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。     

面向彩色有机微显示的有机白光顶发射器件

以比铝、银等金属材料透光性更好的铜作为白光有机顶发射器件的顶电极,将其应用到基于Al底电极的蓝、黄互补色顶发射白光有机电致发光器件(TEWOLED),通过合理设计器件结构,制备出的器件具有较低的驱动电压和较高的效率,4V下亮度超过1 000cd/m2、功率效率达到28.5lm/W,效率滚降较小。我们利用p型电学掺杂结构和电子注入缓冲层结构分别解决了铝和铜电极功函数同空穴传输层的HOMO能级和电子传输层的LUMO能级不匹配问题,并通过TcTa光学覆盖层的调节作用使器件具有较好的光谱稳定性。基于Cu顶电极的TEWOLED与采用Al作为互连金属的CMOS工艺兼容,我们将该器件与硅基CMOS驱动电路结合,获得了SVGA白光有机微显示器件,为彩色有机发光微显示的实现奠定了基础。     

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.     

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...     

Chemical degradation processes of highly stable red phosphorescent organic light emitting diodes

We investigate the chemical degradation processes of highly stable red organic light emitting diodes (OLEDs) based on the triplet emitter tris(1-phenylisoquinoline)iridium(III) by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The analysis of LDI-TOF spectra, collected on OLEDs driven at different current densities, shows a direct correlation between the lifetime of the devices and the formation of the three different reaction products: a BPhen dimer, an adduct of BPhen dimer with cesium, and the complex [BAlq₂ + Al(Me-q)₂]⁺ as well. Additionally it was possible to identify another degradation product, whose chemical structure is related to the α-NPD molecule as well to the fluorine of the used p-dopant. This product is only observable in devices aged at very high current densities.     

Contactless charge carrier mobility measurement in organic field-effect transistors

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes.     

Polymer-based scattering layers for internal light extraction from organic light emitting diodes

Efficient light extraction for organic light emitting diodes (OLED) using scalable processes and low-cost materials are important prerequisites for the future commercialization of OLED lighting devices. The light-extraction technology exhibited in this paper uses polymer-based high-refractive index scattering layers processed from solution. The scatter matrix formulation incorporates two types of nanoparticles for refractive index tuning and scattering, respectively. Planarization by the same material in order to reduce surface defects was critical for achieving highly increased device yield. Highly efficient and defect-free large-area (1.8 cm²) white OLED devices were fabricated on top of the scattering layer in a bottom emitter configuration. Light extraction enhancement leads to an overall efficiency gain of up to 81% for luminances of 5000 cd m⁻².     

Device performances of third order micro-cavity green top-emitting organic light emitting diodes

Results on the device performances of 3rd order micro-cavity effects in green top emitting organic light emitting diodes (TEOLEDs) with optical spacer layer are reported. The performances of fabricated 3rd order micro-cavity green devices using fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃] green phosphorescent emitter and magnesium silver alloy semi-transparent cathode are systematically compared with the 2nd order cavity device. The optical and electrical simulation results of the 3rd order cavity device with emissive layer (EML) located near the semi-transparent cathode shows a current efficiency (CE) of 71.0 cd/A (70% value of the 2nd order device) and more serious color coordinates shift as compared to the 2nd order cavity device. With the center EML positioned 3rd order cavity device similar color variation behavior is observed but the CE value is 73.8 cd/A (73% value of the 2nd order device). The fabricated 3rd order TEOLEDs exhibit about 65–75% CE values of the 2nd order device as the simulation. Additionally, the viewing angle characteristics are significantly improved in the 3rd order center EML TEOLEDs as the simulation. The performances of fabricated TEOLEDs with 3rd order micro-cavity conditions are in quite good agreement with the optical and electrical simulation results.     

Enhancement in light extraction efficiency of organic light emitting diodes using double-layered transparent conducting oxide structure

We examined double-layered transparent conducting oxide (TCO) anode structures consisted of zinc-doped indium oxide (IZO) over the gallium-doped zinc oxide (GZO), and IZO over the GZO with electrochemical treatment. In bottom type OLEDs, power efficiency and current efficiency were enhanced by a factor of 1.50 and 1.14 at a current density of 10 mA/cm² in IZO/GZO anode structure, compared to the only IZO anode structure. Due to the reduced sheet resistance of the IZO/GZO TCO surface, the operating voltage of the OLED with IZO/GZO anode structure was lowered, leading to mostly enhance power efficiency. More enhanced in power efficiency and current efficiency by a factor of 1.21 and 1.25 at a current density of 10 mA/cm² were achieved in IZO/GZO anode structure with electrochemical treatment, compared to the IZO/GZO anode structure due to the change of the surface morphology of the GZO and the existence of the nanoporous layer beneath the GZO surface by an electrochemical treatment. In total, double-layered IZO/GZO anode structure with electrochemical treatment was revealed at an enhancement factor of 1.80 in power efficiency and 1.42 in current efficiency, compared to the only IZO anode structure.     

Full color organic light emitting diodes with laser-patterned optical path-length compensation layer

In this paper, we describe a promising way to fabricate micro-cavity OLEDs (organic light emitting diodes) with the concept of “super top emission” with soluble optical path-length compensation layer (OPCL) formed by Laser Induced Thermal Imaging (LITI) technology. OPCL is an additional layer for setting the cavity length corresponding to intrinsic wavelength of emitter. The resultant OLEDs gave a saturated three primary colors (RGB) after transmitting through common color filters by micro-cavity effect. By this approach, we could simplify the fabrication process of full color OLEDs by reducing conventional color patterning step, fine metal mask method. We also suggested the blending system of polymer and small molecule as transfer layer and this could improve the quality of laser patterning by controlling the surface energy adequately. Also, this system could be a solution to LITI problem of chronic contaminant because transferred materials could remain after cleaning process. Devices fabricated by soluble/LITI process showed comparable or even better performances and the device characteristics could enhance by further study of optimizing several parameters such as materials property and laser patterning condition. Also, we expect that this concept could lead to develop the fabrication process of large and high-resolution OLED displays.     

高效暖白光器件的廉价制备及其相关材料研究

低色温光源由于其对抗黑变激素具有较低的抑制作用而成为生理友好照明的首选。同时,高的能量效率对于节能也至关重要。本课题采用温和的溶液旋涂方法分别制备了含互补色、三基色和四基色磷光染料的单层有机白光二极管(WOLED)。经过优化WOLED的结构,实现了宽亮度范围内100~10 000cd/m2的低色温(low-CCT)白光发射。CCT低至2 500K以下、显示指数(CRI)高达到83、电流效率在亮度为1 000cd/m2时达到了17.8cd/A,与传统的白炽灯功效相当。高发光性能、廉价制备成本及生理友好的特性表明,本工作制备的器件是益于人类健康的照明光源尤其是夜间照明光源的理想选择。     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

Analysis of alternating current driven electroluminescence in organic light emitting diodes: A comparative study

The alternating current (AC) responses of double-injection and double-insulated organic light-emitting diodes (OLEDs) were investigated and compared. To reveal the electroluminescent (EL) processes in these devices, the AC voltage and frequency dependence of the EL intensity and capacitive current were studied in the time domain with a focus on phase difference analysis. It was found that the voltage-dependent transit time and frequency-dependent carrier distribution were important for the AC-driven performance of the double-injection OLEDs. In contrast, although the double-insulated OLEDs shared some similarities with the double-injection OLEDs, they had some unique characteristics, which were the absence of resistive current and phase shift of EL profiles. It was revealed that the EL in the double-insulated OLEDs was driven by the displacement current generated by the ionization of the doped layers, which, however, formed space charge regions and undermined the EL emission. The space charge redistributed the electric field across the devices after the initiation of EL, making the EL maintain for a limited time interval. This effect was significant under low frequency and high AC voltage. Comparing the phase difference between both devices, it was indicated that the space charge effect was responsible for the observed EL phase shift and the asymmetric EL profiles at low frequency and high AC voltage in the double-insulated OLEDs. The proposed model was also of help to understand the EL saturation phenomena with AC frequency and voltage in those devices.     

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.     

Influence of the applied charge on the electro-chemical degradation in green phosphorescent organic light emitting diodes

The influence of the current density on the chemical degradation processes of a phosphorescent OLED based on Ir(ppy)₃ emitter is investigated by laser-desorption/ionization time-of-flight mass spectrometry. Comparing the mass spectra collected for unaged and aged OLEDs, the formation of different chemical degradation products could be detected and are identified as dimer and trimer products of BPhen as well as Cs-adducts of these polymers and the well-known emitter-BPhen-adduct ([Ir(ppy)₂BPhen]⁺). In this work, we will show that the formation of [Ir(ppy)₂BPhen]⁺ depends strongly on the amount of the charge flowing through the device, where the other degradation products show a much different behaviour.     

Improved out-coupling efficiency of organic light emitting diodes by manipulation of optical cavity length

The relationship between thickness of electron transport layer (ETL) and device performance of organic light-emitting diodes (OLEDs) was investigated. Especially, we prepared various OLEDs by varying the thickness of ETL to investigate the difference of device performance. Very interestingly, the device efficiency of green phosphorescent organic light emitting diodes (PHOLEDs) was significantly improved when the thickness of ETL was optimized even though we did not change any materials for such devices except that we applied highly conductive Li doped ETL. This means that the only one factor which is associated with an improvement of device efficiency could be originated from the constructive optical interference. As a result, the simple modification of PHOLEDs only by changing the optical thickness condition causes a dramatic improvement of current efficiency (up to 82.4 cd/A) as well as external quantum efficiency (EQE, up to 23.8%), respectively. Those values correspond to the much more improved ones (by ∼34.4%) compared to those obtained from the normal devices with thin ETL as a reference.     

Thermo-cleavable poly(fluorene-benzothiadiazole) to enable solution deposition of multi-layer organic light emitting diodes

The design of solution processable multi-layer organic light emitting diodes (OLEDs) is often hampered by the choice of solvents. To avoid the dissolution of the emission layer upon the subsequent deposition of further functional layers, we synthesize and investigate thermo-cleavage in poly[2,7-(3-(9-methyl-9H-fluorene-9-yl)propyl (2-methylhexane-2-yl) carbonate)-alt-4,7-(benzo[c][1,2,5]thiadiazole)] (c-F8BT). Employed in OLEDs, the non-cleaved polymer yields about the same current efficiency as state-of-the-art F8BT. During pyrolysis at 200 °C, the polymer releases its solubility groups, fully maintaining the device efficiency but becoming insoluble. This feature allows to enhance the OLED performance by applying an additional bathophenanthroline hole blocking layer from solution or by incorporating a low-molecular weight electron transport moiety into the affixing polymer matrix.     

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.     

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.