layer (Alq₃ or Bphen) have been investigated. With 9,10-bis (2-naphthyl)-2-t-butylanthracene (TBADN) doped with 3% P-bis (P-N, N-diphenyl-aminostyryl) benzene (DSA-ph) and 0.05% 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyl-julolidy-9-enyl)-4H-pyran (DCJTB) as white emitting layer, the MoO₃/ /Bphen based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.43 V and 4.54 lm/W respectively, which is independently reduced 40% and improved 57% compared with those of the m-MTDATA/ / Alq₃ based one, respectively. The energy level diagram of the devices and single-carrier devices are studied to explain the reasons for the improvement. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.