Bistable Liquid Crystal Displays - Springer

7.3.5 Bistable Liquid Crystal Displays Cliff Jones 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509

2 2.1 2.2 2.3 2.4

Infinite Multiplexibility and Rapid Frame Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509 The 0-2p Bistable Twisted Nematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1510 Other Early Bistable Nematic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512 Ferroelectric Liquid Crystals and the tVmin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1514

3 3.1 3.2 3.3 3.4 3.5

Ultralow Power Reflective Mode Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519 Surface Stabilized Ferroelectric Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1521 Scattering Smectic A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1522 Weak Anchoring and the 0-p Bistable Twisted Nematic . . . . . . . . . . . . . . . . . . . . . . . . . . . 1523 Zenithal Bistable Nematic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1526

4 Plastic, Flexibility, and Color in Reflective Bistable Displays . . . . . . . . . . . . . . . . . . . . 1531 4.1 Bistable LCDs Using Retardation Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1531 4.2 Bistable Cholesteric Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1532 5

Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1538

Janglin Chen, Wayne Cranton, Mark Fihn (eds.), Handbook of Visual Display Technology, DOI 10.1007/978-3-540-79567-4_7.3.5, # Springer-Verlag Berlin Heidelberg 2012

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7.3.5

Bistable Liquid Crystal Displays

Abstract: Bistable liquid crystal displays offer many benefits, including the ability to display high levels of image content using passive matrix addressing and without thin-film transistors; ultralow power reflective displays with image storage that only consume power with changes to image; and flexible plastic displays capable of showing color images. The topic is diverse, involving nematic, smectic, and cholesteric liquid crystals; retardation, anisotropic absorption, scattering, and selectively reflecting optical modes; dielectrically, ferroelectrically, and flexoelectrically driven electrooptic effects; bistable textures stabilized by monostable surfaces or smectic layers and bistable surfaces; and applications ranging from electronic skins to high-definition television. Many different bistable display modes have been suggested over the past four decades, and this chapter concentrates on the bistable twisted nematic, surface stabilized ferroelectric liquid crystals (FLCs), scattering smectic A, grating aligned zenithal bistable display, and bistable cholesteric displays (BCDs). List of Abbreviations: BCD, Bistable Cholesteric Display; BiNem™, Trade Name for 0-p Bistable Nematic Mode marketed by Nemoptic; BTN, Bistable Twisted Nematic often used for the 0-2p metastable nematic display mode; C1 and C2, FLC Chevron smectic layer orientation directions defined with respect to the parallel surface alignment directions; CMOS, Complementary Metal-Oxide Semiconductor/Silicon; DMOS, Double Diffused Metal-Oxide Semiconductor/Silicon; DRAMA, Defence Research Agency Multiplexed Addressing Scheme used for tVmin FLC; ESL, Electronic Shelf-Edge Label used in retail for automatically displaying pricing and other product information; FLC, Ferroelectric Liquid Crystal formed in chiral tilted smectic liquid crystals, but usually taken to mean chiral smectic C∗; FLCD, Ferroelectric Liquid Crystal Display; HAN, Hybrid-Aligned Nematic, usually taken to mean homeotropic alignment on one surface, and homogeneous or low pre-tilt alignment on the opposite surface; HDTV, High-Definition Television usually corresponding to 1,920 1,080i (interlaced) or 1,920 1,080p (progressive) pixels; ITO, Indium Tin Oxide, the transparent conducting oxide layer most commonly used by the display, touch screen, and solar panel industries; LCD, Liquid Crystal Display; N and N∗, Nematic and Chiral Nematic Liquid Crystal Phases, where the pitch of the chiral nematic is arbitrarily taken to be much longer than the wavelength of light, to distinguish it from the cholesteric phase; NTSC, National Television System Committee that defined the standards for US color television; OLED, Organic Light Emitting Diode; PEDOT, Poly (3,4- ethylenedioxythioiphene), a polymeric transparent conductor; PES, Polyethersulfone polymer film that can be made without birefringence; PET, Polyethylene Terephthalate, polymer film; Ps and Pf, Electric Polarization, either spontaneous (ferroelectric) or elastically induced (flexoelectric); QVGA, Quarter-Video-Graphics Array, 320 240 pixels; RGBW, Red-Green-Blue and transparent (White) color filter system; RMS, Root Mean Square; SiOx, Silicon Oxide Layer, usually evaporated onto glass surface to induce director alignment; SmA, SmC, and SmC∗, Smectic A, Smectic C, and Chiral Smectic C Phases; STN, Supertwist Nematic Display, taken to include foil compensated STN; TDP, Triangular Director Profile for SmC and FLC chevron structures; TFT, Thin-Film Transistor, usually meaning one or more such elements at each pixel; TN, Twisted Nematic; VAN, Vertically Aligned Nematic, where both surfaces are homeotropically aligned; ZBD, Zenithal Bistable Display/Device; Dn, The birefringence of the anisotropic liquid crystal phases, given as the difference between extraordinary ne and ordinary no refractive indices; tVmin mode, FLCD Mode of operation for low Ps and highly positive dielectrically biaxial FLC


1

7.3.5

Introduction

Bistable displays exhibit electrooptic memory. They can be switched between two stable optically distinguishable states with an appropriate electric field. The states have equivalent or similar energies and are separated by a much higher energy state, or energy barrier. The barrier ensures that the desired state for a pixel is retained after the switching pulse without further electrical excitation: The pixel is then said to have been latched. The property of bistability has several potential benefits for a display technology. The most obvious advantage is the provision of ultralow power operation because the display does not require constant updating and only consumes power when the image content is changed. The first market to gain significant traction that takes advantage of this is the electronic book reader. Such devices often incorporate a bistable electrophoretic ink–based display, providing sufficiently high-image content but with a battery that needs charging only after many pages. However, unlike bistable liquid crystal displays (LCDs), this technology does not have a well-defined threshold and therefore necessitates a thin-film transistor (TFT) element at each pixel to prevent previously written rows from being affected by subsequent data signals. This adds to the display cost. Even with the TFTs, the display update is slow and can be distracting. Because of the relatively high tooling costs, TFTs have less design flexibility and tend to be available only in the formats dictated by large markets: Niche markets are better served by passive matrix displays. However, existing low-cost passive matrix displays without TFTs, such as the Supertwist Nematic (STN), provide neither sufficient optical quality nor the low power often required by the market. For these applications, a bistable LCD uniquely combine the low cost with superior optical performance and ultralow power. There are a variety of different bistable LCDs each with its own merits. Displays have been produced using conventional nematic liquid crystals familiar from many consumer applications in the market today, but also utilizing either cholesteric [1] or smectic [2] liquid crystal phases. The history of bistable LCDs is almost as long as that of the LCD itself. For much of this time, the technological development was driven by the need to display high-image contents, ultimately providing full color and video speed. Now that need is largely satisfied by the success of TFT driven displays, it is the advantages of cost, low power, and good image quality that are more important. However, bistable LCD also offer advantages in developing markets, such as those using flexible plastic substrates. The bistable liquid crystal displays suited for each of these three types of application will be reviewed in turn.

2

Infinite Multiplexibility and Rapid Frame Response

2.1

Introduction

With conventional passive matrix displays, each row is addressed line by line and combined with a synchronous data signal on the columns that either increases or decreases the rootmean-square voltage applied to a given pixel when averaged over the frame time. The degree of discrimination provided by the signal depends on the fraction of the total signal represented by the selected row: That is, the data provide decreasing degrees of discrimination as the number of rows increases. High levels of multiplexing require the electrooptic threshold to be steep, but even the steep threshold of STN limits displays to about 240 rows in practice. Higher levels of multiplexibility required either a separate nonlinear element such as a thin-film transistor

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(TFT) or a bistable response. For this reason, much of the initial development of bistable LCDs targeted high-image content displays, and advantages such as power or image storage were rather secondary. Indeed, the displays were often designed in transmissive mode operating with a backlight that dominated power consumption. A bistable electrooptic response allows unlimited image content to be displayed because each row can be addressed independently and the image built up line at a time. Usually, a pulse of voltage Vs and duration t is applied to a row while appropriate data Vd applied to the columns. The voltages and times are arranged so that the pixel resultant, given by row minus column voltage, causes latching to the opposite state if jVs þ Vd j, whereas no latching occurs for the resultant jVs Vd j because it is lower than the latching threshold. During this operation all of the remaining rows are held at ground, so that the unwritten resultant signals are Vd , which is far too low to cause any latching, particularly if Vs 2Vd. Each frame takes at least n t seconds to address for a display of n rows. However, this minimum is rarely realized practically. Separate addressing signals are required for each transition: Usually a reset or blanking signal is applied to prepare each row for the addressing signal, and then discrimination is done using a separate select signal. Also, liquid crystals are sensitive to deterioration when DC is applied, and so bipolar addressing signals may be used. This means that a very fast latching response is needed if a complex image is to be refreshed rapidly, for example to enable moving images or cursors to be displayed. The frame time depends not only on the latching time taken to address each row, but also on the optical response time of the liquid crystal itself. Bistable LCDs with fast frame response include the 0-2p bistable twisted nematic and the surface stabilized ferroelectric liquid crystal display.

2.2

The 0-2p Bistable Twisted Nematic

Among the first bistable LCD to be invented was the 0-2p bistable twisted nematic (BTN) [3]. Interest in this mode was reinvigorated in the mid-1990s when Seiko Epson produced working demonstrators suitable for use in backlit graphic equalizer displays [4, 5] and a 5.700 diagonal eight-color transmissive QVGA display. The bistable display mode was chosen because it offered a very fast optical response as well as a high level of multiplexing. Conventional twisted-nematic (TN) LCDs have orthogonal rubbing directions and a long cholesteric pitch P to bestow a single handedness to the monostable twist. With the 0-2p BTN, parallel rubbed surfaces are spaced at cell gap d and the inherent chiral pitch P is reduced to give a monostable p twist across the cell (i.e., d/P = 0.5) and two metastable states with 0 and 2p twist. If the surfaces have pre-tilt, the director profile of the p-twist state is splayed whereas the splay is negligible in the metastable 0- and 2p-twist states, > Fig. 1. This broadens the range of d/P over which the metastable states are formed and device operation becomes practical. > Figure 1 also illustrates that the p-twist state is topologically distinct from the two metastable states: There is no continuous transition from this state to the others, being mediated by the creation and movement of disclination loops. On the other hand, the metastable 0- and 2ptwist states are topologically equivalent and the transition between these states occurs without the creation of defects. Application of a sufficiently high transverse field Ereorients the director in the bulk of the sample to a vertical orientation when using a positive De nematic. If the field is removed quickly from this state, backflow causes the tilt in the cell mid-plane to initially increase, causing the 2p-twist state to be formed. Removing the field gradually allows the director profile


7.3.5

π-twist (splayed)

Defect

Defect

2π-twist (un-splayed)

0-twist (un-splayed)

Vertical

E Gradual decrease of E

Sudden decrease of E

. Fig. 1 0-2p BTN

to relax into the 0-twist state because the backflow is then lessened. Strictly speaking, the device is not genuinely bistable and the metastable 0-twist and 2p-twist states relax back to the p-twist state a few seconds after removal of the applied field. The inter-pixel gaps remain in the p-state throughout, and defects spread from these regions across the latched pixel. Optimizing the mixture d/P can help increase the image retention time. Frequently updated displays such as those of reference [4] used a low frequency signal below the Freédericksz threshold voltage but sufficient to prevent the spread of the p-state and ‘‘hold’’ the image. Other methods have been suggested for preventing the p-state formation in the inter-pixel regions altogether, thereby isolating each pixel from relaxing out of the desired state. These include locally reducing the cell gap [3], phase separation of a polymer network into the inter-pixel region [6], or patterning the alignment in that region to increase pre-tilt or twist [7]. Long-term bistability in these systems proved difficult to maintain, because microscopic irregularities within the pixel eventually nucleate the more stable p state. Optical contrast was provided using polarizers either side of the LCD. Crossed polarizers were oriented at 45 to the rubbing directions such that 0-twist state has the transmission T of a simple birefringent retarder given by [8]: T 1 2 pDn:d ¼ sin ð1Þ I0 2 l where the illuminating intensity is I0 and the polarizer is assumed to be perfect, leading to 50% of the light being transmitted at the half-wave plate condition Dn:d ¼ 1=2l. The tilt in both

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states is low. The transmission of the 2p state is given to the generalized expression for a twisted birefringent layer [8]: 2 pffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffi 2T 1 ¼ cos f 1 þ a2 cosðf þ ’1 ’2 Þ þ pffiffiffiffiffiffiffiffiffiffiffiffiffi sin f 1 þ a2 sinðf þ ’1 ’2 Þ I0 1 þ a2 pffiffiffiffiffiffiffiffiffiffiffiffiffi a2 2 þ sin f 1 þ a2 cos2 ðf ’1 ’2 Þ 1 þ a2 ð2Þ where a ¼ p:Dn:d =f:l is related to the twist angle of the director from the input to output substrates f, and ’1 and ’2 are the input and output polarizer directions with respect to the input director, respectively. Given the polarizers and retardation are set to maximize the transmission for the 0p-twist state (i.e., ’1 ¼ 0 , ’2 ¼ 90 , and a ¼ 1=4) (> Eq. 2) shows that the 2p state has a transmission given by: rffiffiffiffiffi! 2T 16 2 17 ¼ 0:035 ð3Þ ¼ sin 2p I0 17 16 and hence appears dark. The chromaticity of this dark state is reduced when the device is operated in a reflective mode. Because both states have a low tilt and the resulting displays have an excellent viewing angle characteristic, with contrast approaching 30:1 over a 100 viewing cone. For a typical liquid crystal with Dn = 0.14, these conditions are met for a cell gap of close to 2 mm. The time taken for the director to respond to the change in electric field is related to the anisotropic viscosities. Ignoring backflow effects, the optical response times are the same as a conventional TN and can be related to the rotational viscosity g1 through the simple expressions: ton ¼

g d2 g d2 1

; toff ¼ 1 2 2 2 e0 DeVth e0 De V Vth

ð4Þ

Both the field driven ‘‘on’’ response time and the field independent ‘‘off ’’ time strongly depend on cell gap. A typical material with a De 20 and g1 0.2 Pa.s has a total optical response time below 5 ms. Together with line-address times below 100 ms, a QVGA 240 360 pixel device could be updated at 60 Hz.

2.3

Other Early Bistable Nematic Displays

During the same period that Berreman and Heffner were developing the 0-2p BTN mode, researchers at the same laboratories were investigating bend-splay bistable modes [9, 10], where the director latches between ‘‘vertical’’ (V) and ‘‘horizontal’’ (H) states. The upper and lower internal surfaces of a nematic LCD were patterned with alternating high tilt stripes of + yS and –yS, as shown in > Fig. 2. The boundaries between the different surface conditions produced pinning sites for ½ surface disclination if the pre-tilt was between 22.5 and 67.5 . Unlike the 0-2p BTN described in the previous section, this device was truly bistable because the two textures are topologically distinct and separated by an energy barrier. Transitions between the states require the movement of the defects from one stripe to the next. Latching between the states was induced using interdigitated electrodes on top and bottom surfaces, where each electrode coincided with the boundary between alternating alignment. A vertical electric field could be applied between upper and lower surfaces and a horizontal field

7.3.5


Splay “H”

topologically distinct

Bend “V”

+

–

+

– inter-digitated electrodes

Disclinations

+θs

+

–

+

–θs

+θs

–θs

–

+θs

–θs

S

. Fig. 2 The splay/bend mode

applied using adjacent electrodes on the same surface. These fields couple to the positive De of the material to induce the Vand H states, respectively. Optimum behavior was found where the width of each stripe s was the same as the cell spacing d. For d = s = 50 mm, switching occurred at 70 V for a duration of 20 ms for the transition from V to H, and 80 ms for H to V. Optical contrast was produced using a pleochroic dye doped into the nematic liquid crystal. Rather than using monostable surfaces, an alternative approach is to create alignment layers that are inherently bistable, and possess two or more favored alignment orientations. This has a number of potential advantages, such as the ability to induce bistable behavior in standard nematic mixtures, insensitivity of the written image to pressure induced flow and greater device design freedom. The first bistable surface alignment layer was formed by evaporating SiOx at a precisely controlled thickness and angle to impart two tilted and a single un-tilted alignment state each of which imparted different azimuthal orientations to a contacting nematic [11, 12]. Evaporated layers are not suited for large-scale manufacturing due to the difficulties of batch processing and variations of evaporating angle over large areas. Instead, Bryan-Brown et al. [13] used a bi-grating to impart azimuthal bistable orientations at 45 to the modulation directions. One of the modulations was blazed substantially, to induce a high director pre-tilt in that state. More recently, azimuthal bistability has been demonstrated using bi-gratings with square features or pillars [14], square wells [15], and compartments with sawtooth-shaped sidewalls [16]. A number of latching mechanisms were proposed for use with the azimuthal bistable surfaces, including in-plane electrodes and chiral ions. One effective device configuration used the flexoelectric polarization inherent to nematic liquid crystals to induce in-plane latching between azimuthally bistable states using a transverse electric field [17], as shown in > Fig. 3. Opposing bistable surfaces are arranged so that the tilted state on one surface is aligned with the un-tilted state on the other. This leads to two states with opposite senses of splay and bend. The resulting flexoelectric polarization is then either in an ‘‘up’’ or ‘‘down’’ state. Each pixel is addressed by first applying a pulse sufficient in strength to break the surface anchoring,

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T1

PH

PLAN VIEW SiOx 45°

θs

T1 SiOx

T2 PH T2

a PLAN VIEWS SiOx

SIDE VIEWS PH

E

θs T2 E Pf

E Pf

T1 PH

Pf Top

E

T1

θs

Pf

SiOx PH

PH2

T1

b Bottom

. Fig. 3 Azimuthal bistable display. (a) Bistable SiOx surface and (b) arrangement for Azimuthal bistable display latching with transverse field [17]

followed by a DC pulse of appropriate polarity to latch the desired state. Latching occurred for pulses of 50 ms duration above threshold fields of 16 V/mm, demonstrated in devices of 1, 2, and 4 mm. However, this type of device was not successfully commercialized due to issues associated with ionic response and image sticking.

2.4

Ferroelectric Liquid Crystals and the tVmin

Such fast response and addressing times still fell short of what is needed for full color television displays driven by passive matrix. Arguably, the most ambitious market targeted by any passive matrix bistable display is large area HDTV. In the mid-1990s, a joint development program between the UK’s Defence Evaluation and Research Agency (DERA), Sharp Laboratories of Europe, and Sharp Corporation of Japan [18] used bistable ferroelectric liquid crystals (FLCs) operating in the tVmin mode [19, 20] to produce 600 [18] and 1700 diagonal [21, 22] prototypes aimed at meeting the HDTV specification. The requirement of 1,080 interlaced lines driven at a 60 Hz frame rate to show 16.8 million colors (256 gray levels) was challenging for a passive matrix display. Eight bits of gray was achieved by combining two bits of spatial dither on the columns (weighted 1:2) with four temporal bits (weighted 1:4:16:64). Even with the use of interlaced lines, achieving a 60 Hz frame rate on a 1,920 1,080 panel set the target lineaddress time to be 15.4 ms and an optical response time target below 4 ms. Operation was required across the temperature range 0–60 C and achieving a very high contrast ratio in excess of 150:1 was critical. Such performance could only be achieved through the combination [20]


7.3.5

of high dielectric biaxiality ferroelectric liquid crystal mixtures operating close to the minimum response time, using monopolar addressing schemes and the C2 chevron alignment, as described below. FLC had been known to provide very fast bistable optical shutters following the original work of Clark and Lagerwall in 1980 [23]. Devices were formed by cooling into the ferroelectric chiral smectic C (SmC∗) phase from a smectic A (SmA) sample with layers aligned normal to the cell walls, as shown in > Fig. 4. The sample spacing was arranged to be sufficiently thin to unwind the helical nature of the chiral nematic and smectic C phases and to operate as a switchable half-wave plate optical shutter in the ferroelectric phase. X-ray studies [24] showed that the smectic C layers tilt into a symmetrical chevron structure, with a layer tilt angle dC typically between 80% and 90% of the smectic C cone angle yC. Bistability results from the orientation of the director at the chevron interface: The liquid crystal n director is continuous across the interface, but constrained to either of two in-plane orientations bi at the interface, given by: cos bi ¼

cos yC cos dC

ð5Þ

This angle is typically between 45% and 60% of yC. The direction of layer tilt with respect to parallel aligned surfaces is dictated by the pre-tilt and the ratio of zenithal and azimuthal anchoring energies. Two layer orientations may form termed C1 and C2 [25] depending on whether the layers tilt in the same or opposite direction to the surface alignment, respectively. The C1 state is the lowest energy state where the pre-tilt yS is relatively high compared to the layer tilt, such as temperatures close to the SmA to SmC(∗) phase transition, or if the azimuthal anchoring energy is low. At intermediate pre-tilts and for

N(*) r θs

SmA

r

SmC(*) Well below Tc (C1)

SmC(*) Immediately below Tc (C1) r

r

layer

r

β1

n

r

SmC(*) Well below Tc (C2)

r

r

r

βs

r

In-plane azimuth angle β

. Fig. 4 Ferroelectric liquid crystal alignment on cooling through the sequence N – SmA – SmC for parallel aligned surfaces, rubbing direction r. C1 and C2 chevron layer textures with low and high tilt and triangular director profiles (TDPs) are shown

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high azimuthal anchoring energies, the C2 state forms a few degrees centigrade below the phase transition. If low tilts are used, both C1 and C2 are formed and the sample is covered with unwanted zigzag defects. The quiescent state director profile is determined both by the chevron interface (> Eq. 5) and the surface alignment. For typical layer tilt angles, the out-of-plane tilt can be ignored and the director profile is approximately triangular from one surface to the other [26], as shown in > Fig. 4. Incident light is extinguished for one of the domains when the device is set between crossed polarizers at the angle bext to the rubbing directions, given by [26]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffii h tan ðbi bs Þ 1 þ 14 a2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tan 2ðbext bS Þ ¼ ð6Þ 1 þ 14 a2 where a = pDnd/fl, as for (> Eq. 2) includes the director twist given by ’¼bi bs, and the effect of the small out-of-plane tilt y is ignored. For thick cells operating close to the full-wave plate condition (Dn:d l), the extinction angle bext diverges and is highly wavelength dependent. At long wavelengths or low cell gaps, (> Eq. 6) predicts that the extinction angle tends toward: b þ bS ð7Þ bext ! i 2 as shown in > Fig. 5 for an FLC with bS = 25 (such as for a high pre-tilt C1 state), bS = 12.5 (i.e., equivalent to bi such as would occur if the azimuthal anchoring energy is negligible), and bS = 0 (such as for the C2 case where yS yC jdC j). For such thin samples, the bext is approximately wavelength independent and equal to the so-called FLC memory angle bm. When the polarizers are oriented at bm the transmission for the other domain is then approximately given by: 2T pDn:d ð8Þ ¼ sin2 ð4bm Þsin2 I0 l Canon launched the first commercial bistable LCDs as computer monitors in the early 1990s [25, 27] using FLCDs operating in the C1 geometry. The C1 texture with high bS, and 24 Polariser Extinction Angle/°

1516

Δn.d = 260 nm

βs 16

25° 12.5° 0°

8

0 400

450

500

600 550 Wavelength/nm

650

700

. Fig. 5 Optical extinction angle bext for chevron quiescent state with the TDP for samples with uC = 22.5º, dC = 19º, and for different surface azimuthal angles bS = 0 , 12.5 , and 25


7.3.5

therefore high memory angle, was achieved using a surface pre-tilt of about yS = 18 . Displays were 1,024 1,280 and either 1500 [25] or 2100 [27] diagonal, operating at between 65 and 150 ms/line, depending on the temperature. Page update rates of 15 Hz were attained by driving two of the 1,020 interlaced rows simultaneously. Any flicker was made less noticeable by scanning each page eight times while addressing every eighth row-pair. Even so, this relatively slow frame rate was not suitable for displaying cursor movement. Hence, the addressing also included the ability to partially update the screen by addressing a limited set of rows at the same time. For example, a 32 32 pixel cursor could readily be addressed at 100 Hz. Operating in the C2 chevron geometry with bS = 0 as shown in > Fig. 4 has a number of advantages for displays. In particular, latching between the states involves no reorientation of the director at the surfaces, being mediated by the transition at the chevron interface only, allowing extremely fast response times to be achieved. However, (> 7) and > Fig. 5 show that the memory angle is significantly less than bm = 22.5 that is required for maximum brightness efficiency from (> 8). The display in applications such as computer television monitors is continually updating and unaddressed rows always have the data waveform applied. This voltage couples to the dielectric biaxiality inherent to smectic C (∗) liquid crystals’ to increase the extinction angle back toward the 22.5 optimum, an effect termed ‘‘AC stabilization.’’ Ignoring any effects due to viscous backflow and considering elastic torques for changes to orientation of the director around the cone ’C only, then the response to an applied electric field is given by [28]: g1 sin2 yC

@ 2 ’C @’ ¼ B1 sin2 ’C þ B2 cos2 ’C cos2 dC þ B3 sin2 dC B13 sin 2dC sin ’C @t @z 2 2 @’C 1 þ ðB1 B2 Þcos2 dC sin 2’C B13 sin 2dC cos ’C 2 @z ð9Þ 2 2 þ PS Ez cos dC sin ’C eo Ez @e sin ’C cos ’C cos dC

1 eo Ez2 De cos ’C sin 2yC sin 2dC sin ’C cos2 dC sin2 yC 4

where De is the uniaxial dielectric anisotropy (= e3 e1); ∂e is the dielectric biaxiality (= e2 e1); B1, B2, B3, and B13 are the elastic constants associated with two-dimensional distortions of ’C for uniform layers; and g1 is the rotational viscosity. At high field strengths or at frequencies too high to cause ferroelectric switching, the effect of the dielectric terms in Ez 2 De and Ez 2 @e dominate. The AC stabilizing effect of the dielectric anisotropies is similar to the switching effect in nematic liquid crystals: The negative De tends to reduce any out-of-plane tilt and stabilize the condition given by (> 5). The dielectric biaxiality is positive and tends to stabilize either ’C = 0 or 180 , where the in-plane component of the director b0 and out-of-plane tilt z0 are given by: 1

b0 ¼ tan

tan yC ; z0 ¼ sin1 ðsin dC : cos yC Þ cos dC

ð10Þ

Typically, b0 is a couple of degrees greater than the cone angle, and z0 a couple of degrees lower than the layer tilt angle. It is clear that the uniaxial and biaxial anisotropies act in

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opposition for chevron geometries. The electrostatic energy is a minimum at a director orientation given by [29]: sin ’AC ¼

De sin yC cos yC tan dC Desin2 yC @e

ð11Þ

Measurements of the biaxial permittivities [29] show that ∂e > De sin2yC and hence the director tends toward the conditions of (> 10). Effective use of AC stabilization requires FLC mixtures with high dielectric biaxiality mixtures to be used [30]. Moreover, the dielectric biaxiality plays a yet more important role in the latching mechanism of FLCDs operating in the tVmin mode. Unlike AC stabilization, the DC ferroelectric response is polarity dependent: An applied field of the appropriate polarity tends to reorient the spontaneous polarization toward the opposite side of the cone, as shown in > Fig. 6. The resulting high gradient of director orientation close to the chevron interface causes a large latching torque that is eventually sufficient to cause the director to swap discontinuously from one allowed state to the other through the formation and movement of a domain wall. After removal of the field, the director relaxes back to a triangular director profile on the opposite sign to the original state. A simple one-dimensional model [31] suggests that the latching transition is a momentary reduction of the smectic cone angle to the condition yC ¼ dC where the director has a single orientation at the interface and the director swaps continuously between the states, > Fig. 6. At low DC field strengths, the coupling to the spontaneous polarization is far stronger than the dielectric effect and the pulse width t required for latching increases linearly with 1/Ez.

chevron interface

+

+

+

+

+

+

+

+

+

+

VDC

VDC latch

. Fig. 6 Ferroelectric liquid crystal latching between ‘‘up’’ and ‘‘down’’ states. (a) Latching in the C1 chevron state with high pre-tilt. (b) Potential model for latching at the chevron interface. (c) Latching in the C2 chevron state with low pre-tilt


7.3.5

At higher fields, the dielectric terms of the switching equation acting in Ez2 increasingly oppose the ferroelectric latching torque PS E. This causes the latching response time to deviate from the 1/Ez behavior and begin to slow its rate of increase. Eventually, a minimum response time is reached, the so-called tV minimum, above which the response slows rapidly with increasing field, diverging at the field where the dielectric and ferroelectric torques balance. Numerical modeling shows that the tV minimum occurs at about 60–64% of this divergence field [32] and is approximately given by: 8 93 =2 PS d < 1 ð12Þ Vmin 0:62 e0 cos dC : Desin2 y @e 23 ðDe cos y sin y tan d Þ23 ; C

C

with the minimum response time approximately:

g sin2 yC @e Desin2 yC tmin 1 PS 2

C

C

ð13Þ

The steep response above the minimum has the potential for high contrast ratios and wide operating windows. Addressing tVmin FLCD is done using monopolar row waveforms, such as in the JOERS/Alvey scheme [19]. Unlike conventional addressing, it is the lower voltage resultant jVs Vd j that causes latching above the minimum, rather than the higher voltage jVs þ Vd j: The display operates in an inverted mode. A two-slot monopolar strobe pulse (0, VS) applied to the rows combines synchronously with select data (Vd, +Vd) and non-select data (Vd, + Vd) to produce the resultants (+Vd, Vs Vd) and (Vd, Vs + Vd), respectively, as shown in > Fig. 7a. This scheme gives both a fast response and wide discrimination between select and non-select resultants even for low data voltages. This is because the two portions of the resultant pulses act in unison: The select pulse Vs Vd is immediately preceded by +Vd that helps latching, but the non-select pulse Vs + Vd is preceded by a pulse of the opposite sign –Vd that hinders switching and thereby improves discrimination. Contrast this with a bipolar strobe pulse where a trailing select pulse is always preceded by a high voltage that slows the response, yet the non-select pulse is preceded by a lower pulse and hence discrimination is reduced. The JOERS/Alvey addressing scheme uses simultaneous blanking pulse several lines ahead of the addressed row to ensure that the correct state is obtained, while minimizing the total frame time. > Figure 7b shows experimental results for a fast FLC mixture operating in the C2 geometry [33]. Assuming each row uses two slots to DC balance the data waveform, the target latching time for HDTV operation is 7.7 ms when operating with conventional 40 V STN drivers. Clearly, the fast response meets the target for high temperatures. However, achieving the response time target at lower temperatures required the use of multiple slot DRAMA schemes [34] to increase the discrimination combined with strobe waveforms extended into the following rows [35]. In this fashion, HDTV performance was obtained from 0 C to 60 C operating temperature [22].

3

Ultralow Power Reflective Mode Displays

3.1

Introduction

By the time passive matrix bistable FLC displays reached the target for full color video imagery, monostable active matrix devices driven by TFTs were being successfully deployed in laptops

1519

7.3.5


Vs+Vd 1000

Slot time τ / μs

0 –Vd

Vs–Vd +Vd 0 100

|Vs+Vd|

|Vs–Vd|

20 1

10 Peak Resultant Voltage/V

a

100

80 20°C 30°C

Slot time τ / μs

1520

10 7.7

40°C

1 10

b

target τ

Maximum Voltage 40

60

100

Pulse Voltage/V

. Fig. 7 The FLC tVmin electrooptic latching characteristic: (a) The principles of tVmin addressing using the JOERS/Alvey scheme for the commercial mixture SCE8. (b) Example response for a low viscosity – high dielectric biaxiality mixture [33]

and computer monitors. Since the turn of the millennium, bistable LCD developments have been concentrated on portable products, where low power and low cost are essential requirements. Such devices operate in a reflective mode and a crucial part of the display design is ensuring excellent reflectivity, high contrast and wide viewing angles are achieved for the bistable quiescent states. Often, the application requires the display to be updated infrequently, and speed of update is only a secondary consideration. Operating voltages might be as high as 40 V, since the total energy consumed is small when the updates are infrequent.


7.3.5

However, standard components and manufacturing methods should be used to ensure that costs are minimized. Other properties that may be advantageous for some applications include stability of the image to shock, wide operating temperature ranges, very high resolution, or the provision of inherent gray scales. Several bistable LCD modes have been considered or used in commercial products requiring low power reflective displays. These applications range from watch and label displays, to large displays for electronic readers. The technologies, their strengths and weaknesses are summarized in the following two sections. This section concentrates on glass-based displays used for portable products, whereas > Sect. 4 looks at plastic substrates and the provision of color.

3.2

Surface Stabilized Ferroelectric Liquid Crystals

FLC displays operating in the C1 chevron geometry can be designed to give good extinction with a memory angle close to the ideal bm = 22.5 using alignment surfaces with high pre-tilt. This ensures that the contrast is retained after the removal of the addressing signals. Recently, Citizen [36, 37] has developed displays using obliquely evaporated SiOx surface alignment with high pre-tilt to produce memory angles in excess of 16 , as predicted for bs = 25 in > Fig. 5. A number of issues arise in this mode when high ferroelectric polarization PS is used. First, image sticking may occur where the latching voltages shift over several hours if the image is not updated. Second, the polar SiOx surface favors the orientation of PS into the surface thereby tending to induce half-splayed states of the director profile which greatly reduce the contrast ratio. This tendency is counteracted if the alignment directions are crossed with respect to each other by up to 20 . Both of these effects are minimized using PS below 16 nCcm2. The principal application for this technology was watches: Not only is ultralow power essential, but also latching below 5 V and high resolution of up to 1,000 dpi was required. FLCDs are perhaps the only bistable LCD capable of operating at such low voltages. Of course, the use of C1 texture, low voltage, and low spontaneous polarization combine to make the update slower than other FLCDs, such as those described in > Sect. 2.4. However, the attractive features offered by this technology also suited other small display applications, such as electronic shelf-edge labels, instrumentation, and electronic dictionaries. For each of these applications, it is important to shield the ferroelectric liquid crystal from shear caused by mechanical pressure to the display. Such shock would permanently damage the smectic layer alignment due to viscous flow in the plane of the cell, thereby disrupting the optical appearance. Photolithographic defined spacers over-coated with adhesive to fix the upper and lower substrates to each other were used to prevent shearing between the substrates and therefore ensure the aligned smectic layers were protected from damage. A different approach was taken by Sharp Laboratories of Europe, who targeted highresolution displays for electronic paper and e-reader applications [38]. Reflective displays using two external polarizers are limited to about 200 dpi due to image shadowing. This effect is caused by parallax due to the separation of the image plane and reflector which are separated by the rear-glass substrate. Higher resolution displays require a single polarizer mode to be used, thereby allowing the rear substrate to include an internal reflector. In such instances, the image is formed at the front polarizer and all parallax is removed. Achromatic black-and-white states are achieved by combining the FLC acting as a switchable half-wave plate with a fixed quarter-wave plate in front of the reflector. The optimum configuration requires the memory

1521

1522

7.3.5


angle between the FLC states to be 22.5 (i.e., bm = 12.25 ), with the polarizer oriented at +7.5 to the rubbing directions and +75 to the slow axis of the wave plate. This condition most closely matches the director profile obtained with the C2 layer orientation. A 300 300 pixel 1.6 mm spaced device was fabricated operating in the tVmin mode. Application of strobe and data voltages of Vs = 15 V and Vd = 3 V respectively, resulted in line-address times of 100 ms.

3.3

Scattering Smectic A

Another early bistable LCD that is receiving renewed attention is the bistable smectic A [39, 40]. The device is shown schematically in > Fig. 8: It latches between a scattering focal-conic texture that appears white due to backscattering, and a transmissive homeotropic state that appears black due to an absorbing layer coated onto the rear substrate. These states are separated by an energy barrier associated with movement of the smectic layers. The smectic layer normal and director of a positive De material are aligned parallel to an applied electric field in a transition analogous to the nematic Freédericksz transition. Unlike the nematic case, the applied field both compresses the layer and induces splay of the n director to cause undulations. The threshold voltage for the transition from planar to homeotropic is related to the cell gap d [41]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p2 k11 d ð14Þ VPH ¼ e0 De:lA where lΑ is a characteristic length related to the layer spacing. Latching to the focal-conic state relies on negative conductivity anisotropy of the smectic phase, wherein there is a high ionic mobility in the direction parallel to the layers. Applying a low frequency field parallel to the layer normal leads to ionic flow in the sample, which tends to create vortices in the smectic centered on surface irregularities and reorient the layers toward the cell plane. The resulting focal-conic texture consists of domains typically between 1 and 10 mm in diameter. Strong backscattering of incident light results if a highly birefringent LC material is employed to create Back-scattered white light

Incident white light

Black Absorber

Transmissive

. Fig. 8 The bistable smectic – A scattering device

Scattering


7.3.5

an attractive white state with an excellent viewing angle. The threshold voltage between uniform homeotropic and scattering focal-conic textures is related to the conductivities sjj and sjj through the relationship: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2p2 k d u 11 ð15Þ VHF ¼ t s e0 ejj 1 s?jj :lA The conductivities are usually enhanced by deliberately doping the liquid crystal with mobile ionic species. At low frequencies, the ions are sufficiently mobile to respond in time to the applied field. However, the disruptive action of the ions is counteracted by the E2 effect of the field coupling to the positive De, and the threshold VHF quickly increases with frequency. A high cell gap of 15 mm < d < 30 mm is needed to provide sufficient scattering centers and create the attractive white state. However, this leads to a trade-off between voltage and reflectance, and typical devices require voltages in excess of 100 V. Recent demonstrations of this technology have been made by PolyDisplay ASA [42] and Halation [43]. Typical room temperature operating voltages are VS = 100 V and Vd = 50 V [43]; such high voltages are supplied using plasma display panel DMOS drivers. Employing a 20 Hz page blank for the low frequency transition to the scattering texture, a 96 96 pixel display is updated within 5 s. This is rather slow for many portable products, but is suitable for electronic signage. Historically, devices also suffered from poor lifetimes due to electrochemical degradation of the liquid crystal. Despite the use of costly row and column drivers and the slow update speed, the device has the advantage of a simple construction without polarizers and produces excellent white coloration, wide viewing angle from the near-Lambertian properties of the scattering, and reflectance of 50%.

3.4

Weak Anchoring and the 0-p Bistable Twisted Nematic

The 0-2p BTN described in > Sect. 2.2 was metastable, since the device would eventually return to a single lowest energy p-twist state. That state differed topologically from the 0p-and 2ptwist states used for switching. True bistability results when the device is arranged to allow latching between the topologically different 0p and p-twist states [44]. From 1998 to 2010, this approach was developed by the French company Nemoptic S. A. and the technology marketed under the tradename BiNem™ [45]. Devices are constructed with parallel aligned surfaces and the cholesteric pitch set approximately to d=P 0:25. One of the surfaces is arranged to have weak zenithal anchoring and switches to homeotropic when a field above a critical value EC is applied [45]: Wy ð16Þ EC pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e0 De < K > where Wy is the zenithal anchoring energy of the weakly anchored surface, the dielectric anisotropy De is positive, and < K > is the average elastic constant. The opposing surface is strongly anchored and usually has a small finite pre-tilt to prevent the formation of reverse tilt domains. The weakly anchored surface has zero pre-tilt, thereby ensuring that the transition to vertical alignment is first order [46]. Once the weakly anchored surface is switched vertical, latching between the bistable states is driven by the backflow of the director in a similar way to the 0-2p BTN, as shown in > Fig. 9. Immediately after the field has been removed the vertical alignment of the weakly anchored surface is at an unstable equilibrium. This means that the

1523

1524

7.3.5


strongly anchored surface

π-twist

0-twist

Topologically distinct weakly anchored surface

E

E

Gradual decrease of E

Sudden decrease of E

Ec

Vertical

. Fig. 9 The 0-p BTN or BiNem™ mode

surface director can relax to either one of the bistable states depending on the viscous flow in the vicinity of the surface. Immediate removal of the field creates a high degree of backflow close to the strongly anchored upper plate. If the cell gap is sufficiently small, this couples hydrodynamically to the director at the weakly anchored plate, inducing a relaxation into the p-twisted state. Alternatively, if the field decreases progressively, or through an intermediate value, the backflow is lower and the uniform 0p texture forms. Once the final state is selected, the surface relaxes rapidly to the un-tilted alignment, usually within 10 ms. Hydrodynamic coupling between the surfaces requires the cell gap to be less than dC, estimated [44] as: sffiffiffiffiffiffi 1 k33 g1 ð17Þ dC < 2 r Wy


7.3.5

The azimuthal anchoring energy must be sufficiently high to maintain the correct twist in the 0- and p-twist states, despite the mismatch of these states with the natural twist of the chiral nematic: d >>

k22 Wb

ð18Þ

Specialized alignment polymers have been formulated [47] that provide weak zenithal anchoring Wy and strong azimuthal Wb anchoring, while retaining the ability to be processed using standard industrial methods. When combined with a proprietary liquid crystal mixture, the zenithal anchoring energy was typically 2 104 Jm2 < Wy < 4 104 Jm2, for which dC 2 mm and the threshold voltage below 20 V. The alignment polymers also satisfied (> Eq. 18) with Wb 0.1 Wy, ensuring that the twist deviation from the 0p and 2p conditions was less than a couple of degrees. Optical contrast is provided using parallel polarizers oriented at 45 to the rubbing direction and the retardation set to the half-wave plate condition (Dn.d = l/2). The 0-twist state then appears dark and the p-twist state transmissive. For the wavelength where the halfwave plate condition is met, (> Eq. 2) then gives: pffiffiffiffiffiffiffiffiffiffiffiffiffi 2T ¼ cos2 p 1 þ a2 ¼ 0:869 ð19Þ I0 Chromaticity of the bright state is readily compensated using a violet filter layer to reduce green light transmission and meet the target white color balance. Optimization of the polarizer angles and retardation give contrast ratios of 15:1, a reflectance of 32%, and a viewing cone of 110 for contrast over 4:1 without compensation layers. The high viewing angle is a consequence of both bistable textures having in-plane director profiles. The most attractive appearance for black-and-white reflective displays requires the inter-pixel gaps to be in the white state. The liquid crystal mixture was arranged with d/P slightly less than 0.25 so that the p-twist state was favored on cooling into the nematic phase. The formation of the p-twist state is also helped by the slightly thicker cell gap that occurs in the inter-pixels gaps, due to the thickness of the transparent electrode. As with any other LCD that requires such a low cell gap, high levels of cleanliness are required during production to ensure satisfactory yields. This thin cell gap, however, gives the display a fast optical response, typically below 10 ms. Nemoptic produced several prototypes to demonstrate different aspects of the technology. A 5.100 diagonal 400 300 reflective color demonstrator attained a 20.5% reflectance with a contrast of 12:1 and an NTSC color saturation of 4.5%. It used an RGBW color filter front-plate, with the color depth designed for a double-pass suitable for a reflective mode [47]. Gray scale was produced by modulating the extent to which the 0-twist texture spread across a pixel from one of the inter-pixel gaps in what was termed a ‘‘curtain’’ effect. Later, a single-polarizer BiNem mode demonstrator was produced by reducing the cell gap still further to form a switchable quarter-wave plate [48]. Good achromatic extinction of light occurred for the 0-twist state when combined with an internal half-wave plate retarder and reflector on the rear substrate, with the polarizer oriented between 8 and 15 to the rubbing direction and the slow axis of the half-wave plate set between 16 and 30 . The use of a single polarizer mode ensured images were free from parallax, while allowing reflectances in excess of 42% to be attained. High viewing angles were maintained using a biaxial compensator built into the achromatic half-wave plate. Recently, this type of display was combined with a TFT back-plane to allow video-frame rates to be achieved for highly complex images [49].

1525

1526

7.3.5 3.5


Zenithal Bistable Nematic Displays

The bistable LCD in this category with the most market success is the Zenithal Bistable Display or ZBD™, which is being sold into the retail sector for market signage and shelf-edge labeling [50]. The device uses a submicron pitch surface relief grating to provide bistable surface alignment [51]. Other than the grating alignment layer, the device shares a construction design of the standard TN display: Thus, it combines low cost construction with infinite mulitplexibility, excellent optical properties, image retention, and the concomitant low power. Deep grating structures provide antagonism between the alignment of the tops and bottom of the grooves and the sidewalls, leading to elastic deformation of the local surface director orientation. This deformation can be reduced by the formation of ½ disclination loops at the surface, close to regions of high surface curvature. The surface is designed to give the correct degree of curvature to ensure both continuous (defect free) and defect-containing states are stable. These states are separated by an energy barrier that represents the energy required to move, create, and annihilate the defects. A typical zenithal bistable surface is provided by a homeotropic grating with a depth AG of about 1 mm and pitch L of 0.8 mm [52]. Although the director is elastically distorted close to the varying surface for the homogeneous and homeotropic continuous states, the deformation decays into the bulk of the cell and becomes constant over distances greater than half the pitch, as shown in > Fig. 10. At this distance, the grating acts as a standard alignment surface with either high, near homeotropic, pre-tilt alignment for the continuous or C state, or low pre-tilt for the defect or D state. The D state pre-tilt yD is dictated by the relative position of the defects, and is strongly dependent on grating shape: yD 90 ð1 2sD =LÞ

C state

ð20Þ

θD

D state

ªL/2 –

a

w

electrode +

hu L

. Fig. 10 Zenithal bistable surface

SD


7.3.5

where sD is the distance between +½ and ½ defects as shown in > Fig. 10, L is the distance between repeating defects (equal to the pitch for a simple, periodic grating), and pre-tilt is defined from the plane of the LCD. Usually, a pre-tilt of less than 4 is ideal and so a near symmetric sinusoid–like grating is preferred. The disclination lines run parallel to the grooves to form defect loops defining areas of D state. At the boundary between D and C states, the +½ and –½ defects detach from the edges to annihilate close to the surface plane. Unless pinned strongly by inherent inhomogeneities of the surface, the defect loop may extend or retract along the grooves if disturbed by external influences, such as changes in temperature, voltage, or viscous flow. Rather than rely on such random defect pinning sites, the grating includes a p phase shift, or ‘‘slip,’’ in the groove structure every few microns along its length to form vertical convex and concave edges. These stabilize the defect state and provide barriers to unwanted defect annihilation. Such structures enable devices with wide operating windows and good shock stability to be maintained from temperatures below 20 C to above 70 C [52]. The energy barrier between the two states ensures that bistability occurs for a wide range of different grating shapes and aspect ratios, even if one state has a lower energy. Indeed, it is often advantageous to deliberately favor one state. The D state always forms on first cooling from the isotropic into the nematic phase. A relatively deep grating is usually chosen to ensure that this state is maintained uniformly across a device at all temperatures in areas that cannot be selectively latched (e.g., in the inter-pixel gaps). An alternative zenithal bistable surface has a locally planar condition, so that the C state has a low near planar surface pre-tilt, and the defect state is higher pre-tilt. If a mono-grating is used, the director simply aligns parallel to the grooves in a monostable configuration. However, if a deep bi-grating is used, the director is forced to deform elastically around the surface features and zenithal bistability is obtained. This type of surface is utilized in the post-aligned bistable nematic device [53]. Numerical modeling of the latching transitions has been developed and compared favorably to experimental results in reference [54]. The model used a two-dimensional Q-tensor formulation of the director field, to allow for the reduction of nematic order at the defect cores. As well as the bulk anisotropic viscoelastic constants, the model included terms for the flexoelectricity, finite surface anchoring, and surface viscosity. > Figure 11 shows snapshots of the director field evolution for latching from C to D and D to C. Both states of a zenithal bistable surface have substantial splay and bend deformations of the director close to the surface, leading to local flexoelectric polarization. Latching between the states occurs if a field of sufficient magnitude, duration, and correct polarity is applied. Where the local surface condition is homeotropic, the applied field couples to a positive De liquid crystal, nucleating a ½ defect pair on the near-vertical sidewall of each groove. If the applied transverse field is positive with respect to the grating surface, the defects separate due to the effect of the flexoelectric polarization close to the surface. This causes the defects to move across the surface until the ½ defect is close to the convex grating top, and the +½ defect is close to the concave grating bottom, where the defects become ‘‘pinned’’ by the surface curvature. Application of a negative field of sufficient impulse causes the defects to detach from the grating edges, and move toward each other across the surface until they annihilate and form the C state. These transitions are reversed if the local surface orientation is planar and latching requires a negative De liquid crystal material. Addressing signals for practical devices use bipolar pulses, with the polarity of the trailing pulse defining the final state. In these instances, the elastic deformation close to the grating is increased by the RMS effect DeE2 of the first portion of the signal.

1527

1528

7.3.5


–

–

–

– – +

+

+

+ +

0 μs

a

497 μs

–

674 μs

900 μs

1.2 ms

30 ms

1.2 ms

30 ms

– –

+

+

b

0 μs

+

497 μs

703 μs

994 μs

. Fig. 11 Numerical simulation of (a) C to D, and (b) D to C transitions [54]

A simple analytical model for a zenithal bistable device has also been derived [55]. This uses a surface polarization and a critical surface torque for discontinuous changes of surface pre-tilt. Simplifying this treatment further [54] leads to an expression for the latching voltage from C to D states: " !#

ejj ejj eg w g1 ls :d 2Wy

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d þ hu þ 1 þ AG : VCD ¼ ðe11 þ e33 Þt ðe11 þ e33 Þ þ e0 DeK33 eg eg ejj w þ ejj L ð21Þ where the pulse duration is t, cell gap is d, bend and splay flexoelectric coefficients are e11 and e33, the bulk twist viscosity is g1, surface viscosity ls, and the zenithal anchoring energy Wy. The expression includes terms for the dielectric effect of the grating approximated to a rectangular shape of amplitude AG, offset hu, dielectric constant eg, full-width half maximum w, and pitch L, as defined in > Fig. 10. The form of this expression fits both numerical simulation and experimental results well, although the grating terms in this expression relate to the dielectric effect of the grating only and do not express the effect of shape differences on the defect dynamics. Both numerical simulation [54] and experimental data [56] give strong direct relationships between the latching voltages, grating pitch L, groove width (1 – w/L), and zenithal anchoring energy Wy. A weak direct relationship with the amplitude AG is also apparent.


7.3.5

Current commercial devices use the grating opposite a conventional rubbed polymer alignment to latch between a HAN and TN state [51, 57] as shown in > Fig. 12a. The grating is aligned with the grooves parallel to the rubbing direction on the opposing surface to give a 90 TN for the low pre-tilt D state. On latching into the high pre-tilt C state, the twist in the cell is removed and the hybrid-aligned HAN is formed. The device is sandwiched between parallel polarizers with a diffusive rear reflector to produce a reflective display operating in the normally white TN mode. The best optical performance uses the grating on the front surface with the polarizer parallel to the grating grooves, and the material used to form the grating is index matched to the ordinary refractive index of the liquid crystal mixture to prevent diffractive losses. The device is either operated at the first Gooch Tarry minimum, or halfway

Polariser

D

TN

Zenithal Bisable surface

C E

E

HAN

Rubbed-polymer Analyser Diffuse reflector

a

b . Fig. 12 ® The ZBD display. (a) Schematic of the device operating in VAN–TN mode. (b) Photograph of a 480 360 pixel 100 dpi 6.800 diagonal ZBD, designed for use in retail signage

1529

1530

7.3.5


between the first and second minima [58] to give good white balance. This arrangement combines the high reflectance of the TN state, with the contrast and viewing angle allowed by the TN–HAN combination. The excellent appearance is achieved without the need for compensation layers, and the tolerance on the 7 mm cell gap is lenient (typically 0.25 mm rather than the 0.05 mm of STN). ZBD latching occurs for pulse magnitudes of several volts and durations around 100 ms, though the optical change associated with this the transition to the D state may take 100 ms. This means that the display is readily addressed using conventional STN drivers, and the image typically takes a few hundred milliseconds to update. Fabrication of the ZBD device is done on a conventional TN LCD production line with negligible equipment outlay. It is inherently low cost, using standard components for TN and STN displays, but without the need for strict tolerances or optical compensation foils. Low-cost fabrication of the grating is done by embossing the structure into a homeotropic photopolymer using an inverse of the desired grating shape on a carrier film. This film is fabricated by first copying a photolithographically defined grating master into nickel, using sputtering and electroforming, which is then used to form the grating into a resin on a PET backing. The process is designed to be self-patterning [59] by arranging for the homeotropic photopolymer to adhere preferentially to the resin, except for region of the display mother glass previously coated with adhesion promoter. The simple fabrication ensures that the device has similar cost to standard STN but with greatly improved optical appearance, higher multiplexibility, and ultralow power consumption. The first application for this technology is retail signage (see > Fig. 12b) and electronic shelf-edge labels [50]. The bistable displays have been combined with an ultralow power RF communications protocol that allows many thousands of labels to operate continuously from two button batteries for between 5 and 7 years when updated several times each day, despite two-way communication for each label with a single transceiver in the store. These attributes are leading to rapidly increasing deployment across Europe, with over a million bistable displays already in operation. Also, this combination of ultralow power consumption display and communications with full graphic information content has begun to attract other sectors, including office signage and manufacturing control. A bistable surface, particularly one with an inherent polar latching mechanism, lends much flexibility to device design. The bistable surface may be used opposite a monostable homeotropic surface to create a bistable device latching between VAN and HAN states, opposite a high tilt surface to form a bistable Pi-cell, or opposite a second bistable surface to give a multi-stable device latching among VAN–HAN1–HAN2–TN states [60]. Typical devices use a periodic grating structure, to ensure a uniform display with constant orientation of the director pre-tilt and azimuthal orientation of the D state. The grating may be varied over length scales much smaller than a pixel, to give analogue gray scale or multi-domain structures [61, 62]. However, because the formation of the defects is dictated by the shape of the grating features, bistable pre-tilts may be stabilized by single isolated features, such as step edges [63], pillars, or wells [64] of the correct aspect ratio. Alternatively, a random or pseudorandom distribution of features may be used to deliberately vary the alignment of the director, resulting in bistable scattering devices [64] for example. Such behavior was also found for a deep homeotropic bi-grating structure, where the defect loops swapped from peak to peak to form random patches of D state. Surface relief is not a prerequisite for producing bistable surface alignment: Two or more stable alignment states can also be induced by patterning the local surface alignment in a periodic manner. For example, bistable pre-tilts of +ys and ys occur for a flat surface patterned to give alternating strips of homeotropic and planar


7.3.5

homogenous surface alignment, where the orientation of the planar alignment is parallel to the direction of the modulation [65] in a similar fashion to the earlier work [9] described in > Sect. 2.3, but with submicron length scales.

4

Plastic, Flexibility, and Color in Reflective Bistable Displays

The glass-based displays summarized in the previous section are beginning to realize the market potential for low power and high-image quality reflective displays. In this section, the potential for flexible plastic displays is described, which promise to help create new markets not currently possible using existing display technologies. Great effort worldwide is dedicated to producing TFT–based plastic displays, either by fabricating the transistors on the plastic substrate, or producing them by conventional means and then transferring to the plastic substrate. Various demonstrators have been made, some of which are likely to make successful entrants to the market over the next year or two. However, such devices are destined to be significantly more expensive than their glass counterparts for the foreseeable future, leaving unfulfilled the requirement for applications where low-cost plastic displays is essential. Bistable displays provide the additional advantage for applications that the image is maintained after updating. This is essential where power is remote from the display, such as for smart cards. Bistability means that a card can be produced without an onboard battery because the image is only updated when the card is inserted into a powered read/write terminal. As passive matrix displays, bistable LCDs are ideally suited for fabrication on plastic substrates, since complex images can be displayed using passive addressing, thereby avoiding the challenges associated fabricating thin-film transistors on plastic substrates. The technologies are all reflective displays, and so do not need backlighting. As LCDs, they are tolerant to low levels of oxygen and moisture ingress and, unlike OLED displays, do not need extensive barrier layers to be added to the plastic substrates.

4.1

Bistable LCDs Using Retardation Modulation

Plastic displays based on ferroelectric liquid crystals [66], Binem BTN [67] and zenithal bistable displays [68–70] have all been demonstrated. Because the devices use external polarizers and modulate the retardation of the liquid crystal, it is important to use substrates that do not add to the retardation effect of the electrooptic medium. Practically, this means that an optically isotropic transparent material should be chosen, such as polyethersulfone (PES). This restricts the process temperatures to a maximum of 170 C. Conventional glass sphere spacers may not be suitable if they impact into the softer plastic substrate material. Instead, photolithographically defined spacers are used, which may be either cylindrical [67, 68] or walls [70]. All bistable displays may lose image if exposed to sufficient shear between the two substrates, but for ferroelectric liquid crystals the resulting alignment damage can be permanent. To meet the requirements for flexing of a smart card, a segmented FLCD was designed wherein all of the inactive areas outside the latching segments were used as spacer [66], thereby furnishing the device with the maximum protection to bending. The design of many of the other components for the display also depends on the degree of flexibility required for the application. Although insensitive to oxygen and moisture ingress, the substrates should still be coated with thin ( Fig. 13. The cholesteric material is contained between electrode-bearing substrates; the front substrate is transparent, but the rear is opaque and absorbent (often black). In the planar state, up to 50% of light in the wavelength band Dl is reflected to appear colored. The focal-conic state is weakly forward scattering, so most of the light incident on areas of this texture is transmitted and absorbed by the black coating. This optical contrast is achieved without the use of separate polarizers, thereby simplifying device construction. The example display shown in > Fig. 14a uses a cholesteric with the pitch tuned to reflect yellow wavelengths, to give a yellow and black appearance of the image. Alternatively, the rear of the display might be coated with a blue material that absorbs transmitted yellow light, but reflects the blue light unaffected by the Incident unpolarised white light LH λ0±½Δλ reflected

RH λ0±½Δλ, LH λ1 and RH λ1 transmitted

Black Absorber

EHP* < E < EFH

Planar

E=0

EHP* < E < EHF

Focal Conic

E > EH

E E < EHP*

Transient Planar

. Fig. 13 Basics of bistable cholesteric display (BCD) operation

Homeotropic

White light transmitted

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0.6 15P

LH Reflectivity of planar state

0.5 Δλ 0.4

8P

0.3 4P

0.2

b

λ0

λ1

0.1 0 400

450

λ1

500 550 600 Wavelength/nm

Initial 1.0 Planar

EPF

EHP*

0.4

0.0

c

700

Final Planar

0.6

0.2

a

650

EFH EHFEH

0.8 LH λ1 Reflectivity

1534

Initial focal conic

0

10

20 RMS Voltage / V

30

. Fig. 14 BCD (a) Photograph of a 6.800 diagonal 800 600 BCD from Kent Displays, Inc. (b) Predicted reflectivity for 2 and 15 turns of a 350 nm pitched cholesteric in the planar state (calculated using MOUSE-LCD simulation software from HKUST, and using the refractive indices for the cyanobipheyl 5CB). (c) Threshold characteristic for the zero-field state after an applied pulse. The latching fields are found from the voltages labeled multiplied by the cell gap used

cholesteric helicity. In this instance, the reflecting state appears white, since the yellow light reflected by the cholesteric combines with the blue light reflected by the rear-coating. The absorbing state then appears blue. Tuning the reflected wavebands by adjusting the pitch of the material and choosing an appropriate contrasting rear absorbing layer allows a variety of different two-color combinations to be offered. Simulated reflection spectra for thick and thin samples are shown in > Fig. 14b for a material with Dn = 0.19 at 550 nm. The number of turns of the helix needed to fully reflect half of the incident light depends on the birefringence of the liquid crystal. Typical materials suitable for displays have 0.17 Dn 0.25, for which only 10–15 turns of the helix are needed. Given a typical average refractive index of n ¼ 1:6a green or yellow reflection band occurs for pitches around 360 nm, giving a high reflectance with a cell gap of only 5 mm, ideal for manufacturability. Good device operation also requires materials to be chosen with no or weak temperature dependence of the pitch. Latching between the states is accomplished by first applying a field above the threshold EH which couples to the positive dielectric anisotropy of the material to unwind the helix and induce the homeotropic orientation [80]: rffiffiffiffiffiffiffiffiffi p2 K22 ð24Þ EH ¼ P e0 De


7.3.5

For the 5 mm cell spacing of our previous example, (> Eq. 24) predicts that the critical unwinding field EH is about 30 V for a typical dielectric anisotropy of De = 20 and twist elastic constant of K22 = 7pN, allowing multiplexing with CMOS driver voltages. The electrooptic characteristic of > Fig. 14c shows the resulting textures that form after pulses of varying applied field to a sample that starts from either the focal-conic or planar texture. The final latched state depends on the rate at which the field is removed. If the field is immediately reduced to below EHP [74, 81, 82]: rffiffiffiffiffiffiffi 2 K22 EH 0:42EH ð25Þ EHP ¼ p K33 then a transient planar state is formed. At the instant the field is removed the director tilt begins to relax into the cell plane, allowing the twist to return by forming a helical conical director profile, > Fig. 13. This transient state has a helix parallel to the cell normal, but with a pitch P∗ that is higher than the intrinsic pitch of the material given by [83]: P ¼

K33 P 2P K22

ð26Þ

The transient state is metastable: All tilt will have decayed within 1 ms typically, but the device will remain in the transient planar state until the more stable planar state nucleates from defects in the sample. This transition is much slower, limiting the latching time of the liquid crystal to 200 ms or longer. The alternative relaxation from the homeotropic into the focalconic texture occurs if the field is first reduced to an intermediate level EHP < E < EHF (where EHF 0.9 EH). At this lower field, the homeotropic state of the director is still favored, but the coupling to the dielectric anisotropy cannot fully maintain the unwound state because the field is lower than EH and the director begins to twist into the plane of the cell, forming the focalconic state. A perfect planar state has a reflectance that both diminishes quickly and changes coloration when viewed off-axis, as predicted by (> Eq. 22). Preferably, the planar state is fractured into micro-domains, for example using a polymer network, to distribute the helical axis about the cell normal. Defects are formed at the domain walls that scatter the incident light across a wider angle range, thereby reducing the reflectance of the planar state. Although the peak reflectance of the device is reduced from a practical maximum of about 45% to between 30% and 35% by introducing the domain structure, devices with a 120 viewing cone and very little off-axis color shift are readily fabricated. The polymer network then tends to stabilize the planar texture, but careful choice of the surface alignment conditions ensures that the wide range of bistability is maintained. Several waveforms for driving the BCD have been suggested, the use of which depends on the application. The interested reader is referred to reference [1] for a more detailed description of the various options. Either bipolar or monopolar schemes can be used, since the display responds to the RMS voltage. If a monopolar scheme is used, the polarity should be reversed regularly to avoid electrolytic issues of the cholesteric material. Conventional addressing [84] is used when image update time is not important. This simple scheme is readily applied using conventional STN drivers, and allows a large number of gray levels to be written. Each row is addressed sequentially, initially with a blank or reset pulse Vb followed by selection using a row voltage Vs (typically 25 V) and synchronous data voltage Vd (typically 5 V) applied on the columns. The resultant pixel voltage (row minus column) switches the pixel either homeotropic or leaves it in the focal-conic state. At the end of the addressing period, the

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voltage reduces to Vd, and the pixels return to the planar state via the transient planar texture. This type of addressing operation takes between 20 and 40 ms per line: Although satisfactory for indicators and signage with low levels of image content, this time is rather long for high content displays such as those used in portable equipment or electronic readers, taking several tens of seconds to update each page. Much faster addressing uses the dynamic addressing method [85]. This scheme is similar to FLCD Malvern schemes [35] with the blanking (or preparation) phase, and the evolution phase being applied during the preceding and trailing rows, respectively to greatly improve the page update time. For example, a 1,000 row electronic reader can be addressed in little over a second using this scheme. There are advantages for use on plastic substrates that particularly suit the use of BCD: (a) Optical contrast is achieved without the cost of polarizers or a diffusing reflector. The construction of a plastic cell is simple, without lamination of these additional components. As well as being attractive in its own right, the resulting decreased thickness also helps improve display flexibility. (b) Birefringent plastic films are applicable, allowing lower cost plastics such as polyethylene terephthalate (PET) or polycarbonate to be used. (c) The lower substrate can be made from an opaque material, and as such can utilize a variety of flexible materials, such as textile or paper. (d) Unlike some bistable LCD modes, the BCD is not sensitive to cell gap and internal flatness of the substrates. (e) Liquid crystals are readily encapsulated into polymer gels. Cholesteric gels can be made in which each droplet retains its bistable nature. As described below, this allows novel fabrication processes to be adapted and employed, such as printing the bistable medium onto a single substrate. Two approaches have been taken to form liquid crystal droplets: phase separation or emulsification. One BCD [79] uses a 20% mixture of prepolymer and cholesteric spread onto a polycarbonate substrate, and laminated against a second substrate with appropriate spacer beads. The display is finished using a 15 min exposure to UV to complete the photo-cure and cutting the desired display from the resulting film. The use of the high level of photopolymer concentration means that the display image is retained when flexed, and the liquid crystal is contained by the polymer at the edges even without the presence of a gasket seal. Careful design of the system is required to ensure that the large droplet size is retained, and hence both bistability and an acceptable level of reflectivity are achieved. Recent displays have been produced using emulsification. Kodak [86] extended the printing methods developed for the photographic industry to cholesteric emulsions. The ITOcoated PET was first coated with a dried gelatine layer to insulate the electrodes from the liquid crystal before coating with the cholesteric dispersion and finally screen printing the opaque electrodes. More recently, Kent Displays Inc. used a transfer method [87] to form a cholesteric substrate-free display device. Uniformly sized droplets of cholesteric are formed in water using membrane emulsification together with an appropriate surfactant to prevent coalescence. Good reflectivity results by achieving 15 mm droplets with only a 2 mm variation. Each droplet is then coated with a 200 nm polymer shell by adding a film-forming polyurethane latex binder into the emulsion before use. The emulsion is then printed onto the electrodebearing substrate where it forms a densely packed array of droplets, and is dried to expel the water. During the drying process, the droplets tend to flatten to form flattened ellipsoids: This enhances the resulting reflectivity toward 30%. After printing the electrodes using PEDOT


7.3.5

conductor, the display is protected by over-coating in a clear polymer layer. A substrate-free device is produced by first coating the glass or plastic preparation substrate with a dark protective layer, and peeling away the completed display from the carrier to provide displays with a total thickness of 20 mm. Each processing step is designed to enable web-based production of the displays, rather than the conventional batch approach used to fabricate glass panels. Such methods have the potential to increase the factory throughput and eventually should begin to reduce the costs associated with making complex displays. The use of selective reflection in BCDs facilitates forming full color displays using a triple stacked system [88], with separate layers tuned to red, green, and blue wavebands. The most attractive appearance is achieved when the red layer is at the rear of the display, and the green layer has the opposite twist-sense to the red and blue layers to improve reflectivity [89]. The most efficient arrangement is that shown in > Fig. 15a, where cholesteric droplets are used to build up three separate cholesteric layers, interspersed with shared transparent PEDOT electrodes. In addition to minimizing parallax between the top and bottom stacks, this construction restricts the number of electrodes traversed by the reflected red light to six or less, thereby significantly reducing absorptive losses. Further enhancements of the flexibility are possible by using the PET layer as a carrier layer during construction, and then removing before use to

Protective polymer B

G

Electrodes

R

Emulsion of Microencapsulated cholesteric Base PET layer

a

b

E

c

d

. Fig. 15 Plastic-based BCD, (a) Shared electrode arrangement for a triple stack using cholesteric emulsions applied to a single substrate. (b) Photograph of a single-substrate BCD triple stack. (c) The electronic skin, providing a latchable color molding to help personalize a mobile phone. (d) A BCD writing pad

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form a substrate-free display. Each panel is addressed sequentially, and so the frame time is increased threefold, but this is outweighed by the appealing look of the resulting full-color fully flexible display, as shown in > Fig. 15b. Glass–based triple BCD stacks are being used in Flepia electronic reader from Fujitsu [90, 91]. The device runs for 40 h while continually showing 260,000 colors on the 800 display with a reflectivity approaching 30%. Multiple stacked cholesteric displays are also used for large area billboard signage by Magink [92]. The daylight readability and low power consumption give the cholesteric displays a competitive edge over large area LED units often used for this application, while allowing the full color gamut required by that application to be achieved. Signs from 6 to 13 m2 are created using tiled units of triple-stack displays. Each 17 17 cm tile has a typical resolution of 3 dpi, which is satisfactory for the typical viewing distances of greater than 10 m. The panels are kept in a temperature-controlled unit to help ensure image uniformity and the width of the inter-tile gap is minimized. Plastic BCD offers distinct advantages for producing reflective color and flexible displays, and this has led to their application in several new markets. The ultrathin and flexible substrate-free displays enable a multitude of novel applications not possible using standard display approaches. An excellent example of such a market is electronic skins, [93]. > Figure 15c shows a mobile phone casing covered with a cholesteric skin. This is effectively a single pixel display molded onto the outer layer of the product to allow the color to be personalized according to the taste of the user. Latching between states is done using very little electronics. Alternatively, > Fig. 15d is an electronic writing pad [94] that takes advantage of the flow-induced change to the planar state caused by a pressure from a writing implement. The electronics in this case are only required to latch from the planar to focal-conic state, being used to erase the whole page and prepare the device for a new page to be written.

5

Discussion and Conclusion

The wide range of different bistable technologies described presents a choice for a potential end-user. Each of the commercially available technologies has individual merits that might suit some markets but not others. > Table 1 takes a target market for each of these technologies, and summarizes the key advantages and disadvantages for that market. For that reason, the table is not a comparison between the different technologies. It also means that the disadvantages cannot be judged against the requirements for other markets. For example, BCDs offer among the brightest reflective color displays available today, yet the requirement for electronic skins demands further improvements. The speed of the FLC update beats all other display technologies, yet the market need is fully met using TFTs and the bistable technology is superseded in that market. Despite a long gestation, there has been a burgeoning of markets requiring the advantages offered by bistability over recent years. This is most evident for e-book readers, but the demand across many other applications and different sectors continues to grow rapidly. Not only do bistable displays offer ultralow power essential for long battery life, but they also have excellent flicker-free readability, low cost, and are available in plastic. In addition to being a consumer portable equipment, bistable displays are creating new applications in equipment where large numbers of battery-operated units require occasional updates. A good example of this is electronic labeling used for retail, manufacturing operations and postal tracking. Particularly attractive to these markets is the combination of bistable display and low power RF


7.3.5

. Table 1 Summary of particular bistable liquid crystal display (LCD) and the main advantages and disadvantages for the selected target market

Section Technology

Example market

Advantages

Disadvantages

3.3

SmA scattering

E-book reader

Viewing angle

High voltage, slow update

2.4

FLC C2

HDTV

Fast response, multiplexibility

Alignment, temperature range, shock stability, low cell gap

3.2

FLC C1

Watch

Very low voltage and power, ultrahigh resolution

Shock stability

4.2

BCD

Electronic skin

Flexibility, conformability color

Reflectance

2.2

0-2p BTN

Graphic equalizer

Fast response

Low cell gap

3.4

0-p BTN Binem™

E-book reader

Fast response, gray scale

Cell gap, viewing angle

3.5

ZBD

Retail signage and ESL

Cost, temperature range, power

Maximum size (