Method for electrically driving a switchable optical element

Abstract

A method for electrically driving a switchable optical element is provided wherein the state of a liquid-crystalline medium is controlled by an applied electric field. The provided method comprises at least one of a) switching from a scattering state to a clear state, b) switching from a clear state to a scattering state, c) holding a scattering state.

Claims

1. A method for electrically driving a switchable optical element comprising at least one switching layer, wherein the switchable optical element is a window element, wherein the switching layer comprises a liquid-crystalline medium having at least three states, the state of the liquid-crystalline medium being controlled by an applied electric field, wherein the switching layer adopts a clear state when the driving voltage exceeds a first level, wherein when the voltage is lowered from the first level, the clear stage is held until the driving voltage falls below a second level, wherein the second level is lower than the first level, wherein the switching layer adopts a scattering state when the driving voltage is further reduced and falls below a third level, and wherein the switching layer adopts a third state when the driving voltage is further reduced and falls below a fourth level, the method comprising: (a) switching from the clear state to the scattering state by lowering the driving voltage from a second clear voltage V.sub.c2 which is lower than the first level and higher than the second level to a low voltage V.sub.L for a period of time t.sub.2 and then raising the driving voltage to a privacy voltage V.sub.p, the privacy voltage V.sub.p being lower than or equal to the third level and higher than the fourth level and the low voltage V.sub.L being lower than the privacy voltage V.sub.p, and (b) holding the scattering state by alternating the driving voltage between the privacy voltage V.sub.p and the low voltage V.sub.L until the state is switched again, wherein the privacy voltage V.sub.p is maintained for a period of time t.sub.4 and the low voltage V.sub.L is maintained for a period of time t.sub.5.

2. The method according to claim 1, further comprising: switching from the scattering state or the third state to the clear state by raising the driving voltage to a clear voltage Vc1 which is equal to or higher than the first level and maintaining the driving voltage at the clear voltage Vc1 for a period of time t 1 and then lowering the driving voltage to the clear voltage Vc2 with the driving voltage being maintained at the second clear voltage Vc2 until the state is switched again.

3. The method according to claim 1, wherein the driving voltage is a square waveform having a frequency of from 0.1 to 1000 Hz.

4. The method according to claim 2, wherein the period of time t.sub.1 is from 1 ms to 60 s.

5. The method according to claim 1, wherein the period of time t.sub.2 is from 1 ms to 60 s.

6. The method according to claim 1, wherein the haze of the switching layer is less than 5% when the switching layer is in the clear state.

7. The method according to claim 1, wherein the switching layer has a haze of at least 20% when the switching layer is in the scattering state.

8. The method according to claim 1, wherein the transition from the scattering state to the third state is defined by a reduction of the haze from the maximum haze observed in the scattering state by at least 10%.

9. The method according to claim 1, wherein the liquid crystalline medium has at least a homeotropic state and a multidomain state and wherein the switching layer is in a clear state when the liquid crystalline medium is in the homeotropic state and is in the scattering state when the liquid crystalline medium is in the multidomain state.

10. The method according to claim 1, wherein the liquid-crystalline medium has a positive dielectric anisotropy .

11. The method according to claim 1, wherein the liquid-crystalline medium comprises a chiral dopant, and the amount of the chiral dopant in the liquid-crystalline medium is from 0.1% by weight to 30% by weight.

12. The method according to claim 1, wherein the liquid crystalline medium comprises a polymer fraction.

13. The method according to claim 1, wherein the switching layer has an average refractive index n, and the molecules of the liquid crystal medium have a pitch p, and the product np is in the range from 50 m to 0.8 m.

14. The method according to claim 1, wherein the molecules of the liquid crystal medium have a pitch p which is between 0.5 m and 30 m.

15. The method according to claim 1, wherein the switching layer has a thickness d and the molecules of the liquid crystal medium have a pitch p, and the d/p is between 3 and 10.

16. The method according to claim 1, wherein when the applied voltage is above the first level, the liquid-crystalline medium exhibits a homeotropic state, when the applied voltage is lowered below the second level the liquid-crystalline medium exhibits a mixed state or a transition state, when the applied voltage is lowered below the third voltage the liquid-crystalline medium exhibits a multidomain state, when the voltage is lowered below the fourth level, the liquid crystalline medium exhibits a planar cholesteric state.

17. The method according to claim 1, wherein the first level is the voltage level when a haze H of less than 5% for the liquid-crystalline medium is first observed when the driving voltage is raised from zero voltage, the second level is the voltage level when a haze H of equal to or greater than 5% for the liquid-crystalline medium is first observed when the driving voltage is lowered from the first level, and the third level is the voltage level when a haze H of greater than 20% for the liquid-crystalline medium is first observed when the driving voltage is lowered from the second level.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings show:

(2) FIG. 1 driving method for switching an optical element to the clear state according to the state of the art,

(3) FIG. 2 driving method for switching an optical element to the privacy state according to the state of the art,

(4) FIG. 3 a first embodiment of a driving method for switching an optical element to the clear state and later to the privacy state,

(5) FIG. 4 a second embodiment of a driving method for switching an optical element to the privacy state,

(6) FIG. 5 a diagram showing at which voltage levels switching of states of the optical element occurs,

(7) FIG. 6 the preparation of control layers for a switchable optical element,

(8) FIG. 7 a schematic view of a switchable optical element,

(9) FIG. 8 a microscope image of the clear state,

(10) FIG. 9 a microscope image of the privacy state at a first time,

(11) FIG. 10 a microscope image of the privacy state at a second time and

(12) FIG. 11 a microscope image of the privacy state at a third time.

(13) In FIG. 1 the driving scheme for switching a switchable optical element to the clear state according to the state of the art is plotted. The plot shows the root mean square value (RMS) of the AC driving voltage versus time.

(14) For switching of the switchable optical element from the privacy state II to the clear state I, the driving voltage is raised to a clear voltage V.sub.c. As long as the clear state is to be maintained the driving voltage is held at the constant level V.sub.c.

(15) In FIG. 2 the driving scheme for switching a switchable optical element to the privacy state according to the state of the art is plotted. The plot shows the root mean square value (RMS) of the AC driving voltage versus time.

(16) For switching of the switchable optical element from the clear state I to the privacy state II, the driving voltage is lowered to a privacy voltage V.sub.p. As long as the privacy state is to be maintained the driving voltage is held at the constant level V.sub.p.

(17) FIG. 3 depicts a first embodiment of the proposed driving scheme. In the plot of FIG. 3 the RMS value of the AC driving voltage is plotted versus time t.

(18) For switching of the switchable optical element from the privacy state II to the clear state I, the driving voltage is first raised to a first clear voltage V.sub.c1. This applied voltage is maintained for a first period of time t.sub.1. After lapsing of the first time period t.sub.1, the driving voltage is lowered to a second clear voltage V.sub.c2. As long as the clear state is to be maintained the driving voltage is held at the constant level V.sub.c2.

(19) For switching of the switchable optical element from the clear state I to the privacy state II, the driving voltage is first lowered to a low voltage V.sub.L which is preferably set to 0 V. The lowering is performed gradually over a second time period t.sub.2. At the end of the time period t.sub.2, the applied driving voltage is at the low Voltage V.sub.L and is from there raised again to the privacy voltage V.sub.p. The raising of the driving voltage is also done gradually over a time period t.sub.3. After the time period t.sub.3 has expired, the driving voltage is maintained at the constant level V.sub.p until the next switch occurs.

(20) FIG. 4 depicts a further embodiment of the proposed driving scheme. In the plot of FIG. 4 the RMS value of the AC driving voltage is plotted versus time t.

(21) For switching of the switchable optical element from the clear state I to the privacy state II, the driving voltage is first lowered to a low voltage V.sub.L which is preferably set to 0 V. The lowering is performed gradually over a second time period t.sub.2. At the end of the time period t.sub.2, the applied driving voltage is at the low voltage V.sub.L and is from there raised again in a single step to the privacy voltage V.sub.p. The driving voltage is maintained at the privacy voltage V.sub.p for a fourth time period t.sub.4. After the time period t.sub.4 has expired, the driving voltage is lowered to the low voltage V.sub.L in a single step. The low voltage is maintained for the fifth time period t.sub.5. After the fifth time period has expired the driving voltage is again raised to the privacy voltage V.sub.p. The driving voltage is alternated between the privacy voltage V.sub.p and the low voltage V.sub.L in the described manner until the next switch to the clear state occurs.

(22) FIG. 5 shows a plot of the driving voltage versus time in which the driving voltage is raised linearly to a maximum voltage and then lowered again.

(23) When the driving voltage is raised, the switchable optical element adopts the clear state when the driving voltage exceeds the first level. The voltage corresponding to the first level is marked with V1 in FIG. 5. When the driving voltage is subsequently lowered, the relaxation back to the scattering (privacy) state does not occur at the same level. The optical element remains in the clear state until the driving voltage is lowered below the second level marked with V2 in FIG. 5. This hysteresis is used to define the first clear voltage level and the second clear voltage level. The first clear voltage V.sub.c1 is chosen equal to or slightly above the voltage marked with V1 and the second clear voltage V.sub.c2 is chosen equal to or slightly above the voltage marked with V2 in FIG. 5. The first and second clear voltages are preferably chosen slightly higher than the respective V1 and V2 voltages by adding a safety margin to ensure that the first clear voltage is always high enough to safely switch the optical element into the clear state and that the second clear voltage is high enough to safely maintain the clear state.

(24) FIG. 6 shows the preparation of control layers for a switchable optical element. Two glass substrates are cut to the shapes depicted in FIG. 6 by removing two diagonally opposite corners of the rectangular substrate. The glass substrates are coated with an indium-tin-oxide (ITO) transparent electrode. Additionally, an alignment layer is arranged on the ITO electrodes. When arranged in a cell wherein the two glass substrates form a gap for receiving the liquid-crystalline medium, the alignment layers provide a preferred direction for the alignment of the liquid-crystal molecules. In the embodiment of FIG. 6, the alignment layers of the two glass substrates are arranged at a respective angle of the rubbing directions of 90. Alternative angles, such as 30, 40, 50, 60, 70 and 80, and 100, 110, 120, 130, 140 and 150 are also possible.

(25) For each of the two glass substrates, two corners have been cut away. When the two glass substrates are arranged in the cell configuration, the ITO electrodes are facing towards the inside of the formed gap. The cut corners expose parts of the electrodes and thus allow the electrodes to be electrically contacted.

(26) FIG. 7 shows a schematic view of a switchable optical element 1. The switchable optical element 1 comprises in this order a first glass substrate 11, a first transparent electrode 12, a first alignment layer 13, a switching layer 14, a second alignment layer 15, a second transparent electrode 16 and a second glass substrate 17.

(27) The switching layer 14 comprises the liquid-crystalline medium. The state of the liquid-crystalline medium is controlled by an electric field which is generated by a driving voltage applied between the first transparent electrode 12 and the second transparent electrode 16.

(28) FIG. 8 shows a microscope image of the clear state. The structures seen in the microscope image are the structures and imperfections of the glass surface of the substrates. No structures of the switching layer are visible.

(29) FIG. 9 shows a microscope image of the privacy state at a first time. The image depicts the privacy state right after the switching has occurred with the driving voltage first set to the privacy voltage V.sub.p. The image shows an homogenous distribution of the domains.

(30) FIG. 10 shows a microscope image of the privacy state at a second time. The image depicts the privacy state at the end of the fifth time period t.sub.5 according to the driving method shown in FIG. 4. Some areas have relaxed back into the planar cholesteric state but most of the domains are still of a small size so that the privacy state of the optical element is still maintained at the depicted time.

(31) FIG. 11 shows a microscope image of the privacy state at a third time. The image depicts the privacy state at the end of the fourth time period t.sub.4 according to the driving method shown in FIG. 4. The image shows again a homogenous distribution of the domains of the multidomain state.

EXAMPLES

Example 1

(32) Two sheets of conductive ITO (indium-tin-oxide) coated glass are obtained. The sheets are cut and ground in the shapes depicted in FIG. 6.

(33) After washing the substrates, a polyimide alignment layer is printed on their coated side. The substrates are baked in an oven and the polyimide is rubbed to obtain alignment layers with a mutual rotation of 90. Subsequently, the substrates are arranged as a cell with a 25 m cell gap (alignment layers facing inside). When combined into a cell, the cut corner of one substrate is facing the non-cut corner of the other substrate, resulting in four areas where electrical contact can be established.

(34) A mixture is prepared consisting of the nematic liquid crystalline medium LCM-1 (see Table 1 for composition) and 1.2 wt % of the chiral dopant R-5011. Subsequently, the cell is filled with the liquid crystal/chiral dopant mixture using vacuum filling, pressed and placed into an oven for a final curing step. To obtain the switchable window, the liquid crystal cell is combined with a glass sheet into an insulated glass unit. Electrical wiring is attached onto the contact areas by soldering.

(35) An AC power source with a 60 Hz frequency and a square waveform with controllable voltage is prepared and connected to the switchable window.

(36) To obtain the clear state, an RMS (root-mean-square) voltage of 57 V is applied for 10 seconds. Subsequently, the voltage is lowered to 45 V, still holding the clear state. The haze H is measured to be 0.5% and the clarity C is measured to be 99.8% (measurements are done using a BYK haze-gard i instrument, which uses the norm ASTM D 1003-00).

(37) To obtain the privacy state, the voltage is reduced from 45 V to 4 V by gradually reducing the voltage over 3 seconds. The haze H is measured to be 85% and the clarity C is measured to be 29%.

Example 2

(38) A glass cell with a 25 m cell gap is produced similar to example 1. However, this time no alignment layer is applied. A mixture is prepared consisting of the nematic liquid crystalline medium LCM-2 (see Table 2 for composition) and 2.1 wt % of the chiral dopant S-811. Subsequently, the cell is filled with the liquid crystal/chiral dopant mixture. Electrical wiring is attached by soldering and the cell is connected to a variable voltage source.

(39) The cell is switched to a scattering state by lowering the voltage from 25 V to 5 V. The microscope image shown in FIG. 9 depicts the observed multidomain state.

(40) When the cell is switched from 25 V to 0 V, and subsequently kept at 0 V, gradually growing planar domains could be observed as can be seen in the microscope images shown in FIG. 10. This results in a gradual loss of privacy.

(41) The cell is switched from 25 V to 5 V, then to 0 V, and after 30 seconds back to 5 V. After the switch from 25 to 5 V a multidomain state is obtained. At 0 V, planar domains start to grow gradually. The cell is then switched back to 5 V for 30 seconds. After these 30 seconds, a complete multidomain state is again obtained (see FIG. 11). This pattern of varying between 0 V and 5 V is continued, thereby maintaining a scattering window.