Photoconductive Poly Research Articles

Electro-opto-thermal addressing bistable and re-addressable display device based on gelator-doped liquid crystals in a poly(N-vinylcarbazole) film-coated liquid crystal cell.

This paper reports an electro-opto-thermal addressing bistable and re-addressable display device based on gelator-doped liquid crystals (LCs) in a poly(N-vinylcarbazole) film-coated LC cell. The bistability and re-addressability of the devices were achieved through the formation of a rubbery LC/gel mixture at room temperature. The desired patterns were addressed, erased, and re-addressed by controlling the temperature, applied voltage, and UV light illumination. Moreover, grayscales were obtained by adjusting UV light intensity. The initiation, relaxation, rise, and fall times of photoconductive poly(N-vinylcarbazole) via UV light illumination of various intensities were also examined.

Role of triplet states of polymethine dyes in photogeneration of electron-hole pairs in poly(N-epoxypropylcarbazole) films

The external magnetic field effects on the spectral and luminescent properties of film composites based on photoconductive poly(N-epoxypropylcarbazole) doped with cationic polymethine and merocyanine dyes have been studied. The magnetic field effects on the intensity and kinetics of delayed fluorescence and recombination luminescence have been revealed. These effects are explained by participation in the photogeneration of charged pairs of singlet-triplet intersystem crossing in excited dye molecules.

High-performance photorefractive polymer composites based on poly(9-vinyl-3-carbazolcarboxyaldehyde diphenylhydrazone)

Photorefractivity is based on the combined effects of photoconductivity and electro-optic property. Under the illumination of non-uniform light formed by the interference of two coherent laser beams, a spatially oscillating space charge field is formed arising from the generation and the redistribution of photo-induced charges. The refractive index of material is subsequently modulated via an electro-optic effect. The photorefractive materials have potentials applications in the fields such as dynamic holography, 3D display, 3D printer, reversible data storage and information processing. In 1994, remarkable improvement in the performance of PR polymeric material was obtained by using the photoconductive polymer poly(N-vinylcarbazole) (PVK). PVK/2,5DMNPAA/ECZ/TNF=34:50:15:1 wt% composite, showed a large gain coefficient of 220 cm and a diffraction efficiency of nearly 100% at the applied electric field of 90 V/ μm. Many studies have been done since Meerholz and coworkers proposed high-performance photorefractive (PR) polymeric composite. Despite this progress, the search for new photorefractive materials are still required for their widespread utilization in information technologies. In a previous paper, we reported on the synthesis and characterization of a photoconducting poly(siloxane) having pendant diphenylhydrazone. This composite shows a high diffraction efficiency of 81% at an electric field of 40 V/μm. Poly[4-(diphenyl-hydrazonomethyl)-phenyl]-[3-(methoxydimethyl-silanyl)-propyl]-methyl-amine (PSX-HZ) system have advantages including excellent optical quality, long shelf lifetime, the large birefringence, and high diffraction efficiency at low electric field. Despite the various advantages, the PSX-HZ system has suffered from relatively slow PR response time. To solve this problem, we applied the polyvinyl-backbone containing the hole transport moiety that is 9-vinyl-3-carbazolcarboxyaldehyde diphenylhydrazone (CPH). Hydrazone based on aromatic amines and arylaldehyde hydrazone are widely studied as organic hole transport materials due to enhanced hole mobility. Carbazole-containg hydrazone especially, exhibit relatively high hole mobility. We expect the realization of large photoconductivity of new polymer. The chemical structures investigated are shown in Figure 1, although the details of the synthesis and photophysical properties will be published elsewhere. The composition our samples was PVCPH/DB-IP-DC/ BBP/PCBM (54:30:15:1 wt%)-C1 and PVCPH/PDCST/BBP/ PCBM (54:30:15:1 wt%)-C2. Where poly(9-vinyl-3-carbazolcarboxyaldehyde diphenylhydrazone) (PVCPH) are photoconducting material, 2-{3-[(E)-2-(dibutylamino) ethen-1yl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (DBIP-DC) and 2-(4-piperidinobenzylidene)malononitrile (PDCST) are nonlinear optical chromophore, butyl benzyl phthalate (BBP, Tokyo Kasei Kogyo Co.) is plasticizer, phenyl-C61butyric acid methyl ester (PCBM, Solenne Co.) is photosensitizer. The device was prepared by sandwiching the softened composite between two indium tin oxide (ITO) coated glass plates.

Direct detection of electron transfer processes in photoconductive poly(N-vinylcarbazole) solid film doped with electron acceptors: Temperature dependence of femtosecond to microsecond dynamics

Abstract Primary processes of photoinduced electron transfer dynamics in a poly(N-vinylcarbazole) solid film doped with electron acceptors were investigated by means of transient absorption spectroscopy in femtosecond to microsecond time region as well as dichroism measurements. Picosecond transient dichroism measurement revealed that the rate constant of the hole shift reaction was independent of the temperature in the range between 10 and 295 K. The femtosecond transient absorption spectroscopy in the NIR wavelength region showed that the delocalization of the cationic states in sub-ps to several ps time region occurred immediately after the charge separation even at 10 K. These results indicated that the rapid delocalization of the cationic states, probably due to the small fluctuation, took an important role in the large rate constant of the hole shift reaction in PVCz solid film. In μs time region, weak temperature dependence was observed for the dynamic behaviors. This temperature dependence was attributed to the trapping/detrapping and the charge recombination processes, which were temperature dependent. The analysis including these rate constants and the temperature independent hole shift reaction rate constant could reproduce the experimental results in the temperature range of 10–295 K, of which results also supported the temperature independence of the hole shift reaction in PVCz solid film.

Unconditionally stable indole-derived glass blends having very high photorefractive gain: the role of intermolecular interactions

The photorefractivity of an indole derivative and of its polymer blends has been studied at room temperature. The indole derivative 3-[2-(4-nitrophenyl)ethenyl]-1-(2-ethylhexyl)-2-methylindole (NPEMI-E) is a typical low-molecular-weight glass-forming molecule having peculiar nonlinear optics characteristics. It is unconditionally soluble in the photoconductive poly-(N-vinyl-2,3-dimethylindole) so that all the possible blends can be studied for a weight percent (wt. %) content of NPEMI-E ranging from zero to 100. A very high and sharp maximum of the photorefractive optical gain Gamma(2) approximately 2000 cm(-1) was obtained for a NPEMI-E wt. % content of about 90. On the basis of recently published theoretical calculations, we have made the hypothesis that the rapid change of Gamma(2) can also be ascribed to a correspondingly quick variation of the molecular electro-optic parameters of the dissolved chromophore for some well distinguished values of its concentration in the polymer matrix. Differential scanning calorimetry measurements were made and the results carefully analyzed with the aim of obtaining information on the intermolecular interactions. These last measurements also allowed rationalizing the unconditionally stable glass appearance of the obtained blends. Measurements of spectroscopic ellipsometry were also made on blends with different NPEMI-E content.

High-resolution photorefractive gratings in nematic liquid crystals sandwiched with photoconductive polymer film

Photorefractive gratings with high grating resolution were observed in the 20 μm thick low-molar-mass nematic liquid crystal (NLC) cell with a separate photoconductive (PC) poly(N-vinylcarbazole) layer. An orientational grating with a grating spacing of 1.9 μm was produced. It is believed that a space–charge field with small fringe spacing forms in the PC layer and its evanescent component penetrates into the NLC layer. The penetrated evanescent field drives the NLC to reorient, and consequently the orientational grating forms. The model indicates that the modulated field exists in several hundred nanometers near the surface, and thus the orientational grating is not full of the NLC film, which is consistent with the observed phenomena of the multiple diffractions. Besides, asymmetric two-beam coupling of 11.2% was achieved for the grating with a grating spacing of 1.9 μm, and a net gain coefficient of larger than 62 cm−1 was obtained.

Preparation of poly(4-butyltriphenylamine) particles by chemical oxidative dispersion polymerization

Photoconductive poly(4-butyltriphenylamine) particles were prepared by a chemical oxidative dispersion polymerization. The utilization of statistical copolymer of methyl methacrylate with 2-hydroxyethyl methacrylate (30:70) as a dispersant afforded particles with the narrowest distribution when the other experimental conditions such as the rate of monomer feed, and the dispersant concentration were appropriately selected. Porous particles were obtained at 40 °C using poly(vinyl pyrrolidone) as a dispersant.

Light sensitivity of photoconducting polymer systems with polymethine dyes based on derivatives of 2(4)-methylquinoline and indole

The photosensitivity of conducting polymer films with newly synthesized polymethine dyes is investigated. It is demonstrated that the photosensitivity of polymer films based on poly(o-hydroxyamide) with transport indole molecules forming intermolecular complexes with dyes is two or three times higher than the photosensitivity of the polymer films containing indole as a structural component of the dye. The introduction of the indole-containing dye into the photoconducting poly(vinylcarbazole) leads to an increase in the photo-conductivity by one and a half orders of magnitude. In this case, the quantum yield increases to η = 0.085. A correlation is revealed between the electronic structure of the dyes, the characteristics of the photoconduction in the materials under investigation, and the mechanisms of charge relaxation in the samples. An increase in the photoconductivity of the films results in the crossover from the drift mechanism of charge relaxation to the relaxation mechanism associated with intrinsic electrical conduction.

A Highly Photoconductive Poly(vinylcarbazole)/2,4,7-Trinitro-9-fluorenone Sol−Gel Material that Follows a Classical Charge-Generation Model

Highly photoconductive properties are reported for organic-inorganic hybrid sol-gel thin film materials composed of a classical poly(vinylcarbazole)/2,4,7-trinitro-9-fluorenone (PVK/TNF) polymeric mixture, entrapped in a SiO(2) matrix, whose pores have been chemically modified by organic functional groups. The highest photosensitivity obtained, 3.4 x 10(-10) cm Omega(-1) W(-1) at E 22 V microm(-)1, at the optimum molar ratio between the active components, TNF, PVK, and SiO(2), is in the range of the highest values ever reported for any PVK/TNF-based classical photoconductive material. It is demonstrated that the PVK/TNF-based sol-gel films follow Onsager's classical charge-generation model. The analysis of the photocurrent efficiency (Phi) of PVK/TNF-based sol-gel films by such a model provides the primary quantum yield of thermalized pair formation and the initial thermalized pair distance, phi(0) = 0.12 and r(0) = 66.1 Angstrom, respectively, for the optimized sample. As a result of Onsager's analysis, a notorious improvement of the photocurrent generation process was achieved for low TNF concentrations.

Carrier generation in photoconductive poly(N-vinylcarbazole) as revealed by multifrequency time-resolved ESR

Carrier generation in photoconductive poly(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>N</mml:mi></mml:math>-vinylcarbazole) as revealed by multifrequency time-resolved ESR

The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers

In liquid crystal cells with photoconductive poly(N-vinlylcarbazole) polymer layers, an external dc field can be completely screened by surface charge layers that develop at the liquid crystal–polymer interface. Under spatially modulated illumination, surface charge layers can be discharged in bright areas and lead to reorientation and spatially modulated Freedericksz transition. As a result, an asymmetric energy exchange in the photorefractive two-beam coupling process can take place. We propose a model to explain the origin of reorientation and phase shift in the two-beam coupling process, based on the profile and tilt of the refractive index grating. We also show that cells with just one photoconducting layer are more efficient than a typical design with two layers.

A molecular device based on light controlled charge carrier mobility

Electrical properties of binary samples containing photoconducting poly[methyl(phenyl)silylene] with admixed photochromic spiropyran were investigated. A reversible chemical reaction of the photochromic system, driven by light and producing highly polar metastable species, manifests itself in a reversible modification of dark currents and photocurrents. The behaviour of the samples is explained assuming a reversible production and annihilation of dipolar traps associated with the polar molecules produced in the photochromic process. Use of similar materials as elements of molecular electroactive systems is postulated.

Synthesis and characterization of a new photoconducting poly(siloxane) having pendant diphenylhydrazone for photorefractive applications

A new photoconducting polymer, diphenyl hydrazone-substituted polysiloxane, was successfully synthesized by the hydrosilylation method and characterized by FT-IR,1H-NMR, and29Si-NMR spectroscopy. The glass transition temperature (Tg) of the polysiloxane having pendant diphenyl hydrazone was ca. 62 °C, which enabled a component of a low-Tg photorefractive material to be prepared without the addition of any plasticizers. This polysiloxane, with 1 wt% of C60 dopant, showed a high photoconductivity (2.8×10−12 S/cm at 70 V/μm) at 633 nm, which is necessary for fast build-up of the space-charge field. A photorefractive composite was prepared by adding a nonlinear optical chromophore, 2-{3-[2-(dibutylamino)-1-ethenyl]-5,5-dimethyl-2-cyclohexenylidene} malononitrile, into the photoconducting polysiloxane together with C60. This composite shows a large orientation birefringence (Δn=2.6×10−3 at 50 V/μm) and a high diffraction efficiency of 81% at an electric field of 40 V/μm.

All‐optical switching modulation in poly(vinyl carbazole)—based azobenzene blends

AbstractWe report on the all‐optical switching polymer blends based on photoconductive poly(vinyl carbazole) (PVK) and azobenzene dyes. The optical switching effects of the blends were measured by using a 514 nm pump beam and a 632.8 nm probe beam. The blends' photoinduced reversible and repeatable change in the transmittance of the probe beam is attributed to the photoinduced anisotropy of the azobenzene dyes. The influencing factors of the optical switching effect, such as the modulation frequency and pump beam intensity, were studied experimentally.

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.

Transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers

The transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers was investigated systematically. The transport of the photocharges is explained in terms of current paths that are formed along the bright sites of an interference pattern. Our study shows clearly that charge trapping occurs predominantly in the photoconductive poly(N-vinylcarbazole) layers and not in the insulating poly(vinyl alcohol) layers, contrary to a previous report.

Direct Observation of a Distant Ion Pair Generated in a Photoconductive Poly(N-vinylcarbazole) Film with Dopant.

Photoconduction in charge transfer materials apparently occurs through a distant geminate ion pair with an interionic separation of about 8 Å, as demonstrated by time-resolved EPR spectroscopy of a doped poly(N-vinylcarbazole) film.

Direct Observation of a Distant Ion Pair Generated in a Photoconductive Poly(N-vinylcarbazole) Film with Dopant

Der Leitungsmechanismus in Charge-Transfer-Materialien mit photoleitenden Eigenschaften umfasst offenbar ein Ionenpaar mit großem interionischem Abstand (ca. 8 Å), wie durch zeitaufgelöste EPR-Spektroskopie an einem dotierten Poly(N-vinylcarbazol)-Film gezeigt werden konnte (siehe Schema).

ATOP dyes. optimization of a multifunctional merocyanine chromophore for high refractive index modulation in photorefractive materials.

This paper reports synthesis, characterization and structural optimization of amino-thienyl-dioxocyano-pyridine (ATOP) chromophores toward a multifunctional amorphous material with unprecedented photorefractive performance. The structural (dynamic NMR, XRD) and electronic (UV/vis, electrooptical absorption, Kerr effect measurements) characterization of the ATOP chromophore revealed a cyanine-type pi-conjugated system with an intense and narrow absorption band (epsilon(max) = 140 000 L mol(-)(1) cm(-)(1)), high polarizability anisotropy (deltaalpha(0) = 55 x 10(-)(40) C V(-)(1) m(2)), and a large dipole moment (13 D). This combination of molecular electronic properties is a prerequisite for strong electrooptical response in photorefractive materials with low glass-transition temperature (T(g)). Other important materials-related properties such as compatibility with the photoconducting poly(N-vinylcarbazole) (PVK) host matrix, low melting point, low T(g), and film-forming capabilities were optimized by variation of four different alkyl substituents attached to the ATOP core. A morphologically stable PVK-based composite containing 40 wt % of ATOP-3 showed an excellent photorefractive response characterized by a refractive index modulation of Deltan approximately 0.007 and a gain coefficient of Gamma approximately 180 cm(-)(1) at a moderate electrical field strength of E = 35 V microm(-)(1). Even larger effects were observed with thin amorphous films consisting of the pure glass-forming dye ATOP-4 (T(g) = 16 degrees C) and 1 wt % of the photosensitizer 2,4,7-trinitro-9-fluorenylidene-malononitrile (TNFM). This material showed complete internal diffraction at a field strength of only E = 10 V microm(-)(1) and Deltan reached 0.01 at only E = 22 V microm(-)(1) without addition of any specific photoconductor.

INFLUENCE OF A PHOTOCONDUCTIVE POLYMER MATRIX ON THE PHOTOLUMINESCENCE OF CATIONIC POLYMETHINE DYES

Using symmetric cationic indopolycarbocyanines HIC and HIDC as an example, the authors detected the enhancement of their photoluminescence in films of photoconductive polymers poly–N–epoxypropyl carbazole and poly–N–vinyl carbazole as compared to nonphotoconductive polymers, i.e., polyvinyl butyral, polystyrene, and polyethylene. The excitation was carried out on the shortwave edge of the absorption band of the dye and did not affect the absorption region of the polymer. It is shown that the effect of enhancement of the luminescence increases with decrease in the excitation wavelength and becomes weaker with increase in the molecular mass of carbazole–containing polymers. Its enhancement is interpreted as the recombination luminescence of electron–hole pairs formed in photogeneration of charge from the dye molecules.