Liquid crystal display device

Abstract: A liquid crystal display device comprising a liquid crystal display cell (23) arranged between the first and second polarizers (21, 22) is disclosed. In the liquid crystal display device, the applied voltage between electrodes (25, 27) is controlled by a switching element (24) so as to drive picture elements defined at the crossings between the first and second transparent electrodes (25, 27), and the light incident to at least the rim portion of the first transparent electrode (25) and the switching means (24) is shaded by a shading film (32) formed in the liquid crystal display cell (23).

Description: The present invention relates to a projection type liquid crystal display device, and more particularly, to such a device of active matrix type. Fig. 5 is a schematic longitudinal cross-sectional view showing the main portion of a conventional active matrix type liquid crystal display device 50. In the liquid crystal display device 50, a liquid crystal display cell 3 is arranged between two polarizers 1 and 2 arranged so that the axes of polarization thereof are parallel to each other. The liquid crystal display cell 3 is composed of a transparent insulating substrate 6 of glass on which thin film transistors (TFTs) 4 and transparent conductive picture element electrodes 5 for displaying picture elements are formed, another transparent insulating substrate 8 of glass on which transparent conductive electrodes 7 are formed so as to oppose the electrodes 5 of the former substrate 6, and a liquid crystal layer 9 arranged between the transparent substrates 6 and 8. The TFTs 4 are formed in a matrix so as to correspond to the respective picture element electrodes 5. An orientation film 10 is formed on the inner surface of the transparent substrate 6 so as to cover the TFTs 4 and the picture element electrodes 5, and an orientation film 11 is formed on the inner surface of the transparent substrate 8 so as to cover the electrodes 7. Nematic liquid crystal molecules contained in the liquid crystal layer 9 are twisted by 90° between the orientation films 10 and 11 while zero voltage is applied between the electrodes 5 and 7. Namely, the nematic liquid crystal molecules in the layer 9 are set in the twisted nematic mode. Shading films 12 are formed between each of the electrodes 7 and the substrate 8 so as to face the respective TFTs 4, and colour filter layers 13 are also formed on the inner surfaces of the substrate 8 so as to face the respective portions of the picture element electrodes 5 where the TFTs 4 are not formed. In Fig. 5, arrows A and B represent the directions of the polarization axes of the polarizer 1 and the polarizer 2, respectively. Fig. 6 is an enlarged schematic longitudinal cross-sectional view showing the TFT 4 used as a switching element for driving the liquid crystal layer 9 in the liquid crystal display device 50. In Fig. 6, a gate electrode 14 of a metal film is formed on the surface of the transparent substrate 6, and a gate insulating film 15 is formed thereon. Furthermore, an amorphous silicon hydride film 16 functioning as an active layer is formed on the film 15, and n⁺ amorphous silicon hydride films 17 and 18 are formed thereon. In Fig. 6, a numeral 19 denotes an insulating film, and a numeral 20 denotes a metal film. In the aforementioned liquid crystal display device 50 constructed as described above, light is emitted from a light source (not shown) arranged on the side of the outer surface of the polarizer 1 as indicated by arrows P, and the emitted light is converted to a linearly polarized light by the polarizer 1. The linearly polarized light is transmitted through the transparent substrate 6, the electrode 5, the liquid crystal layer 9, the electrode 7, the colour filter layer 13 and the transparent substrate 8, sequentially, and is incident to the polarizer 2. As described above, since the liquid crystal layer 9 is set in the twisted nematic mode and the respective polarization axes of the polarizers 1 and 2 are in parallel to each other, when zero voltage is applied between the electrodes 5 and 7, the polarization axis of the light transmitted through the polarizer 1 from the light source is twisted by 90° according to the rotary polarization characteristic of the liquid crystal layer 9, and therefore, the polarization axis thereof becomes orthogonal to that of the polarizer 2, resulting in that the light is prevented from being transmitted through the polarizer 2. On the other hand, since the rotatory polarization characteristic of the liquid crystal layer 9 is dissolved when a predetermined voltage is applied between the electrodes 5 and 7, the polarization axis of the light incident to the polarizer 2 is coincident with that of the polarizer 2, resulting in that the light is transmitted through the polarizer 2. The voltage to be applied between the electrodes 5 and 7 is controlled by the TFTs 4. The picture elements corresponding to the electrodes 5 and 7 between which the predetermined voltage is applied according to the above operation acquire a transparent state, and a coloured display is effected by the transmitted light coloured by the colour filter layer 13. In the above operation, the light transmitted through the polarizer 1 and the transparent substrate 6 is incident to the TFTs 4 comprising the amorphous silicon hydride film 16 having a high photoconductivity for visible light. In this case, since the gate electrode 14 of metal film functions as a shading film, the increase of the OFF current in the TFTs 4 caused by the photoconductivity of the film 16 is prevented. Particularly, in the case that images etc. are projected on a screen through a projecting lens in a magnification mode with use of the active matrix type liquid crystal display device using the TFTs 4 having the aforementioned amorphous silicon hydride film 16 as switching elements, an extremely strong light is required as the light source in order to produce a bright display. In this case, the illuminance of the light incident to the liquid crystal display device 50 is in a range of several hundred thousand to several million lx. In this case, since the gate electrode 14 of the TFT 4 also functions as the shading film, the increase of the OFF current in the TFT 4 caused by the photoconductivity is prevented, resulting in that the degradation of the display characteristics can be avoided. As described above, in the aforementioned conventional active matrix type liquid crystal display device, the display mode is used in which the light is not transmitted at the time of the application of zero voltage and the light is transmitted by dissolving the rotatory polarization characteristic of the liquid crystal layer 9 when a predetermined voltage is applied. In the display device, because the optical rotatory dispersion is caused in the liquid crystal layer 9, there is a problem that the different electro-optical characteristics are obtained respectively for light having different wavelengths such as red, green and blue. Fig. 7 shows the dependency of the transmittance on the root mean square value of the applied voltage with a parameter of the wavelength of light (red, green and blue) in the aforementioned active matrix type liquid crystal display device 50. In Fig. 7, the vertical axis is the light transmittance and the horizontal axis is the root mean square value of the applied voltage. As is apparent from Fig. 7, in the display mode at the time of the application of approximately zero voltage when the applied voltage is close to the voltage of black, i.e., the liquid crystal layer 9 is in the twisted nematic mode, the leak light caused by the optical rotatory dispersion increases, resulting in that the colour reproductivity is lowered. In the aforementioned active matrix type liquid crystal display device, in the case that the projection display is performed for a relatively long time or the display is performed with use of back lighting apparatus such as a fluorescent lamp, the threshold voltage of the TFT 4 is shifted with lapse of the light projecting time, resulting in the degradation of the display characteristic. Fig. 3 shows the relationship between the light projecting time (hours) and the shift amount ΔV TH (volts) of the threshold voltage of the TFT 4, wherein the visible light having an illuminance of 700,000 lx is projected onto the TFT 4. In Fig. 3, the curve ℓ1 shows the shift amount ΔV TH of the threshold voltage of the TFT 4 in the conventional active matrix type liquid crystal display device 50. As is apparent from the curve ℓ1 of Fig. 3, with lapse of the light projecting time, the threshold voltage of the TFT 4 is shifted gradually. In order to solve the problem of the degradation of the colour reproductivity caused by the optical rotatory dispersion, a method for equalizing the colour reproductivities for the respective lights having the wavelengths of red, green and blue has been proposed. According to the method, the colour reproductivities for red, green and blue colours are equalized by setting respective gaps d of the portions of the liquid crystal cell corresponding to individual picture elements for displaying the red, green and blue colours so as to make the ratio Δn·d/λ constant, wherein Δn is the birefringence of the liquid crystal and λ is the wavelength of light. (See, for example, Hotta et al.: SID 86 Digest. p. 296 (1986)). However, in this method, it is difficult to set the gap d of the liquid crystal cell for every colour of red, green and blue exactly, resulting in that the cost for the mass production extremely increases. On the other hand, it has been found out by the present inventors that the shift of the threshold voltage of the TFT 4 due to the light projection for a relatively long time is caused by a portion 16a of the amorphous silicon hydride film 16 uncovered by the gate electrode 14 (See Fig. 6). However, according to the usual manufacturing process for the TFT, it is extremely difficult to remove the aforementioned projected portion 16a completely without giving a bad influence to the characteristics of the TFT. From EP-A-0 221 361, there is known a liquid crystal display device in which a twisted nematic liquid crystal material is sandwiched between opposing transparent substrates. Respective electrodes are provided on the inner surfaces of the transparent substrates. The voltage applied across the opposing electrodes is controlled by a thin-film transistor formed on the inner surface of one of the substrates. On the outside surfaces of the transparent substrates, there are arranged respective polarizing plates whose polarizing directions are mutually orthogonal. Additionally, a reflecting plate is provided on the outside surface of one of the polarizing plates, thereby realising a reflection type display. An essential object of the present invention is to provide a projection type liquid crystal display device having excellent colour display characteristics without causing changes in colours to be displayed and degradation of the display which may be caused by the light projection for a long time. This invention provides a projection type liquid crystal display device comprising: a first polarizer; a second polarizer; a liquid crystal display cell arranged between said first and second polarizers for displaying an image thereon, said liquid crystal display cell including a liquid crystal layer arranged between mutually parallel first and second transparent insulating substrates; first transparent electrodes formed on an inner surface of said first transparent insulating substrate; second transparent electrodes formed on an inner surface of said second transparent insulating substrate so as to oppose said first transparent electrodes; switching means, arranged on an inner surface of said first transparent insulating substrate, for controlling an applied voltage between said first and second transparent electrodes so as to drive picture elements defined between said first and second transparent electrodes; a light source, arranged on the side of an outer surface of said second polarizer, for emitting light through said second polarizer, said second transparent insulating substrate, said liquid crystal layer, said first transparent insulating substrate and said first polarizer toward a display screen arranged on the side of an outer surface of said first polarizer; and shading means arranged on the second transparent insulating substrate so as to shade said emitted light incident to at least said switching means; the respective polarization axes of said first and second polarizers being set so that the light emitted from said light source is prevented from being transmitted through said second polarizer, said liquid crystal display cell and said first polarizer at the time of the application of a predetermined voltage between said first and second transparent electrodes, and the light therefrom is allowed to be transmitted therethrough at the time of the application of zero voltage therebetween. Accordingly, in the present invention, the light emitted from the light source is transmitted through the first polarizer in the twisted nematic mode, and, when the twisted nematic mode is dissolved by applying a predetermined voltage between the first and second electrodes, the emitted light is prevented from being transmitted through the first polarizer. Therefore, possible changes in the colours due to the optical rotatory dispersion is never caused on the display screen and the colour reproductivity can be improved. The shading means are preferably arranged so as to also shade a rim portion of the first transparent electrode. Then, since the light incident to the switching means and the rim portion of the first transparent electrode from the light source through the second polarizer is shaded certainly by the shading means, the degradation of the display characteristic of the switching means can be prevented and the display with a high contrast can be obtained. In the drawings: Fig. 1 is a schematic longitudinal cross-sectional view showing the main portion of an active matrix type liquid crystal display device of the first preferred embodiment according to the present invention; Fig. 2 is a schematic diagram showing a projecting type display optical system having the liquid crystal display device shown in Fig. 1; Fig. 3 is a graph showing the change of the threshold voltage of TFT in the liquid crystal display device shown in Fig. 1 and the threshold voltage of TFT in a conventional liquid crystal display device shown in Fig. 5 with lapse of the light projecting time; Fig. 4 is a schematic longitudinal cross-sectional view showing the main portion of an active matrix type liquid crystal display device of the second preferred embodiment according to the present invention; Fig. 5 is a schematic longitudinal cross-sectional view showing the main portion of a conventional matrix type liquid crystal display device; Fig. 6 is an enlarged longitudinal cross-sectional view showing a TFT of the liquid crystal display device shown in Fig. 5; and Fig. 7 is a graph showing the relationship between the light transmittance and the root mean square value of the applied voltage in the liquid crystal display device shown in Fig. 5. THE FIRST PREFERRED EMBODIMENT Fig. 1 is a schematic longitudinal cross-sectional view showing the main portion of an active matrix type liquid crystal display device 40 of the first preferred embodiment according to the present invention. In the liquid crystal display device 40, a liquid crystal display cell 23 is arranged between the first and second polarizers 21 and 22 arranged so that the polarization axes thereof are orthogonal to each other. The liquid crystal display cell 23 is composed of the first transparent insulating substrate 26 of glass on which TFTs 24 functioning as a switching element for driving a liquid crystal layer 29 and transparent conductive picture element electrodes 25 for displaying picture elements are formed, the second transparent insulating substrate 28 of glass on which transparent conductive electrodes 27 are formed so as to oppose the electrodes 25 of the first substrate 26, and the liquid crystal layer 29 arranged between the transparent substrates 26 and 28. In the liquid crystal display cell 23, the picture element electrodes 25 cross the electrodes 27, respectively, in a known manner. The structure of the TFT 24 is the same as that of the conventional TFT 4 shown in Fig. 6, therefore, the description thereof is omitted. The TFTs 24 are formed in a matrix so as to correspond to the respective electrodes 25. An orientation film 30 is formed on the inner surface of the transparent substrate 26 so as to cover the TFTs 24 and the picture element electrodes 25, and an orientation film 31 is formed on the inner surface of the transparent substrate 28 so as to cover the electrodes 27. Nematic liquid crystal molecules contained in the liquid crystal layer 29 are twisted by 90° between the orientation films 30 and 31 while zero voltage is applied to the liquid crystal layer 29. Namely, the nematic liquid crystal molecules in the layer 29 are set in the twisted nematic mode, so that the polarization axis of the light incident to the first polarizer 21 after transmission through the liquid crystal layer 29 becomes parallel to that of the first polarizer 21 and the polarization axis of the light incident to the second polarizer 22 after transmission through the liquid crystal layer 29 becomes parallel to that of the second polarizer 22. Shading films 32 are formed between each of the electrodes 27 and the substrate 28 so as to face the whole area of the respective TFTs 24 and the rim portions 25a of the electrodes 25. Colour filter layers 33 are also formed on the inner surface of the substrate 28 so as to face the other respective main portions 25b of the picture element electrodes 25 than the rim portions 25a thereof. Since the aforementioned shading films 32 are formed so as to correspond to the respective TFTs 24 and the respective electrodes 25, the shading films 32 have a lattice shape. The shading films 32 are preferably made of metal such as Cr, Ni, Al and Ni-Cr. Alternatively, the shading films 32 may be made of a gelatine film or an organic high polymer coloured with a dye or a pigment. In Fig. 1, an arrow A having a perpendicular direction to the surface of the paper represents a direction of the polarization axis of the first polarizer 21, and arrows B represent a direction of the polarization axis of the second polarizer 22. Fig. 2 is a schematic view showing a projecting type display optical system 60 comprising the aforementioned liquid crystal display device 40. In the projecting type display optical system 60, a mirror 34, a light source 35, the first condenser lens 36, a filter 37 for cutting heat rays, the second condenser lens 38, a filter 39 for cutting ultraviolet rays, the liquid crystal display device 40, a projecting lens 41 and a screen 42 are arranged sequentially. The liquid crystal display device 40 is arranged so that the second polarizer 22 thereof is directed to the light source 35. In this case, the light source 35 is preferably a halogen lamp, a xenon lamp, or a metal halide lamp in order to produce a bright display. The operation of the aforementioned liquid crystal display device 40 will be described below. In Fig. 2, the light emitted from the light source 35 is condensed by the mirror 34 and the first condenser lens 36. The condensed light is transmitted through the filter 37 for cutting heat rays, and the transmitted light is incident to the second condenser lens 38. The light transmitted through the second condenser lens 38 is transmitted through the filter 39 for cutting ultraviolet rays and is incident to the second polarizer 22 of the liquid crystal display device 40. The light thereof is shown by an arrow P in Fig. 1. In the liquid crystal display device 40, the light incident to the second polarizer 22 becomes a linearly polarized light, and light thereof is transmitted through the second transparent substrate 28, the colour filter layer 33, the electrode 27, the liquid crystal layer 29, the electrode 25 and the first transparent substrate 26, sequentially, and thereafter, the transmitted light is incident to the first polarizer 21. As described above, the liquid crystal layer 29 is set in the twisted nematic mode in which the liquid crystal molecules contained in the layer 29 are twisted by 90° and the first and second polarizers 21 and 22 are arranged so that the polarization axes thereof are orthogonal to each other. Therefore, at the time of the application of zero voltage between the electrodes 25 and 27, the polarization axis of the light transmitted through the second polarizer 22 after emission from the light source 35 is twisted by 90° according to the rotatory polarization characteristic of the liquid crystal layer 29 and is parallel to the polarization axis of the first polarizer 21, resulting in that the light is allowed to be transmitted through the first polarizer 21. On the other hand, at the time of the application of a predetermined voltage between the electrodes 25 and 27, the rotatory polarization characteristic of the liquid crystal layer 29 is dissolved and the polarization axis of the light incident to the first polarizer 21 is orthogonal to that of the first polarizer 21, resulting in that the light is prevented from being transmittted through the first polarizer 21. Thus, when the voltage applied between the electrodes 25 and 27 is controlled by the TFTs 24, the light transmittance of the respective picture elements is changed, and then, the transmitted light coloured by the colour filter layer 33 is transmitted through the projecting lens 41, and the enlarged image is formed on the screen 42 with a coloured display. In the aforementioned operation, at the time of the application of zero voltage, the liquid crystal layer 29 is set in the twisted nematic mode and the phenomenon of the optical rotatory dispersion appears in the layer 29. However, since the light is transmitted therethrough, almost no change of the colour due to the optical rotatory dispersion is caused on the display screen 42. On the other hand, at the time of the application of a predetermined voltage when the light is almost not transmitted therethrough, the twist of the polarization axis of the liquid crystal layer 29 is almost dissolved. Therefore, the phenomenon of the optical rotatory dispersion almost does not appear in the layer 29, resulting in that the leak light due to the optical rotatory dispersion is not caused. Thus, the display with use of the liquid crystal display device 40 has an extremely fine colour reproductivity. Furthermore, since the shading film 32 formed on the second transparent substrate 28 prevents the light from the light source 35 from being incident to the TFT 24, the shift of the threshold voltage of the TFT 24 due to the light projection for a relatively long time is prevented. The curve ℓ2 of Fig. 3 shows the relationship between the light projecting time (hours) of the visible light having an illuminance of 700,000 lx incident to the TFT 24 in the liquid crystal display device 40 and the shift amount ΔV TH (volts) of the threshold voltage of the TFT 24. As is apparent from the curve ℓ2 of Fig. 3, even with lapse of the light illumination time, the threshold voltage of the TFT 24 is almost not shifted. Therefore, degradation of the display characteristic can be prevented. Even when a predetermined voltage is applied to the layer 29, the applied voltage does not influence the shading film 32, and the shading film 32 is also used for shading the light transmitted through the rim portions 25a of the electrodes 25 which cannot shade the light, resulting in that the display with a high contrast can be obtained. It is to be noted that, in the construction of the projecting type display optical system 60 shown in Fig. 2, the colour filter layer 33 is used as a means for separating colours. However, a dichroic mirror or a dichroic prism may be used as the means for separating colours. Furthermore, the arrangement of the colour filter layer 33 is not limited to the case where the colour filter layer 33 is arranged between the second and first polarizers 22 and 21. THE SECOND PREFERRED EMBODIMENT Fig. 4 is a schematic longitudinal cross-sectional view showing the main portion of an active matrix type liquid crystal display device 40a of the second preferred embodiment according to the present invention. The liquid crystal display device of the second preferred embodiment comprises a non-linear type element used as the switching element for driving the liquid crystal layer 29. The other construction thereof is the same as that of the liquid crystal display device 40 of the first preferred embodiment. Therefore, in Fig. 4, the same components as the components of the liquid crystal display device 40 of the first preferred embodiment are denoted respectively by the same numerals as those of the components of the liquid crystal display device 40 of the first preferred embodiment, and the descriptions thereof are omitted. A ring diode is used as the non-linear element 43, wherein the ring diode is composed of two diodes 44 and 45 connected in parallel so that the forward direction of one diode is parallel to the backward direction of another diode, and the diodes 44 and 45 have an amorphous silicon hydride film of the PIN junction type as an active layer. The shading film 32 for shading the light transmission through the whole area of the non-linear type element 43 and the rim portion 25a of the electrode 25 must be an insulating film. The shading film 32 is preferably a gelatine film or a high polymer film coloured with a dye or a pigment. In Fig. 4, the diode 44 is composed of an n-layer 44a, an i-layer 44b and a p-layer 44c, and the diode 45 is composed of an n-layer 45a, an i-layer 45b and a p-layer 45c. The numerals 46 and 47 denote metal films, and the numeral 48 denotes an insulating film. Furthermore, in Fig. 4, an arrow A having a perpendicular direction to the surface of the paper represents a direction of the polarization axis of the first polarizer 21, and arrows B represent a direction of the polarization axis of the second polarizer 32. The non-linear type element 43 may be the aforementioned two diodes 44 and 45 connected in series to each other in a forward direction or a backward direction thereof. The non-linear type element 43 may also be a Schottky junction element, or a so-called Metal Insulator Metal element.

Claims: A projection type liquid crystal display device comprising: a first polarizer (21); a second polarizer (22); a liquid crystal display cell (23) arranged between said first and second polarizers for displaying an image thereon, said liquid crystal display cell including a liquid crystal layer (29) arranged between mutually parallel first (26) and second (28) transparent insulating substrates; first transparent electrodes (25) formed on an inner surface of said first transparent insulating substrate; second transparent electrodes (27) formed on an inner surface of said second transparent insulating substrate so as to oppose said first transparent electrodes; switching means (24;43), arranged on an inner surface of said first transparent insulating substrate, for controlling an applied voltage between said first and second transparent electrodes so as to drive picture elements defined between said first and second transparent electrodes; a light source (35), arranged on the side of an outer surface of said second polarizer (22), for emitting light through said second polarizer, said second transparent insulating substrate (28), said liquid crystal layer (29), said first transparent insulating substrate (26) and said first polarizer (21) toward a display screen (42) arranged on the side of an outer surface of said first polarizer (21); and shading means (32) arranged on the second transparent insulating substrate (28) so as to shade said emitted light incident to at least said switching means (24;43); the respective polarization axes of said first and second polarizers being set so that the light emitted from said light source is prevented from being transmitted through said second polarizer, said liquid crystal display cell and said first polarizer at the time of the application of a predetermined voltage between said first and second transparent electrodes, and the light therefrom is allowed to be transmitted therethrough at the time of the application of zero voltage therebetween. A projection type liquid crystal display device as claimed in claim 1, wherein the shading means (32) are arranged so as to also shade a rim portion (25a) of said first transparent electrode (25). A projection type liquid crystal display device as claimed in claim 2, wherein the switching means comprises a thin film transistor (24). A projection type liquid crystal display device as claimed in claim 3, wherein the thin film transistor (24) comprises a gate electrode (14) formed on the inner surface of said first transparent insulating substrate, an electrically insulating film (15) formed on said gate electrode, a semiconductor film (16) formed on said electrically insulating film, and a source electrode and a drain electrode formed on said semiconductor film, the width of said semiconductor film (16) being greater than the width of said gate electrode (14). A projection type liquid crystal display device as claimed in claim 2, wherein the switching means comprises a non-linear type element (43). A projection type liquid crystal display device as claimed in claim 5, wherein the non-linear type element comprises two diodes (44,45) connected in parallel or in series, or a Schottky junction element, or a Metal Insulator Metal element. A projection type liquid crystal display device as claimed in any one of claims 2 to 6, wherein said shading means (32) includes a metal film made of Cr, Ni, Al, or Ni-Cr. A projection type liquid crystal display device as claimed in any one of claims 2 to 6, wherein said shading means (32) includes a gelatine film or an organic high polymer film coloured with a dye or a pigment.

Inventors: ATARASHI HISASHI, INAMI TAKASHI, TAKAFUJI YUTAKA, ATARASHI, HISASHI, INAMI, TAKASHI, TAKAFUJI, YUTAKA
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G02F1
IPC codes: G02F1, G09F9