Large area, low power reflective displays using bistable Smectic A liquid crystal materials laminated between plastic substrates
Reflective displays have attracted more R&D investment and commercial interest in recent years. This is largely due to the success of E Ink, which has enabled the e-reader market with its electrophoretic (‘e-paper’) frontplane laminates.
Like any other e-paper displays Smectic A (SmA) liquid crystal (LC) devices have low power consumption and good readability in bright light. But the technology also has distinct advantages over competing technologies, including high brightness, perfectly clear transparent state and full-colour in a stacked architecture with subtractive colour modulation (like cyan, magenta and yellow inks in printing systems) without the use of filters and increased optical efficiency due to operation on both light polarisations. Simple passive matrix drive is also possible for many applications.
Developing all-plastic SmA e-paper, compatible with high volume large-area roll-to-roll (R2R) techniques, can produce durable rugged low-cost devices for a range of commercial applications that include outdoor signage and advertising, retail electronic shelf displays, passenger/public information screens, as well as smart glass for radiation control in buildings.
Earlier projects by the Photonics and Sensors Group in the Department of Engineering at the University of Cambridge developed good quality black and white displays on glass using SmA LC materials supplied by industrial partner Dow Corning.
With CIKC funding the group, led by Professor Daping Chu, was able to develop a process for producing reflective displays on plastic substrates for the first time by setting up a lamination facility to encapsulate the SmA LC materials between two plastic sheets to produce a laminated electro-active foil (LEAF). The basic lamination technology and processing conditions were then further optimised. The LEAF project resulted in the demonstration of passive matrix displays with large area single pixels based on SmA LC materials on A5 size plastic substrates by lamination. The project also demonstrated full colour displays, using stacked layers of colour dyed cells in yellow, magenta and cyan. Since SmA LC devices are transparent in one of their optical states much brighter images can be produced. The dyes, made from customised black dye mixtures, can withstand millions of switching operations that are required for commercial applications.
During the course of the project other potential applications, for the construction industry, were also identified. These include switchable window laminates for solar radiation control to improve energy efficiency in buildings by alleviating air-conditioning use in the summer time.
Chu says: ‘While we began this work initially with display applications in mind, through our roadmapping meetings additional opportunities in the construction industry became clear. Our industrial partner, Dow Corning, is very interested in these as the company is already a supplier to the construction industry.
Furthermore, in building applications, the LEAF technology can also be used for retro-fitting existing buildings, a market which is substantially larger than new construction.’
Dow Corning participated in the establishment of the CIKC and seconded an employee, Dr Terry Clapp, as Director for the first two years of the Centre. He says: ‘The Centre’s projects are aligned with Dow Corning’s own R&D activities, and have helped further support our customers’ innovation in areas such as big data computing and sustainable urban development.’
The Technology Strategy Board (TSB)-funded PICWIN (Picture Window) project enabled the production of large size demonstrators for a controllable optical film for the built environment, able to provide solar radiation control and to provide low resolution digital display of information. Partners included the Centre for Process Innovation (CPI), Timsons Ltd, DuPont Teijin Films and Vector Foiltec, which makes lightweight cladding for buildings using ethylene tetra fluoro ethylene (ETFE) films.
The team is working with several industrial partners and a technology transfer agreement concerning some of the technology developed in the LEAF project has recently been signed between the University of Cambridge and a global company to explore this technology in the smart window market.