Version 1.0 July 09 1999
by
Salman Saeed
Bistable displays are displays that only have two distinct optical states. Some of the more promising bistable devices are able to maintain their state without the presence of any electrical field and hence are ideal for portable electronic devices.
Other benefits of the bistable design are
The above properties make this technology ideal for devices such as
In fact this technology is ideal for any device that requires a long battery life and has low display refresh rates.
Bistabilty and Cholesteric Cells.
Bistability in cholesteric LC cells will occur under a number of different conditions.
BCT
The BCT display has a cholesteric LC whose intrinsic pitch is on the same order as the wavelength of visible light. This display with suitable surface treatment will exhibit two stable textures with no electric field present. The first is the focal conic and the second the planar texture, as shown below.
The planar state Bragg reflects circularly polarized light whereas the focal conic lets a majority of light thru. Applying a sequence of voltage pulses the two textures can be switched to create an image. This device however suffers from slow switching speeds as the transition involves the formation of disclinations which is an inherently slow process.
BTC/Up-Down
These devices are similar to STN displays. The Up-Down device the director has a 360 degree twist and exhibits hysterisis in its electro optic curve. This means that there are two director configurations that can exist at one voltage, as shown below. At the coexistence voltage the director is in the DOWN state and in the UP state is aligned with the electric field normal to the plane of the cell.
The Up-Down cell however like the STN suffers from the same performance problems such as the unwanted stripe texture that occurs in the UP state. Also the transition from Up to Down is in the order of ~100ms and therefore unsuitable for video rate displays.
The BTC is constructed with a lower d/p~0.6 than the Up-Down device and relies on the flow properties of liquid crystals to select either the 0 or 360 degree state. The lower d/p inhibits the formation of the stripe defects and encourages the 0 degree twist state. However the 180 degree state is lower in energy than the 0 or 360 degree state and so both the latter states will eventually form the 180 degree state. The BTC however has a number of advantages, it has a high switching speed, wide viewing angle, and simple construction.
POLYMER STABILIZATION
Our lab has been working on stabilizing the 0 and 360 degree states of the BTC using polymer stabilization.
As explained above the BTC has three possible twist states in the case where 0<d/p<1. It has the 0, 180 and 360 twist states as shown below.
The 0 and the 360 states are topologically equivalent and so can continously transform from one to the other without a defect. The 180 degree state is topologically distinct from the 0 and 360 twist states but has a lower energy as shown below
At a high field ~40 volts the display relaxes into either the 0 or 360 degree state. The backflow effect was shown by Berreman to switch between the two states. However the 180 degree state has a lower energy than the 0 or 360 state and so after a short period of time the 180 degree state grows in, thereby erasing the information present on the display. This problem can be overcome by refreshing the display every few seconds but this would increase power consumption.
Berreman suggested that a high pretilt area between the pixels would permanently eliminate the 180 degree state. This high pretilt area significantly increases the 180 degree states energy and so inhibits its formation. However the high pretilt area is hard to manufacture and so the idea of photopolymerized polymer walls in the inter pixel region came up.
These polymer walls are created by adding some monomer and photo init to the LC mixture. A mask is placed over the cell with openings where the interpixel region is. The cell is placed under a UV light source and polymerized. Polymer walls form after a few minutes in the interpixel region. It is believed that these walls provide planar alignment for the LC in the cell and so in the interpixel region we have a high pretilt as the LC molecules align alongside the wall. Experimental results show that thes walls do indeed prevent formation of the 180 degree state. No degradation or growth of 180 degree state was observed over several hours indicating that the bistability was truly long term.
We are now looking into using main chain polymers to provide us with better/stronger alignment in the inter pixel regions.