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AssigneeRolic AG
InventorsJurg Funfschilling, Martin Schadt
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The commercial production of DHF (Deformed Helix Ferroelectric) cells, in particular DHF color display cells, has scarcely become established to date. This is in particular because the DHF cells required for this purpose and based on the known prior art have short switching times and a good angular range of view but relatively high voltages and currents are required for their operation. Moreover, for the production of a DHF cell, the liquid crystal mixture must additionally be oriented or formatted by means of an electric field. The present invention relates to a ferroelectric display cell which does not have these disadvantages and in particular is distinguished by the fact that the display is dark at U=0 and bright at .vertline.U.vertline.>0, the current-voltage hysteresis characteristic has two maxima of equal size, the birefringence .DELTA.n in the field-free state is greater than that in the saturated state and the saturation voltage is relatively low even with very small spontaneous polarization P.sub.s (P.sub.s <5 nC/cm.sup.2).
A liquid crystal cell having a ferroelectric, chiral smectic, birefringent liquid crystal layer and at least one polarizer, the cell having a pair of parallel plates which enclose the liquid crystal layer and each of which is provided with a surface structure orienting the molecules of the liquid crystal layer and at least one electrode for generating an electric field with a voltage U, and the liquid crystal layer having a layer thickness d and a saturated switching angle .alpha., wherein the layer thickness d is smaller than the helix pitch p of the liquid crystal layer, wherein the saturated switching angle .alpha. is sufficient to give rise to linear liquid crystal domains and the liquid crystal layer, upon cooling from an isotropic phase, forms a cholesteric N phase before the S.sub.c * phase, wherein the optical path difference .DELTA.n.multidot.d.sub.eff of the cell at U=0 is greater than in the saturated state, .DELTA.n being the birefringence and d.sub.eff the distance covered by the light in the liquid crystal between a polarizer and an analyzer, and wherein the current-voltage characteristic of the cell has two maxima, one for U>0 and one for U<0.
A liquid crystal cell having a ferroelectric, chiral smectic, birefringent liquid crystal layer and at least one polarizer, the cell having a pair of parallel plates which enclose the liquid crystal layer and each of which is provided with a surface structure orienting the molecules of the liquid crystal layer so as to give rise to a helix having a pitch p, and at least one electrode for generating an electric field with a voltage U, and the liquid crystal layer having a layer thickness d and a saturated switching angle .alpha., wherein the layer thickness d is smaller than the helix pitch p of the liquid crystal layer, the current-voltage characteristic of the cell has two maxima, one for U>0 and one for U<0, and the spontaneous polarization does not change in sign from molecular plane to molecular plane.
A liquid crystal cell having a ferroelectric, chiral smectic, birefringent liquid crystal layer and at least one polarizer, the cell having a pair of parallel plates which enclose the liquid crystal layer and each of which is provided with a surface structure orienting the molecules of the liquid crystal layer, and at least one electrode for generating an electric field with a voltage U, and the liquid crystal layer having a layer thickness d and a saturated switching angle .alpha., wherein the optical path difference .DELTA.n.multidot.d.sub.eff of the cell at U=0 is greater than in the saturated state, .DELTA.n being the birefringence and d.sub.eff the distance covered by the light in the liquid crystal between a polarizer and an analyzer, and wherein the current-voltage characteristic of the cell has two maxima, one for U>0 and one for U<0.
Dependencies are inferred from the claim text.