Patent Application
20090322665
The present invention relates to a liquid crystal display element in which a liquid crystal material, particularly, a liquid crystal composition exhibiting a cholesteric phase is driven to display an image, a method of driving the element, and electronic paper utilizing the element.
Recently, various enterprises and universities are actively engaged in the development of electronic paper. The most promising application of electronic paper is electronic books, and other applications include the field of portable apparatus such as sub-displays of mobile terminal apparatus, and display sections of IC cards. One type of display elements used for electronic paper is liquid crystal display elements utilizing a liquid crystal composition forming a cholesteric phase (such a composition is referred to as “cholesteric liquid crystal” or “chiral nematic liquid crystal”, and the term “cholesteric liquid crystal” will hereinafter be used). A cholesteric liquid crystal has excellent features such as semi-permanent display retention characteristics (capability of displaying an image when no electric power is supplied; memory characteristics), vivid color display characteristics, high contrast characteristics, and high resolution characteristics.
In a liquid crystal display element displaying an image by taking advantage of selective reflection at a cholesteric liquid crystal, in order to reset a state of display, the liquid crystal must temporarily be put in a homeotropic state at all pixels by applying a high voltage to the cholesteric liquid crystal. Such a reset process is problematic in that great electric power is consumed to put the liquid crystal at all pixels in the homeotropic state temporarily. The consumption of great electric power for resetting can be a big problem in portable apparatus which are limited in instantaneous electric power.
A method of driving a liquid crystal display element comprising: applying a scan signal voltage to N scan electrodes simultaneously selected from among a plurality of scan electrodes formed in parallel on an inner surface of one of a pair of substrates between which a cholesteric liquid crystal selectively reflecting light having a predetermined wavelength is enclosed for a selection time T and applying a reset voltage for resetting the liquid crystal to a plurality of data electrodes formed on an inner surface of the other of the pair of substrates so as to intersect the plurality of scan electrodes when viewed in the normal direction of a substrate surface, the reset voltage being applied in synchronism with the application of the scan signal voltage, wherein the applying step is repeated while shifting the N simultaneously selected scan electrodes sequentially to perform a display reset for putting the liquid crystal in a homeotropic state by applying the reset voltage to each pixel on each of the scan electrodes N times consecutively, and the display reset is performed using such a selection time T and a simultaneously selected electrode count N that the product T×N satisfies T×N>τ(PL→HT) where τ(PL→HT) represents a response time required for the liquid crystal to change from a planar state to the homeotropic state.
Before describing a liquid crystal display element, a method of driving the element, and electronic paper utilizing the element according to an embodiment of the invention, a description will be made with reference to
The B display portion 46b includes a blue (B) liquid crystal 43b enclosed between a pair of substrates, i.e., a top substrate 47b and a bottom substrate 49b and a pulse voltage source 41b for applying a predetermined pulse voltage to the B liquid crystal 43b. The G display portion 46g includes a green (G) liquid crystal 43g enclosed between a pair of substrates, i.e., a top substrate 47g and a bottom substrate 49g and a pulse voltage source 41g for applying a predetermined pulse voltage to the G liquid crystal 43g. The R display portion 46r includes a red (R) liquid crystal 43r enclosed between a pair of substrates, i.e., a top substrate 47r and a bottom substrate 49r and a pulse voltage source 41r for applying a predetermined pulse voltage to the R liquid crystal 43r. Although not depicted, electrodes for applying the pulse voltages from the respective pulse voltage sources 41 are formed on interface sides of the top substrates 47 and the bottom substrates 49 where the substrates contact the respective liquid crystals 43. An alignment film or an insulation film may be formed in addition to electrodes on each of the interface sides of the top substrates 47 and the bottom substrates 49 where the substrates contact the liquid crystals 43 as occasion demands. A light absorbing layer 45 is disposed on a bottom surface of the bottom substrate 49r of the R display portion 46r.
A cholesteric liquid crystal used in each of the B, G, and R liquid crystals 43b, 43g, and 43r is a liquid crystal mixture obtaining by adding a relatively great amount of chiral additive (also referred to as “chiral material”) to a nematic liquid crystal to a content of several tens percent by weight. When a nematic liquid crystal includes a relatively great amount of chiral material, a cholesteric phase, which is a great helical twist of nematic liquid crystal molecules, can be formed in the liquid crystal. For this reason, a cholesteric liquid crystal is also referred to as “chiral nematic liquid crystal”.
A cholesteric liquid crystal has bi-stability (memory characteristics), and the liquid crystal can be put in any of a planar state, a focal conic state, or an intermediate state which is a mixture of the planar state and the focal conic state by adjusting the intensity of an electric field applied to the same. Once the liquid crystal enters the planar state, the focal conic state, or the mixed or intermediate state, the state is thereafter kept with stability even after the electric field is removed.
The planar state can be obtained by applying a predetermined high voltage between a top substrate 47 and a bottom substrate 49 to apply a strong electric field to the liquid crystal 43 and to thereby reset the liquid crystal 43 to the homeotropic state and thereafter nullifying the electric field abruptly. For example, the focal conic state can be obtained by applying a predetermined voltage lower than the above-described high voltage between the top substrate 47 and the bottom substrate 49 to apply an electric field to the liquid crystal 43 and thereafter nullifying the electric field abruptly. The focal conic state may alternatively be obtained by gradually applying a voltage to a liquid crystal in the planar state.
For example, the intermediate state which is a mixture of the planar state and the focal conic state can be obtained by applying a voltage lower than the voltage to obtain the focal conic state between the top substrate 47 and the bottom substrate 49 to apply an electric field to the liquid crystal 43 and thereafter nullifying the electric field abruptly.
A display principle of the liquid crystal display element 51 utilizing cholesteric liquid crystals will now be described by referring to the B display portion 46b as an example.
In the planar state, light in a predetermined wave band in accordance with the helical pitch of the liquid crystal molecules 33 is selectively reflected by the liquid crystal. The reflected light is circularly polarized light which is either left- or right handed depending on the chirality of the helical structures, and other types of light are transmitted by the liquid crystal. Natural light is a mixture of left- and right-handed circularly polarized light. Therefore, when natural light in the predetermined wave band impinges on the liquid crystal in the planar state, it may be assumed that 50% of the incident light is reflected with the other 50% transmitted.
A wavelength λ at which maximum reflection takes place is given by λ=n·p where n represents the average refractive index of the liquid crystal and p represents the helical pitch.
Therefore, in order to allow blue light to be selectively reflected by the B liquid crystal 43b of the B display portion 46b in the planar state, the average refractive index n and the helical pitch p are determined, for example, such that an equation “λ=480 nm” holds true. The average refractive index n can be adjusted by selecting the liquid crystal material and the chiral material appropriately, and the helical pitch p can be adjusted by adjusting the chiral material content.
In the intermediate state that is a mixture of the planar state and the focal conic state, the ratio between reflected light and transmitted light is adjusted according to the ratio of presence between the planar and focal conic states, and the intensity of reflected light varies accordingly. Therefore, multi-gray-level display can be performed according to intensities of reflected light.
As thus described, the quantity of light reflected by the cholesteric liquid crystal can be controlled by a helically twisted state of alignment of liquid crystal molecules 33. Cholesteric liquid crystals selectively reflecting green and red light rays in the planar state are used in the G liquid crystal 43g and the R liquid crystal 43r, respectively, just as done in the B liquid crystal 43b, and the B display portion 46b, the G display portion 46g, and the R display portion 46r are formed one over another to fabricate a liquid crystal display element 51 capable of full-color display. The liquid crystal display element 51 has memory characteristics, and it is capable of performing color display without consuming electric power except during a screen rewrite.
The liquid crystal display element 51 displaying an image utilizing selective reflection at cholesteric liquid crystals as thus described must involve a display reset process in which a high voltage is applied to temporarily put the cholesteric liquid crystals in the homeotropic state regardless of the state of the cholesteric liquid crystals, i.e., the planar state, the focal conic state, or the mixed or intermediate state.
The illustration of the liquid crystal display element 51 focuses on one pixel or segment of the display to describe a principle of a display operation utilizing cholesteric liquid crystals, and no consideration is needed for the problem of electric power consumed by the display reset process for the illustrated instance. However, when a liquid crystal display element having a plurality of pixels arranged in the form of a matrix or a liquid crystal display element having a plurality of segments is incorporated in a portable apparatus, an increase in power consumption attributable to the display reset process constitutes a problem which cannot be ignored.
For example, in a passive matrix liquid crystal display element, a plurality of pixels are provided by forming a plurality of scan electrodes and data electrodes in vertical and horizontal directions in the form of a matrix. In general, instantaneous electric power available in a portable apparatus for driving such a liquid crystal display element is somewhat limited, and it is therefore difficult to accommodate excessively great power consumption required to put the liquid crystal at all pixels in the homeotropic state at a time when the display reset process is performed.
In order to solve such a problem, a method has been proposed, which includes the step of selecting some of scan electrodes simultaneously and applying a reset voltage to the electrodes in a synchronized manner. The step is repeated with the simultaneously selected scan electrodes sequentially shifted one place each time the step is performed. Thus, the reset voltage is consecutively applied plural times to each pixel on each scan electrode to put the liquid crystal in the homeotropic state (International Publication No. WO2006/103738).
Since the reset process is performed only for the liquid crystal on some of scan electrodes which have been simultaneously selected, instantaneous electric power required for a reset can be kept small, and the reset process can therefore be performed in a portable apparatus which has limited instantaneous electric power. However, the proposed display resetting method neither discloses nor suggests the optimal number of scan electrodes to be simultaneously selected. In order to suppress instantaneous electric power required for a reset, the number of simultaneously selected scan electrodes must be as small as possible. Further, the relationship between a reset process and a subsequent data rewrite must be clarified to allow a display reset process to be performed with stability.
The present embodiment of the invention employs a driving method as described below for when a display reset process is performed.
A selection time T required per scan electrode and a simultaneously selected electrode count N, i.e., the number of scan electrodes to be simultaneously selected, are defined in advance such that the product (T×N) of the selection time T and the simultaneously selected electrode count N satisfies T×N>τ(PL→HT) where τ(PL→HT) represents a response time required for a cholesteric liquid crystal to change from the planar state to the homeotropic state.
Next, a scan signal voltage is applied for the selection time T to N scan electrodes which have been simultaneously selected from among a plurality of scan electrodes formed in parallel on an inner surface of one of a pair of substrates enclosing a cholesteric liquid crystal selectively reflecting light rays having predetermined wavelengths (step S1).
A reset voltage for resetting the liquid crystal is applied in synchronism with the application of the scan signal voltage to a plurality of data electrodes formed in parallel on an inner surface of the other substrate so as to extend across the plurality of scan electrodes when viewed in the normal direction of substrate surfaces (step S2).
The steps S1 and S2 are repeated with the N simultaneously selected scan electrodes sequentially shifted one place each time the steps are performed to apply the reset voltage N times consecutively to each pixel on each scan electrode.
The inventor conducted a close study in search of a step which allows predetermined pixels to be reset to a reflective state after a voltage is applied to a plurality of simultaneously selected scan electrodes. As a result, it was found that predetermined pixels can be uniformly put in a reflective state when T×N>τ(PL→HT) is true as described above.
Thus, a cholesteric liquid crystal can be reliably put in the homeotropic state, and the number of simultaneously selected scan electrodes can be made small as occasion demands. It is therefore possible to minimize instantaneous electric power at the time of a reset, and a display reset can be carried out even on a portable apparatus which is limited in instantaneous electric power.
When data is written at a pixel on a predetermined scan electrode after a display reset, time t spent before the data write is started after the display reset is set to satisfy t>τ(HT→PL) where τ(HT→PL) represents a response time required for the liquid crystal to change from the homeotropic state to the planar state.
The inventor found that such a setting allows a stable state of reflection to be achieved at a predetermined pixel after the application of a reset signal based on simultaneous selection. Thus, a uniform state of display can be achieved.
Response of a cholesteric liquid crystal to the application of a voltage has temperature dependence. Therefore, the response time τ(PL→HT) and the response time τ(HT→PL) also depend on the temperature of the liquid crystal. For this reason, the product T×N and the time t in the present embodiment are determined based on the temperature of the liquid crystal at the time of a display reset. Since the product T×N and the time t can therefore be optimally selected according to the temperature of the liquid crystal, the reset process and display of an image can be stably performed without being affected by fluctuations of ambient temperature.
A liquid crystal display element, a method of driving the element, and electronic paper utilizing the element according to the present embodiment will now be described with reference of
As depicted in
The B display portion 6b has a pair of substrates, i.e., a top substrate 7b and a bottom substrate 9b disposed opposite to each other and a B liquid crystal 3b enclosed between the substrates 7b and 9b. The B liquid crystal 3b is a cholesteric liquid crystal which has an average refractive index n and a helical pitch p adjusted to reflect blue light selectively and has rightward optical rotatory power (rightward chirality). The liquid crystal reflects blue right-handed circularly polarized light and transmits other types of light in the planar state and which transmits substantially all types of light in the focal conic state.
The G display portion 6g has a pair of substrates, i.e., a top substrate 7g and a bottom substrate 9g disposed opposite to each other and a G liquid crystal 3g enclosed between the substrates 7g and 9g. The G liquid crystal 3g is a cholesteric liquid crystal which has an average refractive index n and a helical pitch p adjusted to reflect green light selectively and has leftward optical rotatory power (leftward chirality). The liquid crystal reflects green left-handed circularly polarized light and transmits other types of light in the planar state and which transmits substantially all types of light in the focal conic state.
The R display portion 6r has a pair of substrates, i.e., a top substrate 7r and a bottom substrate 9r disposed opposite to each other and an R liquid crystal 3r enclosed between the substrates 7r and 9r. The R liquid crystal 3r is a cholesteric liquid crystal which has an average refractive index n and a helical pitch p adjusted to reflect red light selectively and has rightward optical rotatory power (rightward chirality). The liquid crystal reflects red right-handed circularly polarized light and transmits other types of light in the planar state and which transmits substantially all types of light in the focal conic state.
The choesteric liquid crystal used as each of the B, G, and R liquid crystals 3b, 3g, and 3r is obtained by adding a chiral material to a nematic liquid crystal mixture to a content of 10 to 40 percent by weight. The chiral material content is a value based on an assumption that the total amount of the nematic liquid crystal component and the chiral material constitutes 100 percent by weight. Various types of known nematic liquid crystals may be used. The cholesteric liquid crystals preferably have refractive index anisotropy Δn having a value satisfying 0.18≦Δn≦0.24. When the refractive index anisotropy Δn is smaller than the range, the liquid crystals 3b, 3g, and 3r have low reflectances in the planar state. When the refractive index anisotropy is greater than the range, the liquid crystals 3b, 3g, and 3r have significant scatter reflections in the focal conic state, and the layers have high viscosity which reduces the speed of response.
The chiral material added in the B and R cholestric liquid crystals and the chiral material added in the G cholesteric liquid crystal are optical isomers of each other in that they are different from each other in optical rotatory power. Therefore, the B and R cholesteric liquid crystals are the same as each other and different from the G cholesteric liquid crystal in terms of optical rotatory power.
As depicted in
The top substrates 7b, 7g, and 7r and the bottom substrates 9b, 9g, and 9r must have translucency. In the present embodiment, pairs of polycarbonate (PC) film substrates cut in longitudinal and transverse sizes of 10 cm×8 cm are used. Glass substrates or film substrates made of polyethylene terephthalate (PET) may be used instead of PC substrates. Such film substrates have sufficient flexibility. In the present embodiment, all of the top substrates 7b, 7g, and 7r and the bottom substrates 9b, 9g, and 9r have translucency, but the bottom substrate 9r of the R display portion 6r disposed at the bottom of the element may be opaque.
As depicted in
As depicted in
In the G display portion 6g, 320 scan electrodes 17g, 240 data electrodes 19b, and G pixels 12g (not depicted) arranged in the form of a matrix having 320 rows and 240 columns are formed in substantially the same manner as in the B display portion 6b. Similarly, scan electrodes 17r, data electrodes 19r, and R pixels 12r (not depicted) are formed in the R display portion 6r. Each set of B, G, and R pixels 12b, 12g, and 12r form one pixel 12 of the liquid crystal display element 1. The pixels 12 are arranged in the form of a matrix to form a display surface.
For example, a typical material used to form the scan electrodes 17b, 17g, and 17r and the data electrodes 19b, 19g, and 19r is indium tin oxide (ITO). Transparent conductive films made of indium zinc oxide (IZO) or the like, metal electrodes made of aluminum, silicon, or the like, or transparent conductive films made of amorphous silicon or the like may alternatively be used.
A scan electrode driving circuit 25 including scan electrode driver ICs for driving the plurality of scan electrodes 17b, 17g, and 17r is connected to the top substrates 7b, 7g, and 7r. A data electrode driving circuit 27 including data electrode driver ICs for driving the plurality of data electrodes 19b, 19g, and 19r is connected to the bottom substrates 9b, 9g, and 9r. A driving section 24 is formed by the scan electrode driving circuit 25 and the data electrode driving circuit 27.
The scan electrode driving circuit 25 selects three predetermined scan electrodes 17b, 17g, and 17r based on a predetermined signal output from a control section 23 and simultaneously outputs scan signals to the three scan electrodes 17b, 17g, and 17r. Based on a predetermined signal output from the control section 23, the data electrode driving circuit 27 outputs image data for B, G, and R pixels 12b, 12g, and 12r on the selected scan electrodes 17b, 17g, and 17r to the respective data electrodes 19b, 19g, and 19r. For example, general-purpose STN driver ICs having a TCP (tape carrier package) structure are used as the driver ICs for the scan electrodes and the data electrodes.
In the present embodiment, since diving voltages for the B, G, and R liquid crystals 3b, 3g, and 3r can be substantially equal to each other, a predetermined output terminal of the scan electrode driving circuit 25 is commonly connected to predetermined input terminals of the scab electrodes 17b, 17g, and 17r. Thus, there is no need for providing a scan electrode driving circuit 25 for each of the B, G, and R display portions 6b, 6g, and 6r, which allows the liquid crystal display element 1 to be provided with a simple configuration. Further, since a reduction can be achieved in the number of san electrode driver ICs, the liquid crystal display element 1 can be provided at a low cost. The output terminal of the scan electrode driving circuit may be shared between the B, G, and R electrodes as occasion demands.
Obviously, each of the electrodes 17b and 19b may be coated with a functional film, e.g., an insulation film or an alignment film for controlling the alignment of liquid crystal molecules (neither of the films is depicted). The insulation film has the function of preventing shorting between the electrodes 17b and 19b, and the film also serves as a gas barrier layer having the function of improving the reliability of the liquid crystal display element 1. The alignment film may be an organic film such as a polyimide resin, a polyamide-imide resin, a polyether imide resin, a polyvinyl butyral resin, or an acryl resin, and an inorganic material such as a silicon oxide or an aluminum oxide may alternatively be used. For example, alignment films are provided throughout the substrates to coat the electrodes in the present embodiment. The alignment films may be also used as insulating thin films.
As depicted in
The structure of the G display portion 6g and the R display portion 6r will not be described because it is similar to that of the B display portion 6b. A visible light absorbing layer 15 is provided on the outer surface (bottom surface) of the bottom substrate 9r of the R display portion 6r. Since the visible light absorbing layer 15 is provided, rays of light which have not been reflected by the B, G, and R liquid crystals 3b, 3g, and 3r can be efficiently absorbed. Therefore, the liquid crystal display element 1 can display an image with a high contrast ratio. The visible light absorbing layer 15 may be provided as occasion demands.
A method of driving the liquid crystal display element 1 will now be described with reference to
An example will now be described, in which a predetermined voltage is applied to a blue (B) pixel 12b (1,1) that is located at the intersection between the data electrode 19b of the first column of the B display portion 6b depicted in
When the selection period T1 ends and a non-selection period T2 starts, voltages of, for sample, +28 V and +4 V having a period equivalent to one half of the selection period T1 are applied to the scan electrode 17b of the first row. On the other hand, predetermined data signal voltages Vd are applied to the data electrode 19b of the first column. In
When the voltage applied to the liquid crystal changes from the voltage VP100 (±32 V) to the voltage VF0 (±4 V) to make the electric field substantially zero abruptly while the liquid crystal molecules are near the homeotropic state, the liquid crystal molecules enter a helical state in which the helical axes are directed substantially perpendicular to the electrodes 17b and 19b. Thus, the liquid crystal enters the planar state, in which rays of light in accordance with the helical pitch are selectively reflected. Since the B liquid crystal 3b at the B pixel 12b (1,1) thus enters the planar state to reflect light, blue is displayed at the B pixel 12b (1,1).
As depicted in
When the voltage applied to the cholesteric liquid crystal changes from the voltage VF100b (+24 V) to the voltage VF0 (+4 V) to make the electric field substantially zero abruptly in the state in which the helical structure of the liquid crystal molecules is not completely decomposed, the liquid crystal molecules enter a helical state in which the helical axes are directed substantially parallel to the electrodes 17b and 19b. That is, the liquid crystal molecules enter the focal conic state in which incident light is transmitted. Thus, the B liquid crystal 3b at the B pixel 12b(1,1) enters the focal conic state to transmit light. As depicted in
The driving voltages and driving method described above are merely examples. When a pulse voltage of 30 to 35 V is applied between the electrodes for an effective duration of 20 to 100 ms at room temperature, the cholesteric liquid crystal of the B liquid crystal layer enters a state for selective reflection (planar state). When a pulse voltage of 15 to 22 V is applied for an effective duration of 20 to 100 ms, the cholesteric liquid crystal enters a highly transmissive state (focal conic state).
A green (G) pixel (1,1) and a red (R) pixel (1,1) are driven in substantially the same manner in which the B pixel (1,1) is driven, whereby color display can be performed at a pixel (1,1) that is formed by the three pixels, i.e., the B, G, and R pixels (1,1) stacked one over another. The scan electrodes constituting the first to m-th rows may be driven in the so-called line sequential mode to rewrite the data voltage at each data electrode of each row (data scan), whereby display data can be output to all of pixels (1,1) to (m, n) to achieve color display of one frame (display screen).
The intermediate state that is a mixture of the planar state and the focal conic state can be obtained to enable full color display be applying a voltage within the two range A or B depicted in
A description will now be made with reference to
First, a response time τ(PL→HT) required for the cholesteric liquid crystals to change from the planar state to the homeotropic state is identified in advance. Based on the response time τ(PL→HT), a selection time T required per scan electrode and a simultaneously selected electrode count N, i.e., the number of scan electrodes to be simultaneously selected, are determined in advance such that the product T×N satisfies T×N>τ(PL→HT).
Let us assume that the scan electrodes 17b, 17g, and 17r on the row in an i-th place (i represents an integer equal to or greater than 1) counted from the first row (which is at the top edge of the display surface D depicted in
The series of N scan electrodes 17bgr(i) to 17bgr(i+N−1) is represented by a region (T×N) which looks like a black horizontal band in
The waveform depicted on the left side of
As thus described, a scan signal voltage is applied for the selection time T to N scan electrodes which have been simultaneously selected from among the entire scan electrodes (step S1), and a reset voltage for resetting the liquid crystals is applied to the data electrodes 19 in synchronism with the application of the scan signal voltage (step S2) as depicted in
Referring to
Data are written in pixels on predetermined scan electrodes after the display reset after the time t (>τ(HT→PL). Thus, the liquid crystal molecules of the liquid crystals are reliably made to enter the planar state within the time t. The driving method depicted in
The response time τ(PL→HT) is a time required for a reflectance change from 100% to 10% where reflectance is 100% in the planar state and 0% in the homeotropic state. Similarly, the response time τ(HT→PL) is a time required for a reflectance from 0% to 90% where reflectance is 0% in the homeotropic state and 100% in the planar state and where the applied voltage is removed or made as low as a few volts in the homeotropic state.
As a result of a decrease in the temperature, the response time τ(PL→HT) changes in the range from about 2 (ms) to about 600 (ms), whereas the response time τ(HT→PL) changes in the range from about 20 (ms) to about 8000 (ms). Thus, the response time τ(HT→PL) undergoes changes in greater amounts.
In consideration to such temperature dependence of response of a cholesteric liquid crystal, it is preferable to allow the product T×N and the time t to be varied based on the temperature of the liquid crystal at the time of a display reset. By allowing the product T×N and the time t to be optically selected according to the temperature of the liquid crystal, a reset process and display of an image can be stably performed without being affected by fluctuations in ambient temperature. While
As depicted in
Specifically, the response times τ(PL→HT) and τ(HT→PL) are 35 ms and 200 ms, respectively, at 0° C. Therefore, at a temperature of 0° C. or higher, the selection time T may be 4 ms, and the simultaneously selected electrode count N may be 10. When the selection time T is 4 ms and the simultaneously selected electrode count N is 10, the time t is 4 ms×(240−10)=920 ms. Therefore, both of the conditions, i.e., the product T×N=40 ms>τ(PL→HT)=35 ms, and time t=920 ms>τ(HT→PL)=200 ms, can be satisfied. Thus, a uniform and high reflectance can be achieved.
Referring to the temperature range between 0° C. and −20° C. (0° C. is excluded), the response times τ(PL→HT) and τ(HT→PL) are 380 ms and 5200 ms, respectively, at −20° C. Let us assume that the selection time T is 20 ms and that the simultaneously selected electrode count N is 20. Then, the product T×N=400 ms>τ(PL→HT)=380 ms, and the product therefore satisfies the condition. However, the time t=20 ms×(240−20)=4400 ms, and the time t therefore becomes shorter than the response time τ(HT→PL) of 5200 ms at −20° C. Thus, time t>τ(HT→PL) is not true, and a sufficient reflectance cannot be achieved.
In the temperature range between 0° C. and −20° C. (0° C. is excluded), it is therefore preferable to set the selection time T at 40 ms and the simultaneously selected electrode count N at 10 than setting the selection time T at 20 ms and the simultaneously selected electrode count N at 20. When the selection time T is 40 ms and the simultaneously selected electrode count N is 10, the product T×N=400 ms>τ(PL→HT)=380 ms. The time t=40 ms×(240−10)=9200 ms>τ(HT→PL)=5200 ms. Therefore, both of the conditions, i.e., the product T×N>τ(PL→HT) and time t>τ(HT→PL), can be satisfied.
As described above, a predetermined pixel can be reset to a reflective state in a stable manner by scanning a plurality of simultaneously selected scan electrodes.
While it is preferable to keep the number of simultaneously selected scan electrodes small from the viewpoint of power consumption, the number of selected electrodes may obviously be increased as occasion demands. It is needless to say that the selection time T, the simultaneously selected electrode count N, and the time t are not limited to the values depicted above.
In the display reset process of the present embodiment improves the display resetting method including the steps of selecting some of scan electrodes simultaneously, applying a reset voltage in synchronism with the selection, and repeating the operation while shifting the simultaneously selected electrodes sequentially one place at each cycle of the operation to put the liquid crystal in the homeotropic state. Specifically, the process allows instantaneous electric power required at the time of a reset to be minimized and allows a reset process to be stably performed in a portable apparatus which is limited in instantaneous electric power.
A method of manufacturing the liquid crystal display element 1 according to this embodiment will now be specifically described.
ITO transparent electrodes are formed using a sputtering process on two PC film substrates 7 and 9 which have been cut to have longitudinal and transversal lengths of, for example, 10 cm and 8 cm. The ITO electrodes are then patterned at a photolithographic step to form electrodes in the form of stripes having a pitch of 0.24 mm (scan electrodes 17 and data electrodes 19) on the respective substrates. Thus, stripe-like electrodes are formed on the two PC film substrates, respectively, to allow QVGA display of 320×240 dots.
Then, a polyimide type alignment film material is applied to the stripe-like transparent electrodes on each of the two PC film substrates to a thickness of about 70 nm using a spin coat process. The two PC film substrates coated with the alignment material are then baked for one hour in an oven at 90° C. to form alignment films.
Then, an epoxy type seal material 21 is applied to a peripheral part of either of the PC film substrates using a dispenser. Next, spherical spacers (manufactured by SEKISUI FINE CHEMICAL) are dispersed on the other PC film substrate i.e., the substrate 9 or 7 to adjust the cell gap (the thickness of the liquid crystal layer) to about 4 μm. Then, the two PC film substrates 7 and 9 are combined and heated for one hour at 160° C. to cure the seal material 21. Then, a B cholesteric liquid crystal LCb is injected using a vacuum injection process, and the injection port is thereafter sealed with an epoxy type sealing material to fabricate a B display portion 6b. G and R display portions 6g and 6r are fabricated using substantially the same method.
Next, the B, G, and R display portions 6b, 6g, and 6r are formed one over another in the order listed from the side of a display surface, as depicted in
Electronic paper is completed by providing the liquid crystal display element 1 thus completed with an input/output device and a control device for exercising overall control of the element (neither of the devices is depicted).
As described above in detail, problems encountered at the time of a display reset of a display device utilizing cholesteric liquid crystals can be solved in the present embodiment. Electronic paper utilizing such a display device can be also provided.
The invention is not limited to the above-described embodiment and may be modified in various ways.
The above embodiment has been described as a liquid crystal display element having a three-layer structure formed by forming B, G, and R display portions 6b, 6g, and 6r one over another as an example. However, the invention is not limited to such an element and may be applied to liquid crystal display elements having a multi-layer structure with two layers or four or more layers. Obviously, the invention may be applied to liquid crystal display elements having a single-layer structure.
The above embodiment has been described as a liquid crystal display element including display portions 6b, 6g, and 6r having liquid crystals 3b, 3g, and 3r for reflecting blue, green, and red rays of light in the planar state, as an example. However, the invention is not limited to such an element and may be applied to liquid crystal display elements including a stack of plural display portions having respective liquid crystals for reflecting cyan, magenta, and yellow rays of light in the planar state enclosed therein.
This application is a continuation of International Application No. PCT/JP2007/054525, filed Mar. 8, 2007.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2007/054525 | Mar 2007 | US |
| Child | 12553641 | US |
