Correlation between photorefractivity,photoconductivity, and dark conductivity in dye-doped,low molecular mass nematic liquid crystal cells

The photoconductivity and photorefractivity of dye-doped low molecular mass nematic liquid crystal homeotropic cells exhibit a threshold at an applied voltage of the order of 1 V (in our samples). At about the same voltage, the dark conductivity of the cells exhibits a change from a cubic current/voltage characteristic to a linear one. We explain these observations by postulating that the cell's conductivity is due to ionic charge carriers generated near the electrodes by some electrochemical processes activated by the injection of electrons and holes. Below the noted threshold, the dark current is limited by a residual ionic space–charge distribution, and grows as the cube of the applied voltage. In this regime, photogenerated charge carriers are screened and are prevented from forming the static space–charge field needed to initiate an orientational photorefractive response. Above that threshold, the residual ionic space–charge disappears, the cell is a weak ohmic electrolyte, and the photogenerated charges contribute to the photoconductivity and photorefractivity of the cell.