Dynamics and steady-state properties of photorefractive poly(N-vinylcarbazole)-based composites sensitized with (2,4,7-trinitro-9-fluorenylidene)malononitrile in a 0–3 wt % range

Abstract

Articles you may be interested inInfluence of chromophore content on the steady-state space charge formation of poly [methyl-3-(9-carbazolyl) propylsiloxane]-based polymeric photorefractive composites Influence of the chromophore ionization potential on speed and magnitude of photorefractive effects in poly(Nvinylcarbazole) based polymer composites Temperature-dependence studies of photorefractive effect in a low glass-transition-temperature polymer composite This paper reports on the characterization of six low-T g poly͑N-vinylcarbazole͒-based photorefractive ͑PR͒ composites sensitized with ͑2,4,7-trinitro-9-fluorenylidene͒-malononitrile ͑TNFM͒ in different concentrations, ranging from 0 to 3 wt %. At 780 nm, two-beam coupling gain coefficients, four-wave mixing diffraction efficiencies, and photoconductivities were measured versus electric field, writing beam intensity, and temperature. Dynamic measurements pointed out that chromophore reorientation is not rate-limiting in any of the six samples. In samples with sensitizer concentrations up to 1.24 wt %, increasing the sensitizer concentration leads to a faster grating buildup through a faster charge generation. The grating buildup in these samples is ratelimited by the photogeneration speed. We provide evidence that the TNFM Ϫ anions, formed by photoreduction of TNFM, can act as a trap, similar to what has been observed in C 60 -sensitized samples. As a result, above 1.49 wt % of TNFM, the larger amount of traps produced by photoreduction of the sensitizer reduces the mobility of the charges. Then, the grating buildup speed becomes mobility limited, and smaller buildup rates are observed. Except for the sample with 3 wt % TNFM, increased writing beam intensities or sensitizer concentrations give rise to a larger dynamic range. The different behavior of the sample with the largest sensitizer concentration is explained theoretically in terms of the trap density. The PR phase shifts were found to decrease with increasing writing beam intensity or sensitizer concentration. This provides evidence that the PR phase shifts are controlled by the charge mobility rather than by the photogeneration efficiency. PR measurements as a function of temperature and electric field evolve as predicted by theory.

New Calculation of Charge Generation Efficiency and Photocurrent in Organic Photoconducting Device

“…The holes would increase with the sensitizer, but the photocurrent shows a maximum at 25% of sensitizer concentration. This agrees with experimental result of (2,4,7-trinitro-9-fluorenylidene)-malononitrile in PVK system by Steenwinckel and et al 32 where the elevated concentration of sensitizer could work as traps to suppress the hole mobility. The space charge field formation was similarly dependent on the disorder of the matrix with dipolar materials at the steady stage.…”

Photorefractive Performance of Poly[methyl-3-(9-carbazolyl) propylsiloxane] Based Composites Sensitized with Poly(3-hexylthiophene) in a 0.2-1wt % Range

“…Increasing the P3HT concentration changes the rate at which the mobile charges are generated in the material through increased absorption. 7 As a result, a faster grating buildup would be expected with increasing sensitizer concentration.…”

“…8,9 After generating free electron-hole pairs in the CT complex, the holes can migrate from one site to another with the aid of a strong electric field and become trapped in the dark region of the illumination pattern. 7 As a result of the trapping process, the trapped charges form a spatially varying space charge field that can be translated into a variation in the refractive index through the Pockels effect using a nonlinear optical chromophore. 7 In this work, we used a poly(3-hexylthiophene) (P3HT) as a sensitizer to achieve a high performance of polymeric PR composite.…”

“…7 As a result of the trapping process, the trapped charges form a spatially varying space charge field that can be translated into a variation in the refractive index through the Pockels effect using a nonlinear optical chromophore. 7 In this work, we used a poly(3-hexylthiophene) (P3HT) as a sensitizer to achieve a high performance of polymeric PR composite. P3HT has been used recently in the fabrication of electronic devices, since it has good stability, reasonably high hole mobility, and a field effect mobility.…”

“…In order to provide photosensitivity at the wavelengths of a commercially available low-cost He-Ne laser (632.8 nm), the photoconducting polymer matrix is often doped with a small concentration of an electron deficient molecule as a sensitizer. 7 The sensitizer assists the PR material in generating photo-charge, which is governed by competition between the recombination of a charge carrier with its parent countercharge, which is termed geminate recombination, and electron-hole pair dissociation. To date, several classes of organic molecules have been used as sensitizers in organic PR materials.…”