Liquid crystal display device

Abstract: A liquid crystal display device minimized in image retention phenomenon, comprising: a first insulating substrate (1) having a plurality of pixel electrodes arranged in a matrix and a first alignment film (7) provided on the inner surface thereof, said insulating substrate (1) comprising a plurality of thin film transistors (6) associated with each of the pixel electrodes; a second insulating substrate (2) disposed opposed to the first insulating substrate, said second insulating substrate having facing electrodes (12) and a second alignment film (13) formed on the inner surface thereof; a liquid crystal layer (3) interposed between the first and the second insulating substrates (1, 2); and at least one of the first and the second alignment films (7, 13) having a thickness of 60 nm or less.

Description: BACKGROUND OF THE INVENTION The present invention relates to a liquid crystal display device, and more particularly, to an improved liquid crystal display device in which the image retention phenomenon is prevented from occurring on the display image plane. A liquid crystal display device having a thin film transistor (TFT) comprises a pair of glass substrates with a liquid crystal sealed therebetween. In the liquid crystal display device of this type, a desired image is displayed on an image plane by driving and aligning the liquid crystal in the particular pixel portion in a predetermined direction, thereby transmitting or cutting off the backlight. In such a type of liquid crystal display device, the pixel electrodes and the TFTs are formed on one of the pair of the glass substrates, and generally, a color filter is formed on the other glass substrate. Furthermore, an alignment film made of a resin material such as polyimide or PVA, or an inorganic material is formed on each of the glass substrates. The alignment film is generally provided at a thickness of about 80 nm, and is subjected to rubbing treatment after forming it on the substrate by means of a printing process and the like. However, a problem of image retention phenomenon is found to occur in a conventional liquid crystal display device having a TFT as described above. That is, in case the display is instantaneously switched off after a same display pattern is displayed continuously for a certain duration of time, the display pattern is found to remain on the image plane even after the display is switched off. The TFTs are driven by alternating current (AC), however, when a still image plane is displayed for a long duration of time, the direct current (DC) component accumulates to disable the liquid crystal shutter. This malfunction occurs presumably because of the charge accumulated in the alignment film. That is, the charge in the alignment film adsorbs the ions inside the liquid crystal that is filled between the two glass substrates and forms an electric double layer. The image retention phenomena in a conventional liquid crystal display device is illustrated in Figs. 2 (A) to 2 (C). Referring to Fig. 2 (A), a first image pattern 2 is continuously displayed in a liquid crystal display image plane 1 for a predetermined duration of time. As is shown in Fig. 2 (B), when the image plane 1 is switched off, the afterimage of the first pattern remains as a retained image pattern 3 in the display image plane 1. When a new image pattern 4 is displayed, as is shown in Fig. 2 (C), the new image pattern is displayed exposed on the still remaining retained image pattern 3 to provide an indiscernible display. Thus, it can be readily understood that the image quality is greatly impaired. The present invention has been accomplished in the light of the aforementioned circumstances. Accordingly, an object of the present invention is to provide a liquid crystal display device having TFTs, said liquid crystal display device characterized in that the image retention phenomenon is prevented from occurring. US-A-5,309,263 discloses a liquid crystal display device which comprises two alignment films, one of which has a thickness of 60 nm or less. The other alignment film is made of polycarbazole. EP-A-0 601 813 describes a method for producing alignment layers for use in a liquid crystal display device. The thickness of these layers is of about 60 mm. JP 4-044 012 discloses alignment layers of polyimide having a thickness between 20 and 300 nm. SUMMARY OF THE INVENTION According to an aspect of the present invention, the object above can be accomplished by a liquid crystal display device as recited in claim 1. The second insulating substrate may comprise a color filter on the inner surface thereof. The first insulating substrate may comprise a color filter on the inner surface thereof. Thus, the alignment film is provided thinner than a conventional one, thereby reducing the charge accumulation and suppressing the generation of image retention phenomenon. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view of a color liquid crystal display device according to the present invention; and Figs. 2 (A) to 2 (C) are an explanatory diagram illustrating the image retention phenomenon. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 is a cross sectional view showing the constitution of a liquid crystal display device according to a preferred embodiment of the present invention. Referring to Fig. 1, the basic constitution of the liquid crystal display device comprises a pair of first glass substrate 1 and a second glass substrate 2 disposed opposed to each other, with a liquid crystal 3 filled therebetween. The liquid crystal 3 is sealed by a sealing resin 4 which also functions as a spacer, provided to the peripheral edge of the glass substrates 1 and 2. The first glass substrate 1 comprises a polarizer sheet 5 provided on the outer surface thereof and a TFT 6 which functions as a switching element for driving the pixel electrode formed on the inner surface thereof. An alignment film 7 is formed on the upper surface of the TFT 6. The alignment film 7 can be provided by using a resin material such as polyimide or PVA, or by using an inorganic material. The alignment film 7 can be formed by a known method such as printing, spin coating, oblique evaporation, LB (Langmuir - Blodgett) method, etc. A color filter 8 is formed on the inner surface of the second glass substrate 2. The color filter 8 comprises a light shielding portion (black mask) 10 and a colored layer 9 consisting of R (red), G (green), and B (blue) color element portions. A transparent electrode (ITO) 12 is formed over the entire surface of the color filter 8 with a protective layer 11 interposed therebetween, and an alignment film 13 is formed further thereon in the same manner as in the first glass substrate 1. The level of image retention as evaluated visually can be improved by changing the alignment films 7 and 13 in such a manner that the thicknesses of the films 7 and 13 are 50 nm each, or that the thicknesses of the films are 60 nm and 50 nm, respectively. Furthermore, the level of image retention is considerably improved by changing the thickness of the alignment films 7 and 13 to 50 nm and 30 nm, respectively, or to 75 nm and 30 nm, or to 45 nm and 30 nm. The film thickness is controlled and selected depending on the type of the liquid crystal display device and the kind of TFT, on the dimension of the components constituting the device, on the use, and on various other conditions so that no image retention phenomenon occurs. Furthermore, the alignment film corresponding to desired regions of the glass substrate may be thinned. A thin alignment film can be readily obtained by diluting the alignment film material, printing the resulting material on the substrate, and then baking the printed material or simply removing the solvent therefrom. The present invention is also effective in the case of a liquid crystal display device comprising a glass substrate, having a color filter formed on the side of the active matrix substrate that is provided thereon with a pixel electrode and a thin film transistor, and a facing glass substrate disposed opposed to said glass substrate. As described in the foregoing, the liquid crystal display device is characterized in that the film thickness of the alignment film is reduced from the conventionally utilized thickness of about 80 nm to 60 nm or less, preferably to 50 nm or less, and more preferably, to 45 nm or less. Accordingly, the charge accumulation in the alignment film can be reduced and the image retention phenomenon can be thereby prevented from occurring. Thus, as a result, a liquid crystal display device with improved image quality can be obtained. Furthermore, the cost of the alignment film material can be lowered. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof.

Claims: A liquid crystal display device comprising: a first insulating substrate (1) having a plurality of pixel electrodes arranged in a matrix and a first alignment film (7) provided on the inner surface thereof, said first insulating substrate (1) comprising a plurality of thin film transistors (6) associated with each of the pixel electrodes, a second insulating substrate (2) opposed to the first insulating substrate (1), said second insulating substrate having facing electrodes and a second alignment film (13) on the inner surface thereof, and a liquid crystal layer interposed between the first and second alignment films, at least one of the first and the second alignment films (7, 13) having a thickness of 60 nm or less, characterised in that each one of the first and second alignment films (7, 13) is made of polyimide or PVA, and the second alignment film (13) is thinner than the first alignment film (7). A liquid crystal display device as claimed in claim 1, wherein at least one of the first and second alignment films (7, 13) has a thickness of 50 nm or less. A liquid crystal display device as claimed in claim 1, wherein at least one of the first and second alignment films (7, 13) has a thickness of 45 nm or less. A liquid crystal display device as claimed in any one of claims 1 to 3, wherein each one of the first and second alignment films (7, 13) is made of a single layer. A liquid crystal display device as claimed in any one of claims 1 to 4, wherein the second insulating substrate (2) comprises a color filter (8) on the inner surface thereof. A liquid crystal display device as claimed in any one of claims 1 to 5, wherein the first insulating substrate (1) comprises a color filter on the inner surface thereof.

Inventors: FUKUMOTO HIROHIDE, KATAOKA HIDEO, NAKANO MINORU, USHIRONO JUNICHI, FUKUMOTO, HIROHIDE, KATAOKA, HIDEO, NAKANO, MINORU, USHIRONO, JUNICHI, Fukumoto, Hirohide, c/o Sony Kokubu Semiconductor, Kataoka, Hideo, c/o Sony Corporation, Nakano, Minoru, c/o Sony Corporation, Ushirono, Junichi, c/o Sony Kokubu Semiconductor
Applicants: SONY CORP, SONY CORPORATION
Application year: 1995
Main classification: G02F1
IPC codes: G02F1