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

Abstract

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.

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

“…17 Finally, we have observed that cells with just one photoconductive layer were as efficient, in terms of two-beam coupling, as cells with two photoconductive layers, the design adopted in all earlier publications. 3,11,12 As expected, our experimental data on two-beam coupling, which we will present in detail in the following section, are in good agreement with previously published results. 3,11,12 Our data are also consistent with the results obtained in cells with nonphotoconducting polymer layers.…”

“…3,11,12 As expected, our experimental data on two-beam coupling, which we will present in detail in the following section, are in good agreement with previously published results. 3,11,12 Our data are also consistent with the results obtained in cells with nonphotoconducting polymer layers. 13 …”

“…They concluded that since charge photogeneration occurs in the liquid crystal bulk, followed by charge trapping at a PVK surface, PVA layers are not necessary. While the models of the mechanism of the space charge field proposed in these papers were quite different, both groups 11,12 assumed that dc electric field alone induced uniform reorientation of the liquid crystal director, followed by spatially modulated deviation of director. Light contributed to the buildup of space charge field and that field caused further reoreintaion of the director.…”

“…PVA was reported as essential for the buildup of the space charge field. Mun and his group, 12 however, studied cells with different combinations of the alignment layers and dopants in both polymer and liquid crystal layers. They concluded that since charge photogeneration occurs in the liquid crystal bulk, followed by charge trapping at a PVK surface, PVA layers are not necessary.…”

“…The minor differences reported were most likely to be due to specific samples and measurement techniques used by the two groups. The experimental results 11,12 included the measurements of the diffraction and two-beam coupling gain. However, such data do not provide any direct evidence for a particular mechanism involved in the formation and change of reorientation grating.…”