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The Photoluminescence Properties of Vinylcarbazole – Vinylimidazole Copolymer Thin Films
Abstract
In our work we obtained the white light emitting material by copolymerization of vinylcarbazole with vinylimidazole. The ratio of vinylcarbazole to vinylimidazole comers in the obtained copolymer (PVKI) was 4:3. This article is concerned with the photoluminescence (PL) study of PVKI thin films deposited on the glass substrate by the dip-coating method. The obtained results were compared with the photoluminescence of thin films of poly (N-vinylcarbazole) (PVK) and poly (N-vinylimidazole) (PVI) deposited by the same technique. All PL spectra have been measured under steady state excitation (λ = 325 nm) in the temperature range from 13 K to 325 K.
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We show that all possible binary combinations of molecules from four different families of organics—a diamine derivative, N,N′-bis(3methylphenyl)-N,N′-diphenylbenzidine, an oxidiazole derivative, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, a substituted thiophene dioxide, 2,5-bis(trimethylsilyl thiophene)-1,1-dioxide, and poly(9-vinylcarbazole)—produce white or near-white emission. We suggest that this is due to exciplex formation, and that this is likely to be a general phenomenon for blends of blue-emitting aromatic organics. This implies that films of spin-coated blends of blue-emitting organics represent a general, simple, and cheap route to white-emitting organic light-emitting diodes. © 2001 American Institute of Physics.
This paper is concerned with the photoluminescence (PL) study of poly(N-vinylcarbazole) (PVK) thin films deposited on the glass substrate by the dip-coating method. The PL spectra have been measured under steady state excitation (He-Cd laser, 325 nm) in the temperature range from 13 to 300 K. All the samples being studied exhibit strong luminescence in broad temperature range. The main emission PL peak has maximum at 410 nm and is attributed to the excimer emission of PVK. We have observed a small red-shift of this peak with an increase of temperature. In all the films under investigation the thermal quenching of PL has been noticed. This behaviour is determined by the closeness of carbazole groups belonging to neighbouring chains because the interaction between them leads to nonradiative transitions. The PL spectra of PVK thin films annealed under iodine atmosphere have also been investigated. We have observed a decrease of PL for these films. We have concluded that the annealing of PVK under iodine atmosphere induces its degradation.
In this paper we present the Photoluminescence emission spectra (PL), photoluminescence excitation spectra (PLE). transmission (TS) and reflection (RS) spectra of tris(8-hydroxyquinoline) aluminium(III) (Alq(3)) layers grown by thermal evaporation deposition Method. All investigated samples exhibit strong luminescence in the wide temperature range from 13 K to room temperature and for different energies of excitation. In our experiments. we have focused on temperature dependence of photoluminescence. The thermal quenching of PL in the measured samples has been found. Me have observed also distinct oscillations from reflection and transmission measurements. The enemies of thermal activation estimated by means of. configuration coordinate diagram and also the layer thickness and refractive index are presented.
The emission properties of poly(N-vinylcarbazole) (PVK), poly(N-ethyl-2-vinylcarbazole), and poly(N-ethyl-3-vinylcarbazole) have been investigated. Dilute fluid solutions of all three vinylcarbazole polymers are observed to exhibit a structureless emission band at energies approximately 5400 cm−1 to the red of the (0,0) band of the normal molecular fluorescence of isolated monomer compounds. On the basis of earlier work appropriate model compounds, this emission is assigned as that from an intrachain excimer state in which two interacting pendant groups, one of which is electronically excited, achieve an overlapping, eclipsed, sandwichlike configuration. The emission spectrum of PVK is characterized by the appearance of a second structureless emission band which appears at higher energy than the intrachain excimer fluorescence common to all three vinylcarbazole polymers. This emission band is unique to PVK and under certain conditions to its model compound, 1,3-bis(N-carbazolyl)propane. This emission is assigned as that from a second intrachain excimer. Contrary to the lower energy intrachain excimer, the polymer conformation appropriate for formation of this state exists prior to the initial excitation step.
The emission properties of poly(N-vinylcarbazole) (PVCz) prepared by radical and cationic polymerization (PVCz(r) and PVCz(c) respectively), the oligomers of N-vinylcarbazole, and poly[2-(N-carbazolyl)ethyl vinyl ether] (PCzEVE) were investigated in solutions. The fluorescence spectrum of PVCz(r) was different from that of PVCz(c). This seems to be attributable to the difference in the tacticity between them. On the basis of this result, the structure of the second excimer site is discussed. PCzEVE, in which the carbazolyl chromophores are widely spaced on the skeletal chains by –O–CH2–CH2– bonds, showed an interesting, new, structureless emission band with a peak at 27000 cm−1. This was different from both the monomeric structured fluorescence and the sandwich-like excimer one which were observed for the model compounds and the polymers. The emission properties of PCzEVE were unique among the vinyl polymers with aromatic chromophores widely spaced on the skeletal chain.
The effects of side-chain length and shape at C-9 position of polyfluorenes on the photoluminescence spectra and the fluorescence lifetimes are investigated for blue light-emitting poly(n-vinylcarbazole)/polyfluorene blends. As the side-chain length of alkyl-substituted polyfluorenes and of bulky cyclohexyl-substituted polyfluorenes increases, the fluorescence lifetime of the blends increase from 1 to 10 ns and the portion of PVK component contributing to the fluorescence lifetime of the blends increases. However, PL intensity of the blend decreases. This provides the side-chain length and shape of polyfluorene could affect the efficiency of energy transfer.
The photoluminescence (PL) and electroluminescence (EL) properties of two novel light-emitting anthracence derivatives containing bis(ethynylphenyl oxadiazole) or bis(vinylphenyl oxadiazole) substituents (ANOs) blended with poly(9-vinylcarbazole) (PVK) are characterized. The energy transfer process in the blends is discussed. By employing 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT) and tris(8-hydroxyquinoline) aluminum (Alq3) as electron transporting layers (ETLs) in the EL devices, remarkable improvements in EL efficiency were observed. The effect of the transporting layer on the EL intensity of the devices is also discussed.
The sandwich structures have been obtained by deposition of organic thin films on commercial tin oxide. Then the organic films were covered by a thin aluminium film. All the films were deposited by evaporation. The organic component was either a thin (150nm) poly(N-vinylcarbazole) (PVK) film or a bilayer constituted of a thin PVK layer covered with a 8-hydroxyquinoline, aluminium (Alq3). When PVK is deposited by classical thermal evaporation there is a shortening effect of the chain length, however, the main properties of the vinyl-carbazole molecules are preserved. These films are photoluminescent with blue light emission. The I–V characteristics of the sandwich structures are typical of such organic sample. They exhibit a rectifying behaviour, with a forward polarisation when the SnO2 electrode is positively biased. They also exhibit electroluminescence. When a single PVK film is used, the signal is quite faint. It is improved when an organic bilayer is used. However, simultaneously there is a red shift of the signal.
Polymer light-emitting diodes (LEDs) have been fabricated from poly(N-vinylcarbazole) (PVK) or blends of PVK and 3-(2-methacryloylethoxycarbonyl)-7-diethylcoumarin (MK). Thin polymer layers needed for the construction of diodes can be prepared by casting from a solution using the spin-coating technique. No subsequent processing or heat treatment is required. Mixtures of PVK and MK, with the molar concentration of the latter equal to 0.5%, 1%, 2%, and 8% respectively, were used. In order to achieve bipolarity in the single polymer emitter layer, PVK was thoroughly mixed with MK. This approach enabled us to improve the efficiencies of the prepared LEDs. It was found that the blend of 99.5% of PVK and 0.5% of MK exhibited the best quantum efficiencies.
The PL and EL properties of the polymer blends of PVK and blue PPV copolymer were studied. Considerable enhancement of both the photoluminescent and the electroluminescent intensity were observed by using the polymer blends as emission layer in the LED devices. The energy transfer process and the formation of exciplex in the polymer blends were also discussed.
The current-voltage (I–V) dependence of PVCz polymer films has been analyzed using transport models for insulating material. Space charge limited current, Schottky current, Poole-Frenkel current and field assisted tunnelling current have been examined for four types of device with different combinations of PVCz and PVCz:perylene layers; the devices with multi-layered structures are: (ITO/PVCz:perylene/ PVCz/Mg-Ag), (ITO/PVCz/PVCz:perylene/Mg-Ag), (ITO/PVCz/PVCz/Mg-Ag) and (ITO/PVCz:perylene/PVCz:perylene/Mg-Ag). Below the threshold voltage of electroluminescence (EL) operation, the current is mainly limited by space charge in both undoped PVCz and perylene doped PVCz emitting layers. Above the EL threshold, the I–V characteristic is controlled by field assisted tunnelling. EL is generated in the polymer layer near the metal (Mg-Ag) interface by carrier injection through the tunnelling limited current. It was made clear that the EL emission in the PVCz multi-layer devices took place at electric fields higher than 106 V/cm.
Carrier mobility and barrier height at negative electrode have been studied by considering temperature dependence of current–voltage characteristics for the electroluminescent (EL) devices fabricated from poly(N-vinylcarbazole) (PVCz) double-layered films doped with Cz. Luminance was more than 3000 cd/m2 at 110 K and 1500 cd/m2 at 300 K without electron transport molecules. Temperature dependence of the hole mobility in Cz doped PVCz has been analyzed and the activation energy for carrier hopping has been derived; 0.02 and 0.05 eV for undoped PVCz and 0.01 eV for Cz doped PVCz. The tunneling barrier height at negative electrode has been analyzed based on the position and applied-voltage-dependent electric field derived from the combination of space charge limited current and tunneling current schemes. The barrier height for undoped PVCz:perylene was 0.46 eV and that for Cz doped PVCz:perylene was 0.38 eV. The intrinsic tunneling has been confirmed from the temperature dependence of barrier height of heavily Cz doped PVCz:perylene device for the first time. In the case of undoped or lightly PVCz:perylene devices, tunneling was observed at lower temperature and charge injection became temperature-dependent with increasing temperature.
Bright single‐layer white light‐emitting organic electroluminescent devices were developed by using dye‐dispersed poly(N‐vinylcarbazole) (PVK). The active layer consists only of one polymer layer that is simply sandwiched between two electrodes, indium‐tin oxide, and Mg:Ag. In order to achieve bipolarity in the single polymer emitter layer, PVK was molecularly dispersed with electron‐transporting additives such as 2‐(4‐biphenyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole. In addition, several fluorescent dyes, having different emission colors, were dispersed as emitting centers. By adjusting the concentration of the fluorescent dyes, white light with a maximum luminescence of 4100 cd/m2 was obtained, which is the brightest white light ever observed for organic electroluminescent devices. © 1995 American Institute of Physics.
Electroluminescent devices were fabricated using poly(N‐vinylcarbazole) (PVK) as a hole‐transporting emitter layer and a double layer of 1,2,4‐triazole derivative (TAZ) and tris(8‐quinolinolato)aluminum(III) complex (Alq) as an electron transport layer. A cell structure of glass substrate/indium‐tin‐oxide/PVK/TAZ/Alq/Mg:Ag was employed. In this cell structure, carrier injection from the electrodes to the PVK layer and concomitant electroluminescence from PVK were observed. Blue emission peaking at 410 nm and a luminance of 700 cd/m2 were achieved at a drive voltage of 14 V.
The interaction between poly(N-vinylcarbazole) (PVK) and rare earth (RE) complexes, such as Eu(thenoytrifluoacetonato) 4-(monopyridinium) (Eu(TTA)4Py). Eu(nitrate)3 (monophenanthroline) (Eu(NO3) 3phen) and Tb(acetylacetonato) 3 (monophenanthroline) (Tb(AcA)3phen), was investigated in solution as well as in films using the photoluminescence (PL) spectrum. In chloroform solution, the fluorescent intensity of the Eu(NO3) 3phen was enhanced by PVK; however, the fluorescent intensity of the Tb complex was greatly quenched by PVK. Strongly characteristic emissions of Eu3+ and Tb3+ were observed in Eu and Tb complex-dispersed PVK films, respectively. The excitation spectra of Eu complex-dispersed PVK films and Tb complex-dispersed PVK film are very similar to that of the pure PVK film, indicating that effective energy transfer occurs from PVK to the RE complexes. Based on the above experimental results, three types of organic electroluminescence (EL) devices with structure of ITO/Eu(NO3)3phen:PVK/OXD-7/Al, ITO/Eu(TTA)4Py:PVK/OXD-7/Al and ITO/ Tb(AcA)3phen:PVK/OXD-7/Al (ITO = indium-tin oxide) were fabricated. Bright red emission can be observed in the first and second devices while bright green light was emitted from the latter device.