Wed, May 26. Along with Dr. Rob Zehner of E Ink, I cochaired Electronic Paper II in the SID Symposium at Display Week today. I also wrote about the talks as they progressed, including my thoughts and notes, to give you a flavor of the progression of a symposium session. So I did double duty as chair and blogger.
Paper 1. First up was Mark Goulding of Merck Chemicals in the UK, discussing Merck's work on dyed polymeric microparticles for electronic paper displays. Rather than using inorganic pigments (which is typical for electrophoretic (EPD) displays such as those used by E Ink), Merck is developing polymeric particles that can serve as pigments in these displays. Colored EPD displays continue to be a challenge, and are not common in today’s marketplace.
Most color EPDs use a black and white shutter, with a color filter overlay. Goulding notes that this approach is limited in performance and may be high in cost. Rather than additive color (RGB color filter), Merck is developing subtractive color modes, similar to those used in newspapers. Stacked subtractive color EPDs should provide better color saturation and performance than single layer additive color filter approaches.
Synthetic polymer particles can be controlled in a wide variety of optical and physical parameters. For example, the zeta potential (the charge on the particle) can be controlled for both positive and negative charges, and for different colors. Uniform particle size can be produced, as a specified size with a narrow size distribution. Uniform particle sizes ranging from 220 nm to 850 nm in diameter were shown in a micrograph. Dye concentration within particles, and particle volume fraction within the EPD, can both be accurately controlled. Density and refractive index can also be dialed in, which affects transport and light scattering, respectively.
Question: How would you actually fill a cell with multiple different colors?
Answer: that’s a current area of interest.
Question : are the particles bistable? Answer: No results to share.
Overall, a good start on an interesting way to push the color EPD forward. Next steps will be to continue to improve performance, and to build some real devices. Color EPD continues to be a challenge, so success here could be quite important to the epaper field.
Paper 2. T. Yoshihara of Fujitsu presented a paper on orientation control of cholesteric LC for epaper. Fujitsu is pushing hard on stacked cholesteric liquid displays for epaper applications. Stacked layers can in principle show good brightness and saturation, though past examples of stacked color have not been particularly bright. This paper was aimed at improving contrast ratio and brightness, through improved orientation control over the cholesteric LCs. Cholesteric panels don’t require an active matrix and can be bistable, both desirable properties.
An alignment layer with rubbing is the approach. The rubbing pattern and rubbing strength are the key elements here. For the rubbing pattern, cross-rubbing (perpendicular on opposing surfaces) provided a wide set of viewing angles. Rubbing density (strength) also was critical – it’s possible to rub too weakly, or too strongly. So, an intermediate rubbing strength is optimal.
The pretilt angle (the angle of the liquid crystal director at the surface) was also examined – this property can be controlled by choice of alignment layer. Both reflectance (brightness) and contrast were considered. A pretilt angle of 10 degrees appeared to be optimal. This material also showed the widest process window in terms of fabrication.
Reflectance of up to 40% was achieved. Fujitsu has improved the appearance of its cholesteric devices (such as the Flepia information device and electronic card holder). To my eye, the photographs did show marked improvement over past efforts by this team.
Question: why is cross rubbing the best? Any physical explanation? No particular insights available from the author.
Paper 3. J.-Y. Kim of Samsung Electronics described the company's 4.8-in. active matrix PDLC display, using printed organic field effect transistors (OFETs).
SAIT (Samsung Advanced Institute of Technology) is performing R&D on printed transistors, due to the interest in flexible displays. Strong market demand for epaper makes this an important area for development. Color, video, and low cost are key areas for focus.
Notes:
-A maskless TFT fab process can reduce cost by 50%, and avoiding vacuum processes reduces costs by 30%.
-Polymer dispersed liquid crystals switch between scattering and transparent state, and don’t require polarizers. They’re good for reflective and for flexible displays.
-For flexible epaper, printed OFETs have low mobility. PDLCs have had low voltage holding ratio, and the color filter layer disposition is complex.
-SAIT is using polymer semiconductors that are printable, and are compatible with flexible semiconductors.
-Newly developed polymer semiconductors – quaterthiophene and bithiazole units with mobility up to 0.5 cm2/VS. Stability data is not available - they’re filing patents!
SAIT has worked on printed OFETs since 2006, showing steady progress. In 2009, it achieved 4.8-in. on plastic, 400x240 pixels @ 100 dpi. Now using inkjet process for color epaper.
UV irradiation is complicated with color filters, since filter affects UV intensity in different colors. Exposure from the backplane side requires high aperture ratio, which complicates pixel design for good optical performance.
Color filter on array – put directly onto the TFT electrode – was described. The PDLC is built on *top* of the color filter/TFT layer – very clever. UV intensity is now extremely uniform and intense. Pixel design with high aperture ratio is achieved.
A seven step process was shown for cell fabrication. First steps were using SiOx gate dielectric over the gate metal. The semiconductor is printed. Mobility is 0.15 cm2/Vs, W/L 200 um/10 um
Gage and data line widths are 20 micron. 80 ppi display was shown – 18% reflect, 4:1 contrast.
In the Q&A, it was stated that a dyed PDLC was used as the display system. UV curing of dyed PDLC has traditionally been very difficult – how did the authors solve? Wait for the next paper!
Paper 4. Research of PDLC structures for flexible displays was presented by G.H. Lee of Samsung Electronics. There are several different PDLC modes, and this paper reviewed their characteristics for color epaper. This was a companion paper for paper 3, focusing on the PDLC portion of the display.
Notes:
-PDLC great for gray scale and video response.
-Normal PDLC – scattering and transparent state. High reflectance requires
-Color PDLC with color filter, shows low reflectance (9%), but good color and contrast. How to improve?
-Dichroic PDLC can show both strong absorbance and scattering, and weak absorbance and clarity. Good contrast and high reflectance.
-Very high UV exposure is required for dye containing PDLC, since the dye absorbs UV light. The color filter on TFT approach (COA) enables high power UV exposure.
-PEN substrate for flexible substrate.
-8:1 contrast, 16% reflectance, 30 Hz driving, 4.8 inch, QVGA.
-Advanced mode: make each pixel with its own dichroic color (RGB or CMY) PDLC, without use of a color filter. RBG can’t generate a good black state. CMY system provides a brighter image with black dye PDLC.
-Black dye with color filter
-RGB- 12-16%
-CMY – 16-22%, with contrast ratio over 5:1.
-22% is very high for reflective color, using black dye with CMY color filter. Apparently there is a ten minute UV cure time, which is a little long.
Overall, this is a very promising approach for a color, video-capable epaper display.
--Paul Drzaic, Drzaic Consulting Services and Past-President SID.
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