PVK Films Research Articles

Luminescence properties and energy transfer in the mixed thin film of Alq3 and PVK

In this paper, the properties of fluorescence were studied of the thin films with different component ratios of polyvinyl carbazole(PVK)and Alq3. The energy transfer between PVK and Alq3 was studied through analysis of absorption spectra, excitated spectra, emission spectra of the thin films. When the content of Alq3 was high enough，the peak of Alq3 was quite evident in the abs orption spectra and excitated spectra, at the same time the emission spectra was the characteristic emission band of Alq3.When the content of PVK was high enough, the peak value of PVK is much greater than that of Alq3. At this time the extraordinary strong characteristic emission band of Alq3 and the feeble charac teristic emission band of PVK was observed. It is proved that the energy can transfer from PVK to Alq3 in a wide concentration range and it is most efficient at the concentration of 1∶7. A model Hamitonian based on single chain model including the dopant was put forward to simulate the experiment .This model can explain the experimental results very well.

Excellent ambipolar photoconductivity of PVK film doped with fluoroperylene diimide

We report a novel bipolar charge transport material in a single-layer photoreceptor. Using the so-called photoinduced xerographic discharge technique, photoconductivity of the material was measured in positive charging and negative charging conditions, respectively. In both cases, it exhibited excellent photoconductivity. The material was obtained by doping poly( N-vinyl carbazole) (PVK) with a novel organic electron acceptor of N, N ′-diperfluorophenyl-3,4,9,10-perylenetetracarboxylic diimide (DFPP). The combined efforts of UV–Vis absorption, DSC, XPS and cyclic voltammetry characterizations demonstrated that D–A complex was formed between DFPP (A) and PVK (D) due to their different electron affinities. The data also suggested that electron transport channels composed of DFPP molecules might exist in the film, providing the film not only high exciton dissociation efficiency but also good electron transport ability. The electronic and structural properties of the DFPP-doped PVK film are also reported.

Synthesis and luminescent properties of novel Eu(III)-chelated complexes and their silicon-based copolymers

We have synthesized several kinds of new Eu(III)-chelated complexes to obtain pure red emission and to improve the carrier-transport properties. The PL intensity of the Eu complex-dispersed PVK film gradually increased with its dopant amount, due to the efficient energy transfer from PVK to Eu (III)-chelated complexes. Also, the first silicon-based copolymers with the Eu-chelated complexes in the main chain were successfully synthesized by Pd-catalyzed Heck reaction. Its PL spectrum shows a strong band at 613 nm in the red region, corresponding to the 5D 0→ 7F 2 transition of the trivalent Eu ion.

Light‐emitting diodes and lasers based on polymer films doped with small organic molecules and rare‐earth complexes

AbstractWe investigated the lasing properties of optically pumped polymer films. Amplified spontaneous emission (ASE) around 400 nm was observed in polymer films of polystyrene (PS) and poly(N‐vinylcarbazole) (PVK) doped up to 20% with the hole‐transporting organic molecule N,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine (TPD). Thus, TPD‐based films are candidates for blue‐emitting organic diode lasers. Films containing several semiconducting organic molecules and polymers and rare‐earth complexes were also investigated. Energy transfer was observed in PVK films doped with various europium and samarium complexes. PS films containing the electron‐transporting organic molecule 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole and small amounts of TPD also showed energy transfer to the europium complexes, but not to the samarium ones. None of these films demonstrated ASE; therefore, they are not appropriate for lasing purposes. However, because rare‐earth ions have very sharp emission spectra, these materials are candidates for very monochromatic light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2706–2714, 2003

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

Improved Efficiency of Single‐Layer Polymer Light‐Emitting Devices with Poly(vinylcarbazole) Doubly Doped with Phosphorescent and Fluorescent Dyes as the Emitting Layer

AbstractWe demonstrate novel organic light‐emitting diode (LED) materials that contain a green phosphorescent dye (dmbpy)Re(CO)3Cl (dmbpy = 4,4′‐dimethyl‐2,2′‐bipyridine), and a red fluorescent dye 4‐dicyanomethylene‐6‐(p‐dimethylaminostyryl)‐2‐methyl‐4H‐pyran (DCM) as dopants and polyvinylcarbazole (PVK) as the host. The photoluminescence (PL) and electroluminescence (EL) properties of these complex materials were studied. The energy transfer efficiency from PVK host to DCM is increased by the (dmbpy)Re(CO)3Cl co‐dopant, which has an emission energy between that of PVK and DCM. The (dmbpy)Re(CO)3Cl, which emits a long‐lived phosphorescence, is used as an energy coupler, providing the possibility to harvest both singlet and triplet energy in the devices. The pure red emission from DCM was observed from PL and EL spectra of (dmbpy)Re(CO)3‐Cl(> 2.0 wt.‐%):DCM(> 0.5 wt. %) doped PVK films, demonstrating an efficient energy transfer from PVK and (dmbpy)Re(CO)3‐Cl to DCM. By optimizing the concentration of DCM and (dmbpy)Re(CO)3Cl in PVK, a maximum EL quantum efficiency of 0.42 cd A–1 at a current density of 9.5 mA cm–2 was obtained. The EL quantum efficiency of the doubly doped device is significantly enhanced in comparison with both a DCM‐only doped PVK device and a DCM‐doped PVK device with the green fluorescent dye Alq3 as co‐dopant. The improvement in the operating characteristics of the phosphorescent and fluorescent dye doubly doped device is attributed to efficient energy transfer in the system, in which both triplet and singlet excitons are used for resultant emission in the polymer device.

Investigation of the Photophysical Processes and Photochemical Reactions Involved in PVK Films Irradiated at λ > 300 nm

This paper is devoted to the investigation of the phototransformation mechanisms of PVK exposed to long-wavelength radiations (I > 300 nm) in the absence and in the presence of oxygen. PVK irradiated as thin films was analyzed by fluorescence, UV-vis, and IR spectroscopies. The photoproducts formed were identified by postirradiation treatments including chemical derivatization reactions and HPLC analysis. The cross-linking was evaluated by gel fraction measurements. PVK is known to give an intense excimer fluorescence and no monomer fluorescence. The phototransformations provoked by light absorption are shown to lead to a dramatic decrease of this fluorescence which can be correlated with the modification of the chemical structure of the macromolecules. The decrease of fluorescence is the result of the diminution of the local mobility of the macromolecular chains resulting from cross-linking reactions provoked by the recombination of the several radicalar species. A general mechanism accounting for the phototransformation of PVK in the presence and in the absence of oxygen is proposed.

Polymer light-emitting devices with phosphorescence and fluorescence dye double-doped polymer as emitting layer

The electroluminescence(EL) properties of a green phosphorescence dye, (dmbpy)Re(CO) 3 Cl, and a red fluorescence dye, DCM, co-doped polyvinylcarbazole(PVK) thin film were studied. The clear and pure red emission from DCM were observed from PL and EL spectra of (dmbpy)Re(CO) 3 Cl(>2.0 wt. %):DCM(> 0.5 wt. %)-doped PVK films. The optimized device ITO/ (dmbpy)Re(CO) 3 Cl (10 wt. %):DCM(1.0 wt.%):PVK/Al shows the highest EL efficiency of 0.42cd / A at current density of 10 mA/cm 2 , which is more than ten times that of the DCM(1.0 wt %): PVK devices.

Synthesis and properties of photoluminescence and electroluminescence of pyrazoline derivatives

Triphenyl pyrazoline derivatives (TPPs) bearing electron withdrawing and pushing substitutents were synthesized. Their photoluminescence (PL) properties in the solution and doped in poly( N-vinylcarbazole) (PVK) thin films were investigated. When TPPs were doped into PVK films the photoluminescence intensity was enhanced with increasing TPPs concentration. It indicated that the energy transfer from PVK to TPPs has happened. Double and three-layer electroluminescence (EL) devices based on PVK doped with TPPs as an active layer were fabricated and investigated and the electroluminescent mechanism was followed by energy transfer from PVK to TPPs. The pyrazoline derivative with both electron withdrawing and pushing substituents was the optimistic candidate for electroluminescent emitter due to higher transfer efficiency from electric energy to light energy as well as larger luminance.

Energy transfer and device performance in phosphorescent dye doped polymer light emitting diodes

Singlet and triplet–triplet energy transfer in phosphorescent dye doped polymer light emitting devices were investigated. Poly(N-vinylcarbazol) and poly[9,9′-di-n-hexyl-2,7-fluorene-alt- 1,4-(2,5-di-n-hexyloxy)phenylene] (PFHP) were selected as the host polymer for the phosphorescent dopants fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] and 2,3,7,8,12,13, 17,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP) because of their high triplet energy levels and long phosphorescence lifetimes. In case of PVK film, efficient triplet energy transfers to both PtOEP and Ir(ppy)3 were observed. In contrast, the triplet energy transfer did not occur or was very weak from PFHP to both PtOEP and Ir(ppy)3 although usual requirements for triplet energy transfer were satisfied. Furthermore, the singlet–singlet energy transfer did not take place from PFHP to Ir(ppy)3 in doped films even though the Förster radius is more than 30 Å. However, the blended film of Ir(ppy)3 with PFHP and PMMA showed the green emission from Ir(ppy)3 via singlet energy transfer. In addition, the solution of PFHP and Ir(ppy)3 (8 wt. %) in p-xylene also showed green emission. The blocking of the energy transfers in the phosphorescent dye doped PFHP films is found to be originated from the formation of aggregates which is evident from the microscopic images taken by transmission electron microscope, atomic force microscope, and fluorescence microscope. The formation of aggregates prevents dopant molecules from being in close proximity with host molecules thereby inhibiting energy transfer processes. The phase separation deteriorates the device performance also. Therefore, the chemical compatibility of a dopant with a host polymer as well as conventional requirements for energy transfers must be significantly considered to fabricate efficient phosphorescent dye doped polymer light emitting devices.

Optical and electroluminescent properties of a new green emitting Ir(III) complex

We have synthesized and characterized a new green Ir(III) complex, Ir(mpp)3, with the ligand 3-methyl-2-phenylpyridine (Hmpp) and fabricated phosphorescent light emitting devices with the complex as a triplet emissive dopant in PVK. The PL and EL spectra of the PVK film doped with the complex confirm an efficient energy transfer from carbazole excimer to Ir complex. The device showed a maximum external quantum efficiency of 4.5% for 2% Ir(mpp)3 doping concentrations, and a peak luminance of 25,000 cd/m2. The device demonstrates the effect brought in by the fine-tuning of the ligand, 2-phenylpyridine in the form of the methyl substitution in the pyridine ring.

Optical Properties of Laser-dye-doped Polyvinylcarbazole Films Coated with Polycarbonate

Spectrophotometry in the ultraviolet-visible-infrared range was used to investigate the optical properties of polymer bilayers obtained by spin coating polycarbonate films about 1 μm thick on top of pyrromethene-doped polyvinylcarbazole (PVK) layers deposited on glass. Due to the light confinement in the PVK plane, the laser-dye photoluminescence collected along the polymer plane was linearly polarized and shifted from green to yellow according to the active film thickness. To laterally contain light, microstructures were imprinted on laser-dye-doped PVK films by soft lithography. The photoluminescence coming out of a microstructured film and the film topography were analyzed by scanning near-field optical microscopy.

PVK-based organic light emitting diodes with low turn on voltage

Poly(N-vinylcarbazole) (PVK) thin films deposited by evaporation have been used in electroluminescent organic diodes. The turn on voltages of the current–voltage (I–V) and electroluminescence–voltage (EL–V) characteristics are not very different and they are smaller than the one obtained in such PVK-based devices, when the PVK film is obtained by spin coating. It is shown that the carrier injection process is dominated by tunneling field effect. During evaporation the chain length of the polymer is shortened and the PVK films contain a high density of dangling bonds. It is proposed that these dangling bonds introduce a broad distribution of localized states in the band gap. Therefore, the carrier injection occurs via hopping among an energetic distribution of localized states rather than by band motion, which is equivalent to a reduction of the energy barriers as experimentally measured.

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 poly(N‐vinylcarbazole) (PVK) doped with the metallofullerene Dy@C82 and the fullerenes C84 and C60

AbstractPhotoconductivity has been measured on films of pure poly(Nvinylcarbazole) (PVK) and PVK doped with the metallofullerene Dy@C82 and the fullerenes C84 and C60. The photo‐induced discharge rate of the PVK film increased dramatically when doped with the metallofullerene or the fullerenes. Comparatively, the film doped with Dy@C82 displayed better photoconductivity than that doped with C84, which is attributed to the Dy@C82 being a better electron acceptor than C84. However, the film doped with C60 showed the best photoconductivity. This is attributed to the better electron accepting ability of excited C60* or the better miscibility of the hollow fullerenes with the PVK polymer and organic solvents.

All-wet fabrication of an organic electroluminescent device

We have demonstrated that organic electroluminescent (EL) devices can be fabricated in air without the need for vacuum equipment. EL devices were fabricated on glass substrates coated with indium–tin oxide (ITO). A dye-doped poly( N-vinylcarbazole) (PVK) film was spin-coated onto each ITO glass substrate as the emissive layer. Silver cathodes were prepared on the film by electroless plating. Metallic lustrous cathodes were obtained when the PVK film adsorbed SnCl 2, after the film was exposed to plasma. These devices exhibited clear carrier injection and light emission. The maximum brightness of one such device was 82 cd/m 2 at 19 Vat 75 mA/cm 2.

Poly(N-vinylcarbazole) (PVK) deposited by evaporation for light emitting diodes thin films structures

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.

Organic solar cell devices based on PVK/porphyrin system

We fabricated organic solar cell based on poly( N-vinylcarbazole) (PVK) mixed with metallo-porphyrin/Alq 3 p–n junction system, which was compared with metalloporphyrin/Alq 3 system. 5,10,15,20-tetra( o-nitro)phenylporphyrinato-zinc (ZnTNPP) was used as a dye. Photovoltaic parameters (open circuit voltage ( V oc), short circuit current ( J sc), fill factor (FF) and conversion efficiency( η)) of ITO/PVK(ZnTNPP)/Alq 3/Al cell and ITO/ZnTNPP/Alq 3/Al cell were compared. ITO/PVK(ZnTNPP)/Alq 3/Al device exihibited V oc=0.605 V, J sc=0.677 μA cm −2, FF=0.312 and η=0.0326%. ITO/ZnTNPP/Alq 3/Al device exihibited V oc=0.631 V, J sc=0.149 μA cm −2, FF=0.296 and η=0.0067%. The cell with porphyrin dye-dispersed PVK film showed higher conversion efficiency.

Electrical conductivity of halogen doped poly( N-vinylcarbazole) thin films

Thin films obtained by thermal evaporation of poly( N-vinylcarbazole) (PVK) powder have been doped with iodine and chlorine either by using a doped powder or by doping deposited films. The doping level of the thin films has been checked by X-ray photoelectron spectroscopy, then the samples have been electrically characterised by current–voltage and room temperature conductivity measurements. It is shown that chlorine reacts with PVK during doping which induces new compounds formation. Three different conductivity domains have been put in evidence, in the case of iodine doping. In the low field range the current is dominated by space charge effect. In intermediary field the current is ohmic. In the high field range the Poole–Frenkel effect is dominating. The evolution of properties of some samples is attributed to iodine ionisation of neutral iodine under high field. These results, obtained on evaporated thin films, are compared to those obtained with spin-coated PVK films.

Effects of polymer concentration and zone drawing on the physical properties of poly(N-vinylcarbazole) thick film

To check the possibility of zone-drawn poly(N-vinylcarbazole) (PVK) thick films as base materials of photorefractive polymers, the effects of the initial polymer concentration and zone drawing on the physical properties of PVK film were investigated. PVK films were prepared from 1,1,2,2-tetrachloroethane (TCE) solutions with different initial concentrations. To investigate the drawing behavior of the PVK films with different solution concentrations, the films were drawn under various zone-drawing conditions. Through a series of experiments, it turned out that the initial concentration of the PVK solution in TCE caused significant changes in the draw ratio of the PVK film, that is, the zone draw ratios of the film at an initial concentration of 9.8 g/dL exhibited their maximum values and gradually decreased at higher or lower concentrations. Thus, it was determined that the initial concentration of 9.8 g/dL is the optimum polymer concentration to produce the maximum draw ratio in this work. The lightness (whiteness) of the zone-drawn PVK film was much higher than that of the hot-drawn PVK film, resulting from diminishing microcrystallite formation, crystallization, and back-folding of molecular chains by zone drawing. Moreover, the mechanical properties of the PVK film were dominantly improved by introducing a zone-drawing technique, maintaining lightness of the zone-drawn film to a similar degree as that of the undrawn film. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1297–1304, 2001