The making of a plastic liquid crystal cell
June 29, 2021
Liquid Crystal cells have been around for decades. The first work on electrically controlled liquid crystal cells for tuneable optical attenuation was done by George Heilmeier in the 1960s. This work inspired the development of flat panel display technology in the 70s and subsequently, the LCD industry; to this day, liquid crystal cells still dominate the $120Bn display market.
Over the years there has been significant advances in liquid crystal chemistry and mechanisms for controlling the liquid crystal alignment as the industry has progressed from nematic, to in-plane, and vertically aligned modes. However, the basic cell construction has remained the same: the liquid crystal layer is sandwiched between two pieces of glass, with spacers defining the cell gap, and it is all held together at the edges with a glue.
From glass to plastic
At FlexEnable, we’ve pioneered the next significant advancement in liquid crystal technology by developing an approach in which the cells are made with a flexible and formable plastic material instead of rigid glass.
To better understand the challenge of making a LC cell with a plastic instead of glass, it is instructive to go back to the start. The first commercial displays were made using a capillary fill technique. This process uses a cell structure fabricated dry with a small gap purposely left in the edge seal. The gap in the cell is then immersed into a pool of liquid crystal and the liquid eventually wicks into the cell.
This process worked well for small displays such as calculators and watches, but wasn’t efficient for larger format products including monitors and televisions. For these products, a One Drop Fill (or ODF) system was used in which a precise dose of liquid crystal is applied directly onto the bottom plate, after which the top plate is added during the cell formation process. Today, this cell assembly processing technique still dominates the LC industry.
Once we’d begun to experiment with plastic, we found that the key to making thin, lightweight plastic cells was to replicate the same assembly techniques developed for glass and use the same processing equipment already present in display fabs. We found that the capillary fill method was not very effective for plastic cells, therefore, in our very early development work, we took advantage of the flexible nature of the top substrate. We used a sheet-to-plate lamination approach to roll the top plate into position at the same time as we established the cell gap.
Streamlining the manufacturing processes
While this approach worked in theory, it didn’t quite work with the infrastructure available at our volume manufacturing partners. Ultimately, we found that the best approach is to develop a technique in which the plastic is initially affixed to a piece of carrier glass before applying the conventional ODF process. This required significant development work on adhesives and demount mechanisms so that the two pieces of glass could be removed without compromising the integrity of the plastic LC cell.
This process was perfected in our labs in Cambridge and today, is being reproduced in many flat panel display factories across the world. Today, not only can we add flexibility to the list of powerful product attributes that plastic liquid crystal cells enable, but also 3D formability to create non-rectangular displays that can wrap around almost any shaped surface.