Low voltage red phosphorescent organic light-emitting devices with triphenylphosphine oxide and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl electron transport layers

We have developed red phosphorescent organic light-emitting devices operating at low voltages by using triphenylphosphine oxide (Ph₃PO) and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) electron transport layers. 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and tris-(1-phenylisoquinolinolato-C²,N) iridium(III) [Ir(piq)₃] were used as host and guest materials, respectively. Small voltage drops across the electron transport layers and direct injection of holes from 4,4′,4″-tris[N-(2-naphthyl)-N-phenyl-amino]-triphenylamine (2-TNATA) hole transport layer into the Ir(piq)₃ guests are responsible for the high current density at low voltage, resulting in a high luminance of 1000 cd/m² at low voltages of 2.8–3.0 V in devices with a structure of ITO/2-TNATA/CBP:Ir(piq)₃/DPVBi/Ph₃PO/LiF/Al.     

Combined host–guest doping and host-free systems for high-efficiency white organic light-emitting devices

Highly efficient white organic light-emitting devices (WOLEDs) with a four-layer structure were realized by utilizing phosphorescent blue and yellow emitters. The key concept of device construction is to combine host–guest doping system of the blue emitting layer (EML) and the host-free system of yellow EML. Two kinds of WOLEDs incorporated with distinct host materials, namely N,N'-dicarbazolyl-3,5-benzene (mCP) and p-bis(triphenylsilyly)benzene (UGH2), were fabricated. Without using light out-coupling technology, a maximum current efficiency (η_C) of 58.8 cd/A and a maximum external quantum efficiency (η_EQE) of 18.77% were obtained for the mCP-based WOLED; while a maximum η_C of 65.3 cd/A and a maximum η_EQE of 19.04% were achieved for the UGH2-based WOLED. Meanwhile, both WOLEDs presented higher performance than that of conventionally full-doping WOLEDs. Furthermore, systematic studies of the high-efficiency WOLEDs were progressed.     

Solution-processed white organic light-emitting devices based on small-molecule materials

We investigated solution-processed films of 4,4′-bis(2,2-diphenylvinyl)-1,1′-bibenyl (DPVBi) and its blends with N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) by atomic force microscopy (AFM). The AFM result shows that the solution-processed films are pin-free and their morphology is smooth enough to be used in OLEDs. We have developed a solution-processed white organic light-emitting device (WOLEDs) based on small-molecules, in which the light-emitting layer (EML) was formed by spin-coating the solution of small-molecules on top of the solution-processed hole-transporting layer. This WOLEDs, in which the EML consists of co-host (DPVBi and TPD), the blue dopant (4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl) and the yellow dye (5,6,11,12-tetraphenylnaphtacene), has a current efficiency of 6.0 cd/A at a practical luminance of 1000 cd/m², a maximum luminance of 22500 cd/m², and its color coordinates are quite stable. Our research shows a possible approach to achieve efficient and low-cost small-molecule-based WOLEDs, which avoids the complexities of the co-evaporation process of multiple dopants and host materials in vacuum depositions.     

Deep blue organic light emitting diode based on anthracene derivative as a dopant

This letter presents a deep blue organic light emitting diode which was fabricated by using 9,10-di(2-naphthyl)anthracene as a dopant and 4,4′-N,N′-dicarbazole-biphenyl as a host. The Commission Internationale de l’Eclairage coordinates of (0.1516, 0.0836) were achieved in the cell, which is very close to the National Television Standards Committee standard of (0.14, 0.08). Meanwhile, maximum luminance over 6500 cd/cm² and maximum current efficiency of 3.5 cd/A were also obtained.     

Screen-printed white OLED based on polystyrene as a host polymer

We demonstrate the use of screen printing in the fabrication of single-layer organic-light-emitting devices (OLEDs). The organic layer is a single-layer of polystyrene, in which we incorporate rubrene for orange emission and α-NPD, DPVBi for blue emission. An appropriate mixing of the two colors produced white emission by incomplete Förster energy transfer. We showed the role of each constituent, α-NPD, DPVBi and rubrene in the emission characteristics of OLEDs. The turn-on voltage of screen-printed white OLEDs was about 10 V with maximum brightness and luminous efficiency up to 1300 cd/m² and 9 cd/A, respectively.     

Optical properties of white organic light-emitting devices fabricated with three primary-color emitters by using organic molecular-beam deposition

White organic light-emitting devices (WOLEDs) with Mg:Ag/Alq₃/Alq₃:DCJTB/Alq₃/DPVBi/α-NPD/ITO and Mg:Ag/Alq₃/DPVBi:DCJTB/Alq₃/DPVBi/α-NPD/ITO structures were fabricated with three primary-color emitters of red, green, and blue by using organic molecular-beam deposition. Electroluminescence spectra showed that the dominant white peak for the WOLEDs fabricated with host red-luminescence Alq₃ and DPVBi layers did not change regardless of variations in the current. The Commission Inernationale de l'Eclairage (CIE) chromaticity coordinates for the two WOLEDs were stable, and the WOLEDs at 40 mA/cm² with luminances of 690 and 710 cd/cm² showed an optimum white CIE chromaticity of (0.33, 0.33). While the luminance yield of the WOLED fabricated with a host red-luminescent Alq₃ emitting layer below 30 mA/cm³ was larger than that of the WOLED fabricated with a DPVBi layer, above 30 mA/cm², the luminance yield of the WOLED fabricated with the DPVBi layer was higher than that of the WOLED with the Alq₃ layer and became more stable with increasing current density. These results indicate that WOLEDs fabricated with a host red-luminescence DPVBi layer without any quenching behavior hold promise for potential applications in backlight sources in full-color displays.     

High-efficiency organic light-emitting diodes (OLEDs) with a mixed layer structure

Improved performance of organic light-emitting diodes (OLEDs) as obtained by a mixed layer was investigated. The OLEDs with a mixed layer which were composed of N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB), tris-(8-hydroxyquinolato) aluminum (Alq₃) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) showed the highest brightness and efficiency, which reached 19048 cd/m² at 17 V and 4.3 cd/A at 10 mA/cm², respectively. The turn-on voltage of the device is 2.6 V. Its Commission Internationale del’Eclairage (CIE) coordinate is (0.497, 0.456) at 17 V, and the CIE coordinates of the device are largely insensitive to the driving voltages, which depicts stabilized yellow color.     

Esterification of sodium 4-hydroxybenzoate by ultrasound-assisted solid–liquid phase-transfer catalysis using dual-site phase-transfer catalyst

The catalytic esterification of sodium 4-hydroxybenzoate with benzyl bromide by ultrasound-assisted solid–liquid phase-transfer catalysis (U-SLPTC) was investigated using the novel dual-site phase-transfer catalyst 4,4′-bis(tributylammoniomethyl)-1,1′-biphenyl dichloride (BTBAMBC), which was synthesized from the reaction of 4,4′-bis(chloromethyl)-1,1′-biphenyl and tributylamine. Without catalyst and in the absence of water, the product yield at 60 °C was only 0.36% in 30 min of reaction even under ultrasound irradiation (28 kHz/300 W) and 250 rpm of stirring speed. When 1 cm³ of water and 0.5 mmol of BTBAMBC were added, the yield increased to 84.3%. The catalytic intermediate 4,4′-bis(tributylammoniomethyl)-1,1′-biphenyl di-4-hydroxybenzoate was also synthesized to verify the intrinsic reaction which was mainly conducted in the quasi-aqueous phase locating between solid and organic phases. Pseudo-first-order kinetic equation was used to correlate the overall reaction, and the apparent rate coefficient with ultrasound (28 kHz/300 W) was 0.1057 min⁻¹, with 88% higher than that (0.0563 min⁻¹) without ultrasound. The esterification under ultrasonic irradiation using BTBAMBC by solid–liquid phase-transfer catalysis was developed.     

An organic electroluminescent device made from a gadolinium complex

A gadolinium ternary complex, tris(1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) (phenanthroline) gadolinium [Gd(PMIP)₃(Phen)] was synthesized and used as a light emitting material in the organic electroluminescent (EL) devices. The triple layer device with a structure of indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) (20 nm)/Gd(PMIP)₃(Phen) (80 nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (bathocuproine or BCP) (20 nm)/Mg: Ag(200 nm)/Ag(100 nm) exhibited green emission peaking at 535 nm. A maximum luminance of 230 cd/m² at 17 V and a peak power efficiency of 0.02 lm/w at 9 V were obtained.     

Tetraalkoxylated-PPV derivative: An efficient and pure green electroluminescent polymer with good solution processability

A new solution-processable tetraalkoxy-substituted poly(1,4-phenylenevinylene) derivative, poly{[2-(3′,7′-dimethyloctyloxy)-3,5,6-trimethoxy]-1,4-phenylenevinylene} (TALK-PPV), was synthesized through a dehydrohalogenation polymerization route, and its light-emitting properties were investigated. The TALK-PPV showed highly blue-shifted UV–visible absorption and PL emission spectra compared to the dialkoxy-substituted PPV derivatives. This is because of the disturbance to the π-conjugation caused by a steric hindered structure. The TALK-PPV thin film exhibited an absorption peak at 446 nm, with an onset at 515 nm. Its PL emission maximum was at 554 nm. Cyclic voltammetric analysis showed the HOMO and LUMO energy levels of the TALK-PPV to be 5.77 and 3.36 eV, respectively. Light-emitting devices were fabricated with an ITO (indium-tin oxide)/PEDOT/polymer/Ca/Al configuration. The TALK-PPV component leads to pure green light emission with a CIE 1931 chromaticity of (0.20, 0.74) at 100 cd/m² brightness, which is very close to the standard green (0.21, 0.71) demanded by the NTSC (National Television System Committee). The maximum brightness of this device was 24,900 cd/m² with an efficiency of 1.45 cd/A.     

Effect of different spacers for performance enhancement on white organic light-emitting devices based on double emissive layers with homogeneous p-/n-type host

White organic light-emitting devices (WOLEDs) based on phosphorescent blue and yellow emitters were fabricated, while p-type di-(4-(N,N-ditolyl-amino)-phenyl)cyclohexane (TAPC) and n-type 2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) were separately utilized as a homogeneous host for both blue and yellow emissive layers (EMLs). Then, various spacers were inserted between the two EMLs for performance characterization. The results showed that for the TAPC-host devices, a device using 4,7-diphenyl-1,10-phenanthroline (Bphen) as the spacer had a maximum current efficiency (CE) of 11.3 cd/A, while stable white light emission with Commission Internationale del’Eclairage (CIE) coordinates of (0.394, 0.435) at a bias of 5 V was observed. Similarly, among the TPBi-host devices, a device using 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as the spacer exhibited a maximum CE of 18.1 cd/A, accompanied by negligible color variation with the CIE coordinates of (0.284,0.333) at 5 V. For the double-EML devices, the improved device efficiency and color stability by introducing proper spacer was attributed to broadened recombination region and efficient energy transfer between the EMLs.     

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.     

Highly efficient flexible organic light-emitting devices utilizing F4-TCNQ/m-MTDATA multiple quantum well structures

Flexible organic light-emitting devices (FOLEDs) based on multiple quantum well (MQW) structures, which consist of alternate layers of 2,3,5,6-Tetrafluoro-7,7,8,8,-tetracyano-quinodimethane (F4-TCNQ) and 4,4′,4″-tris-(3-methylphenylphe-nylamino)tripheny-lamine (m-MTDATA) have been fabricated. The Alq₃-based device with double quantum well (DQW) structure exhibits the remarkable electroluminescent (EL) performances for the brightness of 23,500 cd/m² at 14 V and the maximum current efficiency of 7.0 cd/A at 300.3 mA/cm², respectively, which are greatly improved by 114% and 56% compared with the brightness of 10,958 cd/m² at 14 V and the maximum current efficiency of 4.5 cd/A at 174.0 mA/cm² for the conventional device without MQW structures. These results demonstrate that the EL performances of FOLEDs could be greatly improved by utilizing the novel MQW structures, and the reason for this improvement has also been explained by the effect of interfacial dipole and interfacial doping between F4-TCNQ and m-MTDATA in this article.     

White organic light-emitting diodes based on combined electromer and monomer emission in doubly-doped polymers

We report on white organic light-emitting diodes (WOLEDs) based on polyvinylcarbazole (PVK) doped with 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC) and perylene, and investigate the luminescence mechanism of the devices. The chromaticity of light emission can be tuned by adjusting the concentration of the dopants. White light with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.34) is achieved by mixing the yellow electromer emission of TAPC and the blue monomer emission of perylene from the device ITO/PVK: TAPC: perylene (100:9:1 in wt.) (100 nm)/tris-(8-hydroxyquinoline aluminum (Alq₃) (10 nm)/Al. The device exhibits a maximal luminance of 3727 cd/m² and a current efficiency of 2 cd/A.     

Efficient fluorescent white organic light-emitting devices with a reduced efficiency roll-off based on a blue ambipolar fluorescent emitter

White organic light-emitting devices (WOLEDs) with fluorescent donor-acceptor-substituted spirobifluorene compounds (red 2-diphenylamino-7-(2,2-dicyanovinyl)-9,9′-spirobifluorene and blue 2-diphenylamino-7-(2,2-diphenylvinyl)-9,9′-spirobifluorene) have been fabricated. The optimized WOLEDs shows a maximum current efficiency 5.9 cd/A and very low efficiency roll-off. From the brightness at maximum current efficiency to high brightness of 10000 cd/m², the current efficiency roll-off is only 0.4%. It can be attributed to the ambipolar blue fluorescent emitter with voltage-independnet mobility which makes the device having a broader charge recombination zone and balance of carrier transport.     

Improving the color purity and efficiency of blue organic light-emitting diodes (BOLED) by adding hole-blocking layer

This work demonstrates the fabrication of a bright blue organic light-emitting diode (BOLED) with good color purity using 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi) and bathocuproine (BCP) as the emitting layer (EML) and the hole-blocking layer (HBL), respectively. Devices were prepared by vacuum deposition on indium tin oxide (ITO)-glass substrates. The thickness of DPVBi used in the OLED has an important effect on color and efficiency. The blue luminescence is maximal at 7670 cd/m² when 13 V is applied and the BCP thickness is 2 nm. The CIE coordinate at a luminance of 7670 cd/m² is (0.165, 0.173). Furthermore, the current efficiency is maximum at 4.25 cd/A when 9 V is applied.     

Synthesis and characterization of PPV-based light-emitting copolymer with alkylsilylphenyloxy pendant group for light-emitting diode applications

A novel light-emitting copolymer with high brightness and luminance efficiency was synthesized using the Gilch polymerization method, and its electro-optical properties were investigated. A polymer light-emitting diode (PLED) was fabricated in ITO/PEDOT/light-emitting copolymer/Ca/Al configuration. The turn-on voltage of the PLED was about 5.0 V with maximum brightness and luminance efficiency up to 1420 cd/m² at 16.2 V and 0.5 cd/A at 6.8 V, respectively.     

Study of different roles phosphorescent material played in different positions of organic light emitting diodes

Phosphorescent materials are crucial to improve the luminescence and efficiency of organic light emitting diodes (OLED), because its internal quantum efficiency can reach 100%. So the studying of optical and electrical properties of phosphorescent materials is propitious for the further development of phosphorescent OLED. Phosphorescent materials were generally doped into different host materials as emitting components, not only played an important role in emitting light but also had a profound influence on carrier transport properties. We studied the optical and electrical properties of the blue 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi)-based devices, adding a common yellow phosphorescent material bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2′] iridium(acetylacetonate) [(t-bt)₂Ir(acac)] in different positions. The results showed (t-bt)₂Ir(acac) has remarkable hole-trapping ability. Especially the ultrathin structure device, compared to the device without (t-bt)₂Ir(acac), had increased the luminance by about 60%, and the efficiency by about 97%. Then introduced thin 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host layer between DPVBi and (t-bt)₂Ir(acac), and got devices with stable white color.     

基于量子阱结构的高效磷光有机电致发光器件

采用多重量子阱结构制作了高效红色磷光有机电致发光器件。以4,4'-bis(N-carbazolyl)-1,10-biphenyl (CBP)掺杂bis(1-phenyl-isoquinoline)(Acetylacetonato) iridium(Ⅲ) (Ir(piq)₂(acac))为发光层,4,4'-bis(N-carbazolyl)-1,10-biphenyl(Bphen)为电荷控制层,形成了Ⅱ型双量子阱结构,器件的最大亮度为15 000 cd/m²,最大电流效率为7.4 cd/A,相对于参考器件提高了21%。研究结果表明:以Bphen为电荷控制层形成的Ⅱ型多重量子阱结构能有效地将载流子和激子限制在势阱中,并且使空穴和电子的注入更加平衡,从而提高了载流子复合的几率和器件的效率。     

间隔层对双发光层白色有机电致发光器件性能的影响

采用蓝色bis (FIrpic)和黄色bis iridium(acetylacetonate) 两种磷光染料,制备了双发光层结构的白色有机电致发光器件,器件结构为ITO/TAPC (30 nm)/host: (t-bt)₂Ir(acac) /spacer (x nm)/host: FIrpic (15 nm, 8%)/Bphen (40 nm)/Mg∶Ag (200 nm)。分别选用p型1,1-bis cyclohexane (TAPC)和n型tris borane (3TPYMB)作为主体材料制备了两种类型的器件,通过在两个发光层之间加入一层较薄的间隔层进行器件优化。结果表明,加入间隔层之后,器件性能得到提高,获得了色稳定性较好的白光器件。当主体为TAPC时,使用间隔层后器件取得最大亮度为19 550 cd/m²,最大电流效率为8.3 cd/A;当主体为3TPYMB时,使用间隔层后器件的最大亮度为1 950 cd/m²,最大电流效率为30.7 cd/A。实验结果表明,器件性能的提高,是由于加入了间隔层之后载流子复合区域拓宽,促进了发光层中电子和空穴的平衡。