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1. Single-Substrate Bistable Color Reflective Cholesteric Displays
Cholesteric liquid crystals (ChLCs) in the planar texture possess the unique feature of separating
incident light into its left- and right-handed circular components by reflecting one component and
transmitting the other. 1,2 In a planar aligned ChLC with a preselected helical pitch only a single
wavelength can be Bragg reflected (monochrome). There are two ways to prepare multiple wavelength
(or color) reflecting ChLCs; one method is by stacking multiple layers of ChLCs with different cholesteric
pitches to reflect different wavelengths. In this case, the stacking is normally arranged in a fashion that
ChLC reflecting a shorter wavelength is placed on top and the ChLC reflecting a longest wavelength is
placed at the bottom of the stacked films. Alternatively, multiple (wavelengths) colors reflection ChLCs
can be obtained in sequential arrangement in a single layer with ChLCs reflecting different wavelengths
or colors.
Using inter-digitized electrodes to untwist the helix with in-plane field is an alternative method to
tune cholesteric color. A couple of reported methods on field-induced red shift in a cholesteric liquid
crystal with positive dielectric anisotropy is due to the field-induced pitch dilation.3,4 However, these
methods require a high switching voltage because of the inhomogeneous in field distribution within the
cell and the electric torque applied to different layer of cholesteric liquid crystal to unwinding the helix. A
dramatic loss in reflectivity with increasing voltage is observed in this method due to the reduction in
number of layers and pitch elongation.
Here, we demonstrate a couple of approaches in tuning the cholesteric color; one method is using
the photo-induced pitch change with a tunable chiral additive to allow the adjustment of a pre-selected
ChLC pitch to reflect different colors and the second method is to use electric field to tune a pre-selected
cholesteric color to reflect a second color. 5 A related report on using light control of cholesteric pitch is
reported by doping a CLC with an azobenzene chiral dye.6 As shown in Fig. 1a, a single substrate color
ChLCD panel is fabricated with a photo-induced pitch change (via photo-induced isomerization or
racemization of a chiral additive) in a ChLC. The tuned color pixels are bistableat at zero voltage. The
Bragg reflection spectra of the photo-tuned color pixels is shown in Fig. 1b, where a ChLCD with a blue
color is successively exposed to UV light through a set of photomasks to give a single substrate with
multiple colors. The ChLCD color pixels displays the bistable electro-optical behavior where the color
pixels can be switched either from a reflective state (bright state) or a transmissive state (weak scattering
dark state) with the back of the real panel painted with a black color. The turn on voltages for different
color pixels are around 30, 40 and 50V for red, green and blue pixels, respectively. With an optimized
ChLC mixture and reduction in cell gap, the switching voltages for the color pixels have been reduced to
20, 30 and 40V, respectively. Gray scale switching for the ChLCD has been established by my colleagues
possible through the combination of variation in pulse width and voltages for different ChLC textures
such as preparation, selection and evolution voltage. The challenges for single-substrated ChLCD include
the differential switching voltage for color pixels and brightness of the display, which proportional to the
pixel size.
(a)
(b)
(c)
Fig. 1. (a) A single-substrate bistable reflective ChLCD with pixilated colors, (b) the photo-tuned color pixels and
corresponding reflection spectra, and (c) the electro-optical behavior of the photo-tuned color pixels.
It is recognized quite early that electric-field induced color change in ChLCs can be achieved by
applying a voltage on a ChLC in the direction either parallel or perpendicular to the helix axis. The tuning
of the Bragg reflection is through either extending or compressing the helical pitches or inducing tilt of
helixes.7,8 The mechanism of color tuning is referred to Helfrich deformation, 9 indicating uniformly
periodical layer deformations in the ChLCs in response to external stimuli. The local tilting of helix leads
to shorten of pitch observed from a normal direction, while helix unwinding or dilation in response to
applied voltage leads to a red shift in Bragg reflected wavelength. Both actions, in general, are followed
with the increase in reflection bandwidth.
By using a polymer network formed in the cholesteric, Hikmet10 reported used polymer network
to create liquid crystal gels with the blue-shift and broadening in reflected wavelength with increasing
voltage. A polymer stabilized chiral nematic liquid crystal enables the reversible color switching from
blue to red with the helix axes lie in the plane of the device.11 The electric field is applied perpendicular
to the helix axis which leads to an unwinding of the helices as the field strength is increased.
Here, we present an electrically-switched color cholesteric display with transmissive and
reflective properties in a polymer-stabilized CLC (PSCLC) cell, in which the cell exhibits a color reflection
from one side of the cell (fig.3a), while transmits the light on the other side of the cell (fig.3b).12 The
reflectivity remains in a certain value while the reflected wavelength is electrically switchable. Instead of
broadening, the reflection bandwidth decrease slightly with increasing voltage. According to fig. 3c, the
CLC cell for electrically-switched reflective wavelength has a threshold at approximately 2.0 V/m and
the electrically-tuned color is blue shifted as the increase in applied voltage with a maximum alteration
range of 140 nm. The inserted photos are the color tuning of a cell in response to applied voltage. The
tuning range and critical voltage of the initial reflectivity decreasing are controllable by either varying the
film thickness or localization of the polymer network in the cell. The polymer-stabilized cholesteric
shows good thermal stability of tuned color and the electrically-induced color tuning is reversible. The
electrically tunable wavelength filter may be applied to other spectra such as ultraviolet, infrared, etc.
(a)
(b)
(c)
2o CIE 1931
Increasing in voltage
Fig. 3. (a) The reflection spectra of a polymer-stabilized cholesteric LC (PS-ChLC) cells with anisotropic
reflection behavior, (b) the voltage driven blue-shift in reflective wavelength and reflectance versus
voltage, and (c) the electrically-switched color and color space of a PS-ChLC cell).
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