GB2475535A

GB2475535A - Display with wire grid polarizer capacitively coupled to driving arrangement

Info

Publication number: GB2475535A
Application number: GB0920438A
Authority: GB
Inventors: Allan Evans; Alexandra Baum
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2009-11-23
Filing date: 2009-11-23
Publication date: 2011-05-25
Also published as: WO2011062300A1; GB0920438D0

Abstract

A display, such as a liquid crystal display, comprises an active substrate which carries a driving arrangement 17-19, 23, 25, 26 and an addressing electrode arrangement. The addressing electrode arrangement is formed by a floating wire grid polariser 24 having electrically isolated polarising regions 16, acting as pixel electrodes. Each region 16 is capactively coupled to the driving arrangement 17-19, 23, 25, 26. The LCD may be a an active-matrix LCD using the vertically aligned nematic (VAN) mode or a multiple-domain (or reverse) twisted nematic display mode with vertically aligned liquid crystal.

Description

Display The present invention relates to a display.

Liquid crystal displays require polarisers on either side of the liquid crystal layer. In current displays, the polarisers are typically made from iodine-doped PVA (poly vinyl alcohol) and placed on the outer surface of the glass substrates on which the display is made. A wire grid is a type of polariser formed from an array of very fine wires.

Because of its robustness and small thickness, a wire grid polariser may be placed inside the substrates. This may have advantages including reduced thickness of the display module, easier manufacturing and improved resistance to heat and solvents.

Also, when the the polarisers are placed close to the liquid crystal layer, depolarisation by scattering or birefringence in other layers does not affect the display's contrast.

However the wires in the wire grid polariser are electrical conductors and may interfere with the operation of the pixel electrodes.

Figure 1 of the accompanying drawings is a schematic diagram of a typical active-matrix liquid crystal display. The display is constructed on two substrates 1 and 2, typically glass. External polarisers 3 are placed on the outer surfaces of the substrates.

In most modern displays, they are crossed so that no light passes through unless there is a change in the polarisation state caused by some component in between the two polarisers. The colour-filter substrate 2 has on its inner surface red, green, and blue colour filters 4, followed by a transparent electrode 5 and an alignment layer (often polyimide) 6.

The TFT (thin-film transistor) substrate 1 has electronic driving circuits including TFTs 7 on its inner surface. These are electrically connected by a conductor 8 to transparent pixel electrodes 9. The TFT substrate also has an alignment layer 6.

The two alignment layers are in contact with the liquid crystal 10. The display is illuminated by a backlight 11.

For simplicity, there are a number of components omitted from this diagram, including passivation and insulation layers, other features used to align the liquid crystal, addressing circuits and black masks. Also the details of construction vary from one type of display to another.

Placement of the polarisers on the outer surfaces of the display has a number of disadvantages. They are vulnerable to damage, being softer than the glass substrates.

They are typically placed on the LCD panel after assembly and cleaving of the substrate, requiring an extra, costly manufacturing step.

The most important disadvantage is a possible loss of contrast. Any alteration of the polarisation state of the light between the two polarisers leads to transmission of light through the display. When a pixel is switched on, the liquid crystal layer changes the polarisation state, and this is how the display allows light to pass through. But when a pixel is intended to be opaque and show black, any small amount of birefringence or scattering in other components of the display panel may change the polarisation state of some of the light in the display and cause leakage of light. This makes the black state less dark and lowers the contrast.

This change in polarisation may be caused in a number of different ways. For example, colour filters typically scatter a small amount of the light passing through them, and this scattered light is depolarised. If polymer substrates are used instead of glass, then there is often a small birefringence which changes the polarisation. The edge of opaque features in the driving circuitry, such as the TFTs, may also scatter and depolarise light. In some display designs the pixel electrode has gaps which are used to control alignment of the liquid crystal, and again the edges of these gaps may scatter light.

For all these reasons, it is desirable to place polarisers on the inner surface of the substrates, and as close to the liquid crystal layer as possible. Polarisers on the inner surface of the substrates are known as internal polarisers. The iodine-doped PVA polarisers which are used in current displays are not suitable for this location for two reasons: they are too thick (-20.tm), and they degrade at temperatures around 100C.

The alignment layer 6 is often baked at temperatures up to 240C, so this would destroy an iodine polariser placed underneath.

Wire grid polarisers are thin enough and resistant to high temperatures for this application. A wire grid polariser is a fine array of conducting wires, which transmits light polarised with its electric field perpendicular to the wires and reflects or absorbs light polarised with its electric field parallel to the wires.

The use of wire grid polarizers internally in liquid crystal displays has been considered for more than 20 years, for example by Grinberg (US4688897, 1985), where the wire grid serves as polariser, reflector and electrode in a reflective LCD. More recently, Sergan (J. Opt. Soc. Am. 19, 1872, 2002) used reflective wire grid polarisers in a twisted nematic LCD. Lee et al. demonstrated a stereoscopic LC display based on patterned wire grid polarisers (Society for Information Display Symposium Digest, paper 8.4 2006). Ge et aI. developed a transflective LCD (App!. Phys. Lett. 92, 051109, 2008) and demonstrated light "recycling" from the LCD backlight (App!. Phys. Lett. 93, 121104, 2008) both using the reflections from a wire grid.

Polarisers may be applied on an active substrate, which comprises any display substrate where different pixels are addressed with different voltages. This would include the TFT substrate of an active matrix display, or both substrates of a passive matrix display. Figure 2 of the accompanying drawings shows a number of possible arrangements of polarisers on an active substrate.

Figure 2 (a) shows the arrangement most common in current displays, where an external polariser 3 is placed on the outer surface of the substrate.

In Figure 2(b), an internal wire grid polariser is electrically connected to the driving electronics and used as a pixel electrode 12. Prior art suggesting this method includes US patent application 20080094547 (Sugita) and US patent 7480017 (Fisher). The plan view 13 of the wire grid shows how cross-connections 14 are necessary to allow the whole area of the wire grid for a particular pixel to act as a single electrode.

The main disadvantage of this method is the added manufacturing steps necessary to make the cross-connections 14 and to make an electrical connection 8 to the wire grid.

In Figure 2(c), a uniform wire grid polariser 15 is placed directly on the inner surface of the substrate 1. One difficulty with this method is that manufacturing methods for the pixet driving circuitry often require high temperatures which may disrupt the wire grid structure. Also for high yield, a hard, flat surface is required for manufacturing of the driving circuit: the upper surface of the polariser may not satisfy this requirement. Also, for the reasons mentioned above, it is desirable to place the polariser as close to the liquid crystal layer as possible, so that the contrast of the display is as high as possible.

This method therefore has the disadvantage that the contrast of the display is likely to be lowered by scattering or depolarisation from structures such as the TFTs 7 and conductors 8.

In Figure 2(b) and Figure 2(c), the external polariser 3 is shown in addition to the internal polariser 12 or 15. In this case the internal polariser acts as a clean-up' polariser. This means that the external polariser, which may be a PVAliodine polariser with excellent optical properties, polarises light entering the LCD, and the internal polariser has the function of absorbing light of the wrong' polarisation generated by scattering or birefringence in the layers of the display between the external and internal polarisers.

For example, suppose that the external polariser has an extinction ratio of order 10 000:1 and the internal polariser has an extinction ratio of order 10:1, and that scattering or birefringence results in 1% of the light transmitted by the external polariser being converted to the perpendicular polarisation state. Then with external polarisers only, the contrast of the display is limited to approximately 100:1. Using internal polarisers only would result in a contrast of order 10:1. However when internal and external polarisers are used together, the internal polariser aborbs approximately 90% of the light converted to the perpendicular polarisation state, and the resulting contrast is of order 1000:1.

Internal wire grid polarisers may also be used without additional external polarisers 3.

This has the advantage of reduced cost and thickness and greater robustness: however the contrast of the display then depends strongly on the extinction ratio of the wire grid polariser. Making high-performance wire grid polarisers is possible but expensive.

In summary, the use of wire grid polarisers in in-cell polarisers in liquid crystal displays may provide improved contrast (when used in addition to external polarisers), or reduced thickness and greater robustness (when used without external potarisers).

However additional processing steps are required when the internal polariser is used to replace the pixel electrode, and when the internal polariser is placed beneath the driving circuitry it may be difficult to manufacture the driving circuitry and the resulting contrast is likely to be lower.

Capacitive coupling is the use of a driving circuit which does not have a connection via a conductor to the pixel electrode. The voltage on the pixel electrode is influenced strongly by the capacitance between the electrode and the driving circuit.

Capacitive coupling has been used in liquid crystal displays to provide different voltages to two segments of a pixel. Examples are given in US patent 4840460 (Bernot) and US application 20060268186 (Kamada). Figure 3(a) of the accompanying drawings shows a circuit diagram for this type of pixel and Figure 4(a) of the accompanying drawings shows the layout of the electrical elements on the active substrate. The TFT 17, switched by the gate line 18, controls the voltage supplied to the indium tin oxide (ITO) pixel electrode 19 of the directly driven part 20 of the pixel.

But the TFT is not directly connected to the capacitively driven part 21 of the pixel.

Instead, the capacitance 22 between a coupling electrode 23 and the ITO pixel electrode 24 couples the TFT to this part of the pixel. A storage capacitor 25 enhances the voltage holding capacity of the circuit. In this diagram, the storage capacitor's second electrode 26 is shown connected permanently to the counter-electrode 27 which is placed on the opposing substrate of the liquid crystal display. In fact, in many implementations, the storage capacitor is connected to the gate line of the next row of pixels, which is at the same voltage as the counter-electrode except during addressing of the next row.

The two capacitances 22 and 21 form a voltage divider. The result is that the voltage across the capacitively driven part 21 of the pixel is smaller than the voltage across the directly driven part 20. This causes the two parts of the pixel to transmit different fractions of the light entering them, and also improves the viewing-angle dependence of colours shown on the display.

According to the invention, there is provided a display as defined in the appended claim 1.

Embodiments of the invention are defined in the other appended claims.

Description of the drawings

Figure 1 shows the elements of a typical active-matrix liquid crystal display.

Figure 2 shows a number of ways of arranging electrodes and polarisers on the active substrate of a liquid crystal display, including prior art arrangements (a) and (b) and arrangements (d) and (e) according to embodiments of the invention.

Figure 3 shows (a) the equivalent circuit for a capacitively coupled pixel electrode used for split-pixel driving in the prior art and (b) an equivalent circuit for a pixel including a capacitively coupled wire grid polariser according to an embodiment of the invention.

Figure 4 shows (a) the layout of electrical elements for a capacitively coupled pixel electrode used for split-pixel driving in the prior art, (b) the layout of electrical elements for a floating wire grid electrode which is capacitively coupled to a conductor according to an embodiment of the invention and (c) the layout of electrical elements for a floating wire grid electrode which is capacitively coupled to a transparent electrode according to an embodiment of the invention.

Figure 5 shows the arrangement of elements on the active substrate for a floating wire grid electrode which is capacitively coupled to a transparent electrode according to an embodiment of the invention.

Figure 6 shows the arrangement of elements on the active substrate for a floating wire grid electrode which is capacitively coupled to a conductor according to an embodiment of the invention.

In the following embodiments of the the invention, a wire-grid polariser is disposed on an active substrate of a liquid crystal display so that the segments of the polariser in separate pixels are electrically insulated from one another and act as pixel electrodes to apply an electric field to the liquid crystal layer, and the segment of the polariser in each pixel is coupled capacitively (that is, without a direct conducting connection) to the driving electronics for that pixel. We will refer to this arrangement as a floating wire grid polariser.

An example of an equivalent circuit for a pixel according to an embodiment of the invention is given in Figure 4(b). It differs from Figure 4(a) because there is no directly driven part of the pixel, and because the capacitively driven pixel electrode 24 is a wire grid polariser.

Advantages of this arrangement are that the polariser may be placed very close to the liquid crystal layer, so that the contrast of the display is maximised, and that no manufacturing steps are necessary to provide electrical connections between the driving electronics and the wire grid.

The floating wire grid polariser 16 for each pixel may be capacitively coupled to a conductor, as shown in Figure 2 (d). Figure 5(b) shows the corresponding layout of electrical elements on the active substrate. In this case the conductor is the source line 23 of the TFT. The whole of the active pixel area 21 is capacitively coupled.

This arrangement has the additional advantage that it minimises the number of manufacturing steps. It is not necessary to fabricate a conducting connection between the driving circuit and the wire grid polariser.

The floating wire grid polariser 16 for each pixel may be capacitively coupled to a transparent electrode such as an indium-tin oxide (ITO) electrode, as shown in Figure 2(e). Figure 5(c) shows the corresponding layout of electrical elements on the active substrate. The ITO electrode 19 is electrically connected to the TFT 17, as in the standard pixel design. The coupling capacitance 22 in Figure 3 (a) is now the capacitance between the ITO electrode 19 and the floating wire grid polariser 16 in Figure 5(c).

This arrangement has the additional advantage that it requires few changes from the current, highly refined manufacturing process. The floating wire grid polariser is simply added to the existing stack of elements, with no electrical connections between it and other parts needed. Also, since the coupling capacitor 22 covers the entire pixel area, its capacitance may be made large, leading to strong coupling and a voltage on the floating wire grid electrode which is close to the TFT source voltage.

The first embodiment of the invention is an active-matrix liquid crystal display using the vertically aligned nematic (VAN) mode. The layout of electrical components on the active substrate is shown in Figure 4(c). The display is made using the manufacturing methods currently used for liquid crystal displays, but with the addition of an extra step where a wire grid polariser 16 is added. The wire grid polariser is therefore coupled capacitively to a transparent ITO electrode.

The wire grid polariser is patterned so that portions of the wire grid in different pixels are not electrically connected, as shown in Figure 5. This figure shows the side view of the pixel layout (a), in which the elements are the same as in Figure 4(c). It also shows a top view, where for clarity part (b) shows the TFT 17, source line 18, the ITO electrode 19 and the connection 28 between the source line and the ITO electrode, and part (c) shows in addition the wires 29 of the floating wire grid electrode, while part (d) also shows the black mask 30 which is used to make some parts of the pixel opaque.

In this diagram the wires of the wire grid are shown much larger than in the real device to make the diagram clear.

A number of schemes are used to control the alignment of VAN displays in the white state. These include protrusions on the alignment layer and slits in electrodes (E.g.

Yoshida, Society for Information Display Symposium Digest 2004 paper 3.1), polymer layers deposited from liquid crystal solution (e.g. Hanaoka, Society for Information Display Symposium Digest 2004 paper 40.1), photoalignment, where polarised UVlight is used to control the alignment direction (e.g. Kim, Society for Information Display Symposium Digest 2001 paper P-67), and grid or grating alignment, where a relief grating on the surface facing the liquid crystal controls the alignment direction (e.g. Konovalov, Society for In formation Display Symposium Digest 2002 paper 383).

In the case where slits in electrodes are used, corresponding slits must be made in the floating wire grid polariser, and these slits will reduce its effectiveness in improving contrast. However all other methods may be used with the floating wire grid polariser.

The second embodiment of the invention uses a muttiple-domain (or reverse) twisted nematic display mode with vertically aligned liquid crystal, as described for example in US patent 6856368 (Terashita; Sharp). This type of display has a black state similar to a VAN display, with molecules aligned vertically, and a white state where the liquid crystal molecules are in a horizontally aligned, twisted state. Each subpixel is divided into four domains, with the alignment in each domain controlled by a multiple-rubbing process or a photo-alignment process, as described in U56856368.

In this embodiment, a metal conductor is used as the coupling electrode, as shown in Figure 2(d). Figure 4(b) shows the layout of electrical elements in the subpixel. The metal conductor 23 extends from the source of the TFT 18, and is capacitively coupled to the floating wire grid electrode 16. Figure 6 shows the layout of other elements on the active substrate. The side view in Figure 6(a) is the same as in Figure 4(b). For clarity, Figure 6(b) shows the TFT 17 and the source line 18, while Figure 6(c) shows also the floating wire grid polariser 16 and Figure 6(d) shows all these elements and in addition the black mask which is used to make some parts of the pixel opaque. In this diagram the wires of the wire grid are shown much larger than in the real device to make the diagram clear.

The embodiments described above are examples to illustrate the invention and do not exhaust the range of possibilities which may be covered by the invention.

For example, the vertically aligned twisted nematic mode may be used with the floating wire grid polariser coupled capacitively to an ITO electrode. Other liquid crystal modes, such as in-plane switching (IPS) may be used. Polymer, rather than glass, substrates may be used.

The wires of the wire grid may be aligned in a direction which is not parallel to the pixel rows.

A cross-connection may be made between the wires of the wire grid, as shown in the right-hand side of Figure 2(b), to ensure that they are maintained at the same voltage.

Claims

CLAIMS: 1. A display comprising an active substrate which has formed thereon a driving arrangement and an addressing electrode arrangement exclusively comprising a wire grid polariser, the wire grid polariser comprising a plurality of electrically isolated polarising regions, each of which is exclusively capacitively coupled to the driving arrangement.
2. A display as claimed in claim 1, in which the active substrate comprises an active matrix substrate.
3. A display as claimed in claim 2, in which each of the regions defines a pixel or a sub-pixel of the display.
4. A display as claimed in claim 3, in which each of the regions constitutes a first plate of a capacitor.
5. A display as claimed in claim 4, in which the driving arrangement comprises a control element for each pixel or sub-pixel.
6. A display as claimed in claim 5, in which each control element is connected to an electrode which is disposed in a same layer as the control element and which constitutes a second plate of the capacitor.
7. A display as claimed in claim 5, in which each control element is connected to an electrode which is disposed in a layer between the control elements and the wire grid polariser and which constitutes a second plate of the capacitor.
8. A display as claimed in any one of the claims 5 to 7, in which each control element comprises a thin film transistor.
9. A display as claimed in any one of the preceding claims, in which the wire grid polariser is disposed between the driving arrangement and a light modulating layer of the display.
10. A display as claimed in claim 9, in which the light modulating layer is a polarisation modulating layer.
11. A display as claimed in claim 10, in which the polarisation modulating layer is a liquid crystal layer.
12. A display as claimed in claim 11, in which the wire grid polariser is separated from the liquid crystal layer only by an alignment layer.
13. A display as claimed in claim 11 or 12, arranged to operate in a reverse twisted nematic display mode.
14. A display as claimed in any one of the preceding claims, in which the substrate is disposed between the wire grid polariser and an external polariser.
15. A display as claimed in any one of the preceding claims, in which all of the regions have the same polarising direction.