White emission polymer light-emitting devices with efficient electron injection from alcohol/water-soluble polymer/Al bilayer cathode

We demonstrate highly efficient white emission polymer light-emitting diodes (WPLEDs) from multilayer structure formed by solution processed technique, in which alcohol/water-soluble polymer, poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) was incorporated as electron-injection layer and Al as cathode. It was found that the device performance was very sensitive to the solvents from solution of which the PFN electron-injection layer was cast. Devices with electron-injection layer cast from methanol solution show degraded performance while the best device performance was obtained when mixed solvent of water and methanol with ratio of 1:3 was used. We attribute the variation in device performance to washing out the electron transport material in the emissive layer due to rinse effect. As a result of alleviative loss of electron transport material in the emissive layer, the optimized device with a peak luminous efficiency of 18.5 cd A^?1 for forward-viewing was achieved, which is comparable to that of the device with same emissive layer but with low work-function metal Ba cathode (16.6 cd A^?1). White emission color with Commission International de I’Eclairage coordinates of (0.321, 0.345) at current 10 mA cm^?2 was observed.     

Effect of electron injection and transport materials on efficiency of deep-blue phosphorescent organic light-emitting devices

We investigate the performance of FIr6-based deep-blue phosphorescent organic light-emitting devices (PHOLEDs) with three different electron transport materials, bathocuproine (BCP), 4,7-diphenyl-1,10-phenanthroline (BPhen), and tris[3-(3-pyridyl)mesityl]borane (3TPYMB), and study the effect of doping alkaline metals (Li and Cs) into these charge transport materials. External quantum efficiency (η_EQE) of (20 ± 1)% and peak power efficiency (η_P) of (36 ± 2) lm/W were achieved maintaining Commission Internationale de L’Eclairage (CIE) coordinates of (x = 0.16, y = 0.28) in p-i-n dual-emissive-layer (D-EML) deep-blue PHOLEDs with 3TPYMB as the electron transport material and 3TPYMB:Cs as the electron injection layer. The high efficiencies are attributed to the high triplet energy of 3TPYMB as well as the increased conductivity of 3TPYMB:Cs.     

Electron injection in inverted organic light-emitting diodes with poly(ethyleneimine) electron injection layers

Electron-injection mechanisms from the air-stable metal-oxide cathode to light-emitting polymer layer are studied. The device configuration is aluminum (Al) doped zinc oxide (AZO)/poly(ethyleneimine) (PEI)/poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)/molybdenum trioxide/Al, known as an inverted organic light-emitting diode (iOLED). PEI reduces the electron injection barrier between AZO and F8BT by 0.4 eV, and blocks holes at AZO(PEI)/F8BT interface in iOLEDs. The accumulation of holes at the interface greatly enhances the electron injection because of the Fowler-Nordheim type tunneling injection, leading to high current efficiency of iOLEDs.     

High performance top-emitting organic light-emitting diodes with copper iodide-doped hole injection layer

Efficient top-emitting organic light-emitting diodes were fabricated using copper iodide (CuI) doped 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) as a hole injection layer and Ir(ppy)₃ doped CBP as the emitting layer. CuI doped NPB layer functions as an efficient p-doped hole injection layer and significantly improves hole injection from a silver bottom electrode. The top-emitting device shows high current efficiency of 69 cd/A with Lambertian emission pattern. The enhanced hole injection is originated from the formation of the charge transfer complex between CuI and NPB.     

Low sublimation temperature cesium pivalate complex as an efficient electron injection material for organic light-emitting diode devices

Cesium pivalate ((CH₃)₃CCOOCs) has been synthesized and applied as an electron injection material for organic light-emitting diodes, which showed low sublimation temperature of 180 °C. Typical bilayer structure of ITO/NPB (60 nm)/Alq₃ (50 nm)/EIL/Al was used to evaluate the electron injection efficacy of (CH₃)₃CCOOCs, the results showed (CH₃)₃CCOOCs/Al exhibits better electron injection than LiF/Al cathode and the power efficiency was improved by about 19% at current density of 50 mA/cm². More interestingly, in the typical three layer OLED structure ITO/2-TNATA (60 nm)/NPB (10 nm)/Alq₃:2% C545T (40 nm)/MADN (15 nm)/(CH₃)₃CCOOCs (2 nm)/Al, the maximum current efficiency is up to 20 cd/A with Commission Internationale d’Eclairage (CIE_x,_y) color coordinates of (x = 0.30, y = 0.65) at current density of 140 mA/cm², which indicates that the non-aromatic alkali metal complex can also have good match with the chemically stable compound and exhibit good electron injection properties.     

Bright green organic light-emitting devices having a composite electron transport layer

A bright green organic light-emitting device employing a co-deposited Al-Alq₃ layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq₃)/ Al-Alq₃/Al. In this device, Al-Alq₃ is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm². At optimal condition, the brightness of a device at 20 mA/cm² is 2195 cd/m² achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al-Alq₃ layer is suitable for the ETL of organic light-emitting devices (OLEDs).     

Bright organic electroluminescent devices with double-layer cathode

Electroluminescent devices were fabricated using a diamine derivative and tris (8-quinolinolato) aluminum (III) complex as the hole transport layer and the emitting layer, respectively. The glass substrate/anode/hole transport layer/emitting layer/cathode cells structure was employed. The anode was indium-tin-oxide (ITO) transparent electrode, and the cathode was a double layer consisting of first layer of Mg or Li and the second layer of Ag. Intense bright green emission with luminance of 40400 cd/m² was achieved at 18-V with a current density of 330 mA/cm² for the cell with the Al complex doped with 1 mol.% of coumarin 6 and Li/Ag as the cathode     

Lithium phenolate complexes for an electron injection layer in organic light-emitting diodes

We synthesized π-conjugated lithium phenolate complexes, lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(2′, 2′′-bipyridine-6′-yl)phenolate (LiBPP), and lithium 2-(isoquinoline-1′-yl)phenolate (LiIQP). These complexes showed lower sublimation temperatures of 305–332 °C compared to 717 °C of LiF. The organic light-emitting devices (OLEDs) using these complexes as an electron injection layer exhibited high efficiencies which are comparable to that of the device using LiF. Especially, a 40-nm thick film of LiBPP or LiPP was effective as an electron injection material, providing low driving voltages, while such a thick film of LiF serves as a complete insulator, resulting in high driving voltages.     

Role of hole injection layer in intermediate connector of tandem organic light-emitting devices

In this work, we demonstrate three kinds of intermediate connectors (ICs) having a general configuration of “LiNH₂-doped 4,7-diphenyl-1,10-phenanthroline (BPhen)/hole injection layer (HIL)/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB)”, in which the HIL is 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN), MoO₃ or MoO₃-doped NPB, respectively. Tandem organic light-emitting devices (OLEDs), vertically stacking two electroluminescence units, are fabricated using these intermediate connectors in between. The results show that, higher power efficiency is achievable in the cases of utilizing HAT-CN and MoO₃-doped NPB as HILs in the intermediate connectors versus MoO₃, whereas higher current efficiency can be obtained in the cases of using MoO₃ and MoO₃-doped NPB versus HAT-CN. We use the current density–voltage and low frequency differential capacitance–voltage measurements and find that the HILs primarily influence the voltage drop and the charge generation capability of intermediate connectors. The correlation between the effectiveness of intermediate connectors and the performances of tandem OLEDs is established, which can shed light on choosing suitable component materials to optimize the intermediate connectors.     

Small-sized Al nanoparticles as electron injection hotspots in inverted organic light-emitting diodes

Al nanoparticles, with small size and ultralow coverage on ITO, can play a key role as the electron injection hotspots in both the inverted fluorescent and phosphorescent organic light-emitting diodes. The presence of the hotspots greatly reduces the operational voltage and improves the current efficiency of the devices, which are strongly dependent on the hotspot size. The microscopic and spectroscopic characterization demonstrate that the small-sized hotspots have a minor influence on the surface roughness, transparency and work function of ITO. The hotspot effect is ascribed to the highly efficient electron injection at the Al nanoparticles enhanced by the local electric field, and a physical model is proposed to clarify this mechanism. The finding indicates a promising strategy by design and craft of the injection hotspots in nanoscale to facilitate carrier injection in organic thin film devices.     

Highly enhanced electron injection in organic light-emitting diodes with an n-type semiconducting MnO2 layer

Highly enhanced electron injection is demonstrated with a thin manganese dioxide (MnO₂) electron injection layer (EIL) in Alq₃-based organic light-emitting diodes. Insertion of the MnO₂ EIL between the Al cathode and Alq₃ results in highly improved device characteristics. In situ photoelectron spectroscopy shows remarkable reduction of the electron injection barrier without significant chemical reactions between Alq₃ and MnO₂, which could induce Alq₃ destruction. The reduction of the electron injection barrier is due to the n-type doping effect, and the lack of strong interfacial reaction is advantageous with regards to more efficient electron injection than a conventional LiF EIL. These properties render the MnO₂, a potential EIL.     

Green phosphorescent organic light-emitting devices based on different electron transport layers combining with fluorescent sub-monolayer

We report a small molecule host of 4,4(-N,N)-dicarbazole-biphenyl (CBP) doped with 8% tris(2-phenylpyridine) iridium (Irppy3) for use in efficient green phosphorescent organic light-emitting devices (PHOLEDs) combined with different electron transport layers of Alq and BAlq. The PHOLEDs exhibit maximum current efficiency and power efficiency of 19.8 cd/A and 6.21 lm/W, respectively. The high performance of such PHOLEDs is attributed to the better electron mobile ability of BAlq and sub-monolayer quinacridone (QAD) as carrier trapping layer and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, which can reduce the triplet-triplet annihilation and improve the efficiency of the device. #$TABThis work has been supported by the Major Project of Science and Technology Office of Fujian Province of China (No.2014H0042), the Natural Science Foundation of Fujian Province of China (No.2015J01664), the Project of Science and Technology Research of Quanzhou in Fujian Province of China (Nos.2013Z125 and 2014Z137), and the 2016 Annual National or Ministries Preparatory Research Foundation Project in Quanzhou Normal University (No.2016YYKJ21). E-mail:yanghuishan1697@163.com      

Electromagnetic modeling of organic light-emitting devices

Based on the rigorous electromagnetic wave theory, a numerical model for simulating the radiation characteristics of organic light-emitting devices (OLEDs) is developed. In particular, a novel method for overcoming the numerical difficulty in taking the thick glass substrate into account is proposed. The numerical results confirm the importance of the effects of the thick glass substrate. The algorithms based on the numerical model are then used for evaluating the dependencies of OLED radiation characteristics on various parameters, including the thickness of different device layers and the cathode metal variety. In the study of the effect of emission layer (EML) thickness, it is found that the radiation spectral peak red shifts with increasing EML thickness. This trend is consistent with the experimental result.     

Structure optimization of organic light-emitting devices

A triple layer organic light-emitting diode (OLED) with two heterostructure of indium-tin oxide (ITO)/N,N’-diphenyl-N, N’-bis(1-naphthyl) (1,1’-biphenyl)-4,4’-diamine (NPB)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/ 8-Hydrox- yquinoline aluminum (Alq3)/Mg:Ag has been fabricated by using the vacuum deposition method. The influence of different film thickness of BCP layer on the performance of the OLEDs has been investigated. The results show that when the thickness of the BCP layer film gradually r...     

一种新的多发光层白色有机电致发光器件

采用多发光层结构,将一种新型的黄橙色荧光染料2-溴-4-氟苯乙烯-8-羟基喹啉锌(BFHQZn,(E)-2-(2-bronw-4-fluorostyryl)quinolato-Zinc)与蓝色9,10-二-2-蔡蔥(ADN)组合在一起实现白光.研究了插入4,4-N,N-二咔唑联苯(CBP)对器件色度的影响,通过改变发光...     

Effect of Al:Ag alloy cathode on the performance of transparent organic light-emitting devices

Transparent organic light-emitting devices (TOLEDs) based on a stacked alloy cathode of LiF/Al:Ag are investigated. The devices have a structure of indium-tin-oxide (ITO)/4,4′,4′′-Tris[2-naphthyl(phenyl)amino]triphenylamine (2T-NATA) (25 nm)/N,N''-Di-[(1-naphthyl)-N,N''-diphenyl]-1,1''-biphenyl-4,4''-diamine (NPB) (40 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (50 nm)/LiF (1 nm)/Al:Ag (1:3) (x), where the thicknesses of cathode metal layers (Al:Ag) are adjusted, respectively, from 70 nm to 100 nm. In the experiment, it is found that the LiF (1 nm)/Al:Ag (1:3) (75 nm) has good electron injection efficiency. Compared with an Al-only cathode, the turn-on voltage is lowered. At the voltage of 10 V, the luminances for bottom emission from ITO anode side and top emission from metal cathode side are 2 459 cd/m2 and 1 729 cd/m2, respectively. Thanks to electron injection enhancement by using Al:Ag cathode, we can obtain a better energy level matching between the cathode and the organic layer, thus the devices have lower turn-on voltage and higher luminance. The total transmittance of the devices can achieve about 40% at the wavelength of 550 nm.     

通过引入电子阻挡层的高效率的有机磷光白光器件

以CBP作为母体材料,绿色磷光染料Ir(ppy)3作为敏化剂,以荧光染料rubrene作为受主,制备了结构为ITO/2T-NATA(25 nm)/ NPBX (25-d nm)/ CBP:5%Ir(ppy)3:0.5%Rubrene(8 nm)/NPBX(d nm)/DPVBi(30 nm)/TPBi(20 nm)/Alq(10 nm)/LiF(1 nm)/Al的白光器件.在器件中,敏化剂Ir(ppy)3、荧光染料rubrene的浓度分别为5.0 wt%和0.5 wt%,发光层的厚度选择8 nm,通过调整两层NPBX的厚度来改善器件的性能,得到了比较理想的白光发射.当d的厚度为10 nm 时,器件在7 V的电压下最大电流效率达到11.2 cd/A,在17 V的电压下其最大亮度达到28 170 cd/m2,色坐标为(0.37,0.42),处于白光区.     

具有阶梯势垒的低压有机电致发光器件

在2-t-butyl-9,10-di-(2-naphthyl)anthracene(TBADN)/tris(8-hydroxyquinoline)aluminum(Alq3)界面及TBADN/4'7-diphyenyl-1,10-phenanthroline(Bphen)界面上插入Gaq薄膜作为阶梯势垒,使有机电致发光器件的电子注入得到改善.由于Gaq(2.9 eV)的LUMO(分子最低空余轨道能级)位于Alq3(3.1 eV)(或 Bphen(3.0 eV))的LUMO和TBADN的LUMO(2.8 eV)之间,形成了从Alq3(或Bphen)经Gaq到TBADN的势垒阶梯,提高了电子注入,进而提高了器件效率.实验表明:与没有阶梯势垒的器件相比,无论是单一电子器件还是完整器件,在相同电流密度下,具有阶梯势垒的器件的电压都有所下降.在电流密度为20 mA/cm2时,当电子传输层为Alq3时,单一电子器件的电压从7.9 V降到4.9 V,完整器件的电压从7 V降到5.8 V;当电子传输层为Bphen时,单一电子器件的电压从4.2 V降到3.1 V,完整器件的电压从6.2 V降到5.1 V.在电流密度为200 mA/cm2,Alq3为电子传输层时,亮度从1 992 cd/m2升到3 281 cd/m2,最高亮度达到3 420 cd/m2,Bphen为电子传输层时,亮度从1 745 cd/m2 升到2 876 cd/m2,最高亮度达到3 176 cd/m2.本文运用能级隧穿理论对上述现象进行了解释.     

Enhancing the electron injection in polymer light-emitting diodes using a sodium stearate/aluminum bilayer cathode

In this contribution the molecule sodium stearate (NaSt) is used for the first time as electron injection layer in combination with the fluorescent polymer phenylene substituted poly (para-phenylenevinylene) (Ph-PPV) in organic light-emitting diodes (OLEDs). The fabricated devices show current efficiencies up to 8.4 cd/A, indicating that the employed NaSt/aluminum (Al) bilayer cathode has adequate electron injection capabilities in conjunction with Ph-PPV and, therefore, NaSt has the potential to become a non-toxic alternative to the widely-used alkali halide lithium fluoride (LiF).Numerical simulations of the device structure are performed which are in good agreement with the experiments. Additionally, it is shown that the NaSt/Al cathode of the presented device cannot be simply modeled by using a low work function contact, as it is commonly done for the LiF/Al cathode in simulations of multilayer devices. Instead, an alternative approach is introduced in which an insulator in combination with the Fowler–Nordheim tunneling and the direct tunneling model is chosen to describe the charge carrier injection through the NaSt layer.     

Dipyrenylpyridines for electron-transporting materials in organic light emitting devices and their structural effect on electron injection from LiF/Al cathode

3,5-Dipyrenylpyridine (PY1) and 2,6-dipyrenylpyridine (PY2) were synthesized to achieve not only efficient electron injection from cathode but also high electron mobility. The both of compounds showed much higher electron mobilities than that of Alq₃, and have a similar ionization potential and electron affinity. However, the barrier height of electron injection from cathode to PY1 in an organic light emitting device was much smaller than that to PY2, probably due to the steric hindrance to the central pyridine group. These results suggest that the chemical affinity of electron-transporting materials with cathode is more important than their own electron affinity to improve the electron injection.