Doped PVK Research Articles

Eletroactive Properties of Pure and Malachite Green Doped PVK Samples

Eletroactive Properties of Pure and Malachite Green Doped PVK Samples

The impact of 1 wt% of Ir(ppy)3 on trapping sites and radiative recombination centres in PVK and PVK/PBD blend seen by thermoluminescence

Thermoluminescence (TL) of poly(N-vinylcarbazole) (PVK) and PVK+40wt% of 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) blend, both doped with 1wt% of fac-tris(2-phenylpiridine) iridium (Ir(ppy)3), were studied. TL spectra, registered in temperature range 15–320K, reveal that trapping sites localised on the matrices and on Ir(ppy)3 molecules exist in both investigated systems. The traps localised on Ir(ppy)3 have depth about 0.4eV and they dominate in the doped PVK. At the same concentration of the dopant molecules in the PVK/PBD matrix, the traps located on the matrix dominated. Contribution of deep traps on Ir(ppy)3 is much smaller. After doping, a shift of TL peak, associated with the release of carriers from the matrix traps, to higher temperature was observed. It indicates on the presence of slightly deeper traps as compared with those in the neat matrices. The effect is caused by the interaction of trapped carriers and randomly oriented high permanent dipoles of Ir(ppy)3. In addition, the experiments of spectrally resolved TL (SRTL) provide evidence that the presence of the dopant creates a new channel of energy transfer. Even at concentration of 1wt% of Ir(ppy)3, these molecules act as emission centres which effectively compete with other centres of radiative recombination present in PVK and PVK/PBD systems.

Ligand exchange leads to efficient triplet energy transfer to CdSe/ZnS Q-dots in a poly(N-vinylcarbazole) matrix nanocomposite

Upon exchanging long chain alkylamine ligands with a carbazole terminated fatty acid as 6-(N-carbazolyl)-hexanoic acid (C6) and 11-(N-carbazolyl) undecanoic acid (C11), efficient photoluminescence (PL) of CdSe/ZnS colloidal quantum dots (QDs) was observed upon excitation in the absorption band of the carbazole moiety at 330 nm. This effect, which occurred both in solution and in a poly(N-vinylcarbazole) (PVK) matrix doped with the QDs, is attributed to sensitization of the QDs by PVK and the ligands. More efficient energy transfer was observed in solution for the shorter ligand (C6) capped QDs, due to a shorter average distance between the donor (carbazole) and the acceptor (QD). The binding of C6 and C11 to the QDs was confirmed by 1H solution nuclear magnetic resonance, which showed line broadening of the carbazole signal due to a decrease of the mobility of the carbazoles upon binding to the QDs compared with the sharp lines observed for the free molecules in solution. In doped PVK films, the significant enhancement of the energy transfer to the QD core could also be related to a better miscibility between the QDs and the PVK as confirmed by optical transmission and confocal microscopy images. In contrast to the experiment in solution, the overall energy transfer in the doped films was found more efficient for QDs capped with C11. To study in more detail the energy transfer between the carbazole moieties and the QDs, time-resolved fluorescence measurements were performed for solutions of C6 and C11, capped QDs and PVK films doped with the QDs. In contrast to the large enhancement of the QD emission indicated by steady-state PL spectra, the latter experiments suggested only a relatively low efficiency (19.6% and 10.8%) for singlet transfer from the carbazole ligands to the QDs. This suggests that the enhancement of the QD emission must be largely due to triplet transfer.

Highly Efficient Phosphorescent White Organic Light-Emitting Devices with a Poly(N-vinylcarbazole) Host Layer

We have fabricated phosphorescent white organic light-emitting devices (WOLEDs) with a spin-coated poly(Nvinylcarbazole) [PVK] host layer. Iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,<TEX>$C^{2'}$</TEX>]picolinate (FIrpic), tris(2-phenylpyridine)iridium(III) [<TEX>$Ir(ppy)_3$</TEX>], and tris(2-phenyl-1-quinoline)iridium(III) [<TEX>$Ir(phq)_3$</TEX>], were used as the blue, green, and red guest materials, respectively. The PVK was mixed with FIrpic, <TEX>$Ir(ppy)_3$</TEX>, and <TEX>$Ir(phq)_3$</TEX> molecules in a chlorobenzene solution and spin-coated in order to prepare the emission layer; 3-(4-biphenylyl)-4-phenyl-5-(4-tertbutylphenyl)-1,2,4-triazole (TAZ) was used as an electron transport material. The resultant device structure was ITO/PVK:FIrpic:<TEX>$Ir(ppy)_3:Ir(phq)_3$</TEX>/TAZ/LiF/Al. The electroluminescence, efficiency, and electrical conduction characteristics of the WOLEDs based on the doped PVK host layer were investigated. The maximum current efficiency of the three wavelength WOLED with the doped PVK host was 19.2 cd/A.

Enhanced emission from Alq3 in PVK/Alq3 blend films based on resonance energy transfer

AbstractIn this study, we present the optical properties of the poly(N‐vinylcarbazole) (PVK) films doped with tris(8‐hydroxyquinoline) aluminum (Alq3). It has been found that the photoluminescence (PL) spectrum of the PVK film overlaps well with the absorption band of Alq3. When excited at the absorption maximum of PVK, the doped PVK films show enhanced emissions from the Alq3 component. The PL enhancement is considered to be due to energy transfer from PVK to Alq3 in the doped PVK films by analyzing the PL, PL excitation, and time‐resolved fluorescence spectra. The energy transfer efficiency is increased with increasing the concentration of Alq3 in the doped PVK film according to time‐resolved fluorescence spectra. The energy transfer process has been discussed according to the Förster mechanism. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1772–1777, 2009

A CORRESPONDING RELATION BETWEEN ELECTROLUMINESCENT INTENSITY AND THE ENERGY AND CHARGE TRANSFER IN DOPED ORGANIC ELECTROLUMINESCENT DEVICES

In doped organic electroluminescent devices (OLEDs), the changes of the fluorescence spectra occurred due to the energy and charge transfer from guest to host. A quantitative relation between the energy and charge transfer for doping OLEDs was studied by means of a Hamiltonian model. It was found that there is a corresponding relation between the EL intensity of doped PVK and the amount of the transferred charge and the change of the energy derived from charge transfer with dopant concentration, and the more the amount of the transferred charge and the decreasing of the energy from charge transfer, the higher the EL intensity in doped OLEDs. We can deduce that the efficient energy transfer results from the transferred charge between PVK and perylene. Based on the transport mechanism of carriers in conjugated polymer, an expression of conductivity was presented. Calculated results indicated that there is also a corresponding relation between the EL intensity of doped PVK and conductivities with dopant concentration. The larger the conductivities, the higher the EL intensity in doped OLEDs.

Spectral Characteristics of White Organic Light-emitting Diodes Based on Novel Phosphorescent Sensitizer

White organic light-emitting diodes were fabricated by using a novel phosphorescence bis(1,2-dipheny1-1H-benzoimidazole)iridium(acetylacetonate)[(pbi)2Ir(acac)] as sensitizer and a fluorescent dye of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) codoped into a car-bazole polymer of poly(N-vinylcarbazole) (PVK). Through characterizing the UV-Vis absorption spectra, the photoluminescence spectra of (pbi)2Ir(acac) and DCJTB, and the electroluminescence spectral properties of the WOLEDs, the energy transfer mechanisms of the codoped polymer system were deduced. The results demonstrate that the luminescent spectra with different intensity of (pbi)2Ir(acac) and DCJTB were co-existent in the EL spectra of the blended system, which is ascribed to an incomplete energy transfer process in the EL process. The efficient Forster and Dexter energy transfer between the host and the guests enabled a strong yellow emission from (pbi)2Ir(acac) and DCJTB, where (pbi)2Ir(acac) plays an important role as a phosphorescent sensitizer for DCJTB. With the blue emitting-layer of N,N-diphenyl-N,N-bis(1-naphthyl)(1,1-biphenyl)-4,4-diamine, the codoped system device achieved white emission. The codoped system showed that its Commissions Internationale de 1Eclairage coordinates were more independent of the variation of bias voltage than those of phosphorescent doped PVK systems.

Photoluminescence and electroluminescence of methoxy and carboethoxy derivatives of 1,3-diphenyl-1 H-pyrazolo[3,4- b]quinoline

A series of methoxy (MO) and carboethoxy (CE) derivatives of 1,3-diphenyl-1 H-pyrazolo[3,4- b]quinoline ([DPPQ]) are characterized by spectroscopic methods. All dyes show the photoluminescent spectra which are highly solvatochromic. In the case of 6MO[DPPQ] and 6CE[DPPQ] the emission bands are broad and shifted to the red with increasing of solvent polarity whereas the dyes 6MO1pMO[DPPQ] and 6MO13pMO[DPPQ] exhibit a reverse solvatochromism. The large difference between the excited- and state-dipole moments indicates a strong electron transfer effects in all dyes. The EL spectra are obtained for the fabricated OLEDs with a general structure of EL device ITO/PVK:6X[DPPQ]/Ca/Al. The blue emission originating from PVK host matrix appears to be quenched in EL spectra of doped PVK matrix giving rise to emission in blue, blue-green or green spectral regions. The obtained results demonstrate that a series of newly synthesized methoxy and carboethoxy [DPPQ]-derivatives may be considered as promising materials for electroluminescent applications.

Electroluminescence of 6-R-1,3-diphenyl-1 H-pyrazolo[3,4- b]quinoline-based organic light-emitting diodes (R = F, Br, Cl, CH 3, C 2H 3 and N(C 6H 5) 2)

A series of 6-substituted-1,3-diphenyl-1 H-pyrazolo[3,4- b]quinoline, 6-R-DPPQ[R = F, Br, Cl, CH 3, C 2H 3, N(C 6H 5) 2] has been synthesized and characterized by spectroscopic methods. The photoluminescence (PL) is measured in the organic solutions of different polarities and found to be highly solvatochromic. The large difference between the excited- and state-dipole moments indicates on strong intramolecular charge transfer. The EL spectra are obtained for the fabricated OLEDs with a general structure of EL device ITO/PVK:6-R-DPPQ/Ca/Al. The blue emission originating from PVK matrix appears to be quenched in the EL spectra of doped PVK:6-R-DPPQ layer giving rise to emission in blue-green or green spectral regions. The obtained results demonstrate that a series of newly synthesized DPPQ-derivatives may be considered as prospective materials for electroluminescent applications.

Energy transfer and triplet exciton confinement in polymeric electrophosphorescent devices

AbstractEnergy transfer and triplet exciton confinement in polymer/phosphorescent dopant systems have been investigated. Various combinations of host‐guest systems have been studied, consisting of two host polymers, poly(vinylcarbazole) (PVK) and poly[9,9‐bis(octyl)‐fluorene‐2,7‐diyl] (PF), blended with five different phosphorescent iridium complexes with different triplet energy levels. These combinations of hosts and dopants provide an ideal situation for studying the movement of triplet excitons between the host polymers and dopants. The excitons either can be confined at the dopant sites or can flow to the host polymers, subject to the relative position of the triplet energy levels of the material. For PF, because of its low triplet energy level, the exciton can flow back from the dopants to PF when the dopant has a higher triplet energy and subsequently quench the device efficiency. In contrast, efficient electrophosphorescence has been observed in doped PVK films because of the high triplet energy level of PVK. Better energy transfer from PVK to the dopants, as well as triplet exciton confinement on the dopants, leads to higher device performance than found in PF devices. Efficiencies as high as 16, 8.0, and 2.6 cd/A for green, yellow, and red emissions, respectively, can be achieved when PVK is selected as the host polymer. The results in this study show that the energy transfer and triplet exciton confinement have a pronounced influence on the device performance. In addition, this study also provides material design and selection rules for the efficient phosphorescent polymer light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2681–2690, 2003

Polymer phosphorescence device using a new green emitting Ir(III) Complex

We have synthesized a new green Ir(III) complex fac‐tris‐(3‐methyl‐2‐phenyl pyridine)iridium(III) Ir(mpp) 3 and fabricated phosphorescent polymer light‐emitting device using it as a triplet emissive dopant in PVK. Ir(mpp) 3 showed absorption centered at 388 nm corresponding to the 1 MLCT transition as evidenced by its extinction coefficient of the order of 10 3 . From the PL and EL spectra of the Ir(mpp) 3 doped PVK film, the emission maximum was observed at 523 nm, due to the radiative decay from the 3 MLCT state to the ground state, confirming a complete energy transfer from PVK to Ir(mpp) 3 . The methyl substitution has probably caused a red shift in the absorption and emission spectrum compared to Ir(ppy) 3 . The device consisting of a 2 % doped PVK furnished 4.5 % external quantum efficiency at 72 cd/m 2 (current density of 0.45 mA/cm 2 and drive voltage of 13.9 V) and a peak luminance of 25,000 cd/m 2 at 23.4 V (494 mA/cm 2 ). This work demonstrates the impact of the presence of a methyl substituent at the 3‐position of the pyridyl ring of 2‐phenylpyridine on the photophysical and electroluminescence properties.

Photoconductivity of [60]fullerene derivative doped PVK

1,2-(1′, 1′,2′,2′-tetracyanomethanoxymethano) [60] fullerene has been synthesized. It exhibits stronger ability of accepting electron than pure [60]fullerene. A film of PVK doped with 1.6% by weight of this C60-derivative was prepared and characterized. The measured photoinduced discharge curves and the photoconductivity spectra of the film demonstrate that the photoconductivity of PVK doped with this [60]fullerene derivative is higher than that of PVK doped with pure C60.

Flexible Blue Light Emitting Diodes Made from Dye Doped Poly(N-Vinylcarbazole) with Multilayer Structure

Flexible blue light emitting diodes (LEDs) were fabricated by using poly(N-vinylcarbazole) (PVK) doped with high fluorescence dye, 1, 1, 4, 4-tetraphenyl-1,3-butadiene (TPB), as a emitter layer, a layer of tris(8-quinolinolato)aluminum (III) (Alq3) as an electron-transporting layer, and a layer of 2-(4-biphenylyl)-5-(4-tertbutypheny)-1-3,4-oxadiazole (PBD) as hole-blocking layer; a cell structure of flexible substrate/indium-tin-oxide /PVK:TPB/PBD/Alq3/Al was employed. In this cell structure, carrier injection from the electrodes to the doped PVK layer and concomitant electroluminescence from doped PVK were observed at room temperature with dc bias voltage of 4 V. Blue emission peaks at about 465 nm. This kind of LED is about 200 μm thick and can be curled or bent repeatedly at sharp angles without failure.

In situ IR spectroscopic study of poly(N-vinylcarbazole) film during electrochemical doping

In situ IR spectroscopic study was made on the electrochemical doping of poly(N-vinylcarbazole) (PVK) by the subtractively normalized interfacial Fourier transform technique. The results show that the doping of ClO4- anion is limited by crosslinking of the polymer. The structure of doped PVK film depends on electrode potential and reaction time. Maximum doping of PVK may be achieved using a low oxidation potential for a proper reaction time. (C) 1996 John Wiley & Sons, Inc.