Looking back to go forward – Dye doped liquid crystals for automotive smart windows and AR optics

Our Technical Director discusses Liquid Crystal cells, and how past designs can inform the future

Mike Banach, Technical Director

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Liquid crystals (LC) have been the dominant display technology for over thirty years, primarily because their unique properties have been continuously exploited to consistently improve product experience.  For example, improved viewing angles and better display contrast have come from gaining better control of the liquid crystalline microstructure.  Also, liquid crystal displays have been getting thinner and thinner as manufacturers have improved their ability to handle thin glass.

Transitioning from glass to bioplastic substrates

At FlexEnable we are pioneering the next revolution in LC display form factors by transitioning from glass to ultra-thin bioplastic substrates – this allows designers to bend or shape displays into their products as opposed to fitting the concept around a rigid display, whilst adding almost no weight and thickness to the device

When innovating, the common belief is that there is a need to look forward, over the horizon, to find ways to challenge the status quo.  However, sometimes you can take steps forward by looking back!

LC displays – creating contrast

Let’s start by explaining how conventional liquid crystals displays work.  The common structure is to put a LC cell between polarisation films (polarisers) that are perpendicular to each other.  Without the LC cell, the two polarisers would absorb both polarisation states of light resulting in a completely black appearance.  When the LC cell is inserted and a voltage is applied it has the ability to twist the polarisation state of light.  Therefore, light that passes through the first polariser can evade the absorption of the second, thus allowing light to emit.  The voltage applied controls this twist, which therefore gives good control of light intensity.

LC cells for displays did not always work in this manner.  In 1965 George Heilmeier, working at RCA labs in Princeton, pioneered a liquid crystal mode in which dye molecules were added to the liquid crystal mixture.  In this case the liquid crystal material was not twisting the light, but instead twisting the dye molecule when voltage was applied to the cell.  This twisting of the dye switched it from an absorbing (dark) to transmissive (clear) state, creating the contrast required to make a display.  Some of the first LC displays in the 1970s were based on this approach.

Eventually, the display industry moved to polariser-based designs because they offered greater potential for increasing the contrast between the dark and clear states.  However, all the LC modes that require a polariser are limited to only 50% transmission in the clear state. This is not a problem for a backlit display, but quite limiting for example, for tintable window applications.

Dye-doped liquid crystals for dynamic light control

At FlexEnable we are now using Heilmeier’s dye-doped (commonly known as guest-host) liquid crystals to make electrically tintable LC films with a transmissive state beyond the 50% transmission of polarised based modes, that can rapidly switch from clear to dark and anything in between.

The most obvious application of this technology is in tintable smart windows for the automotive industry, which can be used for a wide range of aesthetic and comfort features. In this case it is important that the LC cell can be biaxially curved so it can be glazed within the curved glass of modern vehicles.  Another exciting application is for augmented reality – in which the virtual image needs to compete with the brightness of the outside world. Ultrathin, lightweight, ambient or pixellated dimming cells that can be included in optics to reduce external brightness on demand are essential to achieving this.

If you’d like more information about our flexible LC technology and potential applications, contact us at info@flexenble.com.