The electroluminescence mechanism of non-doping PhOLEDs based on CBP/Ir(ppy)3 investigated by delayed EL measurements

In this paper, the non-doped PhOLED based on 4,4′-bis(9-carbazolyl)-1,1′-biphenyl(CBP) and fac-tris(2-phenylpyridine) iridium(Ir(ppy)₃) with the emitting layer of [CBP/Ir(ppy)₃]_n/CBP in which n is equal to 2,3,4 and 5, respectively, are prepared. The electroluminescence and the luminance-voltage-current density characteristics of devices are detected. The wavelength range of these devices is varied from 517 nm to 540 nm. And the delay EL measurements are used to elucidate the carrier recombination and light emission mechanism in non-doped PhOLEDs. In this technique, the devices are driven using a square pulse driving scheme, with a forward bias pulse width of 1 ms, which is sufficiently long for prompt EL to reach its steady-state intensity. The delay EL results show that changing the number of the non-doped EML leads to marked changes in the charge-trapping and host–host TTA patterns, which suggests that the carrier transport and recombination processes depends on the number of non-doped EML. When the number of non-doped EML is less than 3, only the trapped carrier recombination signal are detected in the delay EL measurement. For the devices with more non-doped EML than 3, both the trapped carrier recombination and host–host TTA signal are detected. All these results are discussed and give the evidence for the electroluminescence mechanism of prepared devices.     

基于Ir(ppy)2acac的掺杂浓度对 OLED器件发光性能影响研究

制备了结构为:ITO/MoO3(40nm)/NPB(40nm)/TCTA(10nm)/CBP:Ir(ppy)2acac(x%)(30nm)/BCP(10nm)/Alq3(40nm)/LiF(1nm)/Al(100nm)的器件,Ir(ppy)2acac为绿色磷光染料,x分别为4%、6%、8%、10%。通过调节绿色磷光染料的掺杂浓度,对器件的发光性能进行了研究,发现在掺杂浓度为8%,亮度为490cd/m2,器件获得最高电流效率为69.43cd/A,相比4%的器件高出27.5%。分析原因是掺杂浓度越高,载流子在绿色染料上复合的几率越高;CBP与Ir(ppy)2acac的LUMO能级均为2.5eV,注入主体CBP上的电子可以直接传递给掺杂染料,避免电子对掺杂染料传递过程中的能量损失;较高的掺杂浓度更有利于载流子的传输。然而,较高的掺杂浓度会引起三线态激子的猝灭效应。另外,由于TCTA、BCP为载流子阻挡材料,具有较高的三线态能量,可以将载流子和激子限制在发光层内。     

基于无线指挥的有机电致发光器件的特性及实验

深入探究了多层有机电致发光器件的特性及优势。理论上分析了器件的电场与局域态的特性关系,设计了类似夹层式的两次堆叠双层有机层的实验结构,采用了电子传输材料Bphen来进行相关器件的制作。实验结果表明:叠层器件的开启电压几乎高出单层器件的开启电压值的一倍;叠层器件相比单层器件的电流效率高出1.5倍以上;叠层器件会产生微腔效应,设计内部连接层结构十分便捷有效。上述结果对于有机半导体器件的在无线指挥中的发展具有一定的理论和实践意义。     

Temperature dependence of electroluminescence degradation in organic light emitting devices without and with a copper phthalocyanine buffer layer

Temperature dependence of electroluminescence degradation was investigated in two types of organic light emitting devices (OLEDs) based on tris(8-hydroxyquinoline) aluminum (AlQ₃) emitter molecule, one without and another with copper phthalocyanine (CuPc) buffer layer at the hole-injecting contact interface. Electroluminescence degradation in time was measured for devices operated at 22 and 70 °C. Results unexpectedly showed that devices without the CuPc buffer layer demonstrated negligible change in half-life when operated at 22 or 70 °C, while devices with the CuPc layer showed the expected decrease in half-life when the temperature was increased. The results are explained within the framework of recently proposed OLED degradation mechanism, which identifies AlQ₃ cations as unstable, leading to device degradation.     

利用Ir(ppy)_3敏化CzHQZn提高OLED的性能

用CzHQZn作为受主,利用磷光敏化的方法制备了有机电致黄光和白光器件。黄光器件采用Ir(ppy)3掺杂4,4-N,N′-=咔唑基联苯(CBP),敏化新的黄光材料CzHQZn作为发光层,当发光层厚度为18nm时器件性能最好,最大发光效率为3.26cd/A(at10V),最大发光亮度为17560cd/m2(at10V);白光器件采用多发光层结构,结合ADN的蓝光复合发光,同时加入了电子阻挡层(NPBX)和空穴阻挡层(BCP),获得的白光器件最大发光效率为2.94cd/A(at8V),最大亮度为11089cd/m2(at13V)。     

(t-bt)2Ir(acac)超薄层厚度对有机电致发光器件性能的影响

以新型铱配合物黄光磷光染料bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2']iridium(acetylacetonate)[(tbt)2Ir(acac)]为超薄层,制备了结构为indium tin oxide(ITO)/N,N'-bis(naphthalen-1-yl)-N...     

Enhanced efficiency of organic light emitting devices (OLEDs) by control of laser imaging condition

Laser Induced Thermal Imaging (LITI) is a laser addressed thermal patterning technology with unique advantages such as excellent uniformity of transfer film thickness, capability of multilayer stack transfer, high resolution and scalability to large-size mother glass. However, the deterioration of the device performance during imaging process has been an obstacle to use it as a commercial technology. To investigate a possibility of thermal deformation of organic materials as a transfer layer and a receptor layer during imaging process, we executed a preliminary annealing test by using standard green devices at various temperatures. By comparison of these results with those obtained from LITI devices, we found that the main reason of device deterioration could be originated from the mobility change of the organic layers. Hence, we developed the dwell time control technology to suppress the thermal impact during LITI process and we finally obtained current efficiency which is quite equivalent to that obtained from the standard evaporation devices.     

Driving current and temperature dependent magnetic-field modulated electroluminescence in Alq3-based organic light emitting diode

Driving current and temperature dependences of magnetic-field modulated electroluminescence (EL) in tris(8-hydroxyquinoline) aluminum (Alq₃)-based OLEDs have been thoroughly investigated. At low temperatures, the applied magnetic-field induces a sharp increase of the EL in low field regime (B ? 35 mT) and a slow but apparent decrease at high fields (35 ? B ? 500 mT). The low-field increase in EL (LFE) survives at all working temperatures while the high-field decrease (HFE) gradually disappears as temperature is increased. At a given temperature, the higher the current level, the smaller LFE and stronger HFE are observed. To explain the observed MFEs a composite model based on magnetic-field dependent singlet-to-triplet conversion of electron-hole pairs and magnetic-field mediated triplet–triplet annihilation process is proposed in this paper.     

Light emitting devices based on Si nanoclusters： the integration with a photonic-crystal and electroluminescence properties

We present the properties and potentialities of light emitting devices based on amorphous Si nanoclusters. Amorphousnanostructures may constitute an interesting alternative to Si nanocrystals for the monolithic integration of optical andelectrical functions in Si technology. In fact, they exhibit an intense room temperature electroluminescence (EL). The ELproperties of these devices have been studied as a function of current and of temperature. Moreover, to improve theextraction efficiency of the light, we have integrated the emitting system with a 2D photonic crystal structure opportunelyfabricated by using conventional optical lithography to reduce the total internal reflection of the emitted light. The extractionefficiency in such devices increases by a factor of 4 at a resonance wavelength.     

掺杂浓度和无序对柔性有机发光二极管载流子迁移率的影响

基于电荷分布的泊松方程和载流子输运方程,考虑最低能量非占据态和最高能量占据态的载流子态密度服从双高斯分布的模型,自洽地研究了掺杂浓度和无序对Alq3柔性有机发光二极管载流子迁移率的影响.发现掺杂浓度低于1×1020cm-3时,掺杂浓度对迁移率影响很小,迁移率几乎保持不变;当掺杂浓度大于1×1020cm-3时,载流子迁移...     

High efficiency warm white phosphorescent organic light emitting devices based on blue light emission from a bipolar mixed-host

In this study, we demonstrate a high-efficiency and low turn-on voltage warm white phosphorescent organic light emitting devices (PH-WOLEDs) based on a blue mixed-host emission layer (EML) and an orange ultrathin layer. The device has a simple structure and would simplify the fabrication process and reduce fabrication costs. The concept is based on the design a high-efficiency blue mixed-host EML, using an electron-transport material, 4,6-Bis(3,5-di(pyridin-4-yl) phenyl)-2-(3-(pyridin-3-yl) phenyl) pyrimidine (B4PYMPM) to enhance the carrier balance ability of the hole-transport material 1,3-Bis(carbazol-9-yl) benzene (MCP) which operates as the mixed-host and when the MCP: B4PYMPM ratio in the mixed-film was 4:1 got better effects. Based on the blue EML, we realized WOLEDs, characterized by a peak power efficiency of 71.3 lm/W at 3.1 V and a low turn-on voltage of 2.65 V. The mixed-host blue EML exhibited a much higher performance compared to the MCP host. Stable warm white light emission with Commission International de L'Eclairage (CIE) coordinates from (0.37, 0.45) to (0.38, 0.47) for a luminance value ranging from 1000 to 10,000 cd/m² was obtained.     

Effects of substrate topography on current injection and light emission properties of organic light emitting devices

Substrate topography plays a critical role in the function of nano-scale materials and devices. We study small molecular organic light emitting devices (OLEDs) deposited onto non-planar substrates, where the substrate’s radius of curvature in some regions approaches the thickness of the active device layers. As a result, the electric field profile inside the organic charge transport layers is modified, influencing carrier injection, transport, and light emission properties. Experiments and numerical modeling suggest that charge balance and electroluminescence efficiency potentially can be improved in electron injection-limited OLED architectures via substrate geometry. These findings elucidate the optoelectronic behavior (and degradation) of OLEDs on imperfect substrates, and suggest a strategy based on substrate topography for controlling device behavior.     

掺杂浓度和无序对柔性有机发光二极管载流子迁移率的影响

基于电荷分布的泊松方程和载流子输运方程,考虑最低能量非占据态和最高能量占据态的载流子态密度服从双高斯分布的模型,自洽地研究了掺杂浓度和无序对Alq3柔性有机发光二极管载流子迁移率的影响.发现掺杂浓度低于1×1020cm-3时,掺杂浓度对迁移率影响很小,迁移率几乎保持不变;当掺杂浓度大于1×1020cm-3时,载流子迁移率非线性增加,与实验结果很好一致.考虑不同无序度时,发现无序度的增加,相应的迁移率下降;低掺杂浓度下迁移率与无序度的大小关系不大,当掺杂浓度较高的时,无序度较小的材料迁移率增加较快.最终给出了柔性发光二极管器件的发光功率密度与电压关系,表明载流子浓度和无序相关的迁移率结果对制备高性能的柔性有机发光二极管具有指导意义.     

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.     

Fabrication of large-scaled organic light emitting devices on the flexible substrates using low-pressure imprinting lithography

This paper demonstrates an approach to fabricate large-scaled (70 /spl times/70 mm) patterned organic light-emitting devices (ITO/CuPc/NPB/Alq3/LiF/Al) on the flexible polyethyleneterephthalate substrates using low-pressure imprinting lithography. The patterns of the pixel array were defined in crossed-strip style with anode patterned by imprinting techniques followed by wet chemical etching and cathode strips deposited using metal mask. The measured results were: The turn-on voltage of the device was 7.5 V; the luminous efficiency reached 1.13 lm/W (3.04 cd/A) at a luminance of 3.8 cd/m/sup 2/ and its maximum luminance was 2440 cd/m/sup 2/, which were comparable to the performances of the devices patterned by conventional photolithography.     

Double co-host emitter based top emitting white organic light emitting diodes with enhanced brightness and efficiency

quninolinato)-4-phenylphenolate alu-minium (BAlq) as HTL which is responsible for blue light emission is found to have best characteristics when compared to other simulated devices. It has a maximum luminance of 10 000 cd/m2 and current efficiency of 15.25 cd/A, respectively, and CIE coordinates are at (0.329, 0.319). The device is found to be compatible to be used in solid state lighting applications because of the low driving voltage of the device.     

电子传输层中掺杂Ir(ppy)3改善白光OLED的效率

采用真空热蒸镀技术,制备了结构为ITO/NPBX(40nm)/rubrene(0.2 nm)/NPBX(5nm)/DPVBi(30nm)/TPBi:x%Ir(ppy)3(30nm)/LiF/Al的白光器件。利用Ir(ppy)3掺杂到电子传输层TPBi中,在掺杂层中提高了电子的迁移率,调整了空穴和电子的平衡,从而改善了白色有机电致发光器件的效率。当Ir(ppy)3的掺杂浓度为6%时,器件的电流效率最高,在驱动电压9 V时最大电流效率为10.66 cd/A,此时色坐标为(0.36,0.38);当电子传输层TPBi中不掺杂Ir(ppy)3时,白光器件的效率最低,在驱动电压10V时最大电流效率为1.69 cd/A,此时色坐标为(0.31,0.30)。掺杂浓度为6%的白光器件的电流效率是不掺杂白光器件的电流效率的6.3倍。     

Temperature dependence of electroluminescence in a tris-(8-hydroxy)quinoline aluminum (Alq3) light emitting diode

Organic electroluminescent devices using tris-(8-hydroxy) quinoline aluminum (Alq₃) as the emissive layer and N,N'-diphenyl-N,N' bis (3-methylphenyl)-[1-1'-biphenyl]-4-4'-diamine as the conventional hole transport layer have been fabricated. The temperature- and field-dependent quantum efficiency have been investigated over the temperature range from 1 to 300 K using a model developed by Shen et al. to explore the physics at the organic heterointerface in the present device structure with the formation of an accumulation layer. It has been observed that electron luminescence intensity decreases with decreasing temperature down to 160 K, then saturates in the low-temperature region. The quantum efficiency increases with decreasing temperature and finally reaches an almost constant value. From the analysis, it is seen that the model can explain the luminescence behavior of the device satisfactorily down to 120 K but fails to explain the low-temperature behavior. The efficiency has also been studied with voltage and it is seen that there is an optimum voltage required to get the maximum efficiency     

Influence of light absorption on the metallic nanotextured reflectors of GaN-based light emitting diodes

Metallic nanotextured reflectors have been widely used in light emitting diodes (LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however, only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference time-domain (FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon (SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.     

利用常规工艺提高光子晶体GaN LED的出光效率

计算了GaN二维光子晶体的能带结构,并利用常规工艺在国内首次制备出了GaN基二维平板结构的光子晶体蓝光LED。经过器件测试表明,与没有制作光子晶体的器件相比,光子晶体使器件的有效出光效率达到了原来的1.5倍以上。另外,还对感应耦合等离子体刻蚀(ICP)的制备光子晶体LED的刻蚀工艺进行了分析。