Efficient deep-blue emitters based on triphenylamine-linked benzimidazole derivatives for nondoped fluorescent organic light-emitting diodes

Two triphenylamine-substituted benzimidazole derivatives were synthesized for use as efficient deep-blue emitters in nondoped fluorescent organic light-emitting diodes (FLOLEDs). The molecular design of 4′,4′′-(1H-benzo[d]imidazole-1,2-diyl)bis(N,N-diphenylbiphenyl-4-amine) (T2B) to limit the molecular packing density enabled T2B-based devices to suppress the exciton quenching by a bulky three-dimensional structure. Nondoped FLOLEDs fabricated using T2B as a blue emitter exhibited an external quantum efficiency of 4.67% with color coordinates of (0.15, 0.08).     

Efficient nondoped blue organic light-emitting diodes based on phenanthroimidazole-substituted anthracene derivatives

A series of new blue materials based on highly fluorescent di(aryl)anthracene and electron-transporting phenanthroimidazole functional cores: 2-(4-(anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (ACPI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-1-p-henyl-1H-phenanthro[9,10-d]imidazole (1-NaCPI), 2-(4-(10-(naphthalen-2-yl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (2-NaCPI) were designed and synthesized. These materials exhibit good film-forming and thermal properties as well as strong blue emission in the solid state. To explore the electroluminescence properties of these materials, three layer, two layer and single layer organic light-emitting devices were fabricated. With respect to the three layer device 4 using ACPI as the emitting layer, its maximum current efficiency reaches 4.36 cd A⁻¹ with Commission Internationale del’Eclairage (CIE) coordinates of (0.156, 0.155). In the single layer device 10 based on ACPI, maximum current efficiency reaches 1.59 cd A⁻¹ with Commission Internationale del’Eclairage (CIE) coordinates of (0.169, 0.177). Interestingly, both device 4 and 10 has low turn on voltage and negligible efficiency roll off at high current densities.     

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.     

Wide color-range tunable and low roll-off fluorescent organic light emitting devices based on double undoped ultrathin emitters

We demonstrated a novel wide color-range tunable, highly efficient and low efficiency roll-off fluorescent organic light-emitting diode (OLED) using two undoped ultrathin emitters having complementary colors and an interlayer between them. The OLED can be tuned to emit sky blue (0.22, 0.30), cold white (0.29, 0.33), warm white (0.43, 0.42) and yellow (0.40, 0.45) according to the Commission Internationale de L’Eclairage (CIE) 1931 (x, y) chromaticity diagram. The device fabrication was simplified by eliminating doping process in the emission layers. The influence of interlayer thickness on luminous efficiency, efficiency roll-off and color tuning mechanism is thoroughly studied. The recombination zone is greatly broadened in the optimized device, which contributes to stable energy transfer to both emitters and suppressed concentration quenching. With a threshold voltage of 2.82 V, the color tunable organic light emitting diode (CT-OLED) shows a maximum luminance of 39,810 cd/m², a peak external quantum efficiency (EQE) 6% and the efficiency roll-off as low as 11.1% at the luminance from 500 cd/m² to 5000 cd/m². This structure of CT-OLED has great advantages of easy fabrication and low reagent consumption. The fabricated CT-OLEDs are tunable from cold white (0.30, 0.36) to warm white (0.43, 0.42) with correlated color temperature (CCT) 6932 K and 3072 K, respectively, demonstrating that our proposed approach helps to meet the need for lighting with various CCTs.     

基于红外热像与电学测试法的OLED热学分析

采用红外 热成像与瞬态热学测试技术,测试并对比分析了有机电致发光二极管(OLED)的光电热学特性 。研究表明,OLED结温的升高会导致输入 电压的降低。在输入电流为100mA时,整个面板上呈现非 常明显的温度梯度,最高点与最低点 温差可高达30℃。在相同的工作条件下,采用红外热像测试获得的峰值温度比采用瞬态热 学测试获 得的温度要高,且温度梯度会随输入电流的大小发生变化。结合红外热像与电学测试法固有 特点的分析,指出两者的结合可以为OLED面板的热学设计与分析提供更具有指导意义的信息 。     

Optical performance of efficient blue/near UV nitropyridine-conjugated anthracene (NAMA) based light emitting diode

9-[(5-nitropyridin-2-aminoethyl) iminiomethyl]-anthracene (NAMA) was synthesized for the first time from the reaction between 9-antracene carboxaldehyde and 2-(2-aminoethylamino)-5-nitropyridine under mild reaction conditions. The structure of the nitropyridine-conjugated anthracene (NAMA) was characterized using ¹H-NMR, ¹³C-NMR and elemental analysis techniques. Furthermore, electroluminescent behavior of the NAMA was investigated by using a material based on polycyclic aromatic hydrocarbons (PAHs) in organic light-emitting diodes (OLEDs). It was observed that the NAMA exhibits blue/near UV emission at 410 nm with a high density signal. The optimized device structure, ITO/CuPc/α−NPD/NAMA/Alq₃/LiF/Al is characterized by blue/near UV electroluminescence (EL) and a high current density with 7735 cd m⁻² maximum brightness at approximately 10.4 V. The emitting color of the device showed the blue/near UV emission (x, y) = (0.14, 0.10) at 277.1 mA cm⁻² in CIE (Commission Internationale de l’Eclairage) chromaticity coordinates with a long operational lifetime (180 h).     

Random nano-structures as light extraction functionals for organic light-emitting diode applications

In this study, we demonstrated a nano-structured random scattering layer (RSL) as an internal light extraction method to improve the light extraction efficiency of organic light-emitting diodes (OLEDs). Using dewetted Ag droplets as a hard mask, we textured the glass surface to have a scattering layer of the random structure. OLEDs equipped with the RSL showed more that 50% improvement in the external quantum efficiency (EQE) and luminance efficacy (LE) compared to OLEDs without the RSL. This improvement can be understood by the scattering effect which reduces the optical loss at wave-guided modes. Also, by combining the RSL and an external light extraction micro-lens array (MLA), it was possible to achieve further improvements of 105.8% and 92.06% in the EQE and the LE, respectively.     

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.     

Chemical degradation mechanisms of highly efficient blue phosphorescent emitters used for organic light emitting diodes

The stability and the degradation processes of two highly efficient blue-emitting phosphorescent materials, iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) and bis(2-(4,6-difluorophenyl) pyridyl-N,C2′)iridium(III)picolinate (FIrpic), which are commonly used as emitters in organic light emitting diodes (OLEDs), are investigated. Using single layers devices, the optical response and the half-lifetime behavior of the materials are investigated. Layers of FIr6 exposed to UV-light show the formation of a red emitting degradation product. We analyze the chemical reactions of the materials using laser desorption/ionization time-of-flight mass spectrometry. Several products related to the chemical dissociation of the FIr6 molecule as well as charge complex formation between the emitter and the emitter dissociation products are detected. FIr6 and FIrpic are also compared by lifetime studies on commonly used OLED structures. We show that single layers and OLEDs based on FIrpic exhibit higher stability than those based on FIr6. An explanation for this behavior can be found by considering the chemical structure of the molecules.     

基于BAlq的有机电致发光器件的磁效应

研究了ITO/NPB(40nm)/BAlq(60nm)/BCP(5nm)/LiF(0.8nm)/AI有机电致发光器件(OLED)的磁效应.实验结果表明,磁场在10mT时,器件的效率最大增加了34%,这一结果是由于三线态激子与三线态激子间的相互淬灭产生激发单线态激子从而使单线态激子比率增加,致使电致发光(EL)增强.当磁...     

High-efficiency deep blue fluorescent emitters based on phenanthro[9,10-d]imidazole substituted carbazole and their applications in organic light emitting diodes

A series of highly efficient deep blue emitters comprising of carbazole and phenanthro[9,10-d]imidazole moieties are designed and synthesized. These compounds present deep blue emission, narrow FWHM, high quantum yields, high thermal and morphological stabilities. Among them, the design strategy of 2:1 ratio of phenanthro[9,10-d]imidazole and carbazole unit affords M2 with more balanced carrier injection and transporting properties. OLEDs using M2 as emitting layer is observed to deliver a truly deep blue CIE of y < 0.06 with a highest external quantum efficiency of 3.02%. By taking the full advantage of these deep blue emitters, they are further served as excellent hosts for fluorescent and phosphorescent dyes. High-performance green phosphorescent device based on M2/Ir(ppy)₃ is attained with a maximum current efficiency of 33.35 cd A⁻¹, a power efficiency of 22.99 lm W⁻¹ and a maximum external quantum efficiency of 9.47%. When doped with an orange fluorescent material, upon careful tuning the doping proportion, the two-emitting-component white OLED is successfully fabricated with a maximum current efficiency of 5.53 cd A⁻¹ and CIE coordinates of (0.313, 0.305). Both the non-doped and doped devices exhibited high operational stability with negligible efficiency roll-off over the broad current density range.     

Efficiency enhancement of organic light emitting diode via surface energy transfer between exciton and surface plasmon

Organic light emitting diodes (OLEDs) with surface plasmon (SP) enhanced emission have been fabricated. Gold nanoclusters (GNCs) deposited using thermal evaporation technique has been used for localization of surface plasmons. Size of GNCs and distance of GNCs from the emissive layer have been optimized using steady state and time resolved photoluminescence (PL) results. 3.2 Times enhancement in PL intensity and 2.8 times enhancement in electroluminescence intensity of OLED have been obtained when GNCs of size 9.3 nm has been introduced at a distance of 5 nm from emissive layer. Distance dependence of energy transfer efficiency between exciton and SPs was found to be of 1/R⁴ type, which is typically the dependence for dipole-surface energy transfer.     

具有凹凸界面结构的有机发光器件的性能研究

通过采用毫米尺度的凹凸界面结构实现了有机发光器件(Organic Light Emitting Device,OLED)发光效率的提高。在制备OLED器件过程中使用双狭缝模板,在发光层的界面处构建了高度为10nm的凸起,获得最大功率效率为23.9lm/W,最大电流效率为45.6cd/A,与传统的平直界面的OLED相比,分别提高了70%和36%。经过实验数据分析与理论模拟得出初步结论:OLED发光效率提高主要归因于金属界面处局域表面等离子体共振激发,提高了金属阴极界面的远场散射,从而提高OLED的出光效率;另外适当凹凸深度也改善了器件的电学性能。     

Efficient and stable red organic light emitting devices from a tetradentate cyclometalated platinum complex

An efficient and stable red phosphorescent organic light emitting diode was developed using a tetradentate cyclometalated platinum complex. Devices employing the phosphorescent molecule, platinum(II)-9-(4-methylpyridin-2-yl)-2-(3-(quinolin-2-yl)phenoxy)-9H-carbazole (PtON11Me), yielded high external quantum efficiencies and high operational lifetimes. A maximum EQE of 12.5% and color coordinates CIE (x = 0.61, y = 0.36) was achieved in devices employing efficient hole blocking and transporting materials and a high operational lifetime of T_0.97 ∼ 3200 h was achieved in devices utilizing electrochemically stable hole blocking and transporting materials.     

Molybdenum oxide anode buffer layers for solution processed,blue phosphorescent small molecule organic light emitting diodes

In this work we present solution processed organic light emitting diodes (OLEDs) comprising small molecule, blue phosphorescent emitter layers from bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium doped 4,4′,4″-tris(carbazol-9-yl)-triphenylamine and molybdenum trioxide (MoO₃) anode buffer layers. The latter were applied from a molybdenium(V)ethoxide precursor solution that was thermally converted to MoO₃ at moderate temperatures. The high work function MoO₃ facilitated hole injection into the emission layer. The MoO₃ layer properties were investigated by means of energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy. MoO₃ buffer layers performed superior to the commonly used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and enabled an enhanced OLED device efficiency.     

Impact of temperature on the efficiency of organic light emitting diodes

Organic light emitting devices (OLEDs) are known to heat up when driven at high brightness levels required for lighting and bright display applications. This so called Joule heating can in the extreme case lead to a catastrophic failure (breakdown) of the device. In this work, we compare the effect of Joule heated and externally heated OLEDs by their electrical and optical response. A reduction in resistance is observed at elevated temperatures, both, for Joule heating, and for externally heated samples driven at low current density. In both cases, we attribute the change in resistance to a higher mobility of charge carriers at the elevated temperatures. Additionally, we observe a quenching of the emission efficiency in heated single layers as well as in OLEDs, treated with an external heat source as well as on Joule heated samples.     

Efficient silicon based light emitters

Recent progress on electrically driven silicon based light emitters is reviewed, with emphasis on our work on light emitting pn diodes (LED) and MOS devices doped with rare-earth elements. The LEDs were fabricated by high-dose boron implantation, producing nanoscale modifications in the material. The electroluminescence (EL) efficiency increases with temperature, reaching 0.1% (wall plug efficiency) at room temperature for optimized conditions. Such devices were integrated into a microcavity. In the MOS devices, the oxide was implanted with various rare-earth elements, resulting in strong EL in the visible (Tb) and ultraviolet (Gd). External quantum efficiencies in excess of 10% are reported.     

Novel benzothiadiazine 1,1-dioxide based bipolar host materials for efficient red phosphorescent organic light emitting diodes

In this work, three novel bipolar host materials TPA-SA, 3CBZ-SA and 4CBZ-SA have been designed and synthesized by incorporating triphenyl amine and carbazole as donor and benzothiadiazine 1,1-dioxide as an acceptor. These molecules exhibit moderately high triplet energies and bipolar carrier transport characteristics (ambipolarity) which is useful for the exothermic energy transfer to the dopants and also for the balanced carrier injection/transport in the emissive layers. These materials exhibited good performances in PhOLEDs and furnished external quantum efficiency in the range of 10.0–15.0%. Notably, a red phosphorescent device using TPA-SA as the host doped with Ir(pq)₂(acac) exhibited a maximum EQE, power efficiency and current efficiency of 15.0%, 16.0 lm/W, and 25.3 cd A⁻¹, respectively.     

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 highly conductive PEDOT:PSS film with the dipping treatment by hydroiodic acid as anode for organic light emitting diode

A highly conductive, transparent and uniform poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) film has been developed by dipping treatment with hydriodic acid (HI) solution. The HI-treated PEDOT:PSS film can reach a sheet resistance of 68 Ω per square and a transmittance of 87% at 550 nm. The conductivity enhancement for the HI-treated film is ascribed to the permeation of proton and iodine anion of HI into PEDOT:PSS film, resulting in the separation of PSS and PEDOT chains. The phase separation of PSS and PEDOT can provide more conductive pathways for carriers to improve conductivity of the film. Using the optimized HI-treated PEDOT:PSS film as anode, we have fabricated indium tin oxide (ITO)-free organic light emitting diode (OLED), which shows better performance than the device with ITO as anode. This proves that such PEDOT:PSS film with the dipping treatment by HI solution is a promising alternative to ITO for low cost, transparent and flexible OLED application.