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.     

Suppressing molecular aggregation in solution processed small molecule organic light emitting diodes

Solution processing of low-molecular weight organic materials for optoelectronic devices is a challenging task due to often strong molecular aggregation. We present a facile and universal route for suppressing the aggregation of molecules during wet-deposition of emission layers for organic light emitting diodes by incorporating electronically inactive polymers. Moderate polymer concentrations of about 10 wt.% lead to only minor changes of the electrical performance while at the same time improving the film formation and consequently the device luminance significantly. The device performance matches the performance of vacuum processed devices with the same device architecture.     

蓝光磷光微腔有机电致发光器件特性的研究

使用典型蓝色磷光材料Firpic作为磷光金属微腔 有机电致发光器件(OLED)的发光层, 以高反射的Al膜作为阴极顶电极和半透明的Al膜作为阳极底电极,其结构为 Glass/Al(15nm)/MoO₃(30nm)/NPB(40nm)/mCP:Firpic(30nm,x%)/BCP(10nm)/Alq(20nm)/LiF (1nm)/Al(100nm),x%为Firpic的掺杂 质量分数,分别为4%、6%、10%、12%和14%。实验 制备了不同的OLED,比较了测量角度和不同掺杂浓度对OLED发光特性的影响。结 果显示,对发光面积为0.8cm²的器件,测量角度的不同导致蓝光 辐射波长蓝移,色坐标发 生变化,器件的510nm和472nm两个峰值变化 不相同,随着角度的增大, 较大的峰值不断衰减,而较小的峰值不断增强;并且,当掺杂浓度为12%时,OLED得 到最好的发光性能,12V电压驱动下有最大亮度18870cd/m²,说明此时的主客体间能量转移最充分。     

Correlation between interlayer thickness and device performance in blue phosphorescent organic light emitting diodes with a quantum well structure

We systematically examined the effects of interlayer (ITL) thickness variation in an emission layer (EML) on electrical and optical characteristics of blue phosphorescent organic light-emitting diodes. The EML consisted of a quantum well structure using a hole transport material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as an ITL. This ITL facilitated the confinement of charge carriers in the recombination zone (RZ), adjusted the charge carrier balance in the EML, and prevented the triplet exciton loss to adjacent transport layers. The thickness variation in the ITL greatly influenced the size and location of the RZ and the exciton density (ED), which is related to charge balance and exciton diffusion in the EML. A micro-cavity effect around 500 nm and the corresponding redshift/blueshift in the electroluminescent spectrum arose from different ITL thicknesses. Remarkably, the device having a 5-nm-thick TAPC ITL showed better current and power efficiencies than those of any other devices because of the rearrangement of the locations of excitons and ED through control of the hole/electron charge density.     

Efficient white organic light emitting diodes with solution processed and vacuum deposited emitting layers

The white light emitting diode based on small molecules, which have double emitting layers, was fabricated by combining a solution process and a vacuum deposition process. The organic light emitting diode that we have prepared utilizes a three phosphorescent guest materials, iridium(III)bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate, fac tris(2-phenylpyridine) iridium (III) and tris[1-phenylisoquinolinato-C2,N] iridium (III), exhibits a high color rendering index of over 82. Despite its ease of fabrication, the peak power efficiency was about 16 lm/W. The high efficiency was attributed to the distribution of the emitting region over the two emitting layers, which is caused by energy transfer between the phosphorescent guests.     

Efficient red phosphorescent organic light emitting diodes based on solution processed all-inorganic cesium lead halide perovskite as hole transporting layer

An efficient red phosphorescent organic light emitting diode (PhOLED) has been realized by utilizing a composite hole transporting layer comprised of all-inorganic cesium lead halide perovskite CsPbBr₃ via spin-coating and 1,3-bis(9-carbazolyl) benzene (mCP) by vacuum depositing, in which CsPbBr₃ film is used as a hole transporting layer and mCP plays a dominant role in electron and exciton blocking. And this PhOLED shows a saturated red emission coordinated at CIE (0.65, 0.33) driven at 7.5 V, a maximum brightness of 20,750 cd/m², and a maximum current efficiency of 10.64 cd/A, which is as 1.87 times as that 5.68 cd/A of the reference PhOLEDs based on traditional small organic molecular hole transporting material N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzi (NPB). The electroluminescent (EL) spectra and the energy level alignment of different PhOLEDs are investigated. The enhanced EL performances are ascribed to improved hole injecting and transporting behaviors, and better electron and exciton confinements by introducing the composite hole transporting layer CsPbBr₃/mCP.     

Efficient hole-transporter for phosphorescent organic light emitting diodes with a simple molecular structure

N,N-diphenyl-4-(quinolin-8-yl)aniline (SQTPA), which composes a triphenylamine group and a quinoline group, has been synthesized and employed as a hole-transporter in phosphorescent OLEDs. It has been proved that SQTPA has efficient hole-transport property with a hole-mobility of 3.60 × 10⁻⁵ cm²/V s at the electric field of 800 (V/cm)^1/2, which is higher than that of NPB (1.93 × 10⁻⁵ cm²/V s). Blue, orange and green phosphorescent OLEDs have been fabricated based on FIrpic, Ir(2-phq)₃, Ir(ppy)₃ with typical structures by using SQTPA as the hole-transporter. The SQTPA-based devices show maximum external quantum efficiencies and power efficiencies of 17.5%, 32.5 lm/W for blue, 12.3%, 20.5 lm/W for orange and 20.3%, 64.5 lm/W for green. The performances of SQTPA-based devices are much better than that of NPB-based phosphorescent OLEDs with similar structures. Thought of its very simple molecular structure and easy synthetic route, SQTPA should be an efficient hole-transporter for phosphorescent OLEDs.     

Multiple exciton generation regions in phosphorescent white organic light emitting devices

We demonstrate a white electrophosphorescent organic light emitting device (WOLED) with a three-section emission layer (EML) where excitons are formed in the multiple emission regions. The EML consists of a stepped progression of highest occupied and lowest unoccupied molecular orbital energies of the ambipolar hosts. Analysis shows that (36 ± 6)% of the excitons form in the blue emitting region, while (64 ± 6)% form in the green emitting region at 100 mA/cm². The doping of the red, green and blue phosphors, each in its own host, allows for efficient utilization of excitons formed in these multiple regions. Based on this architecture, the WOLED has an internal quantum efficiency close to unity. The WOLED has total external quantum and power efficiencies of η_ext,t = (26 ± 1)% and η_p,t = (63 ± 3) lm/W at 12 cd/m², decreasing to η_ext,t = (23 ± 1)% and η_p,t = (37 ± 2) lm/W at 500 cd/m². When an undoped electron transport layer is used, the peak efficiency is η_ext,t = (28 ± 1)%. Due to the distributed exciton formation in the EML, the WOLED exhibits higher total efficiency than monochromatic devices employing the same red, green and blue dopant–host combinations.     

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).     

Star-shaped hexaaryltriindoles small molecules: Tuning molecular properties towards solution processed organic light emitting devices

We present a series of differently substituted star-shaped hexaaryltriindoles with tunable light-emitting properties. The deep blue emission is unchanged by donor peripheral substituents while an increasing acceptor character produces a reduction of the optical gap, an increased Stokes shift and eventually leads to the appearance of a new electronic level and to the simultaneous deep blue (413 nm) and green (552 nm) emission in solution. Quenching by concentration increases with the acceptor character but is lower as the tendency of these compounds to aggregate is stronger. Solution processed thin films present optical and morphological qualities adequate for device fabrication and similar electronic structure compared to solutions with an emission range from 423 nm up to 657 nm (red), demonstrating the possibility of tuning the energy levels by chemical functionalization. We have fabricated and characterized single-layer solution processed organic light emitting diodes (OLED) to investigate the influence on transport and emission properties of the substituting species. We analyzed the I–V response using a single-carrier numerical model that includes injection barriers and non-uniform electric-field across the layer. As a result, we obtained the electric field dependence of the mobility for each device. Best results are obtained on the most electron rich derivative functionalized with six donor methoxy groups. This material shows the highest emission efficiency in solid state, due to aggregation-induced enhancement, and better transport properties with the highest mobility and a very low turn-on voltage of 2.8 V. The solution processed OLED devices produce stable deep blue (CIE coordinates (0.16, 0.16)) to white (CIE coordinates (0.33, 0.3)) emission with similar luminous efficiencies.     

加入电子阻挡层的黄色磷光有机电致发光器件

使用绿色磷光材料GIr1和红色磷光材料R-4B作 为掺杂剂,制备了一种黄色磷光有机电致发光 器件(OLED),其结构为ITO/MoO₃(60nm)/NPB(40nm)/TCTA(x nm,x＝0、5、10和15)/CPB:GIr1:R -4B(30nm,14%,2%) /BCP(10nm) /Alq₃(40nm)/LiF(1nm)/Al( 100nm)。其中x＝0,5,10,5nm。通过在发光层与空穴传输层之间增 加电子阻挡层TCTA,使器件的效率得到提高。当TCTA厚为10nm时, 起亮电压为4V左右,器 件的最大发光效率为20.2cd/A,最高亮度可以达到21840cd/m²,器件的色坐标 为(0.42,0.53)。器件的EL主峰位于524nm 和604nm。并且当电流 密度为2.49mA/cm²时,10nm厚的TCTA 电子阻挡层的器件发光效率是不加入TCTA的器件发光效率的2倍。发光效率的提高是由于电 子阻挡层的加入限制了空穴传输层NPB的发光,从而使更多的激子在发光层中复合。     

Fluorene-based cathode interlayer polymers for high performance solution processed organic optoelectronic devices

A hydrophilic polyfluorene-based conjugated polyelectrolyte (CPE) Poly[9,9-bis(4′-(6″-(diethanolamino)hexyloxy) phenyl)fluorene], PPFN-OH (Scheme 1) has been synthesized and utilized as cathode interlayer for both polymer light emitting diodes (PLEDs) and solar cells (PSCs). For comparison, another CPE namely Poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH) has also been investigated. They comprise the same polyfluorene backbone structures with, respectively, diethanolaminohexyl (PFN-OH) and diethanolaminohexoxyphenyl (PPFN-OH) substituents attached to the C9 carbon of the fluorene repeat unit. In comparison to reference devices with more reactive Ca/Al cathodes, utilizing these CPEs as interlayers allowed an Al cathode to be used for blue light emission PLEDs, yielding 51% and 92% enhancement of maximum luminous efficiency (LE) for PFN-OH and PPFN-OH, respectively. The PLEDs with PPFN-OH showed both higher maximum LE and maximum luminance (L) (LE = 2.53 cd/A at 6.2 V, L = 9917 cd/m² at 8.3 V) than devices with PFN-OH (2.00 cd/A at 4.1 V, 3237 cd/m² at 7.2 V). The PPFN-OH PLEDs also showed no significant roll-off in efficiency with increasing current density up to 400 mA/cm², indicating excellent electron injection ability and stability for this interlayer. The insertion of alkoxy-phenyl groups at the C9-position in PPFN-OH is clearly advantageous. This simple modification significantly improves the CPE cathode interlayer performance. Parallel investigations of the electron extraction properties of PPFN-OH in inverted architecture PSCs with PCDTBT:PC₇₀BM bulk heterojunction active layers demonstrated a power conversion efficiency enhancement of ∼19% (from 4.99% to 5.95%) for indium tin oxide cathode devices compared with reference devices using Ca/Al cathodes. These results confirm PPFN-OH to be a promising interlayer material for high performance solution processed organic optoelectronic devices.     

Indenocarbazole based bipolar host materials for highly efficient yellow phosphorescent organic light emitting diodes

We report bipolar host materials with robust indenocarbazole and biphenyl moiety as hole-electron-transporting unit for phosphorescent yellow organic light-emitting diodes (OLEDs). New host materials demonstrated an excellent morphological stability with high glass transition temperature of 207 °C. Simultaneously, it also revealed appropriate triplet energy of about 2.6 eV for ideal triplet energy transfer to yellow phosphorescent dopant. A phosphorescent yellow OLED with new host ICBP1 (and ICBP2) and conventional yellow dopant iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (Ir(tptpy)₂acac) shows a low driving voltage of 3.4 (and 3.6 V) at 1000 cd/m², and maximum external quantum efficiency as high as 26.4%. Such efficient performance of phosphorescent yellow OLEDs is attributed to a good charge balance and high electron transport properties of host materials.     

以iridium磷光为红绿蓝染料的白光器件研究

以磷光染料iridium (III) bis [(4,6-di-fluoropheny)-pyridinato-N,C2′] picolinate(Flrpic)掺在4.4′-bis (9-carbazolyl)-2,2′-dimethyl-biphenyl(CDBP)中作为蓝光发光层,tris (2-Phenylpyridine) iridium(Irlppy)3和bis (1-phenyl-isoquinoline) acetylacetonate iridium (III)(Ir(piq)2)(acac)共掺在4,4′-N,N′-dicarbazole-biphenyl(CBP)中作为绿光和红光发光层,制备了高效白光器件.通过控制染料的浓度和发光层的厚度调节颜色,实现白光发射.器件的最大亮度为17 V时37 100 cd/m2,最大效率为5 V时7.37 lm/W.当亮度从1 000 cd/m2 到30 000 cd/m2色坐标由(0.41,0.42) 变到(0.37,0.39).     

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.     

Highly efficient inverted top emitting organic light emitting diodes using a horizontally oriented green phosphorescent emitter

We report a highly efficient phosphorescent green inverted top emitting organic light emitting diode doped with Ir(ppy)₂tmd [bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)] as the horizontally oriented emitter in an exciplex forming co-host system. The device showed a maximum current efficiency of 120.7 cd/A, a maximum external quantum efficiency (EQE) of 27.6% and the power efficiency of 85.9 lm/W at 1,000 cd/m². Moreover the efficiency roll off was small long-lasting to 20,000 cd/m² with EQE’s and current efficiencies of 26.0% and 113.7 cd/A at 10,000 cd/m² and 24.5% and 107.6 cd/A at 20,000 cd/m², respectively. Optical analysis of the efficiencies and emission spectra of the device is also reported.     

有机电致发光器件的热学特性分析

采用COMSOL有限元分析软件的固体传热模块,对有机电致发光器件（OLED）的热学特性进行了仿真,发现器件温度随着输入功率成线性增大。在驱动电流为150 mAcm-2时,仿真结果表明,Alq3发光层的最高温度为82.994 3℃;玻璃基板下表面的最高温是77.392 6℃;器件阴极表面中心区域的最高温度为82.994 2℃,其平均温度为78.445℃。通过改变功能层热传导率、功能层厚度、对流换热系数、表面发射率等参数模拟其对OLED器件热学特性的影响,结果表明,当增加基板的热传导率时,OLED器件温度显著下降而且表面及内部温度梯度大幅减小;提高空气对流换热系数及基板的表面发射率,OLED的温度可以大幅减小。而其他参数则对其影响并不明显。     

异质结堆叠发光层红色磷光有机发光二极管

本文采用主客体交错结构的发光层,即发光层是 由多组主体材料CBP和客体材料Ir(piq)₂(acac)异质结堆叠构成的。为了改善器件的性能 ,分别优化 了单主体层和单客体层的厚度。研 究表明,单主体层厚度为3~4 nm,单客体层厚度为0.3 nm时,器件能够获得的最大电流效率为3.92 cd/A,色纯度 和发光稳 定性俱佳,1mA工作电流下的CIE色坐标为(0.669,0.308),当工作电流从0.1 mA变化 到1mA,色度坐标的变化值(Δ(x,y)) 仅为(0.004,0.002)。所采用的 主客体交错发光层的制备方法,工艺简单,且因为能分别调整主客体层的厚度而改善因客体 分子聚集或因长程偶极子间相互作用对发光效率的影响,为非掺杂磷光有机发光二极管的制 备提供了思路。     

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.     

Enhancing the efficiency of simplified red phosphorescent organic light emitting diodes by exciton harvesting

High efficiency red phosphorescent organic light emitting diode (PHOLED) employing co-doped green emitting molecule bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)₂(acac)] and red emitting molecule bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)₂(acac)] into 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host in a simplified wide-bandgap platform is demonstrated. The green molecule is shown to function as an exciton harvester that traps carriers to form excitons that are then efficiently transferred to the Ir(MDQ)₂(acac) by triplet-to-triplet Dexter energy transfer, thereby significantly enhancing red emission. In particular, a maximum current efficiency of 37.0 cd/A and external quantum efficiency (EQE) of 24.8% have been achieved without additional out-coupling enhancements. Moreover, a low efficiency roll-off with the EQE remaining as high as 20.8% at a high luminance of 5000 cd/m² is observed.