Single-layer Polymer Light-emitting Diodes Research Articles

Influence of Gaussian disorder and exponential traps on charge carriers transport and recombination in single layer polymer light-emitting diodes based on PFO as emitting layer

In this paper, we study the transport and recombination mechanisms of charge carriers in a single layer PLED device based on PFO as emitting layer with considering Gaussian disorder, exponential distribution of traps, Poole–Frankel type field dependent hopping mobility and field independent effective transport hopping energy models. For this purpose, we use the one-dimensional and time-independent drift–diffusion model for simulation of electrical and optical characteristics of the PFO based PLED devices such as current density–luminance–voltage (J–L–V), recombination rate and singlet exciton density profile in steady-state and room temperature conditions. Also, we study the influence of the characteristic temperature and density of electron traps on the performance of devices at different voltages. Another topic is analyzing the current density–voltage characteristics of uni-polar devices (hole only or electron only) by using the well known expressions for trap-limited current and space charge-limited current and comparing them with simulation results.

Highly efficient single-layer blue polymer light-emitting diodes based on hole-transporting group substituted poly(fluorene-co-dibenzothiophene-S,S-dioxide)

We developed a series of high-performance blue light-emitting polymers that contain hole-transport moieties comprising carbazole or triphenylamine substituents in the side chains of random copolymer poly(fluorene-co-dibenzothiophene-S,S-dioxide) (PFSO).

Tuning the emission color of a single-layer polymer light-emitting diode with a solution-processed external layer

The emission color of a single-layer polymer light-emitting diode (PLED) was tuned using an external layer. We combine an electroluminescent polymer, known as poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT), embedded in a diode-like structure (ITO/PEDOT-PPS/F8BT/Ca/Al), and a film of poly(vinyl alcohol) (PVA) containing one of the xanthene dyes, either Erythrosin B (ERB) or Rose Bengal (RB), used as an external photoluminescent layer. The emission color is tuned from the CIE coordinates (0.34, 0.64) for the bare F8BT-diode to (0.39, 0.57) using a ∼42μm PVA:RB film and to (0.35, 0.61) using a ∼55μm PVA:ERB film. It was obtained based on trivial photophysical processes: xanthene dyes partially absorb the electroluminescence (EL) emissions coming from the F8BT-diode, decaying radiatively and emitting light by photoluminescence (PL). This method is versatile and may be extended to other combinations of EL and PL materials, tuning the color output and giving different CIE color coordinates. Because both the EL and PL layers are polymer-based systems, they can be used in the development of flexible displays and illumination sources. Furthermore, the PL layer is a self-supported film formed by water-soluble materials, with the advantage of being environmentally friendly, commercially available and low-cost. The PLED delivered a brightness of 123cd/m2 at 9V.

Morphology-dependent electroluminescence in poly(N-vinyl carbazole)-based multi-component single emissive layer polymer light-emitting diodes

We investigate the electroluminescent performance of single-layer polymer light-emitting diodes with the emissive layer comprised of poly(N-vinyl carbazole) (PVK): electron-transporting molecules (ETMs):iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C2]-picolinate (FIrpic) trinary components. A series of ETMs with comparable energy levels is tentatively utilized to modulate the morphology. The morphology of the emissive layer is studied using the transmission electron microscopy and atomic force microscopy in combination with the interfacial energy calculation between these components, with purpose to understand the multi-component miscibility and morphological properties of these emissive layers. It is found that the multi-component miscibility has dramatic influence on the morphology of the multi-component emissive layer and the final electroluminescent performance of the devices. The results reveal that the aggregation of ETMs mainly results in an increase of driving voltages, while the aggregation of FIrpic emitters would lead to the reduction in the luminous efficiencies as a result of their self-quenching effects. The PVK:1,3-bis[(p-tert-butyl)phenyl-1,3,4-oxadiazoyl]benzene (OXD-7): FIrpic blend demonstrates a more uniform morphology resulting in an optimal luminous efficiency of 15cdA−1.

Synthesis, Characterization, and Electroluminescent Properties of Pyridinylene Vinylene-Modified Phenylene Vinylene Copolymers

Two random copolymers of phenylene vinylene and pyridinylene vinylene, ED-co-PyV-PPV and EM-co-PyV-PPV, were synthesized according to the Gilch route. The polymers possess good solubility in common solvents, and high thermal stability with 5% weight loss temperature above 378°C. The weight-average molecular weight (Mw) and polydispersity index of ED-co-PyV-PPV and EM-co-PyV-PPV are 3.12 × 105 and 1.40, and 2.30 × 105 and 1.96, respectively. Single-layer polymer light-emitting diodes (PLEDs) with indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS)/polymer/Ca/Al configuration were fabricated. The turn-on voltages of the PLEDs based on ED-co-PyV-PPV and EM-co-PyV-PPV were approximately 4.0 V and 2.8 V, respectively. The ED-co-PyV-PPV PLED device exhibited the maximum luminance of ca. 4380 cd/m2 with maximum electroluminescence (EL) efficiency of 6.35 cd/A.

Iridium phosphors with peripheral triphenylamine encapsulation: highly efficient solution-processed red electrophosphorescence

The peripheral triphenylamine-encapsulated red-emitting iridium(III) complexes have been designed and synthesized. External quantum efficiency over 15% has been realized in single-layer polymer light-emitting diodes, which is the highest ever reported for solution-processed red phosphorescence.

Pure blue electroluminescent poly(aryl ether)s with dopant-host systems

A series of novel arylene ether polymers (P5F-BCzVFs) containing both pentafluorene (5F) and distyrylarylene derivative (BCzVF) units in the side chains for efficient pure blue light emission were prepared by a facile, metal-free condensation polymerization. The emission spectra indicated that color tuning could be achieved through efficient Förster energy transfer from the deep-blue 5F host to the pure-blue BCzVF dopant. Single-layer polymer light-emitting diodes (PLEDs) based on P5F-BCzVFs (ITO/(PEDOT:PSS)/polymer/Ca/Al) exhibited voltage-independent and stable pure blue emission with a Commission International de L'Eclairage (CIE) coordinate of (0.15, 0.15), a maximum brightness of 3576 cd/m2, and a maximum luminous efficiencies of 2.15 cd/A, respectively. As most polymers with dopant-host systems, the luminous efficiencies of all P5F-BCzVFs surpassed that of the host-only polymer (P5F), due to the energy transfer and charge trapping from the host to the dopant. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Single-layer white polymer light-emitting diodes based on an iridium (III) complex containing alkyltrifluorene picolinic acid

To explore the relationship between the electronic properties of a host/dopant system and obtain a high-efficiency single-dopant white polymer light-emitting device two novel blue-emitting cyclometalated iridium (III) complexes of (dfppy) 2Ir(Tfl-pic) and (dfppy) 2Ir(Brfl-pic) have been synthesized and characterized, where dfppy is 2-(2,4-difluorophenyl)pyridine, Tfl-pic and Brfl-pic are picolinic acid derivatives containing trialkylfluorene and dibromoalkylfluorene units bridged with an alkoxy chain, respectively. Both iridium (III) complexes exhibited blue emission in dichloromethane solution and their neat films, and possessed good dispersibility and thermal properties. Two different devices using (dfppy) 2Ir(Tfl-pic) as a single component emitter and a blend of poly( N-vinylcarbazole) and 2-(4-biphenyl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole as the host matrix were fabricated. Improved white emission was obtained by adjusting the electron injection layer leading to efficient exciplex emission.

Reduced molybdenum oxide as an efficient electron injection layer in polymer light-emitting diodes

We report a significant improvement in the performance of single layer polymer light-emitting diodes (PLEDs), based on the green emitting copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2, 1′,3}-thiadiazole)], upon inserting a very thin layer of partially reduced molybdenum oxide (MoOx, where x=2.7) at the polymer/Al cathode interface. Both fully oxidized (x=3) and partially reduced (x=2.7) thin molybdenum oxide layers were investigated as electron injection layers and their influence on PLED device performance was examined. Improved current density, luminance, and efficiency was achieved only in the case of devices with a thin partially reduced MoO2.7 film as electron injection layer, as a result of improved electron injection and more facile transfer at the modified polymer/Al interface.

Thermal aging of single-layer polymer light-emitting diodes composed of a carbazole and oxadiazole containing copolymer doped with singlet or triplet emitters

An electron-transporting monomer was synthesized that was structurally and energetically similar to the small molecule 2-biphenyl-4-yl-5-(4- tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD). The monomer was copolymerized with 2-(9 H-carbazol-9-yl)ethyl 2-methylacrylate and the resulting copolymer was utilized in organic light emitting devices which employed fluorescent coumarin 6 (C6) or phosphorescent tris(2-phenylpyridine)iridium(III) [Ir(ppy) 3] emitters. The copolymer devices exhibited a mean luminance of ca. 400 and 3552 cd/m 2 with the C6 and Ir(ppy) 3 emitters, respectively, that was stable with thermal aging at temperatures ranging from 23 °C to 130 °C. Comparable poly(9-vinyl-9 H-carbazole)/tBu-PBD blend devices exhibited more pronounced variations in performance with thermal aging.

Enhanced Brightness and Efficiency in Single-Layer White Polymer Light-Emitting Diodes (PLEDs)

In this paper, the improvements in luminance and efficiency have been demonstrated in a white polymer light-emitting device (PLED) with the structure of ITO/ poly (N-vinylcarbazole) (PVK): 1,1,4,4- tetraphenyl-1,3-butadiene (TPB):5,6,11,12-tetrapheny lanpthacene (rubrene) /LiF(1 nm)/Ca(10 nm)/Al(100 nm). The luminance of the white PLED is up to 4940 cd/m2 at 17 V. The current efficiency and Commission Internationale d’Eclairage (CIE) coordinates is 1.66 cd/A and (0.325, 0.326), respectively. The enhancement of the luminance and efficiency can be attributed to an improved hole-injection ability and balance carrier recombination through two ways: (1) enhancing the hole-injection from ITO by using O2 plasma treatment, and (2) balance the recombination of hole and electron by produce a buffer layer with σ-bonded segments of polymer between the ITO and the polymer layer. Besides, orange-emitting rubrene provides that direct recombination of holes and electrons occurred on the dopant molecules and trapping significantly enhanced the luminance and efficiency.

Study on carrier mobility measurement using electroluminescence in frequency domain and electrochemical impedance spectroscopy

In this paper, the two methods, electroluminescence in frequency domain and electrochemical impedance spectroscopy, have been applied to investigate the carrier mobility in single layer polymer light-emitting diode employing the polymer MEH-PPV (Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) as the light-emitting layer. The carrier mobility μ is 1.64 × 10 −6 cm 2/V s under the electric field 8.3 × 10 5 V/cm measured by the method of electroluminescence in frequency domain. The electrochemical impedance spectroscopy results indicate that the carrier mobility μ is 1.08 × 10 −6 cm 2/V s under the electric field 7.5 × 10 5 V/cm. A significant advantage of the two methods is that both of them can be applied to measure the carrier mobility in the thin film.

Polarized Emission from Donor-Acceptor Liquid-Crystalline Polymers Containing Oxadiazole and Carbazole Moieties

We have synthesized a class of blue-emitting polymers based on a donor-acceptor architecture composed of oxadiazole and carbazole moieties containing various substituted groups in the same side chain. Depending on the substituted group at carbazole, they showed different thermodynamic properties: two polymers containing N-methylcarbazole showed liquid-crystalline behavior. The single-layer polymer light-emitting diodes using these polymers exhibited blue emission with peaks from 430 to 440 nm. The device using the liquid-crystalline polymer with an alignment layer emitted polarized blue light with a dichroic ratio of 5.1.

Effect of Organic Salt Doping on Electron Injection in Single-Layer Polymer Light-Emitting Diodes

We report the effect of organic salt doping on the carrier injection in single-layer polymer light-emitting diodes (PLEDs). From the anomalous behavior in the luminance-voltage characteristic of the salt-doped devices with LiF layer, it was found out that the electron injection through LiF layer occurs competitively with the electrochemical n-doping near the cathode. After the completion of n-doping near the cathode, even without LiF layer the salt-doped devices exhibit the performance comparable to that of the conventional devices with LiF layer. At high operating voltages the salt-doped devices showed the higher brightness and better device stability compared with the conventional devices. We also report for the first time that the salt doping does not affect the device stability appreciably.

Single-layer triplet white polymer light-emitting diodes incorporating polymer oxides: Effect of charge trapping at phosphorescent dopants

This paper describes the effects of charge trapping on the device performances of triplet polymer light-emitting diodes (PLEDs) after the cathode contact had been improved through the blending of poly(ethylene glycol) (PEG) into the active layer. The external quantum efficiency (EQE) was enhanced when the dopant tended to trap electrons. In contrast, we observed no EQE enhancement for the device featuring a hole-trapping dopant. Because PEG promoted electron injection, more electrons were trapped in the triplet molecules, thereby enhancing the probability of recombination. Finally, after incorporating PEG, we further achieved white PLEDs exhibiting both high EQE and high power efficiency.

Numerical simulation of single layer polymer light-emitting diodes

A numerical model for the electrical properties for polymeric light-emitting diodes (PLEDs) is presented which accounts for drift- and diffusion transport, recombination and re-emission processes. The current–voltage characteristics of single layer polymer light-emitting diodes are found to be dominated by the space-charge limited bulk hole-conductivity. The device efficiency is found to increase with bias as electron current is strongly reduced by injection barrier. The operating voltage increases with increased thickness of polymer layer. The understanding of these characteristics will facilitate the further optimization of the performance of polymer LEDs.

All‐organic optocouplers based on polymer light‐emitting diodes and photodetectors

AbstractIn this work, we demonstrate all organic flexible polymeric optocouplers by utilizing a donor‐acceptor bulk heterojunction polymer photodetector (PD) as the output unit and a polymer light‐emitting diode (PLED) as the input unit. The input unit is a single‐layer PLED on a glass or a plastic (PET) substrate utilizing a green emitting polyfluorene‐benzothiadiazole copolymer in the active layer. The output unit is a single‐layer PD on a glass substrate utilizing a P3HT:PCBM(1:1 by weight) blend, where P3HT is regioregular poly(3‐hexylthiophene) and PCBM is (6,6)‐phenyl‐C61‐butyric acid methyl ester. The electroluminescence spectrum of the PLED peaks at 530 nm and covers a spectral range that coincides quite well with the PD absorption spectrum (between 450 and 650 nm). The current density transfer ratio reaches 0.012% for an optocoupler that operates at 0 V and 15 V for the PD and PLED, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement of radiative efficiency and wavelength-tuning by TPD-doping in organic light emitting diode using Poly [methylmethacrylate-co-(7-(4-trifluoromethyl)coumarin acrylamide)] (PCA)

We fabricated polymer light-emitting diodes (PLEDs) using a low-weight-molecular-doped luminescent material with a ?max in the ultraviolet region. A single layer PLED without low-weight-molecular-doping does not reveal any electroluminescence (EL) signal at all. However, a PLED with doping clearly shows EL signals. This indicates that carrier injection as well as radiative efficiency can be drastically improved by low-weight-molecular-doping. In addition, multilayered PLEDs are found to be very effective to further improve carrier injection.

Study of electroluminescence and electron injection on novel red polyelectrolytes copolymer

Single-layer polymer light-emitting diodes with high work function gold cathode were fabricated using a series of novel conjugated polyelectrolytes and their neutral precursors: aminoalkyl-substituted polyfluorenes with different 4,7-di-2-thienyl-2,1,3-benzothiadiazole contents. At the current density of 38.7mA/cm2, the external efficiency reaches 0.42% with the Commission Internationale de I'Eclairage coordinates x=0.67 and y=0.33 for PFN-DBT10. It has been shown that these polymers also can be used as electron injection layer in double-layer devices with high work function gold cathode, which favors the manufacture of solution processing multilayer devices. Based on single-layer and double-layer device structures, it was found that the improvement of device performance is attributed to the interface effect between gold cathode and polyelectrolytes layer.

Synthesis and Characterization of Conjugated Copolymers, and Their Color Tuning with Molecular Microstructure

Poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene-co-1,4-phenylene vinylene) (MEH-R) and poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene-alt-1,4-phenylene vinylene) (MEH-A50) containing phenylene vinylene (PV) unit and alkoxyphenylene vinylene (MEHPV) unit in the main chain were synthesized by the dehydrohalogenation and Honer-Emmons reaction using potassium t-butoxide, respectively. Single layer polymer light-emitting diodes (PLEDs) were fabricated with synthesized conjugated copolymers as emitting layers, and their emission properties were compared with PPV and MEH-PPV homopolymers. UV-Visible absorption and EL spectra of MEH-Rs were blue-shifted as increasing the amount of PV unit in the main chain. Color coordinates of single layer PLEDs made with the conjugated copolymers were compared with those of standard red, green, blue, and white established by NTSC. Color coordinates of single layer PLEDs made with MEH-Rs and MEH-A50 were shifted from red (MEH-PPV) to green region (PPV) as increasing PV unit in the main chain.