Photoconductive Polymer Research Articles

Electrically controllable and polarization-independent Fresnel zone plate in a circularly symmetric hybrid-aligned liquid crystal film with a photoconductive polymer layer

This work reports a Fresnel zone plate in a circularly symmetric hybrid-aligned liquid crystal (LC) film with a photoconductive polymer layer. An ultraviolet-induced electrodelike pattern of polymer layer under a zone plate photomask results in alternating major and minor portions of external voltage dropping on LC layer in conductive and nonconductive regions, respectively. These effects cause the discrepancy in LC reorientation between adjacent zones, generating a Fresnel zone plate. The focusing of the zone plate is electrically controllable and polarization independent. Additionally, the zone plate has advantages of a zero focusing in the voltage-off state and a very small operating dc field range from 0to0.3V∕μm.

Dependency of Ionization Potentials of Photoconductive Polymers on Photorefractive Response Time

Photorefractive response time has been an important issue to be improved. We investigate the dependency of photorefractive speed against ionization potentials of the photoconductive polymers. The composite with the lowest ionization potential gives the highest photoconductivity. Fast component of the response time is found to be strongly dependent of photoconductivity of the composite.

Excitation of phosphorescence of pyrene implanted into a photoconductive polymer

Methods of populating the lowest excited triplet state T 1 of pyrene implanted into polystyrene (PS) or photoconductive polymer materials polymethylphenylsilane (PMPS), poly-N-epoxypropylcarbazole (PEPC), and poly-N-vinylcarbazole (PVK) are studied. Photoluminescence was excited in the first electron transition band of pyrene and the above photoconductive polymers. It is found that, at concentrations of 0.05–0.50 mol l−1, pyrene is effective in quenching the fluorescence of PMPS, PEPC, and PVK but has only a slight effect on the photoemission efficiency of geminate electron-hole pairs. As a result, the phosphorescence of pyrene in photoconductive polymers is excited both by the intersystem crossing from the first excited singlet state (S 1 ⇝ T 1) and by the capture of triplet excitons created in the recombination of charge pairs. In addition, in PEPC and PVK, the phosphorescence of pyrene is excited by recombination of a captured hole with an electron. For this reason, the ratio of the quantum yields of phosphorescence and fluorescence of pyrene in photoconductive polymers is much larger than that in PS, wherein the T 1 state of pyrene is populated by intersystem crossing S 1 ⇝ T 1 only.

Optical phase conjugation in the hybrid polymer liquid crystal panel

In this paper, we present experimental results of the optical phase conjugation (PC) process in nematic liquid crystal cell with a photoconducting polymer layer (PVK doped with TNF). We analyze and discuss the influence of a few parameters (applied voltage, power of the laser beams, and geometry of an experiment) on conjugation efficiency. We also present experimental results showing possible application of the optical phase conjugation phenomenon in correction of phase distortion and aberration.

Study of self-diffraction process in photoconducting polymer-nematic liquid crystal hybrid structure

The authors present results of the measurements of self-diffraction process inside the nematic liquid crystal cell with a photoconducting polymeric layer poly(N-vinylcarbazole) doped with trinitrofluorenone molecules. The dynamics of diffraction grating fading process versus applied voltage and different Ar+ laser power has been studied. Three-step process of grating’s fading was observed, connected with the charge carrier generation and their mobility in the photoconducting polymer. Theoretical calculations assuming proposed mechanism responsible for the grating decay are in a very good agreement with experimental results.

Space charge field and dynamics of the grating formation in a photorefractive polymer-dispersed liquid crystal based on a photoconducting polysiloxane

A photorefractive system composed of liquid crystal droplets dispersed in a photoconducting polymer is characterized by means of two-beam coupling. The amplitudes and phases of the spatial variations of the refractive index and the absorption coefficient are measured using the moving grating technique. Dynamic measurements indicate that the phase of the refractive index modulation is nearly constant while the amplitude increases gradually. The maximum value of the internal space charge field can be estimated from the amplitude of the phase grating and is found to be of the order of one-hundredth of the external bias field.

All-optical poling properties of new nonlinear fluorene derivatives

The polar orientation of fluorene-derivative molecules incorporated in a photoconducting polymer host is studied by means of all-optical poling. We investigate the influence of the donor and acceptor moieties on the induced polar order, we characterize the dynamics of the orientation process and suggest photoinduced reaction schemes by which an efficient all-optical poling is achieved.

Photorefractivity of poly-N-vinylindole-based materials as compared with that of poly-N-vinylcarbazole-based blends

The asymmetric two-beam coupling technique has been employed to measure the photorefractivity of thin films of polymer blends containing 2,5-dimethyl-4-(p-nitrophenylazo)anisole as the nonlinear optical component. Poly-(1-vinylindole) and poly-(2,3-dimethyl-1-vinylindole) were the photoconductive polymer counterparts. The values of the photorefractivity are reported. It appears that they are comparable with those of similar blends based on the well-known poly-(9-vinylcarbazole) (PVK), here considered as a reference standard. Careful differential scanning calorimetry analyses have been accomplished on the different blends taken into account to rationalize the significantly longer shelf lifetime of the indolyl-based films with respect to the PVK-based blends.

Optoelectronic Switch and Memory Devices Based on Polymer‐Functionalized Carbon Nanotube Transistors

Associating carbon nanotubes with conjugated polymers or molecules has been recently recognized as an efficient way to improve performances of organic electronic devices and to achieve new functionality, in particular for optoelectronic applications. This approach maintains the main advantages of organic electronics for low cost, flexible and/or transparent applications. A particularly interesting example of such a nanotube/polymer combination consists of a carbon nanotube field effect transistor coated with a polymer thin film (1; 2). Indeed, such a device can ideally combine the very high carrier mobility of carbon nanotubes (100000 cm2/V.s) with the excellent optical properties of polymers (or molecules in general). Carbon nanotube transistors can, by themselves, emit or detect photons at wavelengths defined by the nanotube chirality. But, to extend their capabilities in optoelectronics, it is important to be able to tune these wavelengths independently of the nanotube structure. Functionalizing the nanotube with a polymer is an efficient way to achieve such a goal. In the present study (2), we demonstrate that drastic photo-induced modifications of the electrical characteristics of self-assembled (3) nanotube transistors functionalized by photo-conductive polymers can be achieved.

Polymerization of conducting polymers inside carbon nanotubes

Supercritical carbon dioxide is an universal tool to impregnate carbon nanotubes with a wide range of organic molecules. In this Letter, we present our results of carbon nanotubes filled with the photo-conducting polymer poly( N-vinyl carbazole) and the conducting polymer polypyrrole which were prepared by polymerizing the monomers inside the nanotubes. The endohedral nanotubes were characterized by HRTEM and 1H NMR which confirmed that the encapsulated material was indeed the conducting polymer.

Surface Screening Layers and Dynamics of Energy Transfer in Photosensitive Polymer-Liquid Crystal Structures

ABSTRACT The dynamics of energy transfer in photoconductive polymer liquid crystal structures can contain important information on interface effects and surface electric fields contributing to the strength of liquid crystal reorientation gratings. The characteristic, transient effects observed during switching on and off of incident light or electric field can be explained by the presence of surface screening layers. Screening layers play an important role in the reorientation of liquid crystal director in cells with different alignment layers. Strong screening of external DC field is present not only in cells with a photoconductive polymer (56 V), but in standard cells with thicker (0.3 μm) polyimide, aligning layers.

Layered photoconductive polymers: Anisotropic morphology and correlation with photorefractive reflection grating response

We demonstrate that the mesophase morphology of the layered photorefractive polymers has a substantial influence on the photorefractive properties, especially in reflection grating geometries with a minimal grating spacing. The layered morphology of the photoconductive polymers based on poly(p-phenyleneterephthalate) (PPT) with pendent carbazole (CZ) groups can be efficiently controlled by changing their molecular weight. Photorefractive composites based on PPT-CZ polymers with different chromophores, diethylaminodicyanostyrene (DDCST) or piperidinodicyanostyrene (PDCST), show anisotropic morphology induced by the squeezing flow during sample preparation. The contributions of the highest occupied molecular orbital levels of the chromophores and of the degree and anisotropy of the layered crystalline structure to the charge transport and trapping result in a high efficiency of the PDCST composite and a similar response speed in DDCST and PDCST composites in the reflection grating geometry, although of about six times lower photoconductivity in the less-ordered PDCST composite.

Carrier Generation Process on Photoconductive Polymer Films as Studied by Magnetic Field Effects on the Charge-Transfer Fluorescence and Photocurrent

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCarrier Generation Process on Photoconductive Polymer Films as Studied by Magnetic Field Effects on the Charge-Transfer Fluorescence and PhotocurrentFuyuki Ito, Tadaaki Ikoma, Kimio Akiyama, Akira Watanabe, and Shozo Tero-KubotaCite this: J. Phys. Chem. B 2006, 110, 10, 5161–5162Publication Date (Web):February 16, 2006Publication History Published online16 February 2006Published inissue 1 March 2006https://doi.org/10.1021/jp0565857Copyright © 2006 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views297Altmetric-Citations4LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (169 KB) Get e-Alerts Get e-Alerts

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive‐polymer‐stabilized ferroelectric liquid crystals (PPS‐FLCs) was examined. The polymer/FLC samples exhibited two‐beam coupling gain coefficients of about 6∼12 cm−1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer‐stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two‐beam coupling signal was reduced significantly in the PPS‐FLC samples.

Photorefractive nematic liquid crystals with gain of constant sign under alternating voltage

Two-beam coupling gain of constant sign was observed in hybrid planar/homeotropic nematic cells under a nonbiased square-wave ac field. Charge photogeneration was induced by using one layer of photoconducting polymer. With writing beams of equal intensities, gains up to 6% were observed in ∼10μm thick samples when the amplitude of the applied field was of the order of 1Vμm−1 at 10Hz. The gain decreased gradually when the frequency was increased, vanishing above 100Hz.

Analytical evaluation of stereoregularity in poly(2- N-carbazolylethyl methacrylate) by two-dimensional NMR spectroscopy

Poly(2- N-carbazolylethyl methacrylate) with varying degree of tacticities was synthesized by anionic and free radical polymerization. 2,2′Azobisisobutyronitrile (AIBN) and t-C 4H 9MgBr were used as initiators for free-radical and anionic polymerizations, respectively. Molecular weight distribution was determined by gel permeation chromatography (GPC). The samples obtained were analyzed for their stereochemical structure and tacticity utilizing various 1D ( 1H, 13C{ 1H}, DEPT) and 2D{HSQC, TOCSY, HMBC} NMR techniques. Signal assignments to different carbon resonances were done using 13C{ 1H} NMR in conjunction with DEPT-135 spectrum. The peak splittings for α-methyl carbon resonances were used to achieve tacticity assignments up to the pentad level. β-Methylene carbon of the main-chain showed configurational sensitivity up to the tetrad level. 2D heteronuclear single quantum coherence (HSQC) spectrum along with total correlation spectroscopy (TOCSY) further, helped in making complete spectral assignments and strengthening them. The 2D heteronuclear multiple bond coherence (HMBC) studies proved highly beneficial in resolving the configurational sensitivity of the non-protonated carbons viz., the quaternary and carbonyl carbon up to pentad level. The mechanism involved in the anionic polymerization of 2- N-carbazolylethyl methacrylate by the initiator t-C 4H 9MgBr was also investigated with the help of various NMR techniques. The validity of these assignments has been tested and confirmed by calculations using statistical relationships. The glass transition temperature ( T g) of poly(2- N-carbazolylethyl methacrylate) was found to be lower than the traditional photoconductive polymers which meant an easier processing.

Photogeneration and photovoltaic effect in blends of derivatives of hexabenzocoronene and perylene

Photogeneration quantum yield in layers of discotic liquid crystals (DLC, hole transport material), perylene derivative (electron transport material) and of their mixtures were determined by using surface potential decay technique. While in pure DLC only very weak photogeneration in the visible range was detected, and in pure perylene derivative only negative mobile charge carriers were photogenerated, in the mixtures a synergetic effect of photogeneration of both positive and negative mobile charge carriers was observed. This is because the excitons photogenerated at perylene derivative dissociate into electron–hole pairs, and then electrons are transported via perylene derivative while the holes are transferred onto the DLC molecules. The obtained results were correlated with performance of the photovoltaic devices with active layer made of the mixtures of the DLC and perylene derivative and of their blends with photoconducting polymer. It was found, that the power conversion efficiency of the device is dependent on the film morphology.

High performance reflection gratings in nematiclike photorefractive polymers

Photorefractive materials based on the nematiclike photoconductive polymers PPT-TPA consisting of the rigid backbone poly(p-phenyleneterephthalate), PPT, and pendent photoconducting triphenylamine (TPA) groups were studied in reflection grating geometry and compared to the layered photorefractive composites PPT-CZs with photoconducting carbazole (CZ) groups. The order level of the mesophase structure has a crucial influence on the trapping characteristics of the composites. The nematiclike composites although of lower photoconductivity exhibit one order of magnitude faster response than in the layered composites in the reflection grating regime. This is attributed to the increased trapping rate as a result of the specific mesophase structure.

Photoconducting polymer nanocomposites with efficient photogeneration and bipolar transport for optoelectronic applications

Photoconducting polymer nanocomposites with high electrophotographic sensitivity for both positive (S λ +) and negative (S λ −) signs of corona charging (up to 400 m 2/J in 300–620 nm range) and high charge carrier photogeneration quantum yield (β up to 0.6) are developed on the base of p-conducting (S λ + ≫ S λ −) polymer matrices (polyimides (PI), carbazolylcontaining (CzCP) polymers) and n-conducting low molecular mass additive (perylenediimide derivative (PDID)) in aggregated form. For p-composites (PI doped with PDID at content C A up to 50% wt.) the efficient hole photogeneration in PDID absorption band is related to the observed electron transfer from PI donor chain fragments with low ionization potential (I D = 6.8 eV) to the excited PDID aggregates as acceptors (with affinity E A = 1.8–2.0 eV). Low S λ − value for p-composites is likely due to potential barrier formation between PDID particles (aggregates and clusters) involved in electron transport network. For n-composites (S λ − ≫ S λ +) (CzCP doped also with PDID) high S λ − and β values are explained by the efficient electron photogeneration via excited charge transfer complex (exciplex) between excited PDID molecule as acceptor and CzCP carbazolyl pendant as donor (with I D = 7.4 eV) as well as electron transport network formation involving PDID particles. As it is only ionization potentials of polymer donor fragments that differ essentially for p- and n-composite, the conclusion is made that charge transfer donor-acceptor interaction in the ground state may be responsible for potential barrier formation between acceptor particles embedded in donor matrix. It is found that films of bipolar sensitive composites (S λ + ≈ S λ −) (obtained by mixing p- and n-composites in solution) which have microsegregated structure are characterized by the highest S λ +, S λ − and β values. The photovoltaic effect is investigated for sandwich cells with (Al, ITO) electrodes. The best parameters are found for bipolar composite films (0.5–1.0 μm thick).

Nematic-like mesophase photoconductive polymer for photorefractive applications

A new nematic-like mesophase photoconductive polymer PPT–TPA consisting of wholly aromatic rigid backbone of poly( p-phenyleneterephthalate), PPT and pendent hole-transporting triphenylamine (TPA) groups attached to the ends of oxydecyl spacers has been synthesized. The photorefractive composite contains the photoconductor PPT–TPA, the chromophore diethylaminodicyanostyrene (DDCST), and the photosensitizer C 60. Although no plasticizer was added, the glass transition temperature T g of the composite is 15 °C, which characterizes it as a low- T g photorefractive material. We investigate the correlation between the mesophase structure and its optical/physical properties by X-ray diffraction, photoconductive and photorefractive experiments. The new composite and its properties are compared to PPT–CZ composites with only a different charge transporting agent (carbazole, CZ) but a much more ordered mesophase structure, which were studied previously and have shown very good photorefractive properties. Despite of a lower photoconductivity of the new photorefractive composite PPT–TPA ( n=10):DDCST:C 60 this material shows a higher photorefractive sensitivity S n2 of 2±0.2 cm 2/kJ at E=50 V/μm than the previously synthesized composite PPT–CZ ( n=10):DDCST:C 60.