eSheet LCD Technology



Electroded sheets, eSheets are thin flexible and rollable polymer substrates with embedded wire electrodes, which are electrically connected to patterned transparent conductive stripes.  The wires carry the electric current along the length of the display and the transparent conductive stripes spread the voltage across the surface or pixel width.  The conductive wires solve the RC time constant addressing problems allowing for long-line, high-speed addressing, as well as fine control of grayscale across the display. 


 ESheets are simply formed by embedding the wire electrodes into the surface of a thin polymer substrate. The transparent conductive electrodes are solution-coated and patterned.  The coated and patterned eSheets are then run through a final flattening process to achieve the tight surface flatness specification (<0.5 μm).   The thin polymer substrate material is presently available in rolls over 5 feet wide.  There are no size limits on the wire electrodes or on the process of embedding them in the polymer sheets.  The transparent conductive electrodes can be deposited on the eSheets using a simple low-cost process, which can easily be scaled to large substrate sizes.



 Monochrome reflective eSheet LCDs are produced by simply sandwiching a liquid crystal material between two orthogonal eSheets.



    Cholesteric LC materials (Ch. LC) from Kent Displays was integrated between eSheet substrates to make reflective, bistable LCDs.  Bistable means that once the image is written onto the display Zero Power is required to indefinitely sustain the image.  The displays are multiplex-addressed using a passive addressing scheme, therefore a transistor is not needed at each pixel.  In order to address large LCDs the electrodes in the display have to be sufficiently conductive to overcome the capacitive (C) loading of the liquid crystal line.  Indium Tin Oxide (ITO) is used by most of the industry for electrodes, but ITO alone is not a solution for making large displays because its resistivity is too high.  Resistive lines (R) lead to a high time constants, τ (τ=RC).  A high time constant (τ) means that when a voltage is applied to the end of a line the far end takes a long time to come up to the same voltage.  The cholesteric liquid crystal materials in the LCDs are very sensitive to time at voltage.  If the electrode lines are not conductive enough, the display will have addressing problems (latching to the reflective Planar state), image non-uniformity, crosstalk, slow addressing speeds, and inability to provide grayscale images.

 
 
The cholesteric liquid crystal is a helical structure composed of a nematic liquid crystal doped with chiral molecules.  The mixture of the nematic and chiral molecules create a long "corkscrew"-like structure that Bragg reflects circularly polarized light if the centerline of the helical axis is normal to the display substrate, as shown below.  This reflective state is called the Planar State and only reflects the color of light with a wavelength equal to the pitch (length of a 360° rotation) of the cholesteric molecule.  The reflective Planar State is stable at zero voltage across the cell.  Applying over 2 V/μm across the cell make the cholesteric LC molecule rotate such that the centerline of the helical axis is in the plane of the display.  When the voltage is removed, the cholesteric LC molecules stay in this rotated stable Focal Conic State.  In this Focal Conic State any light incident on the pixel will get forward scattered through the LC cell.  Therefore, the cholesteric LC cell has two stable states, the reflective Planar State and the forward scattering, "transparent", Focal Conic State.  In order to revert the cell back to the reflective Planar State, the cell must be first taken to the Homeotropic State by applying about 5 V/μm across the cell.  This high voltage rips the nematic LC molecules loose from the chiral molecules and completely aligns them with the electric field.  If the voltage or electric field is quickly reduced to zero, then the cell will evolve to the lower energy reflective Planar State. Whereas, if the voltage is reduced slowly (>5 msec) then the cell will relax into the "transparent" Focal Conic State.  Therefore, it is critical to have conductive electrodes to create fast switching speeds, thus allowing the cholesteric LC to be latched to the reflective Planar State.


   The Ch. LCDs are matrix addressed by applying a scan voltages to the display's row wire electrodes while applying the data signals to the column electrodes.  The first row in the panel is addressed by applying a voltage to the 1st scan line, whose voltage is midway between the onset of the Homeotropic state and the full Homeotropic state, while applying data voltages that are in-phase (Focal Conic State) or out-of-phase (Planar State) with the scan voltage.  When the next scan line in the display is selected the pixels in the previous scan line (now at 0 V) will evolve to there corresponding states.  As long as the data (column) voltages are less than the onset of the switch from the Planar to Focal Conic states (~1 V/μm) then the scan line can be address without affecting any of the pixels in the non-select scan lines (rows).
The amount or type of chiral dopant mixed with the nematic liquid crystal can be altered to change the pitch of the cholesteric LCD or the reflective color.  Red, yellow, green and blue cholesteric LC materials were integrated between orthogonal eSheets to create different color panels, as shown below.  Each of these four separate panels had a black back coated eSheet to produce the respective color on black display.






The only effective way to create a reflective full-color LCD is by stacking red, green, and blue color panels one on top of the next, as depicted below.  A three-layer color stack is capable of reflecting the entire light incident on the pixel. This is opposed to using a color filter where the three colors are placed side-by-side and 2/3 of the incident light is lost.  The stacking method requires that the electro-optic materials are modulated from a transparent state to a reflective red, green or blue state.  The cholesteric liquid crystal material is the only known material system that can be modulated from a transparent to a reflective R/G/B state.

Three red, green and blue primary color eSheet LCDs were stacked together to form a full-color display.




One advantage of the eSheet technology is that the electrode lines can be very transparent because the transparent conductive electrode only has to spread the voltage over 1/2 of the pixel width.  Very transparent eSheet substrates are required when fabricating three-layer colored stacked panels because light reflecting off of the bottom ‘red’ reflective LC panel will have to travel through a total of 10 electrode layers (5 down and 5 back out).  Note that the bright lines between pixels is a result of scoring the transparent conductive electrodes with a razor blade.  Therefore, there is no transparent electrode above or below the liquid crystal at these isolation lines to switch the liquid crystal material.  The bright lines will disappear when the transparent electrode is laser scored or printed directly to reduce the isolation width between adjacent transparent conductive electrodes.

The driving scheme has the option of creating many different levels of reflective intensity (grayscale).  The images of the single panel yellow and blue Ch. LCDs, as well as, the RGB stacked eSheet Ch. LCD, shown below were written using eight shades of “gray”.  In order to produce a grayscale panel with high speed addressing, analog data drivers will be required for fine control of the data voltages.




If you are interested in any of Nupix's assets please contact Chad Moore at cmoore@nupix.com.


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