Investigating the Role of Emissive Layer Architecture on the Exciton Recombination Zone in Organic Light‐Emitting Devices

An experimental approach to determine the spatial extent and location of the exciton recombination zone in an organic light‐emitting device (OLED) is demonstrated. This technique is applicable to a wide variety of OLED structures and is used to examine OLEDs which have a double‐ (D‐EML), mixed‐ (M‐EML), or graded‐emissive layer (G‐EML) architecture. The location of exciton recombination in an OLED is an important design parameter, as the local optical field sensed by the exciton greatly determines the efficiency and angular distribution of far‐field light extraction. The spatial extent of exciton recombination is an important parameter that can strongly impact exciton quenching and OLED efficiency, particularly under high excitation. A direct measurement of the exciton density profile is achieved through the inclusion of a thin, exciton sensitizing strip in the OLED emissive layer which locally quenches guest excitons and whose position in the emissive layer can be translated across the device to probe exciton formation. In the case of the G‐EML device architecture, an electronic model is developed to predict the location and extent of the exciton density profile by considering the drift, diffusion, and recombination of charge carriers within the device.     

Thermally cross-linkable host materials for enabling solution-processed multilayer stacks in organic light-emitting devices

A thermally cross-linkable host material, i.e., two vinylbenzyl ether groups containing a carbazole derivative (DV-CBP), was developed for solution-processed multilayer organic light-emitting devices (OLEDs). DV-CBP was thermally cross-linked at styrene end-groups through curing at approximately 180 °C in the absence of a polymerization initiator. This cross-linking reaction rendered the emissive layer insoluble and enabled the subsequent solution deposition of an upper electron-transporting layer. Furthermore, photoluminescence quantum efficiencies of the emissive layer were maintained at greater than 75% throughout the cross-linking reaction. A solution-processed small-molecule electron-transporting layer on top of the cross-linked emissive layer led to lower driving voltages and higher efficiencies in the OLEDs compared to those of a device with a vacuum-deposited Ca electrode on the emissive layer.     

Two-step fabrication of thin film encapsulation using laser assisted chemical vapor deposition and laser assisted plasma enhanced chemical vapor deposition for long-lifetime organic light emitting diodes

A thin film encapsulation layer was fabricated through two-sequential chemical vapor deposition processes for organic light emitting diodes (OLEDs). The fabrication process consists of laser assisted chemical vapor deposition (LACVD) for the first silicon nitride layer and laser assisted plasma enhanced chemical vapor deposition (LAPECVD) for the second silicon nitride layer. While SiN_x thin films fabricated by LAPECVD exhibits remarkable encapsulation characteristics, OLEDs underneath the encapsulation layer risk being damaged during the plasma generation process. In order to prevent damage from the plasma, LACVD was completed prior to the LAPECVD as a buffer layer so that the laser during LACVD did not damage the devices because there was no direct irradiation to the surface. This two-step thin film encapsulation was performed sequentially in one chamber, which reduced the process steps and increased fabrication time. The encapsulation was demonstrated on green phosphorescent OLEDs with I–V-L measurements and a lifetime test. The two-step encapsulation process alleviated the damage on the devices by 19.5% in external quantum efficiency compared to the single layer fabricated by plasma enhanced chemical vapor deposition. The lifetime was increased 3.59 times compared to the device without encapsulation. The composition of the SiNx thin films was analyzed through Fourier-transform infrared spectroscopy (FTIR). While the atomic bond in the layer fabricated by LACVD was too weak to be used in encapsulation, the layer fabricated by the two-step encapsulation did not reveal a Si–O bonding peak but did show a Si–N peak with strong atomic bonding.     

Realization of blue,green and red emission from top-emitting white organic light-emitting diodes with exterior tunable optical films

We developed an approach to realize blue, green and red emission from top-emitting white organic light-emitting diodes (OLEDs) through depositing exterior tunable optical films on top of the OLEDs. Three primary colors for full color display including blue, green and red emission are achieved by controlling the wavelength-dependent transmittance of the multilayer optical films overlaid on the emissive layer. The advantage of such a device configuration is that the emissive color of the OLEDs can be tuned via the exterior optical films which do not affect the electrical characteristics of the device. This may provide a way to realize full color display by using white top-emitting OLEDs.     

有机发光器件中空穴注入对负电容的影响

对不同结构的有机发光器件(OLED)进行了电容-电压(C-V)特性测量,研究了不同空穴注入结构对OLED负电容的影响。结果表明,负电容的产生与OLED内部电场的分布有着密切的关系,负电容开始出现的频率与电压的平方根呈指数关系。与超薄的单层空穴注入层相比,掺杂的空穴注入层不仅能降低器件的驱动电压,而且其载流子传输特性和出现负电容时的初始电压对频率有着更强的依赖性。     

Blue Single‐Layer Organic Light‐Emitting Diodes Using Fluorescent Materials: A Molecular Design View Point

Since the beginning of organic light‐emitting diodes (OLEDs), blue emission has attracted the most attention and many research groups worldwide have worked on the design of materials for stable and highly efficient blue OLEDs. However, almost all the high‐efficiency blue OLEDs using fluorescent materials are multilayer devices, which are constituted of a stack of organic layers to improve the injection, transport, and recombination of charges within the emissive layer. Although the technology has been mastered, it suffers from real complexity and high cost and is time‐consuming. Simplifying the multilayer structure with a single‐layer one, the simplest devices made only of electrodes and the emissive layer have appeared as an appealing strategy for this technology. However, removing the functional organic layers of an OLED stack leads to a dramatic decrease of the performance and achieving high‐efficiency blue single‐layer OLEDs requires intense research especially in terms of materials design. Herein, an exhaustive review of blue emitting fluorophores that have been incorporated in single‐layer OLEDs is reported, and the links between their electronic properties and the device performance are discussed. Thus, a structure/properties/device performance relationship map is drawn, which is of interest for the future design of organic materials.     

单层有机电致发光器件的电流传导机制的数值拟合分析

采用真空蒸镀的方法制备了以八羟基喹啉铝(Alq₃)为功能层的单层同质结有机电致发光器件,器件结构为indium-tin-oxide(ITO)/tris-(8-hydroxylquinoline)-aluminum(Alq₃)(x nm)/Mg:Ag.通过改变有机功能层的厚度,采用陷阱电荷限制电流(TCLC)理论对器件电流的数值拟合方法具体地研究了不同薄膜厚度的有机半导体器件内部电流的传导机制,验证了实验结果和理论推导的一致性.结果表明,Alq₃层厚度较低的单层器件随外加电压增大,器件电流经历了从欧姆电导区、TCLC区到TCLC-空间电荷限制电流(SCLC)过渡区三个区域的变化;而对于Alq₃层厚度较高的单层器件,Alq₃层中的陷阱机构增多,导致电流－电压曲线的SCLC区域消失.     

有机电致发光器件薄膜封装研究进展

有机电致发光器件(OLEDs)对水汽和氧气非常敏感,渗入OLEDs内的水汽和氧气会腐蚀有机功能层及电极材料,严重影响器件寿命。文中根据OLEDs对封装材料的要求,分析了目前最有前景的OLEDs封装技术———薄膜封装,重点介绍了薄膜封装的分类和研究现状。     

硫系玻璃薄膜封装层对OLED寿命的影响

在高真空条件下(3×10-4Pa),利用硫系玻璃(Se,Te,Sb)薄膜封装材料对有机电致发光器件(OLED)进行原位封装,有效避免了传统封装方法难以避免的水、氧危害,以达到延长器件寿命的目的。实验对比了正常封装与增加Se、Te、Sb薄膜封装层后器件的性能,对比实验中封装过程都未加干燥剂。研究发现Se、Te、Sb薄膜封装层分别可以使器件的寿命延长1.4倍,2倍,1.3倍以上;采用封装层对器件的电流-电压特性、色坐标等光电性能几乎不产生影响,但影响了器件散热,薄膜封装层使器件的击穿电压、最高亮度等参数稍有下降。     

Investigating Morphology and Stability of Fac‐tris (2‐phenylpyridyl)iridium(III) Films for OLEDs

Stable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)₃] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)₃ in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)₃:CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo­luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.     

Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light‐Emitting Diodes

Efficient blue‐, green‐, and red‐light‐emitting organic diodes are fabricated using binuclear platinum complexes as phosphorescent dopants. The series of complexes used here have pyrazolate bridging ligands and the general formula C^∧NPt(μ‐pz)₂PtC^∧N (where C^∧N = 2‐(4′,6′‐difluorophenyl)pyridinato‐N,C^2′, pz = pyrazole ( 1 ), 3‐methyl‐5‐tert‐butylpyrazole ( 2 ), and 3,5‐bis(tert‐butyl)pyrazole ( 3 )). The Pt–Pt distance in the complexes, which decreases in the order 1 > 2 > 3 , solely determines the electroluminescence color of the organic light‐emitting diodes (OLEDs). Blue OLEDs fabricated using 8 % 1 doped into a 3,5‐bis(N‐carbazolyl)benzene (mCP) host have a quantum efficiency of 4.3 % at 120 Cd m^–2, a brightness of 3900 Cd m^–2 at 12 V, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.11, 0.24). Green and red OLEDs fabricated with 2 and 3 , respectively, also give high quantum efficiencies (～ 6.7 %), with CIE coordinates of (0.31, 0.63) and (0.59, 0.46), respectively. The current‐density–voltage characteristics of devices made using dopants 2 and 3 indicate that hole trapping is enhanced by short Pt–Pt distances (< 3.1 Å). Blue electrophosphorescence is achieved by taking advantage of the binuclear molecular geometry in order to suppress dopant intermolecular interactions. No evidence of low‐energy emission from aggregate states is observed in OLEDs made with 50 % 1 doped into mCP. OLEDs made using 100 % 1 as an emissive layer display red luminescence, which is believed to originate from distorted complexes with compressed Pt–Pt separations located in defect sites within the neat film. White OLEDs are fabricated using 1 and 3 in three different device architectures, either with one or two dopants in dual emissive layers or both dopants in a single emissive layer. All the white OLEDs have high quantum efficiency (～ 5 %) and brightness (～ 600 Cd m^–2 at 10 V).     

Luminescent Enhancement of Heterostructure Organic Light-Emitting Devices Based on Aluminum Quinolines

High performance organic light-emitting devices （OLEDs） have been investigated by using fluorescent bis （2-methyl-8-quinolinolato）（para-phenylphenolato）aluminum（BAlq） as an emissive layer on the performance of multicolor devices consisting of N, N＇-bis-（1-naphthyl）-N,N＇diphenyl- 1,1＇-biphenyl-4,4＇- diamine （NPB） as hole transport layer. The results show that the performance of heterostructure blue light-emitting device composed of 8-hydroxyquinoline aluminum （Alq3） as an electron transport layer has been dramatically enhanced. In the case of high performance heterostructure devices, the electroluminescent spectra has been perceived to vary strongly with the thickness of the organic layers due to the different recombination region, which indicates that various color devices composed of identical components could be implemented by changing the film thickness of different functional layers.     

Enhancement of hole injection with an ultra-thin Ag2O modified anode in organic light-emitting diodes

The interface between the organic layer and the Indium Tin Oxide (ITO) layer of an organic light-emitting diode (OLED) is crucial to the performance of the device. An ultra-thin Ag₂O film, used as an anode modification layer, has been employed on ITO surface through the UV-ozone treatment of Ag films. The insertion of this thin film with higher work function enhances the hole injection in the organic light-emitting diode and improves the performance of the devices effectively. The maximum electroluminescence (EL) efficiency of the device with the Ag₂O film is 4.95 cd/A, it is about 60% higher than that of the device without it.     

Straight-forward control of the degree of micro-cavity effects in organic light-emitting diodes based on a thin striped metal layer

We investigated the control of micro-cavity (MC) effects in organic light-emitting diodes (OLEDs) with the introduction of a striped thin metal layer between the indium tin oxide (ITO) layer and the hole transporting layer (HTL). With an enhanced MC effect obtained through the inserted metal layer, the forward emission of the OLED became stronger and the angular distribution became more forward-directed, leading to a current efficiency (CE) that was nearly 1.45 times higher than that of the reference device without the inserted metal layer. The net CE of the OLEDs with a striped metal layer was found to be determined by the area-weighted average of the CE’s of full-cavity-enhanced OLEDs and non-cavity OLEDs. It was also observed that the trade-off between resonance enhancement in efficiency and angle-dependent color stability, often found problematic in MC-based OLEDs, could be mitigated in a straight-forward manner by changing the relative portion of the metal-covered area.     

Light extraction enhancement in organic light-emitting diodes through polyimide/porous silica hybrid films

In this work, light extraction efficiency of organic light-emitting diodes (OLEDs) with a spin-coated polyimide/porous silica hybrids were enhanced. The polyimide/porous silica thin films (C1 and C2) were accomplished by spin coating a hybrid film composed of a polyimide-silica composite blended with various amount of porous silica nanoparticles into the opposite site of ITO glass. The optical, thermal, and morphology properties were controlled by adding various amount of porous silica. The incorporation of light extraction layer improved the maximum external quantum efficiency (EQE) by as much as 24.8% based on integrating sphere measurement condition when compared to that of a reference device without a light extraction layer. Furthermore, the device utilizing the light extraction layer showed identical EL spectra as the reference device did. Additionally, the optical emission distribution of the device was close to the Lambertian profile. The polyimide/porous silica hybrids demonstrated in this work are useful and efficient for OLED device for the enhancement of EL performance, indicating the potential in a wide range of applications, such as displays, lightings and so on.     

NPB/Alq3双层有机电致发光器件薄膜厚度与器件性能的优化

研究了以NPB为空穴传输层、Alq₃为发光层的双层异质结有机电致发光器件的薄膜厚度对器件性能的影响.制备了一系列具有不同NPB和Alq₃厚度的器件并测试了其电致发光特性.结果表明,器件电流随Alq₃与NPB厚度变化的关系并不相同.不同有机层厚度双层器件的电流机制符合陷阱电荷限制(TCL)理论,随外加电压的增大,器件电流经历了欧姆电导区、TCL电流区、陷阱电荷限制-空间电荷限制(TCL-SCL)过渡区三个区域的变化.当有机层厚度匹配为NPB(20nm)/Alq₃(50nm)时可以获得性能优良的器件.器件的流明效率-电压关系曲线的变化规律是在低电压区较快达到最大值,然后随电压的增加逐渐降低.     

Bismuth Trifluoride as a low-temperature-evaporable insulating dopant for efficient and stable organic light-emitting diodes

Bismuth Trifluoride (BiF₃), with a high thermal stability and a low deposition temperature, has been studied as a novel dopant for the conventional hole transporting material of N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB). The efficiency and lifetime of organic light-emitting diodes (OLEDs) have been remarkably improved by using BiF₃ doped NPB. For an optimized green device, a current efficiency of 21.6 cd/A is reached, 40% higher than the control device without BiF₃. And the lifetime is increased from 115 h to 222 h at room temperature. The enhanced efficiency and lifetime are attributed to the improved balance of holes and electrons in the emissive layer. Most importantly, the thermal stability at an elevated temperature of the OLEDs with BiF₃ doped NPB is largely improved, showing an order of magnitude longer lifetime than the control device at 80 °C.     

Two-dimensional-growth small molecular hole-transporting layer by ultrasonic spray coating for organic light-emitting devices

Two-dimensional-growth small molecular organic thin film with high quality is fabricated by ultrasonic spray coating technology (USC) from the toluene solution of 4,4′,4″-tris (carbazol-9-yl) triphenylamine (TCTA). In comparison to the vacuum thermal evaporation (VTE) TCTA film, the USC-TCTA film obtained from liquid-phase solution possesses more uniform surface topography. The differences between the USC-TCTA and the VTE-TCTA in optical property, electrical property and formation mechanism are also studied in detail. Besides, to evaluate the hole transport and electron blocking ability of USC-TCTA film, the organic light-emitting devices (OLEDs) employing USC-TCTA film as hole transport layer are fabricated successfully. Additionally, the green OLEDs based on USC-TCTA film perform as well as the ones with VTE-TCTA film in current density, luminance, efficiency and color stability, and show even better tolerance to high bias voltage.     

Co‐deposited Cu(I) Complex for Tri‐layered Yellow and White Organic Light‐Emitting Diodes

Four compounds 4‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]isoquinoline (TCIQ), 3‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]pyridine (TCPy), 4‐(carbazol‐9‐yl)isoquinoline (4CIQ), and 3‐(carbazol‐9‐yl)pyridine (CPy) containing pyridyl or isoquinolyl were designed and synthesized to co‐deposition with copper iodide (CuI) to form luminescent Cu(I) complex doped film in situ, which could be utilized as the emissive layer in organic light‐emitting diodes (OLEDs). It is found that simple tri‐layered yellow and white OLEDs can be achieved by co‐depositing CuI and TCIQ with tuning ratios. The compound TCIQ serves a dual role as both a ligand for forming the emissive Cu(I) complex and as a host matrix for the formed emitter in yellow OLEDs, and a third role as a blue emitter in white OLEDs.     

用ZnS薄膜作为空穴缓冲层的高效率有机发光二极管

用磁控溅射方法制备的ZnS薄膜作为有机发光器件(OLEDs)的空穴缓冲层,使典型结构的 OLEDs(ITO/TPD/Alq/LiF/Al) 的发光性能得到改善.ZnS 缓冲层厚度对器件性能影响的实验结果表明,当ZnS缓冲层厚度为 5 nm 时,器件的亮度增加了2倍多;当ZnS缓冲层厚度为5、10 nm时,器件的发光电流效率增加40%.研究结果表明 ZnS 薄膜是一种好的缓冲层材料,它能够提高器件的发光效率,改善器件的稳定性.