Photorefractive Composites Research Articles

Synthesis of Polymer Having Thioxanthene Unit in Main Chain and Application to Photorefractive Composites

Synthesis of Polymer Having Thioxanthene Unit in Main Chain and Application to Photorefractive Composites

Photoelectron Yield Spectroscopy and Transient Photocurrent Analysis for Triphenylamine-Based Photorefractive Polymer Composites

The photocurrent for poly(4-(dimethylamino)benzyl acrylate) (PDAA) photorefractive composites with (4-(diphenylamino)phenyl)methanol (TPAOH) photoconductive plasticizers was measured to be two orders of magnitude higher than that obtained with (2,4,6-trimethylphenyl)diphenylamine (TAA) photoconductive plasticizers. In this study, to determine the reason for the large difference in the photocurrent measured for PDAA photorefractive composites containing two different photoconductive plasticizers of TPAOH and TAA, the highest occupied molecular orbital (HOMO) level identical to the ionization potential (Ip) and the width of the density of states (DOS) were evaluated using photoelectron yield spectroscopy, and the transient photocurrent was analyzed using a two-trap model. The estimated hole mobility was also rationalized using a Bässler formalism together with the energetic disorder of the width of the DOS and the positional disorder of the scattering situation for carrier hopping.

Enhancement of the response time in organic photorefractive composites using alkoxy-substituted PDCST as a nonlinear optical chromophore: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 6767 (2020)OPLEDP0146-959210.1364/OL.409743.

Enhancement of the response time in organic photorefractive composites using alkoxy-substituted PDCST as a nonlinear optical chromophore.

The ease of the molecular orientation of a chromophore has an important effect on the electro-optical (EO) properties of polymeric photorefractive (PR) composites. A derivative of 4-piperidinobenzylidene-malononitrile (PDCST) with an alkoxy group added as a side branch was synthesized to improve the molecular orientation characteristics. Electrophoresis was performed on the polymeric PR composite to which the improved PDCST had been added. The optical properties and response times were examined to evaluate the effects of the substitution of the alkoxy group. PDCST substituted with the alkoxy group showed enhanced EO properties and a PR grating formation rate.

Facile organic photorefractive (PR) composites with favorable PR performance under no external electric field

Facile photorefractive (PR) composite films were prepared. As evidence of PR effect of the PR composite films, two-beam coupling (TBC) experiment were carried out at 633 nm wavelength. Interestingly, strong energy transfers appeared under zero external electric field (Eex). The highest TBC gain coefficient is up to 230.32 cm−1. The TBC gain coefficient increases with the increase of photosensitizer within certain range. The probable cause of PR effect under non-external electric field were proposed.

Electron spin resonance and photoelectron yield spectroscopic studies for photocarrier behavior in photorefractive polymeric composites

Abstract An electron spin resonance (ESR) spectroscopy as well as a photoelectron yield spectroscopy (PYS) is used to study the photocarrier behavior (generation and transport) for photorefractivity in poly(N-vinyl carbazole) (PVCz) and poly[bis(2,4,6-trimethylphenyl)amine] (PTAA) based photorefractive (PR) polymer composites. ESR spectra directly catch the radical species in PR composites on the laser illumination under applied electric field; the cation radicals and anion radicals induced under light illumination upon applying an electric field were evaluated. PYS was used to evaluate the energy levels and the width of the electron density of states (DOS) of the carrier transport manifolds. The narrower DOS width of the PTAA-based PR composite was significantly related to the faster response time and broader DOS width for the PVCz-based PR composite to the slower response time. Based on all these results, the comprehensive energy diagrams, including perspectives on the energy level and its distribution, the generated charge carriers with spins, and occupied trap density, are proposed.

Photorefractive polymeric composites with high two-beam coupling gain without external electric field

Herein, three photorefractive (PR) composite films were fabricated. They consist of poly (N-vinycarbazole) (PVK, 33 wt %), 2, 5-dimethyl-(4-p-nitrophenylazo) anisole (DMNPAA, 50 wt %), ethyl carbazole (ECZ, 16 wt %) and fullerene derivatives (1 wt %). The three fullerene derivatives [6, 6]-Phenyl-C61-butyric acid isopropyl ester (PCBIPE), [6, 6]-phenyl-C61-sec-butyl butyrate (PCSBB), and [6, 6]-phenyl-C61-phenethyl butyrate (PCPEB) were used as photosensitizer in the PR films. The PR composite films PVK/DMNPAA/ECZ/PCBIPE(33/50/16/1), PVK/DMNPAA/ECZ/PCSBB(33/50/16/1), and PVK/DMNPAA/ECZ/PCPEB(33/50/16/1) (in wt %) with a low glass transition temperature (21.3 ℃) can easily be fabricated into optically transparent films. The PR performances of the three PR composite films were investigated by two-beam coupling (TBC) experiment at 633 nm wavelength. The results illustrate that the TBC signals were observed under no external electric field or pre-poling condition. The maximum TBC gain of 43.90 cm−1 for PVK/DMNPAA/ECZ/PCPEB(33/50/16/1) PR composite films. The reason of TBC gain of PR materials at zero electric field was discussed.

Fast photorefractive response in polymeric composites enabled by the control of chromophore free volume.

The molecular orientation of a chromophore importantly affects the electro-optic characteristics of polymeric photorefractive composites. We designed methyl, ethyl, and isopropyl derivatives of 4-piperidinobenzylidene-malononitrile (PDCST) with the aim of enhancing molecular orientation properties, and investigated the effects of alkyl substitution on the electro-optic properties and response times of polymeric photorefractive composites. The three alkyl-substituted PDCSTs showed enhanced electro-optic responses and photorefractive grating buildup rates.

Photorefractive dynamics in poly(triarylamine)-based polymer composite: an approach utilizing a second electron trap to reduce the photoconductivity

The effects of a second electron trap, tris-(8-hydroxyquinoline)aluminum (Alq3), 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7), or bathophenanthroline (BPhen), on the photorefractive (PR) sensitivity, response time, and diffraction efficiency are investigated for a methyl-substituted poly(triarylamine) (PTAA)-based PR composite. An extremely high PR sensitivity of 1145 cm2 J−1 with a sub-millisecond response time of 397 μs with an internal diffraction efficiency of 73.3% is achieved at an electric field of 60 V μm−1 for the PTAA-based PR composite containing BPhen. The fast, sub-millisecond response time and fourth-order sensitivity are mainly contributed to the reduction of the photocurrent due to the formation of a charge-transfer complex between PTAA and BPhen. Furthermore, the obtained results estimate the response time to be 97 μs for the PTAA-based PR composite containing an appropriate second electron trap with a photoconductivity of 3.3 nS cm−1.

Optimizing the Near‐Infrared Performance of Photorefractive Composites by Chemical Modification of the Sensitizer

AbstractThe influence of structural changes of a naphthalene–bisimide sensitizer on the photorefractive behavior of the respective polymeric composites is reported. By increasing the number of attached thiophene units, the band gap of the sensitizer was gradually reduced, leading to an increased absorption cross‐section of the photorefractive materials. However, the electron‐accepting capabilities as well as the relative content of the sensitizer in the composites remained unchanged for the whole series. The increased absorption had a remarkably beneficial effect on both the steady‐state as well as the dynamic performance of the composites, leading to a material which is able to outperform the former fastest and most sensitive material at the utilized wavelength by a factor of two.

Influence of PCBM on the Glass Transition Temperature (<i>T</i><sub>g</sub>) of Poly(<i>N</i>-Vinylcarbazole)-Based Photorefractive Composite

2,5-dimethyl-(4-p-nitrophenylazo) phenetole (DMNPAPE) was synthesized. And its structure was confirmed with ultraviolet-visible (UV-vis) spectroscopy, Fourier transform infrared (FT-IR) spectra and 1H Nuclear Magnetic Resonance (1H NMR) spectroscopy. The photorefractive composite consist of 33 wt % poly (N-vinycarbazole) (PVK), 50 wt % 2, 5-dimethyl-(4-p-nitrophenylazo) phenetole (DMNPAPE) and 16 wt % ethyl carbazole (ECZ) doped with x wt % (≤1 wt%) [6, 6]-phenyl C61 butyric acid methyl ester (PCBM) was fabricated. The influence of PCBM on the glass transition temperature (Tg) of the photorefractive composite was studied using a differential scanning calorimetric (DSC) method. The active energy of glass transition (Eg) was evaluated by Kissinger’s and Moynihan’s relation. The analysis results indicate that the transition region shifts to higher temperatures with the increasing heating rate, and PCBM content (≤1.0 wt %) can influence Tg of PVK - based PR composite polymers. The Tg first increase and then went down with the PCBM content (below 1.0 wt %) increasing. The possible cause of the influence of PCBM on Tg was proposed.

Practical correction of a phase-aberrated laser beam using a triphenyldiamine-based photorefractive composite

A photorefractive composite based on a triphenyldiamine (TPD) derivative was used to restore a severely phase-aberrated laser beam to a nearly aberration-free state. Here, a forward degenerate four-wave mixing geometry was employed for the elimination of phase distortions and its practical applicability in the transmission of optically encoded data is demonstrated. In addition, it is demonstrated that the experimental geometry is able to effectively restore dynamically updating images. Conventional two-beam coupling and degenerate four-wave mixing experiments were used to characterize the composite subject to the current experimental setup. The two-beam coupling net gain coefficient was 100 cm−1 with an applied external electric field of 70 V/µm. Internal and external diffraction efficiencies of 10 and 6%, respectively, were observed with a similar external electric field. Due to its superior charge-carrier mobility, the TPD-based composite exhibited a response time of 0.28 s, approximately five times faster than traditional PVK-based composites.

Sub-Millisecond Response Time in a Photorefractive Composite Operating under CW Conditions.

Extensive study of photorefractive polymeric composites photosensitized with semiconductor nanocrystals has yielded data indicating that the inclusion of such nanocrystals enhances the charge-carrier mobility, and subsequently leads to a reduction in the photorefractive response time. Unfortunately, the included nanocrystals may also act as a source of deep traps, resulting in diminished diffraction efficiencies as well as reduced two beam coupling gain coefficients. Nonetheless, previous studies indicate that this problem is mitigated through the inclusion of semiconductor nanocrystals possessing a relatively narrow band-gap. Here, we fully exploit this property by doping PbS nanocrystals into a newly formulated photorefractive composite based on molecular triphenyldiamine photosensitized with C60. Through this approach, response times of 399 μs are observed, opening the door for video and other high-speed applications. It is further demonstrated that this improvement in response time occurs with little sacrifice in photorefractive efficiency, with internal diffraction efficiencies of 72% and two-beam-coupling gain coefficients of 500 cm−1 being measured. A thorough analysis of the experimental data is presented, supporting the hypothesized mechanism of enhanced charge mobility without the accompaniment of superfluous traps. It is anticipated that this approach can play a significant role in the eventual commercialization of this class of materials.

Characterization of Carrier Transport and Trapping in Photorefractive Polymer Composites Using Photoemission Yield Spectroscopy in Air

Carrier trapping in organic photorefractive (PR) composites is affected by the bulk state (which can be varied by modifying the molecule structure and processing method used) and the presence of dipolar molecules. Such situations complicate the understanding of carrier trapping in PR composites. The density of states (DOS) provides direct information about carrier trapping in materials. Here, the first attempt to directly measure the DOS in PR composites is performed to investigate the effect of the change of the bulk state on carrier trapping. The distribution of the DOS is measured using photoemission yield spectroscopy in air. The bulk state of the PR composites is tuned based on the crystallization of dipolar nonlinear optical dyes included as a component. Thus, the bulk with a different net dipole (spatially averaged dipole) moment is prepared with the same composition as the PR materials. The edge part of the DOS profile clearly changes upon tuning the bulk state. The diminished net dipole moment achieved by the bulk state tuning causes the suppression of carrier trapping, which is well supported by the dipole trapping model. This result coincides with those obtained using photocurrent and PR grating measurements. image

Enhancement in the photorefractive performance of organic composites photosensitized with functionalized CdSe quantum dots

Enhancement in the photorefractive (PR) performance of organic composites photosensitized by CdSe quantum dots (QCdSe) passivated with the charge-transport ligands, sulfonated triphenyldiamine (STPD), is reported. This enhancement is primarily attributed to the ability of the passivating ligand, STPD, to facilitate the charge-transfer process between the QCdSe and the triphenyldiamine (TPD) charge-transport matrix. The PR composites exhibited a maximum photocharge-generation efficiency of 0.9% and two-beam coupling gain coefficient of 110 cm−1. These figures of merit represent a significant improvement over similar composites photosensitized with more conventional trioctylphosphine oxide-passivated QCdSe (TQCdSe). Moreover, composites photosensitized with SQCdSe had a faster response time of τ = 128 ms at an electric field of 60 V/μm compared with τ = 982 ms for those containing TQCdSe. Because of the molecular similarity between the STPD passivating groups and the TPD-based charge-transport matrix, concentrations of up to 1.4 wt% of SQCdSe are achieved in PR composites without any detectable phase separation, a considerable improvement over the 0.7 wt% for TQCdSe.

Enhanced photorefractivity of a perylene bisimide-sensitized poly(4-(diphenylamino) benzyl acrylate) composite

Enhanced photorefractive (PR) performances are reported for a PR composite of perylene bisimide (PBI)–sensitized poly(4-(diphenylamino)benzyl acrylate) (PDAA), 2-(4-(azepan-1-yl)benzylidene)malononitrile (7-DCST), and (4-(diphenylamino)phenyl)methanol (TPAOH). The addition of a large amount of the TPAOH photoconductive plasticizer was found to produce a preferred hole manifold that reduces the disordered state. A photorefractive performance with a minimum response time of 11 ms, a maximum external diffraction efficiency of 41.6%, a maximum sensitivity of 117 cm2 J−1, and a maximum optical gain of 296 cm−1 were obtained with an external electric field of 55 V μm−1 for PDAA/7-DCST/TPAOH/PBI (30/30/39.9/0.1 by wt.).

Electron dominated grating in a triphenylamine-based photorefractive composite

Electron dominated grating is confirmed in a photorefractive composite using a triphenylamine polymer and moderate electric field.

Photorefractive dynamics in poly(triarylamine)-based polymer composites.

The photorefractive (PR) response and dynamics are investigated in a methyl-substituted poly(triarylamine) (PTAA)-based PR composite. The charge transfer complex between PTAA and an added small amount of second acceptor, (tris(8-hydroxyquinolinato)aluminium) Alq(3), effectively suppresses the photoconductivity, and thus the sample is able to withstand the dielectric breakdown at a high electric field. The resulting PR response is enhanced at a higher electric field. Sub-millisecond PR response times were observed for both optical diffraction and optical amplification: i.e., 350 μs for optical amplification and 860 μs for optical diffraction observed under 532 nm illumination (0.534 W cm(-2)) at 60 V μm(-1). The response time of optical amplification followed the photocurrent response time of 367 μs.

Off-Resonance Photosensitization of a Photorefractive Polymer Composite Using PbS Nanocrystals

The photosensitization of photorefractive polymeric composites for operation at 633 nm is accomplished through the inclusion of narrow band gap semiconductor nanocrystals composed of PbS. Unlike previous studies involving photosensitization of photorefractive polymer composites with inorganic nanocrystals, we employ an off-resonance approach where the first excitonic transition associated with the PbS nanocrystals lies at ∼1220 nm and not the wavelength of operation. Using this methodology, internal diffraction efficiencies exceeding 82%, two-beam-coupling gain coefficients of 211 cm–1, and response times of 34 ms have been observed, representing some of the best figures of merit reported for this class of materials. These data demonstrate the ability of semiconductor nanocrystals to compete effectively with traditional organic photosensitizers. In addition to superior performance, this approach also offers an inexpensive and easy means by which to photosensitize composite materials. The photoconductive c...

Enhanced photoconductivity and trapping rate through control of bulk state in organic triphenylamine-based photorefractive materials

In organic optical semiconductors, it is rather challenging to achieve precise control of photoconductivity and charge trapping, which determines the device performance. This paper reports on enhanced photorefractive response rate through control of the photoconductivity and trapping rate in organic triphenylamine-based photorefractive materials by means of bulk state tuning. The bulk state in organic triphenylamine-based photorefractive composites was controlled through a rapid cooling process from various melting temperatures during sample fabrication. The photoconductivity and trapping rate were determined from photocurrent measurements. Fabrication at lower melting temperatures enhanced the trapping rate for deep traps, whereas it reduced the trapping rate for shallow traps. As a result, a faster photorefractive response was obtained.