Efficient Polymer Light-emitting Diodes Research Articles

P‐167: Characteristics of Polymer Light Emitting Diode with Solution Processed Graphene Oxide Layer

AbstractWe used the graphene oxide (GO) as the hole transport layer (HTL) in polymer light emitting diode (PLED). Driving voltage and current efficiency of device with GO layer are 7.2 V and 5.2 cd/A, respectively. And those of device with PEDOT:PSS HIL are 6.1 V and 7.5cd/A, respectively. The driving voltage and efficiency of PLED with GO layer are much better than those without HIL. Because of its inorganic and neutrality unlike PEDOT:PSS, the PLED using a GO layer can be more stable compared to conventional PLED with PEDOT:PSS.

Highly efficient blue phosphorescent polymer light-emitting diodes by using interfacial modification

Solution processed highly efficient blue electrophosphorescent polymer light-emitting diodes have been fabricated containing an oligo(ethylene oxide) (PEO-DME) additive to enhance the electroluminescence efficiency. The addition of PEO-DME at 5–10 wt % effectively lowers both the electron and hole injection barriers. The maximum current efficiency obtained was 26.5 cd/A at an emission brightness of 2500 cd/m2. The high performance is due to (i) improved charge carrier injection at the interface resulting from specific interfacial interactions between PEO-DME and aluminum and (ii) improved charge carrier transporting ability and high electric field resulting from PEO-DME doping.

具有减少自猝灭效果的铱配合物掺杂型红色聚合物磷光器件

A highly efficient red-emitting phosphorescent polymer light-emitting diode (Ph-PLED) with a new iridium complex of bis(2-benzo[b]thiophen-2-yl-5-trifluoromethyl-pyridinato-N, C3) iridium(III) (acetylacetonate) [(btfmp)2Ir(acac)] using poly (n-vinylcarbazole) (PVK) as host material was demonstrated. The PLED offers a maximum external quantum efficiency of 4.5% with an emission peak of 648 nm. The CIE coordinates are (0.71, 0.29) and independent from driving current density, indicating an efficient reduction of self-quenching effect at high doping concentrations.

Poly(ethylene oxide)-functionalized Al cathodes of tunable electron-injection capabilities for efficient polymer light-emitting diodes

Poly(ethylene oxide)-functionalized aluminum (Al) cathodes of tunable electron-injection capabilities are reported in this study for the fabrication of high-performance polymer light-emitting diodes (PLEDs). The composite cathodes are prepared by thermally co-evaporating poly(ethylene glycol) dimethyl ether (PEGDE, Mnca. 2000) polymer with Al metal simultaneously in vacuum as the cathode buffer. We attribute the formation of the carbide-like/Al complex as characterized by X-ray photoelectron spectroscopy (XPS) to the instant chemical reactions between the ethylene oxide functional groups with Al, which markedly enhances the device performance. The electron-injection capabilities of PEGDE : Al/Al cathodes are correlated with the contents of the carbide-like/Al complex in the buffer layer and can, therefore, be modulated by varying the co-evaporation ratios of PEGDE and Al. The luminous efficiency of green polyfluorene-based PLEDs applying an PEGDE : Al (2 : 1)/Al cathode is of 10.46 cd A−1, which is much higher than that of ∼0.04, 5.19, 6.40 cd A−1 for PLEDs with Al, PEGDE/Al and calcium/Al cathodes, respectively. This research leads to an alternative cathode design, free of salts, additives, complicated synthesis, and of tunable electron-injection capabilities to balance the opposite charge carriers for fabricating high-performance PLEDs.

Efficient multilayer electrophosphorescence white polymer light-emitting diodes with aluminum cathodes

By simply doping Li 2CO 3 in an alcohol soluble neutral conjugated polymer poly[9,9-bis(6′-diethoxylphosphorylhexyl)fluorene] (PF-EP), we demonstrate highly efficient multilayer electrophosphorescence white polymer light-emitting diodes with air-stable Al cathode. From the mechanism studies, we find that the phosphate groups can make Li ions movable in the PF-EP film, resulting in great improvement in the electron injection and transport ability of PF-EP. The hole-blocking ability is also strengthened.

Fabrication of Multilayer Polymer Light-Emitting Diodes with a Spin Coated Hole Transport Layer and Stamp Transfer Printed Emitting Layer

High efficiency multilayer polymer light emitting diodes (PLEDs) were developed using a stamp transfer printing process. Light emitting polymers were stacked on the spin-coated polymeric hole transport layer by using a stamp transfer printing method. The light-emitting efficiency of PLEDs was improved by more than 60% by stacking the light emitting polymers on the hole transport polymers using the transfer printing process.

Synergistic effect on the efficiency of polymer light-emitting diodes upon blending of two green-emitting polymers

Light-emitting diodes based on blends of the two green-emitting polymers, poly(9,9-dioctylfluorene-alt-benzothiadiazole), F8BT, and poly(9,9-dioctylfluorene-alt-bithiophene), F8T2, show efficiencies that lie in between those of the devices based on the neat polymers (with a maximum efficiency of approximately 4 cd/A for the devices with magnesium cathodes based on F8BT), except for the blend with 5% by weight of F8T2, which is more efficient than the device based on neat F8BT (a maximum efficiency of approximately 5 cd/A is obtained). In view of the lower photoluminescence efficiency of F8T2, we attribute this improvement to the improved hole transport brought about by F8T2, though is surprising that 5% by weight, is enough to significantly improve the charge balance within the emissive layer. A detailed photophysics study was carried out for the neat polymers and their blends and no clear evidence for energy transfer between the components was found. This unanticipated devices performance improvement points to the need of a deeper screening of available conjugated luminescent polymers.

Efficient white polymer light-emitting diodes based on the control of a emissive polymer morphology through solvent processing

We present the results of a systematic study on how the solvent processing affects the EL properties of a single emissive layer white PLED based on a polyfluorene derivative polymer. The EL spectrum of the device using solvent processing, peaked at 436, 572 nm, shows a pure white emission and the luminous efficiency is enhanced, which is attributed to the presence of the crystalline polyfluorene induced by solvent processing. The crystalline polyfluorene that acts as the self-dopant enhances the blue emission and increases the purity of the white light. Most importantly, the charge transport is balanced after solvent processing, which favors improving the luminous efficiency. A light-emitting efficiency of 12 cd/A is achieved after solvent processing, about 50% increase compared to the as-spincoated device.

Efficient multilayer red fluorescent polymer light-emitting diodes by host and guest blend system

Efficient red light polymer light-emitting diodes are obtained by energy transfer from host and guest blend system. Two kinds of blend systems, SPB02:(10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1] benzopyrano [6,7,8-ij]quinolizin-11-one)(C545T):4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and SuperYellow (SY):poly(3,3 ′′′-didodecylquaterthiophene) (PQT-12), are used as the active layers. C545T is a green emitter, acting as an assistant dopant to improve the energy transfer from blue SPB02 host to red DCJTB. The efficiency can achieve to 3.3 cd/A for DCJTB and 3.5 cd/A for PQT-12 system by multilayer device structure with 2,2,2 ′′-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TPBi) as electron transport layer. The SuperYellow:PQT-12 device shows great luminance of 16,930 cd/m 2.

Development and characterization of an efficient bio-white polymer light-emitting diode with red and green phosphorescent dyes as dopants

An efficient white polymer light-emitting diode (WPLED) with stable Commission Internationale de l’eclairage (CIE) coordinates is fabricated. A blue electroluminescence (EL)-emitting conducting polymer [poly(9,9-di-n-hexyl-fluorenyl-2,7-diyl)] is used as a host for red [Bis(1-phenyl-isoquinoline)(acetylacetonate)iridium(III)] and green [iridium(III)tris(2-(4-tolyl)pyridinato-N,C2)] phosphorescent dyes. Although efficient triplet energy transfer is not possible in the green phosphorescent dye, the self-trapping mechanism is utilized for the emission of EL in the green region while an efficient triplet exciton energy transfer from the host to the red dye is utilized for EL in the red wavelength region. Concentrations of the three constituents are optimized to obtain pure white light of appropriate CIE coordinates. An efficient electron-blocking layer based on a biomaterial (salmon-DNA) is also incorporated in the WPLED to improve the device performance. The WPLED shows three distinguished peaks for the primary colors and achieved a maximum luminance and luminous efficiency of 350 cd/m2 and 0.86 cd/A, respectively.

Efficient Green Electrophosphorescence with Al Cathode Using an Effective Electron-Injecting Polymer As the Host

Efficient green phosphorescent polymer light-emitting diodes (PLED) based on aluminum (Al) cathode were demonstrated. The device composed of neutral aminoalkyl substituted polyfluorene, poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) as the host and an Ir complex, fac-tris(2-(4-tert-butyl)-phenylpyridine)iridium (Ir(Bu-PPy)(3)), as the dopant. High luminous efficiency of 35.7 cd/A and a luminance of more than 10 000 cd/m(2) were obtained. The device performance was comparable to those of the devices from (Ir(Bu-PPy)(3)), doped into PVK as the host with a low work-function metal as cathode (such as barium and calcium). We found out that the high-performance was due to the effective electron injection from PFN/Al bilayer cathode, improved charge balance as well as significant reduction of back-transfer of excitons from Ir complex to the low-lying PFN triplet level in the PVK/PFN interface.

Efficiency improvement of polymer light-emitting diodes using a quantum dot interlayer between a hole transport layer and an emitting layer

The current efficiency of polymer light-emitting diodes (PLEDs) were improved using a quantum dot interlayer between a hole transport layer and an emitting layer. The quantum dot interlayer played a role of controlling the hole transport in the PLEDs and enhanced the charge balance in the emitting layer. The current efficiency of the PLEDs was increased by more than 20% by the quantum dot interlayer. In particular, the efficiency improvement was significant at high luminance due to reduced efficiency roll-off in the quantum dot-embedded PLEDs.

Highly efficient blue polyfluorene-based polymer light-emitting diodes through solvent vapour annealing

A highly efficient blue polymer light-emitting diode based on poly(9,9-di(2-(2-(2-methoxy-ethoxy)ethoxy)ethyl)fluorenyl-2,7-diyl) (PFOEO3) by solvent vapour annealing the polymer emitting layer is fabricated in a bilayer device with Al cathode. High electroluminescent efficiency is achieved by balancing charge injection. Better contact interface between the polymer and the cathode was formed by making chain conformation reoriented with solvent vapour exposure. The effect of solvent vapour exposure on the potential barrier height of the interface is estimated using the Richardson–Schottky model. The maximum efficiency that was achieved was 2.3 cd A−1 for the vapour exposed device compared with 0.9 cd A−1 for the untreated pristine device.

Improved efficiency of polymer light-emitting diodes by inserting a hole transport layer formed without thermal treatment above glass transition temperature

We have demonstrated a significant improvement in the performance of polymer light-emitting diodes (PLEDs) by inserting the fluorene-triatylamine copolymer as hole transport layer (HTL) without a thermal treatment above the glass transition temperature ( T g). A thin HTL insolubilized by a thermal treatment above T g is often inserted as an interlayer between an anode buffer layer and a light-emitting polymer (LEP) in PLEDs fabricated by using a conventional solution process. The evaporative spray deposition using ultradiluted solution (ESDUS) method has enabled fabricating polymer bilayer structure without an insolublizing procedure. The bilayer PLEDs fabricated by ESDUS without the thermal treatment showed significantly higher and more stable external quantum efficiency than PLEDs having the conventional interlayer. Thermal treatment above T g of the copolymer would induce degradation of its hole injection property. Furthermore, ESDUS bilayer devices showed much higher power efficiency than interlayer devices when calcium was used for cathode. The improvements would be caused by the enhancement of hole injection and the effective electron blocking at the copolymer/LEP interface in the ESDUS bilayer devices.

Efficient Near-Infrared Polymer and Organic Light-Emitting Diodes Based on Electrophosphorescence from (Tetraphenyltetranaphtho[2,3]porphyrin)platinum(II)

The new metalloporphyrin Pt(tptnp), where tptnp = tetraphenyltetranaphtho[2,3]porphyrin, has been prepared and subjected to photophysical and electrooptical device studies. In degassed toluene solution at room temperature Pt(tptnp) features efficient phosphorescence emission with lambda(max) 883 nm with a quantum efficiency of 0.22. The complex has been used as the active phosphor in polymer and organic light-emitting diodes. Polymer light-emitting diodes based on a spin-coated emissive layer consisting of a blend of Pt(tptnp) doped in poly(9-vinylcarbazole) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole exhibit near-IR emission with lambda(max) 896 nm, with a maximum external quantum efficiency (EQE) of 0.4% and a maximum radiant emittance of 100 muW/cm(2). Organic light-emitting diodes prepared via vapor deposition of all layers and that feature an optimized multilayer hole injection and electron blocking layer heterostructure with an emissive layer consisting of 4,4'-bis(carbazol-9-yl)biphenyl (CBP) doped with Pt(tptnp) exhibit a maximum EQE of 3.8% and a maximum radiant emittance of 1.8 mW/cm(2). The polymer and organic light-emitting diodes characterized in this study exhibit record high efficiency for devices that emit in the near-IR at lambda >800 nm.

Lithium salt doped conjugated polymers as electron transporting materials for highly efficient blue polymer light-emitting diodes

Highly efficient blue polymer light-emitting diodes (PLEDs) are fabricated using a conjugated polymer, poly[9,9-bis(2-(2-(2-diethanol-amino-ethoxy) ethoxy) ethyl) fluorene-alt-4, 4′-phenylether] as an electron transporting layer (ETL). It was found that the performance of these blue-emitting devices could be greatly improved if the ETL was doped with LiF or Li2CO3 salts. A bis[(4,6-di-fluorophenyl)-pyridinato-N, C2] (picolinate) Ir(III) (FIrpic) complex based blue phosphorescent PLED exhibited a maximum luminance efficiency of 20.3 cd/A with a luminance of 1600 cd/m2 at the current density of 7.9 mA/cm2 and drive voltage of 8.0 V.

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.

Synthesis and characterization of fluorene-based copolymers containing benzothiadiazole derivative for light-emitting diodes applications

Five new thermally robust electroluminescent fluorene-based conjugated copolymers, including poly[2,7-(9,9-dioctylfluorene)-co-4,7-{5,6-bis(3,7-dimethyloctyloxymethyl)-2,1,3-(benzothiadiazole)}] (PFO-P2C10BT) were synthesized and used to fabricate the efficient polymer light-emitting diodes (PLEDs). The glass transition temperatures of the polymers were found to be higher than that of poly(9,9-dialkylfluorenes) and are in the range 113–165 °C. We fabricated PLEDs in indium-tin oxide/PEDOT/light-emitting polymer/cathode configurations using either double-layer LiF/Al or triple-layer Alq3/LiF/Al cathode structures. The new copolymers were found to have emission colors that vary from greenish blue (491 nm) to green (543 nm) depending on the copolymer composition. The maximum brightness and luminance efficiency of these PLEDs were found to be up to 5347 cd/m2 and 1.51 cd/A at 10 V, respectively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6762–6769, 2008

Polymers Derived from 3,6-Fluorene and Tetraphenylsilane Derivatives: Solution-Processable Host Materials for Green Phosphorescent OLEDs

We have synthesized two fluorene-based polymeric host materials, P1 and P2, through palladium-catalyzed Suzuki couplings of monomers containing tetraphenylsilane and 3,6-disubstituted fluorene frameworks, respectively. The resultant copolymers exhibited high glass transition temperatures (Tg ≥ 215 °C) and excellent thermal stability. The conjugation lengths of polymers P1 and P2 were effectively confined through the presence of the 3,6-linkages and silane units in the polymer skeleton. The structural features endowed these copolymers with UV−violet emissions in the solid state, together with high triplet energies (ET = 2.60 eV), which were sufficiently high for P1 and P2 to serve as appropriate hosts for green-emitting phosphors. In the case of polymer P2, the incorporation of pendent carbazole groups increased the HOMO level markedly and balanced the rates of charge injection and transportation. Employing P2 doped with green-emitting fac-tris(2-phenylpyridine)iridium as the emissive layer, we fabricated highly efficient polymer light-emitting diodes (32 cd/A, 9.2%) that exhibited significantly enhanced EL performance relative to that of the system employing P1 (14.5 cd/A, 4.2%).

Highly Efficient White Polymer Light‐Emitting Diodes Based on Nanometer‐Scale Control of the Electron Injection Layer Morphology through Solvent Processing

Highly efficient white polymer light-emitting diodes (WPLEDs) are demonstrated using neutral conjugated polymer surfactant PFN-OH as the electron-injection layer through solution processing. The electroluminescence spectra and luminance efficiency are strongly dependent on the morphology of the PFN-OH films. The dramatic improvement of device performance is attributed to the efficient hole-blocking ability due to mixed solvent-induced PFN-OH chain aggregation.