Polymer Light-emitting Diodes Devices Research Articles

Semi-orthogonal solution-processed polyfluorene derivative for multilayer blue polymer light-emitting diodes

Abstract Semi-orthogonal solution processing can circumvent interface intermixing issue of construction of multilayer polymer light-emitting diode devices (PLEDs). In this manuscript, we developed a new strategy to construct multilayer PLEDs by the semi-orthogonal solvents based on the difference between the solubilities of a poly(9,9-dioctylfluorene) that end-capped with a triphenylamine derivative (PFO-TF) (processed with p-xylene) and the light-emitting polymer PPF-SO15 or PPF-SO15-BT1 (processed with 1,4-dioxane). As the highest occupied molecular orbital energy level of PFO-TF (−5.70 eV) lies in between PEDOT:PSS (−5.20 eV) and the light-emitting copolymer PPF-SO15 (−5.94 eV), it can serve as the hole transport layer (HTL) to facilitate hole injection. The blue PLED based on PFO-TF as the HTL exhibits lower turn-on voltage and nearly identical electroluminescent spectra as that of the control device without HTL. Of particular importance is that the blue device with PFO-TF as the HTL exhibits the luminous efficiency over 5.0 cd A−1, which is much higher than that observed from a device without an HTL (3.51 cd A−1) or one with traditional poly(9-vinylcarbazole) as the HTL (4.74 cd A−1). Similar trend is also realized for PLEDs based on green-light-emitting polymer PPF-SO15-BT1. These observations indicate that processing of a multilayer device with this specific semi-orthogonal system may be a promising strategy for the construction of efficient PLEDs.

Electrical and Optical Properties of Green Polymer Light Emitting Diodes with Various Structures of Au Nanoparticles.

PSS) organic material. The green PLED devices with a structure of glass/ITO/PEDOT:PSS+Au NPs/PVK:Ir(ppy)3/TPBi/LiF/Al were prepared by conventional spin-coating and thermal evaporation methods. Various concentrations of Au NPs were doped to the HILs to optimize the device's light emitting characteristic. The effects of Au NPs concentrations on the properties of PLEDs were investigated. The doping concentrations of Au NPs were changed ranging from 0.0 to 1.0 vol%. At the optimized Au NPs concentration of 0.5 vol%, we also studied the effects of various film layers with and without Au NPs on the properties of PLEDs. The maximum luminance and external quantum efficiency of the devices were found to be 20,430 cd/m2 and 7.49%, respectively.

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.

PEDOT:PSS/Graphene Nanocomposite Hole-Injection Layer in Polymer Light-Emitting Diodes

We report on effects of doping graphene in poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate), PEDOT:PSS, as a PEDOT:PSS/graphene nanocomposite hole injection layer on the performance enhancement of polymer light-emitting diodes (PLEDs). Graphene oxides were first synthesized and then mixed in the PEDOT:PSS solution with specifically various amounts. Graphenes were reduced in the PEDOT:PSS matrix through thermal reduction. PLED devices with hole-injection nanocomposite layer containing particular doping concentration were fabricated, and the influence of doping concentration on device performance was examined by systematically characterizations of various device properties. Through the graphene doping, the resistance in the hole-injection layer and the turn-on voltage could be effectively reduced that benefited the injection and transport of holes and resulted in a higher overall efficiency. The conductivity of the hole-injection layer was monotonically increased with the increase of doping concentration, performance indices from various aspects, however, did not show the same dependence because faster injected holes might alter not only the balance of holes and electrons but also their combination locations in the light-emitting layer. Results show that optimal doping concentration was the case with 0.03 wt% of graphene oxide.

Synthesis and characterization of thiophene and fluorene based donor–acceptor conjugated polymer containing 1,3,4-oxadiazole units for light-emitting diodes

A new donor–acceptor (D–A) conjugated polymer (PDTOF) containing 3,4-didodecyloxythiophene, fluorene and 1,3,4-oxadiazole units is synthesized by using Wittig reaction methodology. The synthesized polymer is characterized by 1H NMR, FTIR, GPC, and elemental analysis. The optical energy band gap of the polymer is found to be 2.42eV as calculated from the onset absorption edge. The electrochemical studies of PDTOF reveal that, the HOMO and LUMO energy levels of the polymer are −5.45eV and −3.58eV, respectively. The polymer is thermally stable up to 320°C. Polymer light-emitting diode devices are fabricated with a configuration of ITO/PEDOT: PSS/PDTOF/Al using PDTOF as the emissive layer. The electroluminescence (EL) spectrum of the device showed green emission with CIE coordinate values (0.34, 0.47). By current density–voltage characteristics, threshold voltage of the PLED device is found to be 6.5V.

Properties of Blue Polymer Light Emitting Diodes According to the Doping Concentrations of FIrpic Phosphorescence

PFO [poly(9,9-dioctylfluorene-2-7-diyl) end capped with N,N-Bis(4-methylphenyl)-4-aniline] based blue polymer light emitting diodes (PLEDs) that use a FIrpic [iridium(III) bis[4,6-(difluorophenyl) pyridinato-N,C20 picolinate] phosphorescent dopant as an emission layer were fabricated using a host-dopant system in order to achieve high luminance and efficiency properties. To find the optimum doping concentration, the FIrpic was doped at various concentrations, ranging from 1 to 4 wt%, into the PFO host. Blue phosphorescence PLEDs with the structure of ITO/PEDOT:PSS/PFO:FIrpic/TPBI/LiF/Al were prepared. The electrical and optical properties were investigated and compared in the prepared PLED devices with and without the FIrpic dopant. The luminance and the current efficiency were improved by introducing the FIrpic dopant. For the PLED device with a 2 wt% FIrpic concentration, the maximum luminance and current efficiency was found to be 4128 cd/m2 and 0.92 cd/A, respectively. The CIE color coordinates was (x, y = 0.19, 0.28), exhibiting a sky-blue color.

Enhancement of Operating Lifetime and Performance on Polymer Light Emitting Diode by Mg–Zn–F Passivation

Fluorine material MgF2 with Zn has recently been reported to have good encapsulation properties with high optical transmittance. We fabricated a 200 nm Mg–Zn–F passivation film for encapsulation of organic light emitting diodes using a radio frequency magnetron sputter. The water vapor transmission rate of Mg–Zn–F passivation film on poly(ethylene naphthalate) was determined using a calcium degradation test. Heating of the substrate during sputtering enhanced the device performance, and Mg–Zn–F film applied to a polymer light emitting diode device almost doubled its operating lifetime.

Synthesis and electroluminescence properties of white-light single polyfluorenes with high-molecular weight by click reaction

We reported a new way to synthesize single-chain white light-emitting polyfluorene (WPF) with an increased molecular weight using azide-alkyne click reaction. Four basic polymers with specific end-capping, which exhibited high-glass transition temperatures (Tg > 100 °C) and excellent thermal stability, were used as foundations of the WPF's synthesis; a blue-light polymer (PFB2) end-capped with azide groups can easily react with acetylene end-capped polymers (PFB1, PFG1, and PFR1, which are emitting blue-, green- and red-light, respectively) to form triazole-ring linkages in polar solvents such as N,N-dimethylforamide/toluene co-solvent at moderate temperature of 100 °C, even without metal-catalyst. Several WPFs that consist of these four basic polymers in certain ratios were derived, and the polymer light-emitting diode device based on the high-molecular weight WPF was achieved and demonstrated a maximum brightness of 7551 cd/m2 (at 12.5 V) and a maximum yield of 5.5 cd/A with Commission Internationale de l'Eclairage coordinates of (0.30, 0.33) using fine-tuned WPF5 as emitting material. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Controlling Ion Motion in Polymer Light-Emitting Diodes Containing Conjugated Polyelectrolyte Electron Injection Layers

The properties and function of an anionic conjugated polyelectrolyte (CPE)-containing ion-conducting polyethylene oxide pendant (PF(PEO)CO(2)Na) as electron injection layers (EILs) in polymer light-emitting diodes (PLEDs) are investigated. A primary goal was to design a CPE structure that would enable acceleration of the device temporal response through facilitation of ion motion. Pristine PLEDs containing PF(PEO)CO(2)Na exhibit luminance response times on the order of tenths of seconds. This delay is attributed to the formation of ordered structures within the CPE film, as observed by atomic force microscopy. Complementary evidence is provided by electron transport measurements. The ordered structures are believed to slow ion migration within the CPE EIL and hence result in a longer temporal response time. It is possible to accelerate the response by a combination of thermal and voltage treatments that "lock" ions within the interfaces adjacent to PF(PEO)CO(2)Na. PLED devices with luminance response times of microseconds, a 10(5) fold enhancement, can therefore be achieved. Faster luminance response time opens up the application of PLEDs with CPE layers in display technologies.

Integration of organic thin-film transistor and polymer light-emitting diodes

We investigated the technology and the related physical problems for integrating organic thin-film transistor （OTFT） and polymer light-emitting diode （PLED）. The OTFT uses Pentacene as the active medium, Ta2O5 as dielectric layer, Ta as gate electrode and Au as source/drain electrodes. The structure of PLED device is ITO/PEDOT:PEO （polyethylene oxide）/P-PPV or MEH-PPV/Ba/Al. The PEDOT:PEO, P-PPV and MEH-PPV film layers were obtained by using screen printing technology. The transistors have a threshold voltage of -7.1 V and carrier mobility of 0.91 cm2/（V·s）. The luminance of the PLEDs with P-PPV and MEH-PPV emissive layer when were driven by OTFT could reach 124 and 22 cd/m2, and the maximal luminous efficiency are 12.4 and 1.1 cd/A, respectively.

Patterning of polyfluorene based polymer light emitting diodes by reversal imprint lithography

The authors demonstrated the fabrication of patterned arrays of polymer light emitting diodes (PLEDs) with a polyfluorene based emissive layer using reversal imprint lithography. A stack of patterned metal and polymer films is transferred to a glass substrate in a single reversal imprint step. Two different techniques for coating the patterned Si mold are shown, one involving the spin coating of polymer layers directly onto the mold and one involving inking of the polymers onto the mold from a spin-coated PDMS inkpad. Using these techniques, PLED devices with a minimum feature sizes as small as 1μm and luminescence starting at 3.5V have been demonstrated.

Hyperbranched copolyfluorene from a 2,4,7-trifunctional fluorene monomer

A new hyperbranched (P1) and linear copolyfluorene (P2) were prepared from 2,4,7-trifunctional (branching) and 2,7-bifunctional fluorene monomer, respectively, by the Wittig reaction, followed with end-capping by aromatic oxadiazole groups, to study the effect of hyperbranch structure. The weight-average molecular weights (Mw) of P1 and P2, determined by gel permeation chromatography using polystyrene as standard, were 33,000 and 25,700, respectively. The polymers were readily soluble in common organic solvents and exhibited good thermal stability (Td > 400 °C). Optical properties, both in solution and film state, were investigated using absorption and photoluminescence (PL) spectra. In film state, the absorption and PL spectra peaked at 401–425 nm and 480–495 nm, respectively. The P1 showed energy funnel effect and enhanced fluorescence efficiency owing to hyperbranched structure and terminal oxadiazole groups. The HOMO and LUMO levels of P1 (P2), estimated from cyclic voltammograms, are −5.34 (−5.25) eV and −2.94 (−2.94) eV, respectively. Two-layer polymer light-emitting diodes devices (ITO/PEDOT/P1/Ca/Al) exhibited maximal luminance and luminous efficiency of 3630 cd/m2 and 0.78 cd/A, respectively, which are superior to its linear counterpart P2 (598 cd/m2, 0.11 cd/A). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5541–5551, 2007

Photodegradation and wavelength dependency of blue polymer light-emitting diode devices

A systematic study of the wavelength dependency of the photodegradation of blue polymer light-emitting diodes is reported. Clearly two different regions can be distinguished, inside the absorption band, where the strongest and permanent photodegradation is found, and at wavelengths above the edge of absorption, where at least partially reversible photodegradation is observed. Irradiation experiments on samples with and without a cathode show the effect of the cathode-polymer interface and its contribution to the degradation mechanisms involved.

Diffusion voltage in polymer light emitting diodes measured with electric field induced second harmonic generation

We apply electric field induced second harmonic (EFISH) to polymer light emitting diodes (PLEDs) and demonstrate the ability to determine the diffusion voltage in PLED devices. The EFISH signal is proportional to the square of the effective field, which is the sum of the diffusion voltage and the applied voltage. By minimizing the EFISH-signal as a function of the applied voltage, the diffusion voltage is determined by measuring the applied voltage that cancels out the diffusion voltage. The PLEDs are fabricated with indium tin oxide (ITO) as the hole injecting contact and two different electron injecting contacts, namely aluminum and calcium. The diffusion voltage originates from the rearranged charges caused by the difference in Fermi levels in the materials in the PLEDs. Different contacts will thus cause different diffusion voltages. We demonstrate here that the EFISH signal is proportional to the square of the effective field in both reverse and forward bias, and discuss the dependence on contact materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Patterning polymer light-emitting diodes by micromolding in capillary

Abstract Here we describe a method of patterning electroluminescent (EL) polymers using micromolding in capillary (MIMIC). MIMIC is the one of the lithographic methods based on the microfludics among the soft lithography using a patterned poly(dimethylsiloxane) (PDMS) elastomer. The patterned microstructures of poly(dioctylfluorene) (PDOF), poly( p -phenylenevinylene) (PPV), and poly(2-methoxy-5-2′-ethylhexyloxy)- p -phenylenevinylene) (MEH-PPV) were fabricated by MIMIC for polymer light-emitting diodes (PLEDs). In addition, the solution viscosity effect on MIMIC process was studied. Finally, a PLED device was fabricated using the patterned EL polymer microstructure and a patterned emission image was obtained.

Synthesis, Characterization, and Optoelectronic Properties of a Novel Polyfluorene/Poly(p-Phenylenevinylene) Copolymer

A novel conjugated polyfluorene/poly(p-phenylenevinylene) copolymer containing the pendant bis(4-alkoxyphenyl) groups in the C-9 position of every alternating fluorene unit has been synthesized and well structurally characterized. The photoluminescence spectrum of this polymer exhibits strong concentration and excitation wavelength dependence in solution. The excited triplet-state maximum of polymer occurs in the region of 460−540 nm with a lifetime of 65.8 μs. This copolymer displays a minor positive nonlinear absorption at the focus of the laser irradiation, suggesting possible reverse saturable absorption. The stable electroluminescent spectrum of the polymer light-emitting diode device based on this copolymer (device configuration, indium−tin oxide/Au/copolymer/LiF/Al) was obtained with a peak wavelength of 515 nm. The bright-green emission observed over the whole active area of the copolymer closely resembles the photoluminescence of the most concentrated solution (0.5 M) used. This suggests that cha...

Formation of bubbles on electrical contacts to polymer light-emitting diode devices

Bubble formation on calcium/aluminum contacts to bilayer poly(n-vinyl carbazole) (PVK)/poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT-PSS) polymer light-emitting diode (PLED) devices was studied using optical and electron microscopies and Auger electron spectroscopy (AES). The formation of bubbles is shown to correlate with pinholes in the metal contact thin film and with excess absorbed water in the presence of the calcium layer. AES data show oxidation at the Ca/Al interface during this process. Gas evolution and relaxation of compressive stress when voltage is applied to the PLED devices are postulated to cause formation of the bubbles.

Low work function of the (1000) Ca2N surface

Polymer diodes require cathodes that do not corrode the polymer but do have low work function to minimize the electron injection barrier. First-principles calculations demonstrate that the work function of the (1000) surface of the compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs 2.87eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N an interesting candidate to replace calcium as cathode material for polymer light emitting diode devices.

Enhanced luminance in polymer composite light emitting devices

We demonstrate that mixing insulating oxide nanoparticles into electroluminescent polymer materials results in increased current densities, radiances, and power efficiencies in polymer light emitting diode devices. For low driving voltages, an order of magnitude increase in current density and light output is achieved with minimal loss in device lifetime. At 5 V, we achieve radiances of 10 000 cd/m2 with external quantum efficiencies ∼1% for nanoparticle/MEH–PPV composite films.