Polymer Light-emitting Diode Applications Research Articles

Synthesis and Characterization of a Novel Erbium Doped Poly(vinyl alcohol) Films for Multifunctional Optical Materials

Polymer films based on poly vinyl alcohol (PVA) doped with different contents (0, 4, 8, 16, 20 and 28) wt% of Erbium(III) ions were synthesized by the simple solution casting technique. The structural characteristics of the films were studied by X-ray diffraction pattern (XRD), Fourier transform infrared (FT-IR) spectroscopy and atomic force microscopy measurements (AFM) measurement whereas the optical behavior of these films have been investigated using UV–Vis-NIR spectroscopy and photoluminescence measurements. The structural parameters of the interplanar (d) spacing, crystallite length (L) and the average interchain separation determined from the XRD and the absorption bands of the active IR groups recorded in FT-IR spectra indicate bonding of positively charged Er3+ ions with negatively charged OH groups of the PVA chains. The average cluster size (D) of the samples estimated from AFM data shows larger values than that obtained from XRD measurements. The optical gaps obtained from absorption spectra fitting and Tauch relations are approximately equal and slightly decreased by Er3+ ions incorporation in the PVA matrix whereas the high frequency refractive indices estimated according to Moss and Dimitrov formulae showing small increase. Three intense peaks of 607 nm, 733 nm and 783 nm wavelengths due to energy levels transitions of erbium were recorded in the photoluminescence spectra PVA–Er3+. The collected data support consideration of PVA–Er3+ films as a promising optical polymer material in microptics, planar polymer waveguides, polymer optical fiber and polymer light emitting diodes applications.

Microwave-assisted synthesis of FexZn1−xO nanoparticles for use in MEH-PPV nanocomposites and their application in polymer light-emitting diodes

A one-step microwave-assisted polyol method was used to fabricate FexZn1−xO (x = 0.01, 0.05, 0.10) nanoparticles. Zinc acetate dihydrate, iron (III) acetylacetonate, oleic acid and diethylene glycol were placed in a Teflon-lined reaction vessel. The reaction mixture was heated up to 250 °C for 15 min in a microwave reactor. The surface modification with oleic acid prevented agglomeration of the nanoparticles. The X-ray diffraction analysis revealed characteristics wurtzite hexagonal structure of ZnO and successful incorporation of the Fe dopant into the host crystal lattice. Doping of ZnO by Fe led to bandgap modification as estimated by Tauc plot. The as-prepared nanopowders were dispersed in toluene and mixed with a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) polymer to make stable homogenous dispersions. Then, the FexZn1−xO/MEH-PPV nanocomposite thin films were prepared by spin coating and used as thin active layers in polymer light-emitting diodes. The thickness of deposited FexZn1−xO/MEH-PPV film was ca. 30 nm and that of reference neat MEH-PPV film was ca. 25 nm. The electroluminescent spectroscopy study showed that direct blending of MEH-PPV with Fe-doped ZnO nanoparticles is a simple and effective approach to significantly increase the luminance intensity of the diode in comparison to the diode fabricated by neat MEH-PPV.

Enhancement in fluorescence quantum yield of MEH-PPV:BT blends for polymer light emitting diode applications

We have investigated the effect of blending electron deficient heterocycle Benzothiadiazole (BT) on the photo-physical properties of conjugated polymer Poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Quantum yield (QY) value has been found to increase from 37% for pure MEH-PPV to 45% for an optimum MEH-PPV:BT blend ratio of 1:3. This can be attributed to the efficient energy transfer from the wide bandgap BT (host) to the small bandgap MEH-PPV (guest). The FTIR spectrum of MEH-PPV:BT blended thin film indicates suppression of aromatic C-H out-of-plane and in-plane bending, suggesting planarization of the conjugated polymer chains and, hence, leading to increase in the conjugation length. The increase in conjugation length is also evident from the red-shifted PL spectra of MEH-PPV:BT blended films. Single layer MEH-PPV:BT device shows lower turn-on voltage than single layer MEH-PPV alone device. Further, the effect of electrical conductivity of PEDOT:PSS on the current-voltage characteristics is investigated in the PLED devices with MEH-PPV:BT blend as the active layer. PEDOT:PSS with higher conductivity as HIL reduces the turn on voltage from 4.5 V to 3.9 V and enhances the current density and optical output in the device.

Solution Processable Material Derived from Aromatic Triazole, Azomethine and Tris: Preparation and Hole-Buffering Application in Polymer Light-Emitting Diodes

This work presents the synthesis of a new hole-buffering material TAZS and its successful application in polymer light-emitting diodes to enhance device performance. The TAZS is composed of aromatic 1,2,4-triazolylcore linked with three trihydroxy tert-butyl terminals via azomethine linkages. The TAZS forms ashomogeneous film deposited by spin-coating process. The HOMO and LUMO levels of TAZS are -5.23 eV and -2.40 eV, respectively, as estimated from cyclic voltammogram. The current density results of hole-only and electron-only devices confirm strong hole-buffering capability of TAZS layer. Multilayer PLEDs with different thickness of TAZS (ITO/PEDOT: PSS/TAZS (x nm)/SY/ETL/LiF/Al) have been successfully fabricated, using spin-coating process to deposit hole-injecting PEDOT: PSS, TAZS, and emissive SY layers. The PLED with 16 nm TAZS reveals the optimal device performance, with maximum luminance and maximum current efficiency of 19,046 cd/m2 and 4.08 cd/A, respectively, surpassing those without TAZS as HBL (8484 cd/m2, 2.13 cd/A). The hole-buffering characteristic of TAZS contributes greatly to improved charges’ recombination ratio and enhanced emission efficiency.

Polyazomethines based on oxadiazolyl or 1,2,4-triazolyl groups: synthesis and hole-buffering application in polymer light-emitting diodes

Balance in charge carriers is a prerequisite for obtaining high emission efficiency in polymer light-emitting diodes (PLEDs). Polyazomethines P1 and P2 buffer holes effectively to balance carriers and enhance emission efficiency.

Copolyphenylenes with pendant benzimidazolyl and diethanolaminohexyloxy groups: Synthesis and electron-transporting application in PLEDs

Two new electron-transporting copolyphenylenes P1NH and P2NH possessing balanced charges crucial to emission efficiency of polymer light-emitting diodes (PLEDs) have been synthesized and applied as an electron-transporting layer (ETL). The main chain structure is all para-linkage for P1NH and both para- and meta-linkage for P2NH, with the same pendant electron-withdrawing benzimidazolyl and polar diethanolaminohexyloxy groups. Both copolymers possess excellent thermal stability (Td > 300 °C, Tg > 100 °C) due to their rigid backbones. In addition, the pendant groups effectively lower LUMO (∼ −2.70 eV) and HOMO (∼ −5.70 eV) levels, resulting in improved electron-transporting and hole-blocking capabilities. Multilayer yellow-emitting PLEDs with a configuration of ITO/PEDOT:PSS/SY/ETL/LiF/Al were successfully fabricated by the spin-coating process. The maximum luminance and maximum current efficiency of the P1NH-based device were 12,881 cd/m2 and 10.94 cd/A, respectively, superior to the performance of P2NH-based device (4938 cd/m2, 3.70 cd/A) and the device without ETL (8690 cd/m2, 2.78 cd/A). Current results indicate that P1NH is highly effective in enhancing electron transport and device performance. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017

Efficient near-infrared emitting tetradentate bis-cyclometalated platinum (IV) complexes for solution-processed polymer light-emitting diodes

Two novel tetradentate bis-cyclometalated platinum (IV) complexes of TPA-2q-PtCl2 and 3TPA-2q-PtCl2 with CˆN*NˆC cyclometalated ligands and two coordinated chlorine atoms were synthesized and characterized. The crystal structure of TPA-2q-PtCl2 shows a trans-form irregular octahedral geometry by measurement of single-crystal X-ray diffraction, where two CˆN coordination moieties are twisted and two chlorine atoms are located on both sides of platinum atom. A medium absorption band in a range of 450–474 nm is observed, which is assigned to the metal-to-ligand charge-transfer (MLCT) and intramolecular charge transfer (ILCT). And two obvious emission peaks at about 635 and 750 nm are displayed for both platinum (IV) complexes in 2-methyltetrahydrofuran solutions, which originate from the ILCT and MLCT characters to the lowest excited state. Using one of platinum (IV) complexes as a single dopant and a blend of poly(vinylcarbazole) as a host matrix, their single-emissive-layer polymer light-emitting diodes (PLEDs) exhibit an outstanding near-infrared emission with an electroluminescent (EL) peak of 750 nm. The better electroluminescent properties are obtained in the 3TPA-2q-PtCl2 doped devices with an external quantum efficiency of 0.85% and a radiance of 164 μW/cm−2. This is the first reported sample about tetradentate biscyclometalated platinum (IV) complexes used as organic NIR electroluminescent materials for promising application in PLEDs.

Quantum-Dot and Polychalcone Mixed Nanocomposites for Polymer Light-Emitting Diodes

The present work is aimed at improving the efficiency of light-emitting diodes (LEDs) through the amalgamation of polymeric materials and quantum dots (QDs) in nanocomposites. Herein, we report on the polytriphenylamine-based chalcone (PTpC) or polycarbazole-based chalcone- (PCzC-) QDs mixture nanocomposites as emissive layers for polymer LEDs (PLEDs). QDs were evenly dispersed in the polymer matrix and the synthesized PTpC-QDs and PCzC-QDs nanocomposites were able to form smooth thin films. The luminance characteristics of PTpC-QDs and PCzC-QDs nanocomposites were better than those of the pristine QD, PTpC, and PCzC materials, owing to the high charge-carrier transport ability of polymer-QDs nanocomposites. These results indicate that the proposed polymer-QDs nanocomposites could be potential candidates for application in PLEDs.

Three cationic iridium(III) complexes with 1,10-phenanthroline or compounds containing 1,10-phenanthroline unit as auxiliary ligands: Synthesis and application in polymer light-emitting diodes

Three yellow-green phosphorescent cationic iridium(III) complexes [(CBT)2Ir(phen)]PF6, [(CBT)2Ir(BCP)]PF6 and [(CBT)2Ir(o-phen)]PF6 were synthesized (phen: 1,10-phenanthroline, BCP: bathocuproine, o-phen: 2-(4-tert-butylphenyl)-5-(4-(1-ethyl-H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenyl)-1,3,4-oxadiazole, CBT: 2-(9-(2-ethylhexyl)-9H-carbazol-3-yl)benzo[d]thiazole). All of them were successfully applied in polymer light-emitting diodes: ITO/PEDOT: PSS (40 nm)/PVK: PBD: complex (mass ratios 100: 40: x, x = 1.0, 3.0, and 5.0) (80 nm)/CsF (1.5 nm)/Al (100 nm). The maximum luminances were 8733 cd m−2, 8878 cd m−2 and 13019 cd m−2, the maximum luminance efficiencies were 13.7 cd A−1, 12.5 cd A−1 and 17.5 cd A−1 respectively. Because the steric hindrance of two methyls increase the distance between central Ir3+ and BCP, and the conjugated plane of 1,10-phenanthroline unit in BCP are distorted by two benzene rings, the flow of electrons are partly hindered, [(CBT)2Ir(BCP)]PF6 exhibits the lowest electroluminescent performances. Due to the electron-transporting functional group 2,5-diphenyl-1,3,4-oxadiazole and hole-transporting functional group carbazole providing good bipolar property, [(CBT)2Ir(o-phen)]PF6 exhibits the best electroluminescent performances.

Synthesis of polymerizable liquid crystalline monomers and their side chain liquid crystalline polymers bearing azo-ester linked benzothiazole mesogen

A series of azo-ester linked benzothiazole mesogen containing new polymerizable liquid crystalline (LC) methacrylate monomers M1–M4 having different terminal substituents (H, CH3, OCH3, and OC2H5) on the benzothiazole moiety and their side chain liquid crystalline polymers (SCLCPs) P1–P4 were synthesized and characterized. The chemical structures, thermal stability, and LC phase behaviors of monomers and polymers were examined by the usage of different experimental techniques. All the synthesized monomers and polymers exhibited excellent thermal stability. Monomer M1 (without terminal substitution) displayed both nematic and smectic phases while monomers M2–M4 (CH3, OCH3, and OC2H5 substituted) showed only nematic phase. On the other hand, all the prepared polymers exhibited only nematic LC phase. Polymers P1–P4 showed two strong absorption bands in the range of 261–262 and 376–413 nm whereas polymers P1–P4 exhibited strong fluorescence emission in the range of 522–524 nm. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were found to be −4.87 to −4.80 eV and −2.36 to −2.14 eV, respectively. The synthesized polymers could be potential candidate as fluorescent materials in the polymer light-emitting diode applications.

Selectively modulated inkjet printing of highly conductive and transparent foldable polymer electrodes for flexible polymer light-emitting diode applications

High efficient flexible polymer light-emitting devices which composed with highly conductive and transparent foldable polymer electrodes were fabricated. New doping materials, n-methyl-2-pyrrolidone (NMP) and n-methylformamide (NMF), have much improved the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The selectively modulated inkjet-printing method facilitated the PEDOT:PSS’s application to both transparent anodes and highly conductive bus line electrodes. Multiple-time printed PEDOT:PSS electrodes showed a similar performance to Ag bus lines while one-time printed anode showed a much better figure of merit than that of ITO on plastic. Due to the flexible property, high transparency and high work function of the polymeric anode, ITO-free PLEDs showed a high performance with foldable characteristics.

Cubic Zincblende ZnSe Nanowires with an Entangling Structure Grown via Oriented Attachment and Their Application in Organic–Inorganic Heterojunction Light-Emitting Diodes

Cubic ZnSe nanowires with a polymer-like entangling structure and their application in polymer light-emitting diodes (LEDs) are reported in this paper. The entangling nanostructure of ZnSe, which is observed for the first time, is grown on the basis of the oriented attachment mechanism but does not occur completely unidirectionally. This polymer-like entangling structure can hybridize with conductive polymers to form an interpenetrating network that can provide continuous paths for carriers. By incorporating ZnSe entangling nanowires into a poly[2-methoxy-5-((2-ethylhexyl)oxy)-1,4-phenylenevinylene] (MEH-PPV) matrix to form an inorganic–organic p–n heterojunction, the device electroluminescence intensity significantly increases more than 330% and the driving voltage decreases ∼26% for single-layer MEH-PPV LEDs.

Poly(9,9-dialkylfluorene-co-triphenylamine)s – A family of photo-curable hole-transport polymers for polymer light-emitting diodes applications

Hole-transporting photocurable blends of (4-(AcOCH2)C6H4)3N (1) or ((4-(AcOCH2)C6H4)2NC6H4)2 (2), poly(fluorene-co-triphenylamine)s (PFTs) and diphenyliodonium 9,10-dimethoxyanthracene-2-sulfonate (DIAS, a photoacid generator (PAG)) have been successfully formulated. Under UV irradiation (λ>200nm), PAG decomposes to release protons that catalyze the heterolytic decomposition of 1 and 2 to give benzylic cation intermediates. Friedel–Crafts type electrophilic aromatic substitution would then occur at the triphenylamine (TPA) units of PFT, leading to a photo-curing phenomenon. The curing efficiency is therefore highly dependent on the content of TPA in the PFT. This approach can be implemented to PLED applications. Under the optimized conditions, the polymeric light-emitting device of ITO/photo-cross-linkable HTL/PVK-Ir(ppy)3-t-PBD/Mg–Ag, with a turn-on voltage of 9.4V (1cd/m2), a maximum brightness of 7630cd/m2, and a current efficiency of 15.8cd/A was achieved. The lifetime of the device has also been improved by 1.8 times when compared with that of the STD device.

Gold nanoparticles modified ITO anode for enhanced PLEDs brightness and efficiency

We reported a facile and controllable electrostatic adsorption of gold nanoparticles (Au NPs) on an indium tin oxide (ITO) anode, for applications in polymer light-emitting diodes (PLEDs). Au NPs (size 20 nm) were found to be well dispersed on a pre-treated ITO anode, and the density of Au NPs was well controlled from 0 to 60 μm−2 depending on the adsorption time. By using the Au NPs modified ITO as the anode, the optimized PLEDs showed a 60% enhancement in brightness and 40% enhancement in luminous efficiency. Our results showed that the enhancement of devices performance mainly originated from enhanced photoluminescence, the maximum of luminous efficiency appeared when the distance of Au NPs and light emission profile was about 40–80 nm, which indicated that the origin of enhanced luminescence is due to the “far-field” effect of the Au NPs layer rather than a classical localized surface plasma resonance.

Theoretical design study on photophysical property on oligomers based on spirobifluorene and carbazole-triphenylamine for PLED applications

The photophysical properties of five blue light-emitting polymers based on spirobifluorene applied in polymer light-emitting diodes (PLED) materials have been studied by quantum chemistry. In order to understand the intrinsic reasons for the different performances displayed by the polymers, we carried out density functional theory (DFT) and Marcus theory investigations on their oligomers in terms of structure and properties stability, absorption and emission properties, and carrier injection and transport properties. Especially, some important parameters which had not been reported to our knowledge were given in this contribution, such as the ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ), ke/kh (the ratio between the electron transfer rate (ke) and hole transfer rate (kh)), and the radiative lifetimes (τ). The main results indicate that the co-oligomers of PCC-1, PCC-2, and PCC-3 with push-pull interactions produced by the existing D-A segments have better carrier injection and transport properties than the oligomers of PSF and PCF. Especially PCC-2 co-oligomer, its large radiation lifetime (7.46 ns) and well balanced and adequate carrier transport guarantee its champion performance for PLED. The calculated results coincide with the experimental ones. Besides, PNF structurally similar to PCC-2 has similar photoelectric properties to PCC-2 in theory, and the fluorescence emission of PNF co-oligomer is superior to PCC-2 co-oligomer. Therefore, we predict that PNF is a promising candidate for PLED.

Fluorene- and benzimidazole-based blue light-emitting copolymers: Synthesis, photophysical properties, and PLED applications

Blue light-emitting materials are receiving considerable academic and industrial interest due to their potential applications in optoelectronic devices. In this study, blue light-emitting copolymers based on 9,9'-dioctylfluorene and 2,2'-(1,4-phenylene)-bis(benzimidazole) moieties were synthesized through palladium-catalyzed Suzuki coupling reaction. While the copolymer consisting of unsubstituted benzimidazoles (PFBI0) is insoluble in common organic solvents, its counterpart with N-octyl substituted benzimidazoles (PFBI8) enjoys good solubility in toluene, tetrahydrofuran, dichloromethane (DCM), and chloroform. The PFBI8 copolymer shows good thermal stability, whose glass transition temperature and onset decomposition temperature are 103 and 428 degrees C, respectively. Its solutions emit blue light efficiently, with the quantum yield up to 99% in chloroform. The electroluminescence (EL) device of PFBI8 with the configuration of indium-tin oxide/poly(ethylenedioxythiophene):poly( styrene sulfonic acid)/PFBI8/1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl) benzene/LiF/Al emits blue light with the maximum at 448 nm. Such unoptimized polymer light-emitting diode (PLED) exhibits a maximum luminance of 1534 cd/m(2) with the current efficiency and power efficiency of 0.67 cd/A and 0.20 lm/W, respectively. The efficient blue emission and good EL performance make PFBI8 promising for optoelectronic applications. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 2172-2181, 2012

Mesogen-jacketed liquid crystalline polymers: from molecular design to polymer light-emitting diode applications

This critical review covers the recent progress in the chemical structure design and light-emitting performance evaluation of electroluminescent mesogen-jacketed liquid crystalline polymers (MJLCPs). By applying the concept of MJLCPs, two main chromophoric building units, electron-transporting and hole-transporting units with large steric hindrance, are attached to the backbones solely or together. We highlight the structure–properties–performance relationships of electroluminescent MJLCPs. Device performances can be enhanced by the “jacketing effect”, which inhibits the aggregation of conjugated structures, fluorescence quenching, and excimer formation.

Synthesis and optoelectronic properties of thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine)

This paper describes the synthesis of a new thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine) (PFTV) by Suzuki coupling reaction and its application in polymer light-emitting diodes (PLEDs). The characteristics of PFTV were analyzed by 1H NMR, differential scanning calorimetry, optical spectroscopy, cyclic voltammetry, and atomic force microscopy. Its HOMO level lies between those of PEDOT:PSS and poly(9,9-dioctylfluorene), forming a stepwise energy ladder to facilitate hole-injection. Multilayer device with thermally cross-linked PFTV as hole-transporting layer (ITO/PEDOT:PSS/HTL/PFO/LiF/Ca/Al) was readily fabricated by successive spin-coating processes, its maximum luminance efficiency (2.27 cd/A) was significantly higher than that without PFTV layer (0.50 cd/A). In addition, the PFTV was successfully applied as host for red-emitting Ir(piq)2acac to obtain a device with moderate performance (5300 cd/m2 and 2.64 cd/A). The PFTV is a promising hole-transporting material for the fabrication of multilayer PLEDs by wet processes as well as a potential host for phosphorescent PLEDs.

Synthesis and electrochemical polymerization of 9,9-bis(carbazolylalkyl)fluorene and characterization of its conducting polymer film with high tensile strength

Free-standing poly(9,9-bis(carbazolylalkyl)fluorene) (P2Cz-A-F, alkyl = hexyl or dodecyl) films with high tensile strength (185–200 kg cm−2) were electrochemically synthesized by direct anodic oxidation of the monomer 9,9-bis(carbazolylalkyl)fluorene (2Cz-A-F) in a mixed electrolyte of boron trifluoride diethyl etherate containing 30% CH2Cl2 (by volume). The electrochemical and physical properties show that the quality of P2Cz-A-F films are much better than that of polycarbazole (PCz) and polyfluorene (PF) films due to the specially designed structure consisting of stiff bicarbazyl chromophores linked by a flexible long alkyl spacer at the fluorene unit. The structure of free-standing P2Cz-A-F films was investigated by UV–visible and infrared spectroscopy. Thermal analysis shows that the polymer films have good thermal stability. Fluorescent spectral studies indicate that polymer films in the solid state are blue-light emitters, which may meet potential applications in polymer light-emitting diodes.

Photophysics of Fluorene Copolymers: Control of Fluorescence and Charge Separation by the Presence of Carbazole, Oxadiazole, or Biphenyl Units

A set of fluorene copolymers designed to introduce electron donor/acceptor structures in the polymer chain that could eventually produce more efficient charge separation with long lifetimes has been prepared. To validate our approach, herein we report photoluminescence and laser flash photolysis measurements of the series of alternating specially functionalized fluorene copolymers. The copolymers contain 9,9-dioctylfluorene and N-octylcarbazole, oxadiazole, and oxadiazole−octafluorobiphenyl−oxadiazole units. It appears that the copolymer with the longest charge separated state lifetime and the highest charge separation quantum yield of the series is an alternating copolymer of a N-octylcarbazole and trifluorene units in which two fluorenes were attached with hydroxyhexyl side groups and the central fluorene with octyl side group at C-9 positions, respectively. These photophysical data suggest that this polymer a good candidate for photovoltaic cell and polymer light-emitting diode applications.