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

This paper reports on the characterization of six low-T-g poly(N-vinylcarbazole)-based photorefractive (PR) composites sensitized with (2,4,7-trinitro-9-fluorenylidene)-malononitrile (TNFM) 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 rate- limited by the photogeneration speed. We provide evidence that the TNFM- 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. (C) 2001 American Institute of Physics.