GB2024444A

GB2024444A - Liquid crystal display cell

Info

Publication number: GB2024444A
Application number: GB7914526A
Authority: GB
Original assignee: Citizen Watch Co Ltd
Current assignee: Citizen Watch Co Ltd
Priority date: 1978-04-28
Filing date: 1979-04-26
Publication date: 1980-01-09
Also published as: GB2024444B; JPS54143246A

Abstract

A liquid crystal display cell has a refractive index compensating layer (38) formed on at least one substrate (14) inner surface, and a transparent electrode pattern (19) formed over the compensating layer (38), the compensating layer (38) compensating for the difference between the refractive indices of the transparent electrode pattern (19) and the substrate (14) (for example by having an intermediate refractive index), to thereby make less visible those portions of the electrode pattern (19) which are not currently energized by an applied voltage. Electrode 19 and layer 38 are covered by an oblique evaporated silicon monoxide alignment layer. <IMAGE>

Description

SPECIFICATION Liquid crystal display cell The invention relates to liquid crystal display cells, and, in particular, to a liquid crystal display cell having an improved appearance and visibility by utilizing a transparent compensating layer which compensates for a difference between the reflectance and refractive index of a transparent substrate and those of a transparent electrode pattern.

Liquid crystal display cells are in widespread use at the present time, particularly for such portable devices as electronic wristwatches and pocket calculators, due to their capability for operating at low levels of drive voltage, and their low level of power consumption. An increasing number of devices such as electronic wristwatches are being provided with multifunction capabilities, to provide, for example, displays of preset alarm times, etc., in addition to the normal timepiece information consisting of the current hours, minutes, seconds, date, etc.

Provision of such additional functions results in the electrode patterns formed upon the cell substrates becoming extremely complex. While any particular information is being displayed, therefore, a large proportion of the display area will be occupied by portions of the cell substrate electrode pattern which are not energized, i.e. to which a drive voltage is not applied. For optimum display visibility, these non-energized portions of the electrode pattern should be invisible to the user. However, in practice, due to such factors as a difference between the refractive index of the cell substrate on which the electrode pattern is formed and the refractive index of the electrode pattern, the non-energized portions of the electrode pattern will be visible, under certain circumstances.The degree of this unwanted visibility will depend upon the viewing angle, the angle of incidence of light upon the display cell, etc. This problem becomes increasingly severe as the number of functions provided by the device which incorporates the liquid crystal display cell are increased, so that the degree of complexity of the electrode pattern becomes greater.

Various methods have been proposed in order to overcome this problem, such as improved types of polarizers, reduction of substrate thickness, adjustment of electrode pattern film thickness, etc. However completely satisfactory results have not been achieved up to the present.

As stated, one of the major reasons for the above problem is that there is a difference between the refractive indices of the transparent substrate and the transparent electrode pattern.

The present invention therefore provides a means for compensating for this difference in refractive indices consisting of transparent insulating layer having a refractive index which has a value intermediate between the refractive indices of the cell substrate and the transparent electrode pattern. In this way, portions of the display cell corresponding to parts of the transparent electrode pattern which are not energized become virtually indistinguishable from portions of the display cell which are not occupied by the electrode pattern. The clarity of display provided by a liquid cystal cell in accordance with the present invention is significantly enhanced, over a wide range of viewing angles and angles of incidence of light, by comparison with a conventional type of liquid crystal display cell.

According to the present invention, there is provided a liquid crystal display cell comprising: at least one transparent supporting substrate; a transparent electrode pattern formed over said transparent supporting substrate; a refractive index compensating layer formed upon said transparent supporting substrate between said transparent electrode pattern and said transparent supporting substrate, to compensate for differences between the reflectivities and refractive indices of said transparent supporting substrate and said transparent electrode pattern; and a liquid crystal material in contact with said transparent electrode pattern and with portions of said refractive index compensating layer outside said transparent electrode pattern.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view in elevation of a liquid crystal display cell of a known type; Fig. 2 is a view in plan of the display cell of Fig. 1; Fig. 3 is a graph showing the relationship between overall reflectance and pattern electrode thickness in a liquid crystal display cell, for two different thicknesses of alignment layer; Fig. 4 is a partial cross-sectional view in elevation of an embodiment of a liquid crystal display cell according to the present invention; Fig. 5 is a graph showing the relationship between reflectance and refractive index for various thicknesses of transparent insulating layer;; Fig. 6 is a graph showing the relationship between overall reflectance and pattern electrode thickness for various thicknesses of transparent insulating layer; and Fig. 7 is a cross-sectional view in elevation of an embodiment of a liquid crystal display cell according to the present invention.

Referring now to the drawings, Fig. 1 is a crosssectional view in elevation of a type of liquid crystal display cell which is known in the prior art and is widely utilized. This device comprises a closed chamber formed by upper and lower glass substrates 14 and 26 which are held parallel and a slight distance apart by means of a spacer 18 which is sealed between the inner surfaces of substrates 14 and 26 adjacent to the peripheries of these inner surfaces. A transparent electrode pattern, which defines the shapes of numerals, characters etc. to be displayed, is formed on the inner surface of upper glass substrate 14. A portion of this transparent electrode pattern, which will be referred to hereinafter as the pattern electrode, is denoted by reference numeral j 9.

Numeral 17 denotes a connecting lead. A continuous layer of transparent electrode is formed on the inner surface of lower glass substrate 26, and is denoted by reference numeral 24.

Transparent electrode 24 will be referred to hereinafter as the common electrode 24. An alignment layer 16 is provided over pattern electrode 19, connecting lead 17, and the inner surface of substrate 14. An alignment layer 22 is formed over common electrode 24. An upper polarizer 12 is disposed adjacent to the outer surface of upper glass substrate 14, parallel to substrate 14. A lower polarizer 28 is disposed adjacent to and parallel to the outer surface of lower glass substrate 26. A reflector plate 25 is located adjacent to and parallel to the outer surface of lower polarizer 28.

Reference numeral 27 denotes a ray of light which passes through glass cover plate 10, upper polarizer 12, upper glass substrate 14, pattern electrode 19, alignment layer 1 6, liquid crystal 20, alignment layer 22, common electrode 24, lower glass substrate 26, and lower polarizer 28, and which is then reflected back along the same optical path, by reflector 25. Reference numeral 29 denotes a ray of light which passes through glass cover plate 10, upper polarizer 12, upper glass substrate 14, alignment layer 16, liquid crystal 20, alignment layer 22, common electrode 24, lower glass substrate 26 and lower polarizer 28, and which is also reflected back along the same optical path. Both light rays 27 and 29 are,.

assumed to be in a direction normal to the substrate plane of the liquid crystal cell. If pattern electrode 19 9 is not energized, i.e. if no voltage is being applied to electrode 19 through connecting lead 17, then there should be no discernible difference between the light rays 27 and 29 when these are viewed after having been reflected back out of the liquid crystal display cell. However, in fact, there is some degree of difference, due to the fact that light ray 27 has passed through pattern electrode 19, both before and after having been reflected by reflector 25, while light ray 29 has nol passed through pattern electrode 19. While passing through pattern electrode 19, light ray 27 is reflected and refracted to a certain extent.The degree of such refiection and refraction depends upon various factors, such as the angle of incidence of light ray 27 relative to the substrate plane of the display cell, the thickness of pattern electrode 19, the optical properties of pattern electrode 19, and on other factors. For this reason, there will be a visible difference between display areas of the liquid crystal display cell which are occupied by non-energized portions of the transparent electrode pattern (referred to hereinafter as non-energized pattern electrodes) and display areas which are outside the transparent electrode pattern (referred to hereinafter as the background area). In other words, the non-energized pattern electrodes will be visible, to some extent, against the background area, to a degree which will vary according to the viewing angle, angle of incidence of incoming light, etc. This will especially apparent if the liquid crystal cell has a highly complex electrode pattern, as in the case of a display cell used in an electronic device having a multiplicity of functions.

This fact is illustrated in Fig. 2, which is a plan view of part of a liquid crystal display cell such as that of Fig. 1, with some segments of the transparent electrode pattern being energized and denoted by reference numeral 32, while other segments are non-energized, and denoted by reference numeral 34.

The background is denoted by numeral JO. In an ideal liquid crystal display cell, non-energized display segments should be indistinguishable from background area 30, to provide maximum visibility of energized display segments 32. However due to the differences between the reflectances and indices of refraction of pattern electrode 1 9 and alignment layer 16, the amount of light which is reflected back out of the displaycell after passing through the pattern electrode 1 9 and the alignment layer 1 6 is less than the amount of light which is reflected back out of the cell without having passed through pattern electrode 1 9. Thus, non-energized segments 34 of the display are visible against the background area 30, to an extent which varies according to the viewing angle and the angle of incidence of light falling on the display cell. Although this difference between the light reflected back from a non-energized display segment and that reflected back from the background area also occurs in a liquid crystal display cell which does not incorporate an alignment layer, the effect is considerably more apparent in the case of a display cell having an alignment layer.The effect is not solely caused by the differences between the reflectances and refractive indices of the cell substrate, the pattern electrode and the alignment layer, but is affected also by varying optical properties of these and other cell components with respect to the wavelength of light The result of such nonenergized display segments as segment 34 being visible against the background area 30 is that the display has an unsatisfactory appearance, particularly if the transparent electrode pattern has a high degree of complexity, so that there are a large number of non-energized display segments such as segment 34, at any given time.

Fig. 3 is a graph showing the relationshiD between the thickness of the pattern electrode 19 and the difference in overall reflectance of the cell.

The term "difference in overall reflectance" is used herein to denote the difference between the reflectance of a liquid crystal display cell, such as that shown in Fig. 1, with respect to light which passes through upper glass substrate 14 and liquid crystal 20, and light which passes through upper glass substrate 14, pattern electrode 19, and liquid crystal 20, with this difference in reflectances being expressed as a percentage. In the case of the curve denoted by reference numeral 37, the light also passed through an alignment layer 16 formed on upper substrate 14, while in the case of curve 37, no alignment layer was provided. The alignment layer 1 6 consisted of a layer of silicon monoxide with a thickness of 500 AO, having a refractive index of 1.48.The alignment layer was formed by slant evaporative deposition. The liquid crystal material 20 consisted of a nematic liquid crystal having a refractive index of 1.70. The pattern electrode 1 9 consisted of a mixture of indium oxide with 5% of tin oxide, and had a refractive index of 2.00. The substrate 14 consisted of soda lime glass having a refractive index of 1.53.

The procedure used to obtain the data plotted in the graphs of Fig. 3 was as follows. At each value of thickness of pattern electrode 1 9 for which a measurement was performed, the wavelength of the incident light was varied in steps of 200 AO, over the range of 4000 AO to 7000 AO, while the angle of incidence of the incident light with respect to the normal to the substrate plane was varied in steps of 50, over the range from 0 O to 650. The results thus obtained were averaged, to obtain the values plotted in Fig.

3.

As can be understood from Fig. 3, there is a significant variation of the overall reflectance as the thickness of the pattern electrode is varied in the range of 600 AO to 1000 AO. It is generally recognized that when the difference in overall reflectance between a pattern electrode area and a background grea of the display is lessthani.5%, then no visible difference can be perceived between the non-energized pattern electrode areas and the background area. As shown by Fig.

3, if the liquid crystal display cell includes an alignment layer, then the difference in overall reflectance will generally exceed 1.5%, so that the non-energized pattern electrodes will be visible, even if only faintly, against the display background.

Referring now to Fig. 4, a partial crosssectional view is shown therein of a preferred embodiment of a liquid crystal display cell according to the present invention. Reference numeral 38 denotes a refractive index compensating layer which is madeof a transparent insulating material and which is formed on an upper glass substrate 14. A pattern electrode 19 is formed over transpar,ent refractive index compensating layer 38. In this embodiment, the pattern electrode 1 9 is shown as comprising a transparent layer of a mixture of indium oxide with 5% tin oxide.Reference numeral 1 6 denotes an alignment layer comprising a film of silicon monoxide formed by slant evaporative deposition over pattern electrode 1 9 and over areas of transparent compensating layer 38 which are not covered by pattern electrode 1 9. Numeral 37 denotes a light ray which has been reflected from an area of the display occupied by pattern electrode 19, while light ray 39 has been reflected by an area of the display which is not occupied by a pattern electrode.

Fig. 5 is a graph showing the relationship between the difference in overall reflectance of non-energized pattern electrode areas and the background area, and the refractive index of transparent compensating layer 38, for the cell of Fig. 4. Curves 40, 42 and 44 were plotted for thickness of transparent compensating layer 38 of 400 A0, 800 A0 and 1500 A0 respectively. The data for Fig. 5 was obtained in the same manner as described for the graph of Fig. 3, except that the measurements were made at a fixed angle of incidence of 00, i.e. normal to the substrate plane, and with the same cell components as in the case of Fig. 3, except for the fact that transparent compensating layer 38 was incorporated.Fig. 5 clearly shows that a thickness of transparent compensating film of approximately 800 AO provides a minimum amount of difference in overall reflectivity, and that this minimum is obtained at a value of 1.80 for the refractive index of the transparent compensating film.

Tests were also performed to find the optimum thickness of transparent compensating film for pattern electrode thicknesses of 400, 700 and 1400 AO. It was found that in each case, the optimum thickness of transparent compensating film was approximately 800 AO, with a refractive index of from 1.70 to 1.80.

Fig. 6 is a graph showing the results of measurements to determine the relationship between the difference in overall reflectance and the pattern electrode thickness, for various thicknesses of transparent compensating film. The measurements were performed in3he same way as described above for the graph of Fig. 3, but were made up on the cell structure of Fig. 4, i.e.

with the addition of a transparent compensating layer in addition to the alignment layer. The refractive index of the transparent compensating layer for the graph of Fig. 6 was 1.80 in each case.

The thickness of the alignment layer was 500 AO.

Fig. 6 demonstrates that, for a thickness of transparent compensating layer 38 of from 600 to 1000 AO, the difference in overall reflectance is held to 1.5% or less, over a wide range of thicknesses of transparent pattern electrode 19.

With a thickness of transparent compensating layer 38 in the range 600 to 1000 AO, therefore, no visible difference can be perceived between non-energized portions of the pattern electrode and the display background. If the thickness of transparent compensating layer 38 is greater than or less than this range, then the non-energized portions of the pattern electrode will become visible against the display background.

Tests were also performed on a display cell which had a transparent reflecting layer, but not alignment layer. It was found that, for a transparent compensating layer having a thickness in the range 400 to 1400 A0, and a refractive index of 1.80, the difference in overall reflectivity was extremely small. However, if the index of refraction of the transparent compensating layer was set to 1.50 or 2.0, then an extremely high level of difference in overall reflectivity was found.

Fig. 7 shows a cross-sectional view in elevation of an embodiment of a liquid crystal display cell according to the present invention. Numerals 38 and 39 denote transparent compensating layers which are formed on upper and lower substrates 14 and 26 respectively. All other components of the display cell are identical to those of the device shown in Fig. 1. Transparent compensating layer 38 consists of a film of aluminium oxide formed over one entire face of upper substrate 14, by a sputtering technique.Upper substrate 14 is made of soda-lime glass and has a thickness of 0.5 mm, while transparent compensating layer 38 has a thickness of approximately 800 AO. Pattern electrode 19 is one of seven segments of a display pattern for forming numerals of characters, and is- formed, together with connecting lead 17, by vapor deposition through a mask onto transparent compensating layer 38, to a thickness of approximately 500 AO. Similarly, transparent compensating layer 39 consists of a film of aluminium oxide deposited to a depth of 800 AO over the inner surface of lower substrate 26, which is also made of soda-lime glass. A common electrode 24 is formed over transparent compensating layer 39.An alignment layer 1 6 is formed over pattern electrode 1 9 and connecting lead 17, and over areas of transparent compensating layer 38 which are not covered by pattern electrode 19 and connecting lead 17. An alignment layer 22 is formed over the entire surface of transparent compensating layer 39. A spacer 1 8 is sealed around the periphery of the cell to form an enclosed chamber, in which is contained a nematic liquid crystal material 20.

Upper and lower substrates 14 and 26 are separated from each other by approximately 1 Ossu, by spacer 18. An upper polarizer 12 is provided above upper substrate 14, while a lower polarizer 26 is provided below lower substrate 26. A reflector 62 is provided below lower poiarizer 62.

With the embodiment of the present invention shown in Fig. 7, it was found that non-energized pattern electrodes 19 were completely indistinguishable from the display background. A display was therefore provided which was extremely clear and attractive in appearance.

Another preferred embodiment of a liquid crystal display cell according to the present invention was also tested, which utilized transparent polyester plastic substrates having a refractive index of approximately 1.49, pattern electrodes made of a mixture of indium oxide and tin oxide having a refractive index of 2.00, and transparent compensating layers having a refractive index of 1.60. With this embodiment also, the non-energized portions of the pattem electrodes were completely indistinguishable from the display background.

From the results obtained from the embodiments described above, it has become apparent that the refractive index of the transparent compensating layer should have a value which is intermediate between the refractive indices of the pattern electrode and the substrates.

Other tests were performed upon an embodiment of the present invention in which a liquid crystal material from the Schiffs Base family was used, having a refractive index of 1.795, and in which the transmission axis of the polarizers was aligned in parallel with the major axes of the liquid crystal molecules. Other tests were performed upon an embodiment in which the liquid crystal material had a refractive index of 1.535 and in which the transmission axis of the polarizers was aligned with the minor axes of the liquid crystal material.

Further tests were performed upon embodiments of the present invention utilizing biphenyl liquid crystal materials having refractive indices of 1.74 and 1.52, and phenylcyclohexane family liquid crystal materials with refractive indices of 1.627 and 1.493.

In all of the embodiments of liquid crystal display cells according to the previous invention mentioned in the previous three paragraphs, it was found that completely satisfactory results were obtained, and that non-energized portions of the pattern electrode could not be distinguished from the display background. In each case, the previously described transparent compensating layers were utilized and a display having a high degree of contrast and visibility was obtained.

It has been found that tne use of a transparent compensating layer in accordance with the present invention has the further advantage of reducing the effects of sodium ions from the glass substrate, which can deleteriously affect the alignment of the liquid crystal molecules by the alignment layers. Such a transparent compensating layer therefore helps in ensuring a maximum degree of display contrast.

The materials which can be used as the transparent insulating layer are not confined to aluminium oxide, but can also consist of silicon monoxide, silicon dioxide, magnesium fluoride, tungsten oxide, cerium oxide, silicon nitride, cerium bromide, calcium fluoride, boron compound, antimony compound, gallium compound, arsenide, gold compound or phosphoric compound or of a combination of one of these materials with a metallic material.

It should be understood that the values for the refractive index of the transparent compensating layer stated in the descriptions of the preferred embodiments are valid for a nematic liquid crystal material having a refractive index of 1.53.

However a transparent compensating layer in accordance with the present invention can provide the stated effects in liquid crystal display cells which utilize liquid crystal materials of other types, having different indices of refraction, if a suitable value for the refractive index of the transparent compensating layer is selected.

Thus, although the present invention has been shown and described with reference to particular embodiments, it should be understood that various changes and modifications to these embodiments are possible, which fall within the scope claimed for the present invention.

Claims

1. A liquid crystal display cell comprising; at least one transparent supporting substrate; a transparent electrode pattern formed over said transparent supporting substrate; a refractive index compensating layer formed upon said transparent supporting substrate between said transparent electrode pattern and said transparent supporting substrate, to compensate for differences between the reflectivities and refractive indices of said transparent supporting substrate and said transparent electrode pattern; and a liquid crystal material in contact with said transparent electrode pattern and with portions of said refractive index compensating layer outside said transparent electrode pattern.

2. A liquid crystal display cell according to claim 1, wherein said refractive index compensating layer comprises a transparent insulating layer.

3. A liquid crystal display cell according to claim 2, in which said transparent insulating layer has a predetermined refractive index.

4. A liquid crystal display cell according to claim 3, wherein said predetermined refractive index has a value intermediate between that of said transparent supporting substrate.

5. A liquid crystal display cell according to claim 1, wherein said refractive index compensating layer is formed over the entirety of a surface of said transparent supporting substrate.

6. A liquid crystal display cell according to claim 1, wherein said refractive index compensating layer is formed into a predetermined pattern over a surface of said transparent supporting substrate.

7. A liquid crystal display cell according to claim 1, further comprising an alignment laver formed on said transparent electrode pattern and on portions of said refractive index compensating layer situated outside said transparent electrode pattern, for providing alignment of molecules of said liquid crystal material in a predetermined direction.

8. A liquid crystal display cell according to claim 1, wherein said refractive index compensating layer consists of a material selected from the group consisting of aluminium oxide, silicon monoxide, silicon dioxide, magnesium fluoride, tungsten oxide, cerium oxide, silicon nitride, cerium bromide, calcium fluoride, boron compound, antimony compound, gallium compound, arsenide, gold compound or phosphoric compound.

9. A liquid crystal crystal display cell according to claim 3, wherein said predetermined refractive index is in the range between 1.60 and 1.85.

1-0. A liquid crystal display cell according to, claim 1, wherein said refractive index compensating layer has a thickness in the range 500 AO to 1000 Ab.

11. A liquid crystal display cell substantially as shown and described with reference to the accompanying drawings.