By David Barnes
A presentation at SID’s 50th Anniversary in September revealed that blue phase LCD are coming. I was impressed with early demonstrations of blue phase technology at DisplayWeek 2010 but there were several practical problems to solve and I pushed the topic to the back of my mind. Dr. Wu’s talk at the anniversary surprised me. Advances in liquid crystal formulations, electrode structures and optical films suggest that faster, lower-power LCD may arrive sooner than I thought.
Dr. Wu’s students at CREOL (University of Central Florida) have worked with researchers at ITRI and with Taiwanese engineers at AU Optronics (AUO) and Chimei Innolux (CMI) to develop several solutions to the problem of LCD power consumption. In conventional LCDs, less than 10 percent of the backlight or sidelight power reaches our eyes. This poses problems for larger LCD TVs. For example, U.S. Energy Star 5.3, which became effective a year ago, limits TV power consumption to 108 watts for any HD display sized 50 inches or larger. Use of LED instead of CCFL backlights have helped brands meet that spec. A number of 50- and 55-inch sets earn Energy Star ratings today. Earning the Energy Star tag becomes more difficult as panel sizes increase, however. An 84-inch set would have to consume two-thirds less energy per square inch than a 50-inch set would to get the tag.
And power efficacy is important in smaller displays also. A number of technologies have tried to take share from conventional LCD in applications such as e-Readers or other portable devices. While there has been some success, notably by E Ink, there have been more disappointments. Field-sequential color (FSC) systems for LCD that avoid the light lost in color filters have suffered from slow backlight modulation and from slow liquid crystal response. The advent of LED backlights enables FSC but conventional LC materials impede it. In addition, early advances in lower-power reflective or transflective LCD have been stymied by consumer preference for bright, full-color video.
Polymer-stabilized blue-phase LCD promises to solve many of the problems that face large and small panel makers today. As Dr. Wu explained, clever mixtures of nematic host molecules, chiral dopants and stabilizing monomers can stretch the temperature range of double-twist blue phase LC operation to more than 70º C. More important, they’re fast: Switching time can be less than a microsecond! If discrete RGB (or multi-primary RGBY or even RGBW) backlights are applied, there is plenty of bandwidth for rainbow-trail-free FSC. That would eliminate need for sub-pixels. Effective aperture ratios would increase and color filter losses would be eliminated (along with their costs!). Power efficacy could increase four-fold. That would be really good news for brands planning 84-inch Quad-HD products or long-battery-life portables.
In collaboration with Dr. Wu, CMI engineers invented a way to improve the optical switching capabilities of blue phase LC by forming trapezoidal pixel electrodes. Early attempts to construct IPS-type pixels with blue phase LC faced problems because of higher voltage requirements and cell-gap sensitivities. As shown in the diagram from student Linghui Rao’s paper (doi:10.1063/1.3271771), interleaved electrodes with a trapezoidal shape (a 1 micron plateau on a 2 micron base) put the blue phase crystals in the proper shape to enable fast switching at 10 Volts. The cell gap sensitivity declines also, which is always a good thing for manufacturers. As Dr. Wu explained, such electrode structures are possible, but they may be difficult to control in mass production.
An AUO-sponsored project took a different approach. Rather than shape the electrodes, they chose to shape the input and output angles of light through the display cell. This method, called vertical field switching (VFS) uses optical films to guide light into the cell at an acute angle and turn it with a top film that mitigates the internal reflection problem to extract light efficiently. While it looks complex (see doi:10.1109/JDT.2011.2164236), it shifts the manufacturing problem from one of fabrication to one of film specification, which is a much easier problem to solve.
That’s not all, other student have invented ways to control blue phase materials electrophoretically. Such methods could enable very low-power, color-reflective displays. I was even more surprised during Q&A on a blue-LED paper by a student of Nakamura at UCSB when Dr. Wu pointed out that polarized LED devices could reduce need for polarizer films and increase LCD efficacy even more.
I almost hear Blind Lemon Jefferson playing “Ain’t Singin’ Them Blue-phase Blues No More!”