Patent Publication Number: US-2023148148-A1

Title: Single cell liquid crystal device comprising an interstitial substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application No. 63/012,524 filed Apr. 20, 2020, the content of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure relates generally to liquid crystal devices comprising at least one interstitial substrate, and more particularly to liquid crystal windows comprising at least two liquid crystal layers separated by an interstitial substrate. 
     BACKGROUND 
     Liquid crystal devices are used in various architectural and transportation applications, such as windows, doors, space partitions, and skylights for buildings and automobiles. For many commercial applications, it is desirable for liquid crystal devices to provide high contrast ratio between the on and off states while also providing good energy efficiency and cost effectiveness. Higher contrast ratio can be achieved using greater amounts of liquid crystal material and/or light absorbing additives. However, as the layer of liquid crystal becomes thicker, it becomes harder to control the orientation of the crystals, which can negatively impact the optical effectiveness and contrast ratio of the overall device. As such, achieving a high contrast ratio using a single liquid crystal cell design has been challenging to date. 
     Liquid crystal devices including a double cell structure, e.g., two side-by-side liquid crystal cell units, have conventionally been used to obtain desired high contrast ratios. However, double cell structures also have various drawbacks, such as increased overall weight and thickness of the unit and higher manufacturing cost and complexity due to the presence of additional glass layers and electrode components. The additional glass interfaces may also result in optical losses across the double cell structure. 
     As such, there is a need for lighter and/or thinner liquid crystal devices that provide an acceptable contrast ratio for commercial applications. It would also be advantageous to reduce the cost and complexity of manufacturing such a liquid crystal device. It would further be advantageous to improve the energy efficiency and optical effectiveness of such a liquid crystal device. 
     SUMMARY 
     Disclosed herein are liquid crystal devices comprising first and second glass substrate assemblies, first and second liquid crystal layers, and a third interstitial substrate assembly separating the first and second liquid crystal layers. Also disclosed herein are liquid crystal windows comprising a liquid crystal device as disclosed herein and an additional glass substrate separated from the liquid crystal device by a sealed gap. 
     The disclosure relates, in various embodiments, to liquid crystal devices comprising: a first substrate assembly comprising a first glass substrate, a first alignment layer, and a first electrode layer disposed therebetween; a second substrate assembly comprising a second glass substrate, a second alignment layer, and a second electrode layer disposed therebetween; a third substrate assembly comprising a third alignment layer, a fourth alignment layer, and a third substrate disposed therebetween; a first liquid crystal layer disposed between the first substrate assembly and the third substrate assembly; and a second liquid crystal layer disposed between the second substrate assembly and the third substrate assembly. 
     In non-limiting embodiments, the first liquid crystal layer can be in direct contact with the first alignment layer and the third alignment layer, and the second liquid crystal layer can be in direct contact with the second alignment layer and the fourth alignment layer. A thickness of the first and second glass substrates may independently range from about 0.1 mm to about 4 mm. The first and second glass substrates may be independently chosen from soda-lime silicate, aluminosilicate, alkali-aluminosilicate, borosilicate, alkaliborosilicate, aluminoborosilicate, and alkali-aluminoborosilicate glasses. 
     According to various embodiments, a thickness of the third substrate can range from about 0.005 mm to about 1 mm. In certain embodiments, the thickness of the third substrate can be substantially equal to the thickness of the first or second liquid crystal layer. The third substrate may, for instance, comprise glass, ceramic, or plastic materials. According to further embodiments, the third substrate can comprise at least one of an electrical conductivity of at least about 10 −5  S/m or a dielectric constant of at least about 10. In still further embodiments, at least one of the first glass substrate, the second glass substrate, and the third substrate can comprise at least one surface having a surface waviness of less than 100 nm as measured by a contact profilometer. 
     According to various embodiments, a thickness of the first and second electrode layers can independently range from about 1 nm to about 100 nm. The first and second electrode layers may be independently chosen from transparent conductive oxides, graphene, metal nanowires, carbon nanotubes, and conductive ink layers. In certain embodiments, the first and second electrode layers can comprise interdigitated electrodes. In additional embodiments, the first and second electrode layers can comprise a pattern, such as a plurality of lines or a plurality of square or rectangular pixels. 
     Further embodiments of the disclosure include first and second liquid crystal layers having a thickness independently ranging from about 0.001 mm to about 0.2 mm. The first and second liquid crystal layers can, for example, comprise achiral nematic liquid crystal, chiral nematic liquid crystal, cholesteric liquid crystal, or smectic liquid crystal. In some embodiments, the liquid crystal layers can optionally further include at least one additional component chosen from dyes, coloring agents, chiral dopants, polymerizable reactive monomers, photoinitiators, and polymerized structures. Alignment layers can be present in the liquid crystal device and may be in direct contact with the first and/or second liquid crystal layers. A thickness of the alignment layers can independently range from about 1 nm to about 100 nm. Exemplary materials for the alignment layers include, but are not limited to, main chain or side chain polyimides having layer anisotropy, photosensitive azobenzene-based compounds having surface anisotropy, and inorganic thin films having periodic surface microstructures. 
     Also disclosed herein are liquid crystal devices comprising: a first substrate assembly comprising a first glass substrate and a first alignment layer; a second substrate assembly comprising a second glass substrate and a second alignment layer; a third substrate assembly comprising a third alignment layer, a fourth alignment layer, a first interdigitated electrode layer, a second interdigitated electrode layer, and a third substrate; a first liquid crystal layer disposed between the first substrate assembly and the third substrate assembly; and a second liquid crystal layer disposed between the second substrate assembly and the third substrate assembly. In further embodiments, the first interdigitated electrode layer is disposed between the third alignment layer and the first substrate and the second interdigitated electrode layer is disposed between the fourth alignment layer and the third substrate. 
     Further disclosed herein are liquid crystal devices comprising: a first substrate assembly comprising a first glass substrate, a first electrode layer, and, optionally, a first alignment layer; a second substrate assembly comprising a second glass substrate, a second electrode layer, and optionally, a second alignment layer; a third substrate assembly comprising a third substrate and, optionally, one or both of a third alignment layer and a fourth alignment layer; a first liquid crystal layer disposed between the first substrate assembly and the third substrate assembly; and a second liquid crystal layer disposed between the second substrate assembly and the third substrate assembly. 
     According to various embodiments, the liquid crystal devices may include more than two liquid crystal layers, such as three or more, four or more, and so forth. For example, the liquid crystal device may include a third liquid crystal layer and a fourth substrate assembly comprising fifth and sixth alignment layers and a fourth substrate disposed therebetween. Further disclosed herein are liquid crystal windows comprising any liquid crystal device of the above embodiments and a glass substrate separated from the liquid crystal device by a sealed gap. The sealed gap may, in various embodiments, contain air, an inert gas, or a mixture thereof. 
     Additional features and advantages of the disclosure will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide an overview or framework for understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure and together with the description serve to explain the principles and operations of the various embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description can be further understood when read in conjunction with the following drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. It is to be understood that the figures are not drawn to scale and the size of each depicted component or the relative size of one component to another is not intended to be limiting. 
         FIGS.  1 A-B  depict cross-sectional views of liquid crystal devices according to various embodiments of the disclosure; 
         FIGS.  2 A-B  depict cross-sectional views of liquid crystal devices according to additional embodiments of the disclosure; 
         FIG.  3    depicts a cross-sectional view of a liquid crystal device according to further embodiments of the disclosure; 
         FIGS.  4 A-B  depict cross-section views of liquid crystal devices according to still further embodiments of the disclosure; and 
         FIG.  5    depicts a cross-sectional view of a liquid crystal window according to non-limiting embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein are liquid crystal devices comprising: a first substrate assembly comprising a first glass substrate, a first alignment layer, and a first electrode layer disposed therebetween; a second substrate assembly comprising a second glass substrate, a second alignment layer, and a second electrode layer disposed therebetween; a third substrate assembly comprising a third alignment layer, a fourth alignment layer, and a third substrate disposed therebetween; a first liquid crystal layer disposed between the first substrate assembly and the third substrate assembly; and a second liquid crystal layer disposed between the second substrate assembly and the third substrate assembly. Also disclosed herein are liquid crystal devices comprising: a first substrate assembly comprising a first glass substrate and a first alignment layer; a second substrate assembly comprising a second glass substrate and a second alignment layer; a third substrate assembly comprising a third alignment layer, a fourth alignment layer, a first electrode layer, a second electrode layer, and a third substrate; a first liquid crystal layer disposed between the first substrate assembly and the third substrate assembly; and a second liquid crystal layer disposed between the second substrate assembly and the third substrate assembly. 
     Further disclosed herein are liquid crystal devices comprising: a first substrate assembly comprising a first glass substrate, a first electrode layer, and, optionally, a first alignment layer; a second substrate assembly comprising a second glass substrate, a second electrode layer, and optionally, a second alignment layer; a third substrate assembly comprising a third substrate and, optionally, one or both of a third alignment layer and a fourth alignment layer; a first liquid crystal layer disposed between the first substrate assembly and the third substrate assembly; and a second liquid crystal layer disposed between the second substrate assembly and the third substrate assembly. Still further disclosed herein are liquid crystal windows comprising any liquid crystal device disclosed herein and a glass substrate separated from the liquid crystal device by a sealed gap. 
     Embodiments of the disclosure will now be discussed with reference to  FIGS.  1 - 5   , which illustrate various aspects of the disclosure. The following general description is intended to provide an overview of the claimed devices, and various aspects will be more specifically discussed throughout the disclosure with reference to the non-limiting depicted embodiments, these embodiments being interchangeable with one another within the context of the disclosure. 
       FIGS.  1 A-B  and  2 A-B illustrate cross-sectional views of non-limiting embodiments of liquid crystal devices  100 ,  100 ′ ( FIGS.  1 A-B ) and  200 ,  200 ′ ( FIGS.  2 A-B ). The liquid crystal devices disclosed herein may have a single cell configuration, e.g., a single liquid crystal unit controlled by a single pair of electrodes. The liquid crystal unit may comprise two liquid crystal layers, as depicted in  FIGS.  1 - 2   , three liquid crystal layers, as depicted in  FIG.  3   , or more than three crystal liquid layers (not depicted). 
     Referring to  FIGS.  1 A-B , liquid crystal devices  100  and  100 ′ include first and second substrate assemblies  100 A,  100 B. First substrate assembly  100 A comprises a first glass substrate  101  having a first surface  101 A and a second surface  101 B. A first electrode layer  103  is formed on and/or in direct contact with second surface  101 B of first glass substrate  101 . The first substrate assembly  100 A further includes a first alignment layer  106 . The first alignment layer  106  is formed on and/or in direct contact with the first electrode layer  103 . The first electrode layer  103  is thus disposed between the first glass substrate  101  and the first alignment layer  106 , as depicted in  FIGS.  1 A-B . According to various embodiments, no additional layers are present between the first electrode layer  103  and the first substrate  101  or between the first electrode layer  103  and the first alignment layer  106 . In further embodiments, the first substrate assembly  100 A consists of the first substrate  101 , the first electrode  103 , and the first alignment layer  106 . The first substrate assembly  100 A may be referred to interchangeably herein as an “outer” substrate assembly and the first glass substrate  101  may be referred to herein as an “outer” substrate. 
     Similarly, second substrate assembly  100 B comprises a second glass substrate  102  having a first surface  102 A and a second surface  102 B. A second electrode layer  104  is formed on and/or in direct contact with first surface  102 A of second glass substrate  102 . The second substrate assembly  100 B further includes a second alignment layer  109 . The second alignment layer  109  is formed on and/or in direct contact with the second electrode layer  104 . The second electrode layer  104  is thus disposed between the second glass substrate  102  and the second alignment layer  109 , as depicted in  FIGS.  1 A-B . According to various embodiments, no additional layers are present between the second electrode layer  104  and the second substrate  102  or between the second electrode layer  104  and the second alignment layer  109 . In further embodiments, the second substrate assembly  100 B consists of the second substrate  102 , the second electrode  104 , and the second alignment layer  109 . The second substrate assembly  100 B may be referred to interchangeably herein as an “outer” substrate assembly and the second glass substrate  102  may be referred to herein as an “outer” substrate. 
     Liquid crystal devices  100  and  100 ′ also include a third substrate assembly  100 C, disposed between the first and second substrate assemblies  100 A,  100 B. Third substrate assembly  100 C comprises a third alignment layer  107 , a fourth alignment layer  108 , and a third substrate  105 . The third and fourth alignment layers  107 ,  108  are formed on and/or in direct contact with opposing surfaces of the third substrate  105 . The third substrate  105  is thus disposed between the third alignment layer  107  and the fourth alignment layer  108 , as depicted in  FIGS.  1 A-B . According to various embodiments, no additional layers are present between the third substrate  105  and the third alignment layer  107  or between the third substrate  105  and the fourth alignment layer  108 . In further embodiments, the third substrate assembly  100 C consists of the third substrate  103 , the third alignment layer  107 , and the fourth alignment layer  108 . The third substrate  105  may comprise glass, similar to the first and second substrates  101 ,  102 , or may comprise any other suitable material, such as ceramics or plastics. The third substrate assembly  100 C may be referred to interchangeably herein as an “interstitial” substrate assembly and the third substrate  105  may be referred to herein as an “interstitial” substrate. 
     Liquid crystal devices  100  and  100 ′ further include first and second liquid crystal layers  110 ,  111 , which are disposed between the first and third substrate assemblies  100 A,  100 C and the second and third substrate assemblies  100 B,  100 C, respectively. First liquid crystal layer  110  may be in direct contact with the first alignment layer  106  of the first substrate assembly  100 A and in direct contact with the third alignment layer  107  of the third substrate assembly  100 C. According to various embodiments, no additional layers are present between the first liquid crystal layer  110  and the first alignment layer  106  or between the first liquid crystal layer  110  and the third alignment layer  107 . Similarly, second liquid crystal layer  111  may be in direct contact with the second alignment layer  109  of the second substrate assembly  100 B and in direct contact with the fourth alignment layer  108  of the third substrate assembly  100 C. In certain embodiments, no additional layers are present between the second liquid crystal layer  111  and the second alignment layer  109  or between the second liquid crystal layer  111  and the fourth alignment layer  108 . According to further embodiments, the liquid crystal device may consist of the first substrate assembly  100 A, the second substrate assembly  100 B, the third substrate assembly  100 C, the first liquid crystal layer  110  and the second liquid crystal layer  111 . 
       FIG.  1 B  illustrates a non-limiting configuration for a liquid crystal device  100 ′ including first and second seals s 1 , s 2 . First substrate assembly  100 A can be produced, for example, by coating, printing, or otherwise depositing the first electrode layer  103  on the second surface  101 B of the first substrate  101 , and coating, printing, or otherwise depositing the first alignment layer  106  on the first electrode layer  103 . Similarly, second substrate assembly  100 B can be produced by coating, printing, or otherwise depositing the second electrode layer  104  on the first surface  102 A of the second substrate  102 , and coating, printing, or otherwise depositing the second alignment layer  109  on the second electrode layer  104 . Third substrate assembly  100 C can be produced by coating, printing, or otherwise depositing the third and fourth alignment layers  107 ,  108  on opposing surfaces of the third substrate  105 . These substrates assemblies can then be arranged, with the third substrate assembly  100 C between the first and second substrate assemblies  100 A and  100 C, to form two gaps, which can be filled with liquid crystal material to form liquid crystal layers  110 ,  111 . In some embodiments, spacers (not illustrated) can be used to maintain the desired cell gap and resulting liquid crystal layer thickness. The liquid crystal material can be sealed in the cell gaps around all edges using any suitable material, such as optically or thermally curable resins, to form first seal s 1 . A second seal s 2  can optionally be applied to protect the exposed edges of the substrates and/or electrodes and/or any electrical connections within the device from mechanical impacts and exposure to liquids such as water or condensation. In some embodiments, as shown in  FIG.  1 B , the first and second electrode layers  103 ,  104  may be at least partially exposed, e.g., extending outside of seals s 1  and s 2 , to enable electrical connection to a power source. 
     According to non-limiting embodiments, first and second electrode layers  103 ,  104  may include interdigitated electrode layers. Interdigitated electrode layers comprise a pair of electrodes on a single surface that are energized with different voltages. Liquid crystal layer(s) can be controlled by interdigitated electrodes using In Plane Switching (IPS). An electric field starts at the higher voltage interdigitated electrode, travels through any surrounding media (such as an adjacent liquid crystal layer), and terminates at the lower voltage interdigitated electrode. Referring to  FIG.  1 A , electrode layers  103  can comprise interdigitated electrodes on second surface  101 B of first substrate  101 . An applied electric field can then travel from a high voltage interdigitated electrode on second surface  101 B, loop through first liquid crystal layer  110 , and end at a low voltage interdigitated electrode on surface  101 B. Electrode layer  104  can similarly comprise interdigitated electrodes on first surface  102 A of second substrate  102 , to which an electric field can be applied to control the alignment of second liquid crystal layer  111 . 
     The location of the interdigitated electrode layers may not be limited only to the outer substrate assemblies. For example, as shown in  FIGS.  2 A-B , interdigitated electrode layers may alternatively be part of the interstitial substrate assembly. Referring to  FIGS.  2 A-B , liquid crystal devices  200  and  200 ′ include first and second substrate assemblies  100 X,  100 Y. First substrate assembly  100 X comprises a first glass substrate  101  having a first surface  101 A and a second surface  101 B. The first substrate assembly  100 X further includes a first alignment layer  106 . The first alignment layer  106  is formed on and/or in direct contact with the second surface  101 B of first glass substrate  101 . According to various embodiments, no additional layers are present between the first glass substrate  101  and the first alignment layer  106 . In further embodiments, the first substrate assembly  100 X consists of the first glass substrate  101  and the first alignment layer  106 . The first substrate assembly  100 X may be referred to interchangeably herein as an “outer” substrate assembly and the first glass substrate  101  may be referred to herein as an “outer” substrate. 
     Similarly, second substrate assembly  100 Y comprises a second glass substrate  102  having a first surface  102 A and a second surface  102 B. The second substrate assembly  100 Y further includes a second alignment layer  109 . The second alignment layer  109  is formed on and/or in direct contact with the first surface  102 A of second glass substrate  102 . According to various embodiments, no additional layers are present between the second glass substrate  102  and the second alignment layer  109 . In further embodiments, the second substrate assembly  100 Y consists of the second glass substrate  102  and the second alignment layer  109 . The second substrate assembly  100 Y may be referred to interchangeably herein as an “outer” substrate assembly and the second glass substrate  102  may be referred to herein as an “outer” substrate. 
     Liquid crystal devices  200  and  200 ′ also include a third substrate assembly  100 Z, disposed between the first and second substrate assemblies  100 X,  100 Y. Third substrate assembly  100 Z comprises a first interdigitated electrode layer  103 *, a second interdigitated electrode layer  104 *, a third alignment layer  107 , a fourth alignment layer  108 , and a third substrate  105 . The first and second interdigitated electrode layers  103 *,  104 * are formed on and/or in direct contact with opposing surfaces of the third substrate  105 . The third substrate  105  is thus disposed between the first interdigitated electrode layer  103 * and the second interdigitated electrode layer  104 *, as depicted in  FIGS.  2 A-B . According to various embodiments, no additional layers are present between the third substrate  105  and the first interdigitated electrode layer  103 * or between the third substrate  105  and the second interdigitated electrode layer  104 *. The third and fourth alignment layers  107 ,  108  are formed on and/or in direct contact with the first and second interdigitated electrode layers  103 *,  104 *, respectively. The first interdigitated electrode layer  103 * is thus disposed between the third alignment layer  107  and the third substrate  105  and the second interdigitated electrode layer  104 * is disposed between the fourth alignment layer  108  and the third substrate  105 . In certain embodiments, no additional layers are present between the first interdigitated electrode layer  103 * and the third alignment layer  107  or between the first interdigitated electrode layer  103 * and the third substrate  105 . In further embodiments, no additional layers are present between the second interdigitated electrode layer  104 * and the fourth alignment layer  108  or between the second interdigitated electrode layer  104 * and the third substrate  105 . In still further embodiments, the third substrate assembly  100 Z consists of the first interdigitated electrode layer  103 *, the second interdigitated electrode layer  104 *, the third alignment layer  107 , the fourth alignment layer  108 , and the third substrate  105 . The third substrate assembly  100 Z may be referred to interchangeably herein as an “interstitial” substrate assembly and the third substrate  105  may be referred to herein as an “interstitial” substrate. 
     Liquid crystal devices  200  and  200 ′ further include first and second liquid crystal layers  110 ,  111 , which are disposed between the first and third substrate assemblies  100 X,  100 Z and the second and third substrate assemblies  100 Y,  100 Z, respectively. First liquid crystal layer  110  may be in direct contact with the first alignment layer  106  of the first substrate assembly  100 X and in direct contact with the third alignment layer  107  of the third substrate assembly  100 Z. In some embodiments, no additional layers are present between the first liquid crystal layer  110  and the first alignment layer  106  or between the first liquid crystal layer  110  and the third alignment layer  107 . Similarly, second liquid crystal layer  111  may be in direct contact with the second alignment layer  109  of the second substrate assembly  100 Y and in direct contact with the fourth alignment layer  108  of the third substrate assembly  100 Z. In certain embodiments, no additional layers are present between the second liquid crystal layer  111  and the second alignment layer  109  or between the second liquid crystal layer  111  and the fourth alignment layer  108 . According to further embodiments, the liquid crystal device may consist of the first substrate assembly  100 X, the second substrate assembly  100 Y, the third substrate assembly  100 Z, the first liquid crystal layer  110  and the second liquid crystal layer  111 . 
       FIG.  2 B  illustrates a non-limiting configuration for a liquid crystal device  200 ′ including first and second seals s 1 , s 2 . First substrate assembly  100 X can be produced, for example, by coating, printing, or otherwise depositing the first alignment layer  106  on the second surface  101 B of the first substrate  101 . Similarly, second substrate assembly  100 Y can be produced by coating, printing, or otherwise depositing the second alignment layer  109  on the first surface  102 A of the second substrate  102 . Third substrate assembly  100 Z can be produced by coating, printing, or otherwise depositing the first and second interdigitated electrode layers  103 *,  104 * on opposing surfaces of the third substrate  105 , and coating, printing, or otherwise depositing third and fourth alignment layers  107 ,  108  on the first and second interdigitated electrode layers  103 *,  104 *. These substrates assemblies can then be arranged, with the third substrate assembly  100 Z between the first and second substrate assemblies  100 Z and  100 Y, to form two gaps, which can be filled with liquid crystal material to form liquid crystal layers  110 ,  111 . In some embodiments, spacers (not illustrated) can be used to maintain the desired cell gap and resulting liquid crystal layer thickness. The liquid crystal material can be sealed in the cell gaps around all edges using any suitable material, such as optically or thermally curable resins, to form first seal s 1 . A second seal s 2  can optionally be applied to protect the exposed edges of the substrates and/or electrodes and/or any electrical connections within the device from mechanical impacts and exposure to liquids such as water or condensation. In some embodiments, as shown in  FIG.  2 B , the first and second interdigitated electrode layers  103 *,  104 * may be at least partially exposed, e.g., extending outside of seals s 1  and s 2 , to enable electrical connection to a power source. 
     The liquid crystal devices disclosed herein can, in some embodiments, comprise more than two liquid crystal layers, such as three or more, or four or more liquid crystal layers, and so forth.  FIG.  3    depicts one such non-limiting embodiment, which comprises three liquid crystal layers. Liquid crystal device  300  additionally comprises a third liquid crystal layer  113  and a fourth substrate assembly  100 D. Fourth substrate assembly  100 D comprises a fifth alignment layer  114 , a sixth alignment layer  115 , and a fourth substrate  112 . The fifth and sixth alignment layers  114 ,  115  are formed on and/or in direct contact with opposing surfaces of the fourth substrate  112 . The fourth substrate  112  is thus disposed between the fifth alignment layer  114  and the sixth alignment layer  115 , as depicted in  FIG.  3   . In non-limiting embodiments, no additional layers are present between the fourth substrate  112  and the fifth alignment layer  114  or between the fourth substrate  112  and the sixth alignment layer  115 . According to further embodiments, the fourth substrate assembly  100 D consists of the fourth substrate  112 , the fifth alignment layer  114 , and the sixth alignment layer  115 . The fourth substrate  112  may comprise glass, similar to the first and second substrates  101 ,  102 , or may comprise any other suitable material, such as ceramics or plastics. The fourth substrate  112  may comprise the same material as third substrate  105  or may comprise a different material. The fourth substrate assembly  100 D may be referred to interchangeably herein as an “interstitial” substrate assembly and the fourth substrate  112  may be referred to herein as an “interstitial” substrate. 
     The third liquid crystal layer  113  is disposed between the second and fourth substrate assemblies  100 B,  100 D. Third liquid crystal layer  113  may be in direct contact with the second alignment layer  109  of the second substrate assembly  100 B and in direct contact with the sixth alignment layer  115  of the fourth substrate assembly  100 D. According to various embodiments, the liquid crystal device can consist of the first substrate assembly  100 A, the second substrate assembly  100 B, the third substrate assembly  100 C, the fourth substrate assembly  100 D, the first liquid crystal layer  110 , the second liquid crystal layer  111 , and the third liquid crystal layer  113 . 
     It is to be understood that the scope of the disclosure is not limited solely to the embodiments depicted in  FIGS.  1 - 4   . The liquid crystal devices disclosed herein can comprise additional liquid crystal layers and additional substrate assemblies with different configurations, such as any combination of substrate assemblies  100 A,  100 B, and/or  100 C, as depicted in  FIG.  1 A  and substrate assemblies  100 X,  100 Y, and/or  100 Z as depicted in  FIG.  2 A . For example, liquid crystal device  200  in  FIG.  2 A  could be modified to include additional liquid crystal layers and one or more substrate assemblies  100 A,  100 B, or  100 C from  FIG.  1 A . Liquid crystal device  100  in  FIG.  1 A  can similarly be modified to include one or more substrate assemblies  100 X,  100 Y, or  100 Z from  FIG.  2 A . 
     Various components of liquid crystal devices  100 ,  100 ′,  200 ,  200 ′,  300 ,  400 , and  400 ′ will now be discussed in more detail. According to non-limiting embodiments, at least one of the outer (e.g., first and second) substrates, interstitial (e.g., third and fourth) substrates, electrode layers, and alignment layers may comprise an optically transparent material. As used herein, the term “optically transparent” is intended to denote that the component and/or layer has a transmission of greater than about  80 % in the visible region of the spectrum (˜400-700 nm). For instance, an exemplary component or layer may have greater than about 85% transmittance in the visible light range, such as greater than about 90%, or greater than about 95%, including all ranges and subranges therebetween. In certain embodiments, all of the glass substrates, interstitial substrate(s), electrode layers, and alignment layers comprise an optically transparent material. 
     In non-limiting embodiments, the first and second glass substrates  101 ,  102  may comprise optically transparent glass sheets. The first and second glass substrates  101 ,  102  can have any shape and/or size, such as a rectangle, square, or any other suitable shape, including regular and irregular shapes and shapes with one or more curvilinear edges. According to various embodiments, the first and second glass substrates  101 ,  102  can have a thickness of less than or equal to about 4 mm, for example, ranging from about 0.1 mm to about 4 mm, from about 0.2 mm to about 3 mm, from about 0.3 mm to about 2 mm, from about 0.5 mm to about 1.5 mm, or from about 0.7 mm to about 1 mm, including all ranges and subranges therebetween. In certain embodiments, the glass substrates can have a thickness of less than or equal to 0.5 mm, such as 0.4 mm, 0.3 mm, 0.2 mm, or 0.1 mm, including all ranges and subranges therebetween. In non-limiting embodiments, the glass substrates can have a thickness ranging from about 1 mm to about 3 mm, such as from about 1.5 to about 2 mm, including all ranges and subranges therebetween. First and second glass substrates  101 ,  102  may, in some embodiments, comprise the same thickness, or may have different thicknesses. 
     The first and second glass substrates  101 ,  102  may comprise any glass known in the art, for example, soda-lime silicate, aluminosilicate, alkalialuminosilicate, borosilicate, alkaliborosilicate, aluminoborosilicate, alkalialuminoborosilicate, and other suitable display glasses. First and second glass substrates  101 ,  102  may, in some embodiments, comprise the same glass, or may be different glasses. The glass substrates may, in various embodiments, be chemically strengthened and/or thermally tempered. Non-limiting examples of suitable commercially available glasses include EAGLE XG®, Lotus™, Willow®, and Gorilla® glasses from Corning Incorporated, to name a few. Chemically strengthened glass, for example, may be provided in accordance with U.S. Pat. Nos. 7,666,511, 4,483,700, and 5,674,790, which are incorporated herein by reference in their entireties. 
     According to various embodiments, the glass substrates may be chosen from glass sheets produced by a fusion draw process. Without wishing to be bound by theory, it is believed that the fusion draw process can provide glass sheets with a relatively low degree of waviness (or high degree of flatness), which may be beneficial for various liquid crystal applications. An exemplary glass substrate may thus, in certain embodiments, comprise a surface waviness of less than about 100 nm as measured with a contact profilometer, such as about 80 nm or less, about 50 nm or less, about 40 nm or less, or about 30 nm or less, including all ranges and subranges therebetween. An exemplary standard technique for measuring waviness (0.8˜8 mm) with a contact profilometer is outlined in SEMI D15-1296 “FPD Glass Substrate Surface Waviness Measurement Method.” With reference to  FIGS.  1 - 2   , at least one of the first and second surfaces  101 A,  101 B of first glass substrate  101  and/or at least one of the first and second surfaces  102 A,  102 B of second glass substrates  102  can, in some embodiments, comprise a surface waviness as described above, e.g., of less than about 100 nm. Similarly, at least one of the opposing major surfaces of third substrate  105  (not labeled) can, in non-limiting embodiments, also comprise a surface waviness of less than about 100 nm. 
     Third substrate  105  and fourth substrate  112 , if present, as well as any other interstitial substrates that might be present in the liquid crystal device, can comprise a glass material as discussed above with reference to first and second glass substrates  101 ,  102 . In some embodiments, the outer (e.g., first and second) substrates and the interstitial (e.g., third and fourth substrates) may all comprise a glass material, which can be the same or different glass materials. According to other embodiments, the interstitial substrates such as the third and fourth substrates  105 ,  112  may comprise a material other than glass, such as plastics and ceramics, including glass ceramics. Suitable plastic materials include, but are not limited to, polycarbonates, polyacrylates such as polymethylmethacrylate (PMMA), and polyethyelenes such as polyethylene terephthalate (PET). Third and fourth substrates  105 ,  112  (as well as any other interstitial substrates) may, in some embodiments, comprise the same material, or may be different materials. 
     The third substrate  105  and fourth substrate  112 , as well as any other interstitial substrate that might be present in the liquid crystal device, can have any shape and/or size, such as a rectangle, square, or any other suitable shape, including regular and irregular shapes and shapes with one or more curvilinear edges. According to various embodiments, the third and fourth substrates  105 ,  112  can have a thickness of less than or equal to about 4 mm, for example, ranging from about 0.005 mm to about 4 mm, from about 0.01 mm to about 3 mm, from about 0.02 mm to about 2 mm, from about 0.05 mm to about 1.5 mm, from about 0.1 mm to about 1 mm, from about 0.2 mm to about 0.7 mm, or from about 0.3 mm to about 0.5 mm, including all ranges and subranges therebetween. In certain embodiments, the interstitial substrates can have a thickness of less than or equal to 0.5 mm, such as 0.4 mm, 0.3 mm, 0.2 mm, 0.1 mm, 0.05 mm, 0.02 mm, 0.01 mm, or less, including all ranges and subranges therebetween. Third and fourth substrates  105 ,  112  (as well as any other interstitial substrates) may, in some embodiments, comprise the same thickness, or may have different thicknesses. 
     According to various embodiments, during operation of the liquid crystal device, the opposing surfaces of the interstitial substrate(s), e.g., third substrate  105 , may be at the same or substantially the same electrical potential. Without wishing to be bound by theory, it is believed that maintaining a substantially constant electrical potential across the interstitial substrate can reduce the voltage drop across the liquid crystal cell, thereby improving the energy efficiency of the overall device. In certain embodiments, the interstitial substrate(s) may comprise a material with a dielectric constant substantially equal to or greater than the dielectric constant of the liquid crystal material. The liquid crystal dielectric constant can range, in some embodiments, from about 1 to about 100, such as from about 5 to about 90, from about 10 to about 80, from about 15 to about 70, from about 20 to about 60, from about 25 to about 50, or from about 30 to about 40, including all ranges and subranges therebetween. By way of non-limiting example, the dielectric constant of the third substrate  105 , and any other interstitial substrates that may be present in the device, may be greater than or equal to about 1, such as greater than or equal to about 5, greater than or equal to about 10, greater than equal to about 20, greater than or equal to about 50, or greater than or equal to about 100, e.g., ranging from about 1 to about 100, such as from about 5 to about 90, from about 10 to about 80, from about 15 to about 70, from about 20 to about 60, from about 25 to about 50, or from about 30 to about 40, including all ranges and subranges therebetween. In various embodiments, the dielectric constant of the third substrate  105  and any other interstitial substrates present in the device can be greater than or equal to about 10. 
     According to further embodiments, the interstitial substrate(s) may comprise a highly conductive material, for instance, a material having an electrical conductivity of at least about 10 −5  S/m, at least about 10 −4  S/m, at least about 10 −3  S/m, at least about 10 −2  S/m, at least about 0.1 S/m, at least about 1 S/m, at least about 10 S/m, or at least about 100 S/m, e.g., ranging from 0.0001 S/m to about 1000 S/m, including all ranges and subranges therebetween. A substantially constant electrical potential across the interstitial substrate can also be achieved via configurational changes within the liquid crystal device, for example, by providing shorted electrode layers on either side of the interstitial substrate, as shown in  FIG.  4 A , discussed in more detail below. 
     The orientation of liquid crystal material can be described by a unit vector, referred to herein as a “director,” which represents the average local orientation of the long molecular axes of the liquid crystal molecules. The substrates in the liquid crystal device can have a surface energy promoting the desired alignment of the liquid crystal director in a ground or “off” state without applied voltage. A vertical or homeotropic alignment is achieved when the liquid crystal director has a perpendicular or substantially perpendicular orientation with respect to the plane of the substrate. A planar or homogeneous alignment is achieved when the liquid crystal director has a parallel or substantially parallel orientation with respect to the plane of the substrate. An oblique alignment is achieved when the liquid crystal direction has a large angle with respect to the plane of the substrate, which is substantially different from planar or homeotropic, i.e., ranging from about 20° to about 70°, such as from about 30° to about 60°, or from about 40° to about 50°, including all ranges and subranges therebetween. 
     Specific alignment of the liquid crystal can be achieved by coating the surfaces of the substrates and/or electrodes with an alignment layer, for example, alignment layers  106 ,  107 ,  108 ,  109 ,  114 , and  115  as shown in  FIGS.  1 - 4   . Alignment layers can comprise a thin film of material having a surface energy and anisotropy promoting the desired orientation for the liquid crystals in direct contact with its surface. Exemplary materials include, but are not limited to, main chain or side chain polyimides, which can be mechanically rubbed to generate layer anisotropy; photosensitive polymers, such as azobenzene-based compounds, which can be exposed to linearly polarized light to generate surface anisotropy; and inorganic thin films, such as silica, which can be deposited using thermal evaporating techniques to form periodic microstructures on the surface. Organic alignment layers promoting vertical or homeotropic orientation of the liquid crystal molecules may be rubbed to create different pretilt angles other than 90° with respect to the plane of the substrate. The pretilt angle of the liquid crystal molecules with respect to the substrate surface will break the symmetry during switching from vertical orientation and can define an azimuthal direction of liquid crystal switching. 
     Organic alignment layers may be deposited, for example, by spincoating a solution onto a desired surface or using printing techniques. Inorganic alignment layers can be deposited using thermal evaporation techniques. According to various embodiments, the first, second, third, and fourth alignment layers  106 ,  107 ,  108 ,  109 , as well as the fifth and sixth alignment layers  114 ,  115 , if present, and any additional alignment layers, can have a thickness of less than or equal to about 100 nm, for example, ranging from about 1 nm to about 100 nm, from about 5 nm to about 90 nm, from about 10 nm to about 80 nm, from about 20 nm to about 70 nm, from about 30 nm to about 60 nm, or from about 40 nm to about 50 nm, including all ranges and subranges therebetween. The alignment layers  106 ,  107 ,  108 ,  109 ,  114 , and  115  and any other additional alignment layers may, in some embodiments, comprise the same thickness, or may have different thicknesses. 
     While improved alignment of the liquid crystals can be attained through the use of alignment layers, such alignment layers are not required components for the liquid crystal devices disclosed herein. While  FIGS.  1 - 5    depict an alignment layer in contact with both sides of the liquid crystal layers  110 ,  111 , and/or  113 , it is possible to remove one or more of the alignment layers such that no alignment layers are in contact with the liquid crystal layer(s) or only one alignment layer is in contact with the liquid crystal layer. As such, referring to  FIGS.  1 A-B , one or more of alignment layers  106 ,  107 ,  108 , and  109  may be removed from devices  100 ,  100 ′ without departing from the scope of the disclosure. First substrate assembly  100 A can comprise or consist of first substrate  101  and first electrode  103 , i.e., without the presence of first alignment layer  106 . Similarly, second substrate assembly  100 B can comprise or consist of second substrate  102  and second electrode  104 . Third substrate assembly  100 C can comprise or consist of third substrate  105  alone or in combination with only one of third or fourth alignment layers  107 ,  108 . One or more of alignment layers  106 ,  107 ,  108 , and  109  can likewise be removed from devices  200 ,  200 ′ depicted in  FIGS.  2 A-B . One or more alignment layers  106 ,  107 ,  108 ,  109 ,  114 , and  115  can also be removed from device  300  depicted in  FIG.  3   . Finally, one or more alignment layers  106 ,  107 ,  108 , and  109  can be removed from devices  400 ,  400 ′ depicted in  FIGS.  4 A-B . The liquid crystal window(s)  500  can be correspondingly modified to remove one or more alignment layers. 
     As discussed above, liquid crystal devices  100 ,  100 ′,  200 ,  200 ′, and  300  can have a single cell configuration, i.e., electrically speaking, there is a single liquid crystal cell unit, which is controlled by a single pair of electrodes. The single liquid crystal cell unit may have multiple liquid crystal layers, but all of the layers may be controlled by a single pair of electrodes and/or a single power source. According to certain embodiments, liquid crystal devices  100 ,  100 ′,  200 ,  200 ′, and  300  do not include electrodes in addition to the first and second electrode layers  103 ,  104 . In non-limiting embodiments, the liquid crystal devices disclosed herein comprise only two electrode layers. However, in some alternative embodiments, additional electrode layers may be present within the device. By way of non-limiting example,  FIGS.  4 A-B  illustrate devices  400 ,  400 ′ that may comprise additional electrode layers, but that can still function as a single cell, i.e., driven by a single power source. 
       FIG.  4 A  illustrates a non-limiting configuration for a liquid crystal device  400 . Liquid crystal device  400  includes a first substrate assembly  100 A, a second substrate assembly  100 B, a third substrate assembly  100 E, a first liquid crystal layer  110 , and the second liquid crystal layer  111 . Third substrate assembly  100 E is disposed between the first and second substrate assemblies  100 A,  100 B. Third substrate assembly  100 E comprises a third electrode layer  123 , a fourth electrode layer  124 , a third alignment layer  107 , a fourth alignment layer  108 , and a third substrate  105 . The third and fourth electrode layers  123 ,  124  are formed on and/or in direct contact with opposing surfaces of the third substrate  105 . The third substrate  105  is thus disposed between the third electrode layer  123  and the fourth electrode layer  124 , as depicted in  FIG.  4 A . According to various embodiments, no additional layers are present between the third substrate  105  and the third electrode layer  123  or between the third substrate  105  and the fourth electrode layer  124 . The third and fourth alignment layers  107 ,  108  are formed on and/or in direct contact with the third and fourth electrode layers  123 ,  124 , respectively. The third electrode layer  123  is thus disposed between the third alignment layer  107  and the third substrate  105  and the fourth electrode layer  124  is disposed between the fourth alignment layer  108  and the third substrate  105 . 
     In certain embodiments, no additional layers are present between the third electrode layer  123  and the third alignment layer  107  or between the third electrode layer  123  or the third substrate  105 . In further embodiments, no additional layers are present between the fourth electrode layer  124  and the fourth alignment layer  108  or between the fourth electrode layer  124  and the third substrate  105 . In still further embodiments, the third substrate assembly  100 E consists of the third electrode layer  123 , the fourth electrode layer  124 , the third alignment layer  107 , the fourth alignment layer  108 , and the third substrate  105 . The third substrate assembly  100 E may be referred to interchangeably herein as an “interstitial” substrate assembly, the third substrate  105  may be referred to herein as an “interstitial” substrate, and the third and fourth electrode layers  123 ,  124  may be referred to herein as an “interstitial” electrodes. 
     In the embodiment depicted in  FIG.  4 A , third and fourth electrode layers  123 ,  124  are electrically linked to one another, or “shorted,” via connections  125 . As such, the interstitial (e.g., third and fourth) electrode layers  123 ,  124  are not linked to a power source, whereas the outer (e.g., first and second) electrode layers  103 ,  104  are powered. Without wishing to be bound by theory, it is believed that this embodiment may reduce the driving voltage required, by comparison, for the liquid crystal device  200 . Device  400  thus comprises four electrode layers  103 ,  104 ,  123 ,  124 , but is controlled by first and second electrode layers  103 ,  104 , which are linked to a single power source. 
       FIG.  4 B  illustrates a non-limiting configuration for a liquid crystal device  400 ′. Similar to liquid crystal device  400 , liquid crystal device  400 ′ includes a first substrate assembly  100 A, a second substrate assembly  100 B, a third substrate assembly  100 E, a first liquid crystal layer  110 , and the second liquid crystal layer  111 .  FIG.  4 B  further illustrates a different configuration for electrically connecting the electrode layers within liquid crystal device  400 ′. In the depicted embodiment, third and fourth electrode layers  123 ,  124  are electrically linked to first and second electrode layers via connections  126 A,  126 B. First electrode layer  103  may be linked to fourth electrode layer  124  via connection  126 A and second electrode layer  104  may be linked to third electrode layer  123  via connection  126 B. As such, interstitial (e.g., third and fourth) electrode layers  123 ,  124  are linked to opposite outer (e.g., first and second) electrode layers  103 ,  104 , which are connected to a power source (not depicted). Device  400 ′ thus comprises four electrode layers  103 ,  104 ,  123 ,  124 , but these electrodes are all linked to a single power source. 
     Electrode layers in the liquid crystal device, e.g., electrode layers  103 ,  104 ,  103 *,  104 *,  123 ,  124 , may comprise one or more transparent conductive oxides (TCOs), such as indium tin oxide (ITO), indium zinc oxide (IZO), gallium zinc oxide (GZO), aluminum zinc oxide (AZO), and other like materials. Alternatively, the electrode layers may comprise other transparent materials, such as a conductive mesh, e.g., comprising metals such as silver nanowires or other nanomaterials such as graphene or carbon nanotubes. Printable conductive ink layers such as ActiveGrid™ from C3Nano Inc. may also be used. According to various embodiments, the sheet resistance of the electrode layers can range from about 10 Ω/□ (ohms/square) to about 1000 Ω/□, such as from about 50 Ω/□ to about 900 Ω/□, from about 100 Ω/□ to about 800 Ω/□, from about 200 Ω/□ to about 700 Ω/□, from about 300 Ω/□ to about 600 Ω/□, or from about 400 Ω/□ to about 500 Ω/□, including all ranges and subranges therebetween. 
     Electrode layers  103 ,  104 ,  103 *,  104 *,  123 ,  124  can be fabricated using any technique known in the art, such as vacuum sputtering, film lamination, or printing techniques. With reference to  FIGS.  1 A-B , the first and second electrode layers  103 ,  104  can be deposited on the second surface  101 B of the first glass substrate  101  and the first surface  102 A of the second glass substrate  102 , respectively. With reference to  FIGS.  2 A-B , the first and second interdigitated electrode layers  103 *,  104 * can be deposited on opposing surfaces of the third substrate  105 . With reference to  FIGS.  4 A-B , the first and second electrode layers  103 ,  104  can be deposited on the second surface  101 B of the first glass substrate  101  and the first surface  102 A of the second glass substrate  102 , respectively, and the third and fourth electrode layers  123 ,  124  can be deposited on opposing surfaces of the third substrate  105 . The thickness of each electrode layer can, for example, independently range from about 1 nm to about 1000 nm such as from about 5 nm to about 500 nm, from about 10 nm to about 300 nm, from about 20 nm to about 200 nm, from about 30 nm to about 150 nm, or from about 50 nm to about 100 nm, including all ranges and subranges therebetween. 
     In non-limiting embodiments, the electrode layers  103 ,  104 ,  103 *,  104 *,  123 ,  124  can comprise a pattern, such that they produce desired zones or pixels to allow the switching of the entire liquid crystal device or only a desired portion of the device. For instance, the electrode layers can be patterned to form a plurality of lines or stripes having a vertical or horizontal orientation. Such a pattern can be used to configure, e.g., window transmission similar to mechanical shades by turning on alternating stripes or by setting adjacent electrode stripes to different transmission intensities. Alternative patterns are possible and envisioned as falling within the scope of this disclosure, such as a matrix of square or rectangular pixels, which can be used to configure, e.g., window transmission to provide an arbitrary pattern. The width of the patterned lines and/or pixels can range, in various embodiments, from about 1 mm to about 500 mm, such as from about 2 mm to about 400 mm, from about 3 mm to about 300 mm, from about 5 mm to about 200 mm, from about 10 mm to about 100 mm, or from about 20 mm to about 50 mm, including all ranges and subranges therebetween. 
     In a powered “on” state, external voltage applied across the electrode layers generates an electric field within the device that can be used to realign the orientation of the liquid crystals in the device. Additive molecules dissolved in the liquid crystal mixture or otherwise combined with the liquid crystals typically follow the same orientation as the liquid crystals. In an “off” state, the liquid crystals and any additive molecules within the cell will be aligned in an orientation with the smallest amount of free energy. Such a state may be defined by an anchoring force acting on the liquid crystals, e.g., by the alignment layer(s). Voltage applied to the electrodes thus allows the user to change the orientation of the liquid crystals and additive molecules to control the degree of attenuation of light passing through the liquid crystal layer. In a bright/clear state, the geometry and choice of liquid crystal can be chosen to provide equal or substantially equal transmittance to all polarizations of light incident on the cell. Similarly, in a dark/opaque state, the geometry and choice of liquid crystal can provide equal or substantially equal attenuation to all polarizations of light incident on the cell. 
     Liquid crystal devices  100 ,  100 ′,  200 ,  200 ′, and  300 ,  400 ,  400 ′ can include two or more liquid crystal layers, such as first liquid crystal layer  110 , second liquid crystal layer  111 , and third liquid crystal layer  113 , if present ( FIG.  3   ), as well as any other additional liquid crystal layers that might be present in the device. A liquid crystal layer can comprise liquid crystals and one or more additional components, such as dyes or other coloring agents, chiral dopants, polymerizable reactive monomers, photoinitiators, polymerized structures, or any combination thereof. The liquid crystals can have any liquid crystal phase, such as achiral nematic liquid crystal (NLC), chiral nematic liquid crystal, cholesteric liquid crystal (CLC), or smectic liquid crystal, which are operable over a broad range of temperatures, such as from about −40° C. to about 100° C. 
     According to various embodiments, the liquid crystal layers  110 ,  111 , and/or  113 , can comprise a cell gap or cavity that is filled with liquid crystal material. The thickness of the liquid crystal layer, or the cell gap distance, can be maintained by particle spacers and/or columnar spacers dispersed in the liquid crystal layer. The first and second liquid crystal layers  110 ,  111 , and the third liquid crystal layer  113 , if present, as well as any additional liquid crystal layers, can have a thickness of less than or equal to about 0.2 mm, for example, ranging from about 0.001 mm to about 0.1 mm, from about 0.002 mm to about 0.05 mm, from about 0.003 mm to about 0.04 mm, from about 0.004 mm to about 0.03 mm, from about 0.005 mm to about 0.02 mm, or from about 0.01 mm to about 0.015 mm, including all ranges and subranges therebetween. The first and second liquid crystal layers  110 ,  111 , the third liquid crystal layer  113 , if present, and any other liquid crystal layers in the device may comprise the same thickness, or may have different thicknesses. 
     Any liquid crystal switching mode known in the art can be used, such as a TN (twisted nematic) mode, a VA (vertically aligned) mode, an IPS (in plane switching) mode, a BP (blue phase) mode, a FFS (Fringe Field Switching) mode, and an ADS (Advanced Super Dimension Switch) mode, to name a few. An analog switching mode may be desirable in certain embodiments, in which gradual changes in the magnitude of voltage applied to the electrodes allows for variation in transmitted light intensity levels to achieve a gray scale effect. The liquid crystal device may also function in a binary switching mode with only two available light intensity transmission levels—bright/clear (high light transmission) and dark/opaque (low light transmission). One potential advantage for binary mode switching is the ability to function in a bistable fashion, such that electrical power is consumed only during switching between on and off states and is not consumed once these states are reached. 
     Referring to  FIGS.  1 - 2   , a liquid crystal device  100 ,  100 ′,  200 ,  200 ′ comprising two liquid crystal layers  110 ,  111  can allow for three stable optical states. Each bistable liquid crystal layer can be independently switched to a bright/clear state or dark/opaque state. In the first optical state, both the first and second liquid crystal layers  110 ,  111  are switched to a bright/clear state. In the second optical state, both the first and second liquid crystal layers  110 ,  111  are switched to a dark/opaque state. In the third optical state, one of the first or second electrode layers  110 ,  111  is switched to the clear state and the other is switched to the dark/opaque state. 
     Referring to  FIG.  3   , a liquid crystal device  300  comprising three liquid crystal layers  110 ,  111 ,  113  can allow for four stable optical states. In the first optical state, all liquid crystal layers  110 ,  111 ,  113  are switched to a bright/clear state. In the second optical state, all liquid crystal layers  110 ,  111 ,  113  are switched to a dark/opaque state. In the third optical state, one of the electrode layers  110 ,  111 ,  113  is switched to the bright/clear state and the other two are switched to the dark/opaque state. In the fourth optical state, one of the electrode layers  110 ,  111 ,  113  is switched to the dark/opaque state and the other two are switched to the bright/clear state. 
     In some embodiments, dyes or other coloring agents, such as dichroic dyes, can be added to one or more of the liquid crystal layers  110 ,  111 ,  113  to absorb light transmitted through the liquid crystal layer(s). Dichroic dyes typically absorb light more strongly along a direction parallel to the direction of a transition dipole moment in the dye molecule, which is typically the longer molecular axis of the dye molecule. Dye molecules oriented with their long axis perpendicular to the direction of light polarization will provide low light attenuation, whereas dye molecules oriented with their long axis parallel to the direction of light polarization will provide strong light attenuation. 
     A normally bright/clear liquid crystal device with the highest light transmission in the “off” state can, in various embodiments, be achieved by using a homeotropic alignment and a liquid crystal layer comprising liquid crystals with negative dielectric anisotropy and additive dye molecules. In this configuration, the dye molecules will be aligned in a low-absorbing perpendicular orientation in the powered “off” state and will be rotated with the liquid crystals to a highly-absorbing parallel orientation in the powered “on” state. Similarly, a normally dark/opaque liquid crystal device with the highest light transmission in the “on” state can, in certain embodiments, be achieved by using a planar alignment and a liquid crystal layer comprising liquid crystals with positive dielectric anisotropy and additive dye molecules. In this configuration, the dye molecules will be aligned in a highly-absorbing parallel orientation in the powered “off” state and will be rotated with the liquid crystals to a low-absorbing perpendicular orientation in the powered “on” state. 
     Generally, both normally bright/clear and normally dark/opaque liquid crystal devices function in a haze-free or low-haze fashion such that an observer can see through the liquid crystal device with little to no distortion. However, in certain instances, it may be desirable to provide the liquid crystal device with a “privacy” mode such that the image an observer can see through the liquid crystal device is darkened or diffused. Such a privacy mode can be achieved, e.g., by providing a light scattering effect to trap light within the liquid crystal layer such that the amount of light absorbed by the dye is increased. 
     Light scattering effects within the liquid crystal layer can be achieved in several different ways that promote or enhance the random alignment of liquid crystals. One or more chiral dopants may be added to the liquid crystal mixture to form highly twisted cholesteric liquid crystals (CLC), which may have a random alignment that provides light scattering effects, referred to herein as a focal conic texture. Random liquid crystal alignment can also be promoted or assisted by including polymer structures, such as polymer fibers, in the matrix of the liquid crystal layer, referred to herein as polymer stabilized cholesteric texture (PSCT). Random liquid crystal alignment can also be achieved using small droplets of nematic liquid crystal (without a chiral dopant) randomly dispersed in a solid polymer layer or a dense network of polymer fibers, or polymer walls, referred to herein as polymer dispersed liquid crystal (PDLC). 
     According to various embodiments, polymers may be dispersed in the matrix of the liquid crystal layer or on the interior surfaces of the glass and interstitial substrates. Such polymers may be formed by polymerization of monomers dissolved in the liquid crystal mixture. In certain embodiments, polymer protrusions or other polymerized structures may be formed on the interior surfaces of the outer substrates and/or interstitial substrates, such as in a normally clear liquid crystal device with homeotropic alignment layer(s), to define an azimuthal switching direction and to improve electro-optic switching speed. 
     As noted above, chiral dopants may be added to the liquid crystal mixture to achieve a twisted supramolecular structure of liquid crystal molecules, referred to herein as cholesteric liquid crystal (CLC). The amount of twist in the CLC is described by a helical pitch which represents the rotation angle of a local liquid crystal director by 360 degrees across the cell gap thickness. CLC twist can also be quantified by a ratio (d/p) of cell gap thickness (d) to CLC helical pitch (p). For liquid crystal applications, the amount of chiral dopant dissolved in the liquid crystal mixture can be controlled to achieve a desired amount of twist across a given cell gap distance. It is within the ability of one skilled in the art to select the appropriate dopant and its amount to achieve the desired twisted effect. 
     In various embodiments, the liquid crystal layers disclosed herein may have an amount of twist ranging from about 0° to about 25×360° (or d/p ranging from about 0 to about 25.0), for example, ranging from about 45° to about 1080° (d/p from about 0.125 to about 3), from about 90° to about 720° (d/p from about 0.25 to about 2), from about 180° to about 540° (d/p from about 0.5 to about 1.5), or from about 270° to about 360° (d/p from about 0.5 to about 1), including all ranges and subranges therebetween. As used herein, a liquid crystal mixture that does not include chiral dopants is referred to as a nematic liquid crystal (NLC). A liquid crystal that includes a chiral dopant and has a small pitch and a large twist refers to a CLC mixture wherein d/p is greater than 1. A liquid crystal that includes a chiral dopant and has a large pitch and a small twist refers to a CLC mixture wherein d/p is less than or equal to 1. 
     As discussed above, dichroic dyes absorb light more strongly when the long axis of the dye molecules is oriented parallel to the direction of polarized light. Thus, devices comprising nematic liquid crystal layers perform best in cases where there is only one linear polarization of light. However, in certain commercial applications, such as automotive glazings, the light passing through the liquid crystal device is unpolarized. In such instances, it may be advantageous to provide a liquid crystal device comprising two or more liquid crystal layers comprising nematic liquid crystals, and to position the liquid crystal layers with orthogonal orientations (e.g., rotated by 90°) relative to each other to efficiently attenuate the unpolarized light. Alternatively, attenuation of unpolarized light can be achieved using a liquid crystal device comprising two or more liquid crystal layers comprising twisted CLC liquid crystals. For instance, when at least 90° of twist is provided by the CLC across the cell gap thickness, the molecules of dye can absorb substantially all linearly polarized components of the unpolarized light. 
     In the case of planar or homogeneous alignment, in the “off” state a twisted CLC structure will align the dye molecules in a parallel or horizontal orientation, thereby creating a dark/opaque state with minimum light transmission. In the “on” state, the liquid crystal layer will be realigned by the applied electric field to a perpendicular or vertical orientation, thereby creating a bright/clear state with maximum light transmission. Similarly, in the case of vertical or homeotropic alignment, in the “off” state a twisted CLC structure will be suppressed by the alignment layers on either side of the liquid crystal layer, which causes the dye molecules to align in a perpendicular/vertical orientation, thereby creating a bright/clear state with maximum light transmission. In the “on” state, the liquid crystal layer will be realigned by the applied electric field to a parallel/horizontal orientation, thereby creating a dark/opaque state with minimum light transmission. 
     The liquid crystal devices  100 ,  100 ′,  200 ,  200 ′,  300 ,  400 , and  400 ′ disclosed herein can comprise two or more liquid crystal layers and four or more alignment layers. The individual liquid crystal layers in the device may comprise the same or different liquid crystal materials and/or additives, the same or different thicknesses, the same or different switching modes, and the same or different orientations relative to one another. Similarly, the individual alignment layers in the device may comprise the same or different materials, the same or different thicknesses, and the same or different orientations relative to one another. Likewise, the individual electrode layers in the device may comprise the same or different materials, the same or different thicknesses, and the same or different patterns. 
     In certain embodiments, optical effects from a liquid crystal structure can be amplified by assembling the liquid crystal device with alignment layers at specific orientations with respect to each other. For example, the axes of the different alignment layers, which may be defined, e.g., by the direction of rubbing, may be parallel to one another, antiparallel to one another, rotated by 90° with respect to each other, or rotated by another angle relative to each other. 
     Referring to  FIGS.  1 - 2   , first and third alignment layers  106 ,  107  and first liquid crystal layer  110  may produce a first liquid crystal director in the “off” state, whereas fourth and second alignment layers  108 ,  109  and second liquid crystal layer  111  may produce a second liquid crystal director in the “off” state that is different from the first liquid crystal director. Similarly, referring to  FIG.  3   , third liquid crystal layer  113  and the alignment layers that it contacts can produce a liquid crystal director that is the same or different from the directors of the other liquid crystal layers in the device. 
     The liquid crystal devices disclosed herein can be used in various architectural and transportation applications. For example, the liquid crystal devices can be used as liquid crystal windows that can be included in doors, space partitions, skylights, and windows for buildings, automobiles, and other transportation vehicles such as trains, planes, boats, and the like. Referring to  FIG.  5   , a liquid crystal window  500  may, in some embodiments, comprise an additional glass substrate  401 , which is separated from the liquid crystal device  100  by a gap  502 . The additional glass substrate  501  can comprise any suitable glass material having any desired thickness, including those discussed above with respect to first and second glass substrates  101 ,  102 . The gap  502  can be sealed, e.g., by third seal s 3 , and filled with air, an inert gas, or a mixture thereof, which may improve the thermal performance of the liquid crystal window. Suitable inert glasses include, but are not limited to, argon, krypton, xenon, and combinations thereof. Mixtures of inert gases or mixtures of one or more inert gases with air can also be used. Exemplary non-limiting inert gas mixtures include 90/10 or 95/5 argon/air, 95/5 krypton/air, or 22/66/12 argon/krypton/air mixtures. Other ratios of inert gases or inert gases and air can also be used depending on the desired thermal performance and/or end use of the liquid crystal window. 
     In various embodiments, the glass substrate  501  is an interior pane, e.g., facing the interior of the building or vehicle, although the opposite orientation, with glass  501  facing the exterior, is also possible. Liquid crystal window devices for use in architectural applications can have any desired dimension including, but not limited to 2′×4′ (width×height), 3′×5′, 5′×8′, 6′×8′, 7×10′, 7′×12′. Larger and smaller liquid crystal windows are also envisioned and are intended to fall within the scope of this disclosure. Although not illustrated, it is to be understood that the liquid crystal device  500  can comprise one or more additional components such as a frame or other structural component, a power source, and/or a control device or system. It is also to be understood that while  FIG.  5    illustrates a liquid crystal window  500  comprising the liquid crystal device  100  of  FIG.  1 A , any of the liquid crystal devices depicted and/or described herein can also be used in a liquid crystal window application. 
     The liquid crystal devices and windows disclosed herein may have various advantages as compared to prior art devices. For instance, the liquid crystal devices may have a high contrast ratio comparable to that of traditional double cell devices, but with a thinner and/or lighter profile due to the use of less glass in the overall structure. In certain embodiments, the contrast ratio of the liquid crystal devices disclosed herein can be greater than or equal to 1:20, such as greater than 1:30, greater than 1:40, or greater than 1:50, including all ranges and subranges therebetween. Visible light transmission in the dark/opaque state may be about 3% or less, such as about 2% or less, or about 1% or less, including all ranges or subranges therebetween, while light transmission in the bright/clear state may be about 70% or greater, such as about 80% or greater, or about 90 or greater, including all ranges and subranges therebetween. Optical losses may also be minimized due to the reduction in glass interfaces within the device. According to various embodiments, the liquid crystal devices disclosed herein may have a low haze value, such as less than about 1%, less than about 0.5%, or less than about 0.1%, including all ranges and subranges therebetween. 
     While a traditional double cell device comprises four panes of glass, two for each liquid crystal cell, the liquid crystal devices disclosed herein can comprise a single liquid crystal cell having only three substrates, e.g., the first and second (outer) glass substrates and the third (interstitial) glass substrate. Additionally, because the interstitial substrate is not critical in terms of structural stability of the overall device, this substrate can, in some embodiments, have a relatively low thickness as compared to the outer substrates. Thus, even in embodiments where more than one interstitial substrate is present, the overall thickness and/or weight of the device may still be considerably lower than that of a double cell device. 
     Manufacturing complexity and/or cost may also be reduced due to a decrease in the number of components, such as glass substrates and/or electrodes. The liquid crystal devices disclosed herein may comprise only one pair of electrodes, as compared to two pairs of electrodes employed in a double cell device. As such, manufacturing costs can be reduced due to the use of less electrode material, e.g., TCOs, as well as the use of only one electrical drive circuit instead of two. 
     It will be appreciated that the various disclosed embodiments may involve particular features, elements or steps that are described in connection with that particular embodiment. It will also be appreciated that a particular feature, element or step, although described in relation to one particular embodiment, may be interchanged or combined with alternate embodiments in various non-illustrated combinations or permutations. 
     While various features, elements or steps of particular embodiments may be disclosed using the transitional phrase “comprising,” it is to be understood that alternative embodiments, including those that may be described using the transitional phrases “consisting” or “consisting essentially of,” are implied. Thus, for example, implied alternative embodiments to a device that comprises A+B+C include embodiments where a device consists of A+B+C and embodiments where a device consists essentially of A+B+C. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the spirit and scope of the disclosure. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the disclosure may occur to persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims and their equivalents.