SWITCHABLE WINDOW ELEMENT

Abstract

A switchable window element (10) having a layer structure is proposed. The layer structure comprises a switchable layer (20), two polarizers and two optical retarders, wherein a first polarizer and a first optical retarder are arranged in an optical path (40) prior to the switchable layer (20) and a second polarizer and a second optical retarder are arranged in the optical path (40) after the switchable layer (20). Further, the switchable layer (20) is a vertically aligned liquid crystal layer comprising a liquid crystalline medium, wherein the product of the thickness d of the switchable layer (20) and the optical anisotropy Δn of the liquid crystalline medium is in the range of from 0.05 μm to 3.0 μm and the liquid crystalline medium has a clearing point of at least 70° C.

Further aspects of the invention relate to the use of the switchable window element as window for a building or a vehicle.

Claims

1. A switchable window element having a layer structure comprising a switchable layer, two polarizers and two optical retarders, wherein a first polarizer and a first optical retarder are arranged in an optical path prior to the switchable layer and a second polarizer and a second optical retarder are arranged in the optical path after the switchable layer, wherein the switchable layer is a vertically aligned liquid crystal layer comprising a liquid crystalline medium, wherein the product of the thickness d of the switchable layer and the optical anisotropy Δn of the liquid crystalline medium is in the range of from 0.05 μm to 3.0 μm and the liquid crystalline medium has a clearing point of at least 70° C.

2. A switchable window element according to claim 1, the layer structure comprising in this order a first polarizer layer as first polarizer, a first retardation element as first optical retarder, a first electrode layer, a first alignment layer, the switchable layer, a second alignment layer, a second electrode layer, a second retardation element as second optical retarder, and a second polarizer layer as second polarizer.

3. A switchable window element according to claim 1, the layer structure comprising in this order a first polarizer layer as first polarizer, a first retardation element as first optical retarder, a first electrode layer, the switchable layer, a second electrode layer, a second retardation element as second optical retarder, and a second polarizer layer as second polarizer, wherein the switchable layer is a self-aligned vertical alignment liquid crystal layer.

4. A switchable window element according to claim 2, wherein the first retardation element and/or the second retardation element is a layer structure comprising an optically isotropic substrate and a retardation layer.

5. A switchable window element according to claim 2, wherein the first retardation element and/or the second retardation element is a layer structure comprising an optically anisotropic substrate and a retardation layer.

6. A switchable window element according to claim 2, wherein the first retardation element and/or the second retardation element consists of an optically anisotropic substrate.

7. A switchable window element according to claim 4, wherein the optically isotropic substrate is selected from a glass or from a polymer.

8. A switchable window element according to claim 5, wherein the optically anisotropic substrate is selected from polyethylene terephthalate, cellulose triacetate and polycarbonate.

9. A switchable windows element according to claim 1, wherein the first optical retarder and/or the second optical retarder has an absolute value of an out of plane retardation Rth of from 1 nm to 1000 nm and/or an absolute value of an in plane retardation Re of from 1 to 300 nm.

10. A switchable window element according to claim 1, wherein the switchable layer has a thickness d between 1 and 35 μm.

11. A switchable window element according to claim 1, wherein the liquid crystalline medium has an optical anisotropy Δn in the range of from 0.03 to 0.3 for light having a wavelength of 589.3 nm and a dielectric anisotropy Δε of −0.5 to −20.

12. A switchable window element according to claim 2, wherein the first alignment layer and/or second alignment layer is a homeotropic alignment layer.

13. A switchable window element according to claim 12, wherein the homeotropic alignment layer is a polyimide-based layer.

14. A switchable window element according to claim 1, wherein the switchable window is curved in space.

15. A switchable window element according to claim 2, wherein the switchable window comprises at least one further substrate and at least one interlayer, wherein the at least one further substrate is connected to the first polarizer layer and/or second polarizer layer by means of the at least one interlayer.

16. A switchable window element according to claim 1 which is a sunroof of a vehicle and is configured to be normally dark.

17. A switchable window element according to claim 1 which is a windshield of a vehicle or a window of a vehicle or a window of a building, and is configured to be normally bright.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The drawings show:

[0068] FIG. 1 a first embodiment of a switchable window element,

[0069] FIG. 2 a second embodiment of the switchable window element,

[0070] FIG. 3 a third embodiment of the switchable window element,

[0071] FIG. 4a the angle dependence of the dark state for a state of the art window element,

[0072] FIG. 4b the angle dependence of the dark state for an inventive window element,

[0073] FIG. 5a the angle dependence of the bright state for a state of the art window element, and

[0074] FIG. 5b the angle dependence of the bright state for an inventive window element.

[0075] FIG. 1 shows a first embodiment of a switchable window element 10. The switchable window element 10 has a layer structure which comprises in this order a first polarizer layer 12, a first retardation element 14, a first electrode layer 16, a first alignment layer 18, a switchable layer 20, a second alignment layer 22, a second electrode layer 24, a second retardation element 26 and a second polarizer layer 28. The first and second electrode layers 16, 24 are, for example, based on a thin layer of indium tin oxide (ITO).

[0076] Depending on the configuration of the switchable window element 10, the first and second polarizer layers 12, 28 may be arranged in parallel or in crossed configuration. In crossed configuration, the window element 10 is normally dark. In parallel configuration, the window element 10 is normally bright.

[0077] In the embodiment of FIG. 1, the first retardation element 14 and the second retardation element 26 serve as substrates for a liquid crystal cell. The first retardation element 14 carries the first electrode layer 16 and the first alignment layer 18 and the second retardation element 26 carries the second electrode layer 24 and the second alignment layer 22. The retardation elements 14, 26 of the first embodiment are configured as optically anisotropic substrates which provide both mechanical stability and compensation for phase dispersion in a single element.

[0078] The two substrates are arranged such that a liquid crystal cell having a cell gap is formed. The switchable layer 20 is sandwiched between the two substrates, wherein the two alignment layers 18 and 22 are facing towards the switchable layer 20. A seal 30 closes the cell.

[0079] The switchable layer 20 is a vertically aligned liquid crystal layer which comprises a liquid crystalline medium having a negative dielectric anisotropy Δ∈. For achieving the vertical alignment with a pretilt angle of about 90°, the alignment layers 18 and 22 are configured as homeotropic polyimide based alignment layers.

[0080] Light, which passes through the switchable window element 10 along an optical path 40 is first linear polarized by the first polarizer layer 12. The light then passes through the first retardation element 14. Depending on the state of the switchable layer 20, the linear polarization plane of the light is unaffected or rotated by about 90°. After the switchable layer 20, the light passes through the second retardation element 26 and then through the second polarizing layer 28.

[0081] The out of plane and/or in plane retardation of the two retardation elements 14, 26 is selected such that phase dispersion of light passing through the layers 12, 16,18, 20, 22, 24 and elements 14, 26 of the layer structure is compensated. In particular, the out of plane retardation and the in plane retardation of the first and second retardation element 14, 26 are set such that for the switchable layer 20 set to the bright state, light having passed through the first polarizer layer 12, the first retardation element 14, the switchable layer 20 and the second retardation element 26 is linear polarized, wherein the polarization is parallel to the orientation of the second polarizer layer 28. In case the switchable layer 20 is set to the dark state, light having passed through the first polarizer layer 12, the first retardation element 14, the switchable layer 20 and the second retardation element 26 is linear polarized, wherein the polarization is orthogonal to the orientation of the second polarizer layer 28.

[0082] FIG. 2 shows a second embodiment of the switchable window element 10. The switchable window element 10 of FIG. 2 has the same layer structure as the switchable window element 10 of the first embodiment which was described with respect to FIG. 1. The switchable window element 10 of the second embodiment has a layer structure which comprises in this order the first polarizer layer 12, the first retardation element 14, the first electrode layer 16, the first alignment layer 18, the switchable layer 20, the second alignment layer 22, the second electrode layer 24, the second retardation element 26 and the second polarizer layer 28.

[0083] In the second embodiment shown in FIG. 2, the first retardation element 14 is a layer structure comprising a first retardation layer 32 and a first substrate layer 34. Likewise, the second retardation element 26 is a layer structure comprising a second retardation layer 38 and a second substrate layer 36. In the second embodiment, the substrate layers 34, 36 of the retardation elements 14, 26 face towards the switchable layer 20.

[0084] The configuration of the retardation elements 14, 26 as a layer structure allows the use of both optically isotropic and anisotropic substrates. The substrate may be chosen primarily for providing the required mechanical properties as the substrate layers 34, 36 may only provide no or only a part of the required total retardation. The remaining amount of retardation is provided by the first and second retardation layers 32, 38 which must not fulfill mechanical stability requirements by their own and can therefore be chosen only in dependence on the required retardation.

[0085] FIG. 3 shows a third embodiment of the switchable window element 10. The switchable window element 10 of FIG. 3 has essentially the same layer structure as the switchable window element 10 of the second embodiment which was described with respect to FIG. 2. However, the first polarizer element 12 and the first retardation layer 32 are provided in form of a first combined polarizer-retarder element 50 and the second polarizer element 28 and the second retardation element 38 are provided in form of a second combined polarizer-retarder element 52.

[0086] This structure allows the use of optically isotropic substrates 34 and 36 which, in combination with the first and second electrode layers 16, 24, the first and second alignment layers 18, 22 and the switchable layer 20 form a liquid crystal cell. This liquid crystal cell may be prepared in a first step and the combined polarizer-retarder elements 50, 52 may be applied at a later step.

[0087] In addition, the switchable window element 10 of the third embodiment comprises a further substrate 44 and an interlayer 42.

[0088] The further substrate 44 is included in order to provide further mechanical strength. In the embodiment shown in FIG. 3, the further substrate 44 is connected to the second polarizer layer 28 by means of the interlayer 42. Alternatively or additionally, a further substrate 44 may be connected to the first polarizer layer 12. The further substrate 44 is preferably optically transparent and may be selected from a polymer or a glass.

EXAMPLE

[0089] An inventive vertically aligned liquid crystal cell is prepared wherein the product of the thickness d of the switchable layer and the optical anisotropy Δn of the liquid crystalline medium was set to 0.3 μm. The cell gap d was set to 3.45 μm. Two polarizer foils which additionally comprise retardation elements provided by Polatechno Co., Ltd. were used as first and second polarizing layer and first and second retardation elements. The combined polarizer and retarder foils were applied to optically isotropic substrates forming the liquid crystal cell.

[0090] A liquid crystal cell having a Heilmeier configuration was used as comparative example. In a Heilmeier cell, a guest-host system is used as switchable layer which comprises at least one liquid crystal as host and a dichroic dye as guest. When the LC molecules change their orientation due to an applied electric field, the orientation of the dichroic dye is changed as well. The dichroic dye absorbs, or respectively preferentially absorbs, light in one orientation so that light transmission may be modulated by changing the orientation of the dichroic dye. In the comparative example, a configuration using one polarizer and one liquid crystal cell was used.

[0091] The angle dependent transmission of the inventive cell and the Heilmeier cell was determined for the dark state and the bright state. The transmission in the dark state is shown in FIGS. 4a and 4b. FIG. 4a shows the dark state transmission of the Heilmeier cell and FIG. 4b shows the dark state transmission of the inventive switchable window element having a vertically aligned liquid crystal layer. The inventive switchable window element provides an improved dark state having less transmission and less angle dependence than the Heilmeier cell.

[0092] The transmission in the bright state is shown in FIGS. 5a and 5b. FIG. 5a shows the bright state transmission of the Heilmeier cell and FIG. 5b shows the bright state transmission of the inventive switchable window element having a vertically aligned liquid crystal layer. The bright state of the inventive switchable window element is slightly less even than the bright state of the Heilmeier cell. However, the angle having the brightest transmission is large for the inventive switchable window element wherein the angle for the brightest transmission is narrow for the Heilmeier cell.

LIST OF REFERENCE NUMERALS

[0093] 10 switchable window element [0094] 12 first polarizer layer [0095] 14 first retardation element [0096] 16 first electrode layer [0097] 18 first alignment layer [0098] 20 switchable layer [0099] 22 second alignment layer [0100] 24 second electrode layer [0101] 26 second retardation element [0102] 28 second polarizer layer [0103] 30 seal [0104] 32 first retardation layer [0105] 34 first substrate layer [0106] 36 second substrate layer [0107] 38 second retardation layer [0108] 40 optical path [0109] 42 interlayer [0110] 44 further substrate [0111] 50 first combined polarizer-retarder element [0112] 52 second combined polarizer-retarder element