Blue organic light-emitting diodes with low driving voltage and maximum enhanced power efficiency based on buffer layer MoO3

Blue organic light-emitting devices based on wide bandgap host material, 2-(t-butyl)-9, 10-di-(2-naphthyl) anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO₃) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline (BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO₃ (5 nm)/NPB (30 nm)/[TBADN: DSA-Ph (3 wt%)](35 nm)/BPhen (12 nm)/LiF (0.8 nm)/Al (100 nm). It was found that the MoO₃∥BPhen-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA∥Alq₃ is based on, respectively. The J-V curves of ‘hole-only’ devices reveal that a small hole injection barrier between MoO₃∥NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.     

4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺掺杂MoOx作为空穴传输层对有机太阳电池性能的影响

通过采用4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺 (m-MTDATA)掺入MoO_x作为器件的空穴传输层来提高酞菁铜(CuPc)/C₆₀小分子 有机太阳电池的效率. 采用真空蒸镀的方法制备了一系列器件, 其中结构为铟锡氧化物 (ITO)/m-MTDATA:MoO_x(3:1)(30 nm)/CuPc(20 nm)/C₆₀(40 nm)/4,7-二苯 基-1,10-菲罗啉 (Bphen)(8 nm)/LiF(0.8 nm)/Al(100 nm)的器件, 在AM1.5 (100 mW/cm²)模拟太阳光的照射条件下, 开路电压V_oc=0.40 V, 短路电流J_sc=6.59 mA/cm², 填充因子为0.55, 光电转换效率达1.46%, 比没有空穴传输层的器件ITO/CuPc(20 nm)/C₆₀(40 nm)/Bphen(8 nm)/LiF(0.8 nm)/Al(100 nm) 光电转换效率提高了38%. 研究表明, 加入m-MTDATA:MoO_x(3:1)(30 nm)空穴传输层减小了有机层和ITO电极之间的接触电阻, 从而减小了整个器件的串联电阻, 提高了器件的光电转换效率.     

High-efficiency p–i–n organic light-emitting diodes with a novel n-doping electron transport layer

We demonstrate p–i–n organic light-emitting diodes (OLEDs) incorporating an n-doping transport layer which comprises 8-hydroxy-quinolinato lithium (Liq) doped into 4′7-diphyenyl-1,10-phenanthroline (Bphen) as ETL and a p-doping transport layer which includes tetrafluro-tetracyano-quinodimethane (F₄-TCNQ) doped into 4,4′,4″-tris(3-methylphenylphenylamono) triphenylamine (m-MTDATA). In order to examine the improvement in the conductivity of transport layers, hole-only and electron-only devices are fabricated. The current and power efficiency of organic light-emitting diodes have been improved significantly after introducing a novel n-doping (Bphen: 33 wt% Liq) layer as an electron transport layer (ETL) and a p-doping layer composed of m-MTDATA and F₄-TCNQ as a hole transport layer (HTL). Compared with the control device (without doping), the current efficiency and power efficiency of Device C (most efficient) is enhanced by approximately 51% and 89%, respectively, while driving voltage is reduced by 29%. This improvement is attributed to the improved conductivity of the transport layers that leads to the efficient charge balance in the emission zone.     

Reduction of driving voltage in organic light-emitting diodes with molybdenum trioxide in CuPc/NPB interface

A novel structure of organic light-emitting diode was fabricated by inserting a molybdenum trioxide (MoO₃) layer into the interface of hole injection layer copper phthalocyanine (CuPc) and hole transport layer N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB). It has the configuration of ITO/CuPc(10 nm)/MoO₃(3 nm)/NPB(30 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq₃)(60 nm)/LiF(0.5 nm)/Al. The current density-voltage-luminance (J-V-L) performances show that this structure is beneficial to the reduction of driving voltage and the enhancement of luminance. The highest luminance increased by more than 40% compared to the device without hole injection layer. And the driving voltage was decreased obviously. The improvement is ascribed to the step barrier theory, which comes from the tunnel theory. The power efficiency was also enhanced with this novel device structure. Finally, “hole-only” devices were fabricated to verify the enhancement of hole injection and transport properties of this structure.     

Highly efficient and stable organic light-emitting diodes employing MoO3-doped perylene-3, 4, 9, 10-tetracarboxylic dianhydride as hole injection layer

We report highly efficient and stable organic light-emitting diodes (OLEDs) with MoO₃-doped perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) as hole injection layer (HIL). A green OLED with structure of ITO/20 wt% MoO₃: PTCDA/NPB/Alq₃/LiF/Al shows a long lifetime of 1012 h at the initial luminance of 2000 cd/m², which is 1.3 times more stable than that of the device with MoO₃ as HIL. The current efficiency of 4.7 cd/A and power efficiency of 3.7 lm/W at about 100 cd/m² have been obtained. The charge transfer complex between PTCDA and MoO₃ plays a decisive role in improving the performance of OLEDs.     

利用p型掺杂技术制备的高效有机电致发光器件

利用氧化钼(MoO_x)作为p型掺杂剂,以掺杂层4,4'-bis(carbazol-9-yl)biphenyl(CBP):MoO_x作为空穴注入层,制备了一种结构为ITO/MoO_x/CBP:MoO_x/CBP/CBP:tris(2-phenylpyridine)iridium(III)(Ir(ppy)₃)/4,7-diphenyl-1,10-phenanthroline(Bphen)/LiF/Al的有机电致发光器件.器件中CBP同时作为空穴注入层、空穴传输层以及发光层母体材料,这种结构具有结构简单同时能有效降低空穴注入势垒等优点.研究发现,随着空穴注入层厚度的增加,器件的电流密度增加,表明p型掺杂层的引入能够有效增强空穴的注入;通过优化器件空穴注入层与空穴传输层厚度,器件性能有所提高,最大电流效率为29.8 cd/A,可以认为合理的优化空穴注入层和空穴传输层的厚度,使载流子在发光层中的分布更加平衡是提高器件发光效率的主要原因.值得指出的是,从电流效率最大值到亮度为 20 000 cd/m²时,优化后器件的效率衰减仅为17.7%,而常规器件的效率衰减则为62.1%,优化后器件效率衰减现象得到了明显的改善.分析认为优化后的器件中未掺杂的CBP有助于展宽激子形成区宽度,进而减弱了三线态-三线态湮灭、三线态-极化子淬灭现象,激子形成区的展宽是改善效率衰减的主要原因.     

一种改善红光顶发射OLED色纯度及角度依赖特性的方法研究

在Si/SiO₂衬底上生长金属银作为阳极,4,4,4-tris(3-methylphenylpheny-lamino)-triphenylamine(m-MTDATA):MoOx/m-MTDATA/N,N-bis-(1-naphthyl)-N,N-diphenyl-1,1-biphenyl-4,4-diamine(NPB)作为空穴注入及传输层,发光层采用4,4-N,N-dicarbazole-biphenyl(CBP)掺杂磷光染料(1-(phenyl)isoquinoline)iridium(III) acetylanetonate(Ir(piq)₂(acac))的结构,4,7-di-phenyl-1,10-phenanthroline(BPhen)作为空穴阻挡层及电子传输层,阴极为LiF(1 nm)/Al(2 nm)/Ag(20 nm)复合阴极结构.通过在光取出的复合阴极上方生长一层CBP光学覆盖层,有效地改善了复合阴极膜系的透射率,从而改善了顶发射结构的光学耦合输出特性,在提高器件的正向发光效率的同时还使色坐标往深红光区移动.并且生长光学覆盖层结构的器件角度依赖特性明显得到改善,这对于制作高显示质量的显示器件具有重要意义.在原有结构的基础上增加20 nm的NPB掺杂磷光染料Ir(piq)₂(acac)作发光层,从而得到双发光层结构为NPB:Ir(piq)₂(acac)(1%,20 nm)/CBP:Ir(piq)₂(acac)(1%, 20 nm).由于NPB具有较高的空穴迁移率,避免了由于光学厚度的增加而引起器件工作电压的大幅升高,而双发光层的结构有利于增大激子复合区域,提高辐射复合几率,减少非辐射损耗,实现主客体之间高效的三线态能量传递,相对单发光层顶发射结构,双发光层结构不仅提高了器件的发光效率,而且改善了器件的色坐标.     

Low driving voltage in organic light-emitting diode using MoO3/NPB multiple quantum well structure in hole transport layer

Driving voltage of organic light-emitting diode (OLED) is lowered by employing molybdenum trioxide (MoO₃)/N, N'-bis(naphthalene-1-yl)-N,N'-bis(phe-nyl)-benzidine (NPB) multiple quantum well (MQW) structure in hole transport layer. For the device with double quantum well (DQW) structure of ITO/ [MoO₃ (2.5 nm)/NPB (20 nm)]₂/Alq₃(50 nm)/LiF (0.8 nm)/Al (120 nm)], the turn-on voltage is reduced to 2.8 V, which is lowered by 0.4 V compared with that of the control device (without MQW structures), the driving voltage is 5.6 V, which is reduced by 1 V compared with that of the control device at the 1000 cd/m². In this work, the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure, which is attributed not only to the reducing energy barrier between ITO and NPB, but also to the forming charge transfer complex between MoO₃ and NPB induced by the interfacial doping effect of MoO₃.     

Optimized Performances of Thick Film Organic Lighting-Emitting Diodes

The performance of organic light-emitting diodes (OLEDs) with thick film is optimized. The alternative vanadium oxide (V₂O₅) and N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) layers are used to enhance holes in the emissive region, and 4,7-dipheny-1,10-phenanthroline (Bphen) doped 8-tris-hydroxyquinoline aluminium (Alq₃) is used to enhance electrons in the emissive region, thus ITO/V₂O₅ (8nm)/NPB (52nm)/V₂O₅ (8nm)/NPB (52nm)/Alq₃ (30 and 45nm)/Alq₃:Bphen (30wt%, 30 and 45nm)/LiF (1nm)/Al (120nm) devices are fabricated. The thick-film devices show the turn-on voltage of about 3V and the maximal power efficiency of 4.5lm/W, which is 1.46 times higher than the conventional thin-film OLEDs.     

An organic electrophosphorescent device driven by all-organic thin-film transistor using photoacryl as a gate insulator

Organic electrophosphorescent devices have been intensively investigated for using in full-color flat-panel display. Since the quantum efficiency of electrophosphorescent device decreases rapidly as the luminance increases, it is desirable to operate the electrophosphorescent display with active matrix rather than passive matrix. Here we report the study of driving electrophosphorescent diode with all-organic TFT. We obtained the maximum power luminance that was obtained about 90 cd/m². Turn-on voltage is approximately 10 V. Field effect mobility, threshold voltage, and on–off current ratio in 0.5-μm thick gate dielectric layer were 0.13 cm²/V s, −7 V, and 10⁶ A/A. The structure of electrophosphorescent diode is ITO/TPD/BCP:Ir(ppy)₃/BCP/Alq₃/Li:Al/Al. In organic TFT, photoacryl is used as an insulator and pentacene as an active layer.     

Tandem organic light-emitting diode with molybdenum tri-oxide thin film interconnector layer

A 10-nm thickness molybdenum tri-oxide (MoO₃) thin film was used as the interconnector layer in tandem organic light-emitting devices (OLEDs). The tandem OLEDs with two identical emissive units consisting of N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB)/tris(8-hydroxyquinoline) aluminum (Alq₃) exhibited current efficiency-current density characteristics superior to the conventional single-unit devices. At 20 mA/cm², the current efficiency of the tandem OLEDs using the interconnector layers of MoO₃ thin film was about 4.0 cd/A, which is about twice of that of the corresponding conventional single-unit device (1.8 cd/A). The tandem OLED showed a higher power efficiency than the conventional single-unit device for luminance over 1200 cd/m². The experimental results demonstrated that a MoO₃ thin film with a proper thickness can be used as an effective interconnector layer in tandem OLEDs. Such an interconnector layer can be easily fabricated by simple thermal evaporation, greatly simplifying the device processing and fabrication processes required by previously reported interconnector layers. A possible explanation was proposed for the carrier generation of the MoO₃ interconnector layer.     

Isophorone derivative as red dopant for organic electroluminescent devices

The donor–acceptor functionalized molecule, bis(4-(2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)vinyl)phenyl)(1-naphthyl)amine (DPN-4CN), with symmetrical structure, was investigated for its application in optoelectronic devices. Red organic light-emitting diodes (OLEDs) were fabricated by doping DPN-4CN in tris(8-hydroxyquinolino) aluminum (Alq₃) as red emitters, with a structure of ITO/NPB/Alq₃:DPN-4CN/BCP/Alq₃/LiF/Al. The device with a doping concentration of 2.5 wt% showed pure red emission with λ_max at 654 nm and CIE coordinates of (0.62, 0.36), a high brightness of 5080 cd m⁻² at a driving voltage of 12 V, a current efficiency of 2.14 cd A⁻¹ and an external quantum efficiency of 1.07% at a current density of 20 mA cm⁻². The current efficiencies and CIE coordinates of the device were almost constant over a current density from 1 to 200 mA cm⁻².     

Highly efficient and stable organic light-emitting diode by balancing drift current of charge

Highly efficient and stable OLED device in which hole-drift current and electron-drift current are balanced was fabricated. Drift current characteristics according to the thickness of organic layer were examined using the device with ITO/m-MTDATA/NPB/Al structure that can only move the hole and the device with Al/LiF/Alq₃/LiF/Al structure that can only move the electron. Using the result of such examination, green device with balanced drift current was produced. Device with the structure of m-MTDATA (80 nm)/NPB (20 nm)/C-545T (3%) doped Alq₃ (5 nm)/Alq₃ (59 nm)/LiF (1 nm)/Al (200 nm) showed color purity of (0.309, 0.643) and high efficiency of 7.0 lm/W (14.4 cd/A). Most of light emission was observed inside the green emitting layer. Through the result of EL spectrum for the device also including red emitting layer, same result could be obtained. The device with balanced drift current also showed half life-time of 175 h for initial luminance of 3000 cd/m², which is more stable in comparison to the device without balanced drift current.     

Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

Performance improvement of MEH-PPV:PCBM solar cells using bathocuproine and bathophenanthroline as the buffer layers

In this work, bathocuproine (BCP) and bathophenanthroline (Bphen), commonly used in small-molecule organic solar cells (OSCs), are adopted as the buffer layers to improve the performance of the polymer solar cells (PSCs) based on poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction. By inserting BCP or Bphen between the active layer and the top cathode, all the performance parameters are dramatically improved. The power conversion efficiency is increased by about 70% and 120% with 5-nm BCP and 12-nm Bphen layers, respectively, when compared with that of the devices without any buffer layer. The performance enhancement is attributed to BCP or Bphen (i) increasing the optical field, and hence the absorption in the active layer, (ii) effectively blocking the excitons generated in MEH-PPV from quenching at organic/aluminum (Al) interface due to the large band-gap of BCP or Bphen, which results in a significant reduction in series resistance (Rs), and (iii) preventing damage to the active layer during the metal deposition. Compared with the traditional device using LiF as the buffer layer, the BCP-based devices show a comparable efficiency, while the Bphen-based devices show a much larger efficiency. This is due to the higher electron mobility in Bphen than that in BCP, which facilitates the electron transport and extraction through the buffer layer to the cathode.     

基于新型电荷生成层的P-i-N结构白色有机叠层电致发光器件

采用2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline(BCP): 5 wt.% cesium carbonate(Cs₂CO₃)和N, N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine(NPB): 20 wt.% molybdenum oxide(MoO₃)分别作为器件的电子注入层和空穴注入层,研究了N型和P-i-N结构有机电致发光器件的载流子传输特性.载流子传输层中BCP: Cs₂CO₃和NPB:MoO₃的引入有效增强了载流子注入能力,从而降低了器件的驱动电压.基于新型电荷生成层BCP: 5 wt.% Cs₂CO₃/ NPB: 20 wt.% MoO₃制备了色稳定、高效率P-i-N结构有机叠层器件.与单元器件相比,引入新电荷生成层有机叠层器件的最大电流效率增大了2.5倍,表明该电荷生成层可以有效地将电子和空穴分别注入到相邻发光单元中.采用该电荷生成层制备了P-i-N结构白色有机叠层器件,器件的上下发光单元分别为橙光和蓝光发射.当发光亮度从500增加到5 000 cd/m²时,器件的色坐标稳定在(0.33, 0.29)附近,接近白光等能点.利用单色发光单元堆叠制备白色有机叠层器件的方法为实现色稳定、高效率的白色有机电致发光器件提供了一种有效的途径.     

High efficiency top-emitting white organic light-emitting devices with a (metal/organic)2 cathode

White top-emitting organic light-emitting devices (TEOLEDs) were fabricated on a glass substrate with metal/organic multilayer of (Ag/Alq₃)₂ (Alq₃ is tris-(8-hydroxyquinoline) aluminum) as cathode. White TEOLEDs with high efficiency were obtained due to the microcavity effects. And the (Ag/Alq₃)₂ cathode, which adjusted the optical characteristics of the devices, played an important role. In addition, Alq₃–Ag–Alq₃ multilayer could work as a buffer layer, which would simplify the process of encapsulation for devices. We also calculated the electroluminescence spectrum of devices encapsulated with Al₂O₃ (150 nm) and Al₂O₃(75 nm)/ZrO₂(75 nm). And the results indicated that the CIE coordinates is almost the same between with and without encapsulating.     

Improvement of performance of tetraphenylporphyrin-based red organic light emitting diodes using WO3 and C60 buffer layers

One of the porphyrin derivatives, meso-tetraphenylporphyrin (TPP), has been synthesized and examined as an emitter material (EM) for efficient fluorescent red organic light-emitting diodes (OLEDs). By inserting a tungsten oxide (WO₃) layer into the interface of anode (ITO) and hole transport layer N,N′-Di-[(1-napthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (NPB) and by using fullerene (C₆₀) in contact with a LiF/Al cathode, the performance of devices was markedly improved. The current density–voltage–luminance (J–V–L) characterizations of the samples show that red OLEDs with both WO₃ and C₆₀ as buffer layers have a lower driving voltage and higher luminance compared with the devices without buffer layers. The red OLED with the configuration ITO/WO₃ (3 nm)/NPB (50 nm)/TPP (60 nm)/BPhen (30 nm)/C₆₀ (5 nm)/LiF (0.8 nm)/Al (100 nm) achieved the high luminance of 6359 cd/m² at the low driving voltage of 8 V. At a current density of 20 mA/cm², a pure red emission with CIE coordinates of (0.65; 0.35) is observed for this device. Moreover, a power efficiency of 2.07 lm/W and a current efficiency of 5.17 cd/A at 20 mA/cm² were obtained for the fabricated devices. The study of the energy level diagram of the devices revealed that the improvement in performance of the devices with buffer layers could be attributed to lowering of carrier-injecting barrier and more balanced charge injection and transport properties.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.