Blue-green light-emission LECs based on block copolymers containing di(α-naphthalene vinylene)benzene chromophores and tri(ethylene oxide) spacers

The polymer light-emitting electrochemical cells (LECs) were fabricated and characterized with two novel block copolymers as the luminescent polymers. The block copolymers are composed of the rigid segments (chromophores), 1,4-di(α-2-naphthalene vinylene)benzene (DNVB) or 2,5-dimethyloxy-1,4-di(α-2-naphthalene vinylene)benzene (MDNVB) and the flexible segments (spacers), tri(ethylene oxide) (TEO). Efficient blue-green light emission of the LECs was demonstrated with onset voltage lower than 3 V and electroluminescence (EL) efficiency of ∼0.2 cd/A. The a.c. impedance data indicate that the operation mechanism of the LECs agrees with the electrochemical doping model.     

Light-emitting diodes based on alternating copolymers containing fluorene and oligo(p-phenylenevinylene)

We synthesized poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-cyanostyryl)benzene-1,4-diyl)] [P(FOPV-CN)] and poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4′-diphenylaminostyryl)benzene-1,4-diyl)] [P(FOPV-Am)]. These polymers have two axes of longitudinal π-conjugation: one in the direction of the polymer backbone and the other in the direction of the oligo(p-phenylenevinylene) chain. From the spectroscopic study, the emissive color of poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(benzene-1,4-diyl)] changed from blue to green by substituting cyanostyryl or diphenylaminostyryl group at the 2,5-positions of the benzene ring. The electronic structures were significantly influenced by the substituents at the ends of oligo(p-phenylenevinylene) moiety. The diphenylamino substituent which has more extended π-conjugation resulted in red-shifted electronic absorption and emission spectra compared to the cyano substituent groups. In the light-emitting diodes (LEDs) based on these two polymers, the turn-on voltage (ca. 9 V) of the device based on P(FOPV-Am) was lower than that of P(FOPV-CN) (ca. 12 V) due to the electron-donating effect of the amino groups. The maximum brightness of the P(FOPV-Am)-based LED was 3100 cd m⁻² at 22 V which is much higher than that of the P(FOPV-CN)-based LED (27 cd m⁻² at 26 V).     

Poly(1,4-bis[2-(4-hexylthiophene)]-2,5-dimethylphenylene): a new conjugated electroluminescent polymer

We report the synthesis of the conjugated polymer, poly(1,4-bis[2-(4-hexylthiophene)]-2,5-dimethylphenylene), by FeCl₃ oxidation in high yield. The polymer has perfect head-to-head (HH) linkages between the thiophene rings, exhibits good solubility in conventional organic solvents, and exhibits high thermal stability. The optical band gap is ∼2.76 eV. The polymer emits greenish-blue light under UV irradiation with the emission maximum at 507 nm; the photoluminescence (PL) efficiency (solid thin film) is ∼10%. The external electroluminescence efficiency was measured to be 0.0012% for single layer devices using Al as the cathode material and 0.0035% for single layer devices using Ca as the cathode material.     

Effects of chemical modifications on photophysics and exciton dynamics of π-conjugation attenuated and metal-chelated photoconducting polymers

Effects of two types of chemical modifications on photoconducting polymers consisting of polyphenylene vinylene (PPV) derivatives are studied by static and ultrafast transient optical spectroscopy as well as semi-empirical ZINDO calculations. The first type of modification inserts 2,2′-bipyridyl-5-vinylene (bpyV) units in the PPV backbone, and the second type involves metal-chelation with the bpy sites. Photoluminescence (PL) and exciton dynamics of polymers 1 and 2 with PV:bpyV ratios of 1 and 3 were examined in solution, and compared with those of the homo-polymer, poly(2,5-bis(2′-ethylhexyloxy)-1,4-phenylene vinylene) (BEH-PPV). Similar studies were carried out for several metal-chelated polymers. These results can be explained by changes in π-conjugation throughout the polymer backbone. The attenuation in π-conjugation by the chemical modifications transforms a conducting polymer from one-dimensional semiconductor to molecular aggregates.     

Polymers containing alternating 1,4-bis(3-octyl-2,5-thienylene)phenylene and 2,5-pyridyl, 4,4’-biphenylene or phenylene repeating units

Two polymers based on polymer backbone modification, poly[1,4-bis(3-octyl-2,5-thienylene)phenylene-alt-2,5-pyridyl] (PBTYC₈) and poly[1,4-bis(3-octyl-2,5-thienylene)phenylene-alt-4,4’-biphenylene] (PBTBBC₈), were synthesized and characterized in comparison with other two polymers reported earlier: poly[3-octyl-2,5-thienylene-alt-1,4-phenylene] (PBTC₈) and poly[1,4-bis(3-octyl-2-thienylene)phenylene] (PBTBC₈). PBTBBC₈ shows a lower conductivity in comparison with PBTC₈ and PBTBC₈. A trend in absorption peak wavelength (λ_max) of solution samples and in emission peak wavelength (λ_em) of film samples is also noted for the polymers: PBTBBC₈<PBTC₈<PBTBC₈, with the decrease of phenylene content. The optical behavior of PBTYC₈ seems to suggest the existence of charge transfer (CT) and energy charge (ET) phenomenon due to the incorporation of electron-deficient pyridyl units. Interesting electrochemical observation is also obtained, which is supported directly by the spectral change measured in situ.     

Organic electroluminescence devices fabricated with chemical vapour deposited polyazomethine films

We demonstrate the use of chemical vapour deposition polymerization as an effective method to fabricate conjugated polymer films as constituents in organic electroluminescence (EL) devices. This technique provides excellent compatibility with vacuum sublimation deposition of low molecular weight materials for construction of hybrid devices. The specific polyazomethine polymer studied here, namely, poly (1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne), is shown to be an addition to the range of polymer structures that provide electron transport functionality and, in combination with suitable emissive materials, provides the basis for new device structures. From measurements on a series of devices we propose an energy level structure for this material. Despite remaining isoelectronic with poly (1,4-phenylenevinylene) the energy levels of the polyazomethine are strongly shifted through the replacement of a vinylene CH with a N atom.     

Application of aluminum,copper and gold electrodes in a.c. polymer light-emitting devices

Symmetrically configured a.c. light-emitting (SCALE) ‘5-layer’ devices having the configuration M/EB/P/EB/ITO, where M = Al, Cu or Au, EB = polyaniline (emeraldine base), P = poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) or PPV-PPyV, and ITO = indium-tin oxide glass, show electroluminescent properties in both forward and reverse bias modes. In the absence of emeraldine base, in the case of aluminum and copper, electroluminescence is observed only in the forward biasmode; in the case of gold no electroluminescence is observed in either forward or reverse bias modes. The electrical properties of the ‘5-layer’ devices (M = Al, Cu) are most surprising since their total resistance at a given applied voltage is significantly less than that of the corresponding devices in which the two polyaniline insulator layers are not present.     

Synthesis and characterization of red-emitting diketopyrrolopyrrole-alt-phenylenevinylene polymers

Two novel diketopyrrolopyrrole (DPP) and p-phenylenevinylene alternating copolymers, poly(1,4-(2,5-dicyano)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P1) and poly(1,4-(2,5-diethoxy)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P2), were synthesized through Wittig reaction in good yields, and were characterized by FTIR, NMR. Two polymers possessed moderate molecular weights and polydispersities, well-defined structures, and were readily soluble in common organic solvents. In particular, P1 and P2 exhibited excellent thermal stability with T_d = 393 and 398 °C at 5% weight loss, respectively. Both P1 and P2 in solution and in thin films exhibited strong red photoluminescence. Both electroluminescence devices using ITO/PEDOT/polymer/Ba/Al configuration with P1 and P2 as the emitting layer showed nearly pure red emission with the emission peaks at 646 nm for P1 and 648 nm for P2, and exhibited low turn-on voltages of 4.5 and 3.4 V, respectively. The results show that P1 and P2 are promising candidates for red emissive materials in polymer light-emitting diodes.     

Photo- and electroluminescence from hyperbranched phenylene vinylenes

We report photophysical and electroluminescence (EL) properties of three novel hyperbranched phenylene vinylenes (HPVs) with benzene as the core, 2,5-dimethyloxyl substituted phenylene vinylene as the connecting unit, and benzene, dimethylaniline or pyridine as the terminal group, respectively. The absorption maxima of the HPVs blue-shift from solution to the films due to their twisted molecular conformation. The emission spectra in the films red-shift compared to those in solution, which may originate from interchain excimer species. The thin films of the HPVs exhibit pretty high photoluminescence (PL) quantum yields around 50.5–87.8%. Blue–green light-emitting electrochemical cells (LECs) based on the HPVs have been demonstrated with turn-on voltages of 2.5–2.6 V. The ac impedance measurements indicate the operation of the LECs corresponds to an electrochemical doping model.     

Application of polyaniline (emeraldine base,EB) in polymer light-emitting devices

We report the fabrication of several multilayer light-emitting-diode (LED) devices based on a novel conjugated polymer, poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) (PPV-PPy V), involving the use of polyaniline (emeraldine base, EB) as an insulating layer between the emissive polymer layer and the electrodes. In all the above devices with various configurations (‘3-layers’, ‘4-layers’-1’, ‘4-layers-2’ and ‘5-layers’), only the symmetrically configured a.c. light-emitting (SCALE) (‘5-layers’) device shows the capability of operating in both forward and reverse bias modes and in an a.c. mode. The SCALE devices have a typical turn-on voltage of about 4–6 V and work well under both forward and reverse bias modes. It is important to note that the total resistance (R= V/I+) of the four devices at any given applied potential decreases as the number of insulating polymer layers increases, suggesting that the nature of the electrode/polymer interface plays a critical role in determining the characteristics of the devices.     

Photo-, and electroluminescence studies of 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophenes

We have synthesized a thiophene-based compound 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophene (HBDT) and a copolymer poly(2,5-bis(2′-(4″-(6-hexoxy benzyl))-1′-ethenyl)-3,4-dibutyl thiophene-1,6-diisocyanatohexane) (HBDT-PU) consisting of alternating HBDT and urethane spacer units. Absorption and photoluminescence (PL) spectra of the compounds coincide indicating that emission in HBDT-PU occurs from the thiophene containing unit. PL is emitted in the blue–green region with a maximum at 460 nm. Concentration quenching occurs in pure materials: PL efficiency is strongly enhanced when the compounds are dispersed in a polymer matrix like poly(N-vinyl carbozole) (PVK) or poly(methyl metacrylate) (PMMA). Light emitting devices were fabricated using a PVK:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) blend doped with HBDT as the emitter material. Efficient energy transfer from PVK:PBD to HBDT molecules takes place in blended films. The electroluminescence (EL) spectra coincided with the PL spectra of HBDT indicating that EL emission comes solely from HBDT molecules. The influence of the doping concentration on the EL efficiency was found to coincide with the concentration dependence of the PL quantum yield.     

Chiral alkylated poly(m-phenylene)s: Optical activity and thermal stability of helical structures

Chiral poly[4,6-bis(alkylthio)-1,3-phenylene-alt-2-methyl-1,3-phenylene] was synthesized from 1,3-dibromo-2,6-bis(3-dodecyl-2-methylthio)benzene and 2-methyl-1,3-phenylenebis(pinacol borate) as a precursor of chiral poly(thiaheterohelicene). Circular dichroism (CD) spectra that arise from the poly(1,3-phenylene) backbone inverted according to the chirality of the side chains, which indicated that a helical conformation of the polymer was induced by the interaction between the side chains. The CD intensity of the polymer increased in non-polar solvents such as hexane. The decrease in the molar CD intensity and the broadening of a fluorescence band at higher concentrations suggested that the aggregation of the polymer suppressed the formation of the helical structure. The conformational changes were monitored by the CD and the ¹H NMR spectra at different temperatures. In a good solvent such as dichloromethane, the CD intensity increased, and the ¹H NMR signal of benzene protons shifted to lower fields at low temperatures. In hexane, the CD spectra and the ¹H NMR signals were less dependent on temperatures, as a result of the strong interaction between the chiral alkyl chains in the polymer to freeze the helical conformation.     

Sharp and red “single-chain” luminescence from poly[2,5-dialkoxy-1,4-phenylene vinylene] locked in ordered host matrix

We found that the luminescence profile of poly[2,5-di(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (OC₁₀C₁₀-PPV), which originally is an orange-emitting polymer, can red-shift significantly by ca. 30 nm (λ_max: from 585 to 614 nm) and become very narrow (full width at half maximum: 30 nm) when doped into poly{2-[m-(3′,7′-dimethyloctyloxy)phenyl]-1,4-phenylene vinylene} (m-Ph-PPV). Due to the ordered structure of the host polymer, the effective conjugation length of the guest polymer is extended as if the chains were frozen. Surprisingly, the OC₁₀C₁₀-PPV chains can even be more tightly locked in the ordered m-Ph-PPV matrix than in the cooled neat OC₁₀C₁₀-PPV film at 5 K.     

Conjugated copolymers of 2-methoxy-5-2′-ethyl-hexyloxy-1,4-phenylenevinylene and 2,5-dicyano-1,4-phenylenevinylene as materials for polymer light-emitting diodes

A new series of electroluminescent polymers has been synthesized by incorporating the segments of 2,5-dicyano-1,4-phenylenevinylene (DCN-PV) into the backbone of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV). The copolymers have similar optical properties with MEH-PPV. The oxidation potentials of the copolymers are slightly larger than that of MEH-PPV. The copolymers exhibit “pre-reduction” processes, beginning at about −0.9 V (vs. SCE), before the main reduction processes. The “pre-reduction” feature is thought to be helpful in improving electron injection, and thus to be helpful in improving EL performance. The improved EL performance of the copolymers in comparison with MEH-PPV is demonstrated by a single-layer polymer light-emitting diode (PLED) device employing one of the copolymers as the emissive material.     

Poly(2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene): a new material for blue-light-emitting diodes

The electronic and chemical structures of a new polymer, poly(2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene), have been studied experimentally by X-ray and ultraviolet photoelectron spectroscopies, and theoretically by quantum chemical calculations. Optical absorption and photoluminescence spectra were also obtained. In addition, single-layer light-emitting diodes, consisting of calcium electrodes deposited on polymer films which had been spin-coated on ITO glass, were fabricated in ultra-high vacuum, and the electroluminescence spectra as well as current-voltage curves were studied. The polymer was found to be highly fluorescent, emitting light of light-blue/green color. The light emitted from the diodes was turquoise in color and clearly visible in daylight.     

Photoluminescence and electroluminescence properties of coumarin-urea/poly(N-vinylcarbazole) blends

The photoluminescence (PL) properties of a novel coumarin derivative (DCDU) in solution state and in poly(N-vinylcarbazole) (PVK) matrix were investigated. The single-layer electroluminescence (EL) devices fabricated from the DCDU/PVK blends showed the emission peak varying from 445 nm to 472 nm with the different dye doping ratio. When 2,5-bis(5′-tert-butyl-2-benzoxazoly)thiophene (BBOT) was used as an electron transport layer, the ITO/(2 wt.% DCDU+PVK)/BBOT/A1 device displayed the improved characteristics with the low turn-on voltage at 6.3 V, a maximum luminance of 730 cd/m² and an external quantum efficiency of 0.8%.     

Enhanced performance of polythiophene derivative based light emitting diodes by addition of europium and ruthenium complexes

Fabrication of polymer light emitting diodes (PLEDs) from a urethane containing processable polythiophene derivative, poly[2-(3-thienyl) ethanol n-butoxy carbonylmethyl urethane], (PURET) and its composites with luminescent Ruthenium (II) and Europium (III) complexes is discussed. Enhanced electroluminescence (EL) performance was observed, when 1% by weight of europium (III) or ruthenium (II) complexes were added to the PURET polymer. Multi-layered devices with polyaniline as a hole injecting layer and tris-8-hydroxyquinoline-aluminum as an electron injecting layer have also been fabricated. PLEDs from PURET polymer have a threshold voltage of 3.6 V and emit an orange-red light with a brightness of about 500 cd/m² under forward bias of 9 V, upon the addition of 1% of europium (III) thenoyl trifluoroacetonate complex to the polymer.     

Synthesis and spectral characterizations of electroluminescent poly(2,3-di-[p-(2′-ethylhexoxy)phenyl]-1,4-phenylenevinylene)

By means of chemical modification, flexible branched alkoxy chains were successfully attached to the para-site of the side phenyl groups of DP-PPV to obtain a new soluble fully conjugated PPV, named poly(2,3-di-[p-(2′-ethylhexoxy)phenyl]-1,4-phenylenevinylene). Spectroscopic characterizations using UV, photoluminescence (PL) and photoluminescent excitation (PLE) showed only the emission of single chromophore of this polymer in the solution state, indicating that the solubility of poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV) was significantly improved by attaching the alkoxy side chains. In the spin-coated film, the emission of the single chromophore with longer conjugating length was observed. In addition, the spectral feature showed the absence of the emission from the aggregate, which was usually observed in the alkoxy-modified PPV, revealing that the polymer chains were well-separated by the side chains. The performance of the single-layer ITO/PEDOT/p-EHDP-PPV/Ca/Al LED device was also reported. The maximum luminescence and efficiency at 11 V was 3735 Cd/m² and 0.57 Cd/A, respectively.     

Time-resolved Förster energy transfer in polymer blends

Sub-picosecond spectroscopy and pump–probe experiments show Förster energy transfer in blends from larger gap (blue or green-emitting) host polymers poly(2,3-diphenyl-5-hexyl-1,4-phenylenevinylene) (DP6-PPV) or poly[2-(meta-2′-ethylhexoxyphenyl)-1,4-phenylenevinylene) (m-EHOP-PPV) to the smaller gap, red-emitting guest polymer poly(2,5-bis(2′-ethylhexoxy)-1,4-phenylenevinylene) (BEH-PPV). The dynamics of the stimulated emission (SE) and photoinduced absorption (PA) of the blends indicate that 10–20 ps are required for complete energy transfer. Quantitative measurements of energy transfer rates give a Förster interaction range of 3–4 nm, 1.4 times longer than the theoretical values as calculated from the spectral overlap. We attribute this difference to delocalization of the excited state. Insufficient spectral overlap between the emission of the host and absorption of the guest is shown to be the cause for the absence of energy transfer in a blend with poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene) (BCHA-PPV) as the guest polymer.     

Photo-oxidation of polymers used in electroluminescent devices

Two electroluminescent polymers, poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene) (BCHA-PPV) and poly(3-octylthiophene) (P3OT), have been characterized on Au and Al substrates in terms of photo-oxidative stability after exposure to UV and visible irradiation from a Hg lamp. Thin polymer films were analyzed in situ using IR reflection absorption spectroscopy (IRRAS), and ex situ by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection (ATR) IR spectroscopy. The oxidation of BCHA-PPV is quite extensive, resulting in the formation of an ester and volatile aldehyde species. Singlet oxygen, formed by energy transfer from the BCHA-PPV triplet state, appears responsible for at least one photo-oxidation pathway. BCHA-PPV triplet exciton states will also form during electroluminescent device operation (the singlet exciton state is responsible for luminescence in the film) resulting in the formation of reactive singlet oxygen. Thus, it appears that BCHA-PPV will slowly self-degrade in the presence of oxygen when used as the active layer in electroluminescent devices. P3OT shows no detectable oxidation during similar exposure.