Conventional and inverted organic light emitting diodes based on bright green emmisive polyfluorene derivatives (original) (raw)
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Synthesis and Optical Properties of Light-Emitting Polyfluorene Derivatives
Molecular Crystals and Liquid Crystals, 2011
The emitting-polymers, Polyfluorene (PF) and Poly(fluorene-benzothiadiazole-quinoline) [PF-BT-QL], have been synthesized by the Suzuki coupling reactions. The properties of polymers were characterized using UV-Vis spectroscopy, GPC, DSC, TGA, Photoluminescence (PL), Fluoresence (FL), Electroluminescence (EL) spectroscopy. The synthetic polymers were soluble in common organic solvents and easily spin-coated onto the indium-tin oxide (ITO)-coated glass substrates. Light-emitting devices (LEDs) with ITO/PEDOT:PSS/polymer/LiF/Al configuration were fabricated, and the devices using copolymers showed red shift EL spectra relative to that of PF. The turns on voltages of copolymers were lower than that of PF.
A study of an efficient blue light-emitting diode based on a fluorescent aryl polyfluorene (aryl-F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO3 as electron- and hole-injecting electrodes, respectively. Charge-carrier balance and color purity in these structures are achieved by incorporating poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine (TFB) into aryl-F8. TFB is known to be a hole-transporting material but it is found to act as a hole trap on mixing with aryl-F8. Luminance efficiency of ≈6 cd A−1 and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl-F8. Study of charge injection and transport in the single-carrier devices shows that the addition of a small fraction of hole traps is necessary for chargecarrier balance. Optical studies using UV–vis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl-F8, which is the emitting material in these devices. Luminance efficiency of up to ≈11 cd A−1 and EQE values of 5.7% are achieved in these structures with the aid of improved out-coupling using index-matched hemispheres.
Thin Solid Films, 2007
For organic light-emitting diodes (OLEDs) applications, we have investigated novel polymers, using substituted polyacetylenes (PA), poly(1-(fluorophenyl)-2-(alkylcyclohexylphenyl)acetylene) (PDPA-nF) (n = 1 or n = 2) which exhibit air stability, better solubility in common organic solvent and higher luminescence than polyacetylene. In this study, we have used poly[(1-(3,4-difluorophenyl)-2-(4-pentylcyclohexylphenyl) acetylene)] (PDPA-2F) as an emitter in OLEDs and their performance was determined by measuring the current-voltage-luminance characteristic. The devices have a maximum brightness of 827 candela (cd)/m 2 at 12 V and a maximum current efficiency of 0.78 cd/A at 9 V with a maximum luminescence at 536 nm. Influence of the metal electrode on the charge injection was studied using several cathode configurations (Ca, Al and Au) for the devices. Furthermore, the charge injection and transport processes were correlated to the presence of traps inside the polymer, determined by deep level transient spectroscopy (DLTS).
Applied Physics Letters, 2008
We used a water-soluble bis͑fluorinated phenyl azide͒ to cross-link a poly͑ethylene dioxythiophene͒:poly͑styrene sulphonic acid͒ ͑PEDOT:PSS͒, hole-injection layer, with a view to its future use with water-soluble emitters. To enable direct comparison with conventional PEDOT:PSS, we studied the cross-linked films in diodes incorporating the organic-solvent soluble polymer poly͑9,9Ј-dioctylfluorene-alt-benzothiadiazole͒. Kelvin probe characterization of the PEDOT:PSS and electroabsorption measurements of the devices consistently show a 0.2 eV increase of the PEDOT:PSS work function upon cross-linking. We also observe a 70-fold reduction in resistivity, an increase of the current above threshold and a decrease of the "leakage" current below threshold.
Light-emitting properties of photo-curable polyfluorene derivatives
Synthetic Metals, 2009
An alternating copolymer composed of bis-(4-octyloxyphenyl)fluorene and bis((3-hexyloxy-3ethyl)oxetane) fluorene [poly(PF-alt-OXTF)], and a terpolymer composed of the above two monomers with N-hexylphenotiazine [poly(PF-co-OXTF-co-PTZ)] have been synthesized by Suzuki coupling polymerization. The synthesized poly(PF-alt-OXTF) and poly(PF-co-OXTF-co-PTZ) films became insoluble after UV irradiation in the presence of a proper photo-acid generator. The UV-vis absorption and PL emission spectra of the polymers before and after photo-cross-link reaction were almost same. The EL devices were constructed with ITO/PEDOT:PSS/polymer/LiF/Al configuration. The EL devices using the photo-cross-linked polymer films also showed lower operating voltages than the devices using the corresponding polymer films without cross-link. Moreover, the EL device of the poly(PF-co-OXTF-co-PTZ) film with photo-cross-link showed better device performances than the device without cross-link, and the maximum brightness and power efficiency of the device were 4750 cd/m 2 and 0.68 cd/A, respectively. Furthermore, we obtained a well-patterned thin film with poly(PF-co-OXTF-co-PTZ) after a successive lithographic process.
Organic Electronics, 2008
For green-emitting Ir complex-doped polymer devices, high efficiencies have been achieved only when non-conjugated poly(N-vinylcarbazole) (PVK) was used as the host polymer. It is commonly believed that conjugated polyfluorenes (PFO) are not suitable for the use as host of green phosphorescent complexes due to the presence of the low-lying triplet state. In this paper we reported that despite very inefficient PL phosphorescent emission of Ir(Bu-PPy) 3 in the PFO films, strong green electroluminescence (EL) and high device efficiencies have been observed for devices from such blend films when multilayer device with PVK was used as anode buffer. We analyze the PL spectra in steady state and transient response and found that the energy transfer via PVK interlayer plays important role in the efficient EL performance. This fact clearly indicates the efficient electrophosphorescence might be achieved even if the triplet energy of phosphorescence dye is higher than that of the host. These findings could significantly broaden our selection in polymer hosts for phosphorescent devices.
Optical Materials, 2017
Monomers and oligomers containing electronically isolated 4-aryl-7-phenylfluorene fragments have been synthesized by the multi-step synthetic route. The materials were characterized by thermogravimetric analysis, differential scanning calorimetry and electron photoemission technique. The oligomers represent materials of very high thermal stability having initial thermal degradation temperatures in the range of 402e412 C. The glass transition temperatures of the amorphous oligomers were in the rage of 97e129 C. The electron photoemission spectra of thin layers of the oligomeric materials showed ionization potentials in the range of 5.7e6.1 eV. Hole injecting/transporting properties of the electroactive oligomers were tested in the structures of organic light emitting diodes with tris(quinolin-8-olato) aluminium as a green emitter. The device containing hole-transporting material with 4-biphenyl-7phenylfluorene electrophores exhibited the best overall performance with low turn on voltage of 4.4 V, high current efficiency exceeding 3.6 cd/A and maximum brightness exceeding 3200 cd/m 2 .