JPH0220896A - Photoconduction type liquid crystal light valve - Google Patents

Abstract

PURPOSE:To obtain voltage holding effect without spoiling high-sharpness write performance by arranging a couple of transparent electrodes outside a photoconductive layer and a liquid crystal layer and composing the photoconductive layer of an n type semiconductor which has a positive hole trap potential. CONSTITUTION:The liquid crystal layer 11 and photoconductive film 15 are laminated across a light absorbing film 17 and the photoconductive film 15 is composed of the n type semiconductor which has the positive hole trap potential. The light reflecting film 16 reflects projection light which is incident from a readout side and the light absorbing film 17 absorbs leak light from the light reflecting film 16. Transparent conductive films 18 and 19 are arranged as transparent electrodes outside the liquid crystal layer 11 and photoconductive film 15 and those are all sealed by glass substrates 20 and 21. The life of electrons generated by irradiating the photoconductive film 15 with light is prolonged through the operation of positive hole trapping, so the storage time of charges becomes long. Consequently, the voltage holding effect for the liquid crystal layer is obtained without spoiling the high-sharpness write performance and fast optical writing is enabled.

Description

TECHNICAL FIELD The present invention relates to a photoconductive liquid crystal light valve used in a projection type liquid crystal display device.

Background Art A photoconductive liquid crystal light valve has a photoconductive layer and a liquid crystal layer laminated with a light reflecting layer sandwiched between them, a pair of transparent electrodes arranged outside the photoconductive layer and the liquid crystal layer, and a pair of transparent electrodes arranged on the outside of the photoconductive layer and the liquid crystal layer. The structure is such that an alternating current voltage is applied between them, and only when light is irradiated from the writing side of the element, the resistance of the photoconductive layer in the irradiated part decreases, and the resistance in the corresponding part of the liquid crystal layer decreases. A voltage will be applied and an image will be written. That is, a voltage is applied to the liquid crystal layer only during the period when it is irradiated with light. Therefore, when this photoconductive liquid crystal light valve is applied to a projection type liquid crystal display device, some kind of voltage holding function is required in order to hold the image written on the liquid crystal layer.

Conventionally, there has been known a projection type liquid crystal display device that uses a CRT (cathode ray tube) to write an image and maintains the voltage of the liquid crystal layer using the afterimage of the phosphor on the CRT. is shown in Figure 3. In the figure, as a means for writing an image on the photoconductive liquid crystal light valve 1,
An optical fiber tube 2 is used which is coupled with an optical fiber that guides the RT and the image displayed on the CRT to the liquid crystal light valve 1, and the image displayed on the CRT is written on the photoconductive film of the liquid crystal light valve 1 through the optical fiber. . On the other hand, the light emitted from the light source 3 passes through the mirror 4 and the condenser lens 5 and enters the polarizing beam splitter 6. The P-polarized component of the incident light is sent directly to the polarizing beam splitter 6.
The traveling direction of the S-polarized light component is bent at right angles by the polarization beam splitter 6 and enters the liquid crystal light valve 1.

Here, when an image is drawn on the liquid crystal layer of the liquid crystal light valve 1, the reflected light reflected by the liquid crystal light valve 1 locally contains a P-polarized light component in the liquid crystal layer depending on the density of the image. It becomes like this. Only the P-polarized light component in this reflected light passes through the polarizing beam splitter 6 as it is, so that an image of this P-polarized light component is projected onto the screen 8 via the projection lens 7.

In this way, in the projection type liquid crystal display device using the optical fiber tube 2, although the photoconductive liquid crystal light valve 1 has a high definition writing performance, there is a limit to high definition as long as a CRT is used. The drawback was that the resolution was limited by the optical fiber. There is also a method of using optical fiber as a face plate, but it is expensive.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a photoconductive liquid crystal light valve that can obtain a voltage holding effect without impairing high-definition writing performance.

The photoconductive liquid crystal light valve according to the present invention has a structure in which a photoconductive layer and a liquid crystal layer are laminated with a light reflecting layer in between, and a pair of transparent electrodes are arranged outside the photoconductive layer and the liquid crystal layer. The conductive layer is made of an n-type semiconductor having a hole trap level.

Example Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

In FIG. 1 showing an example of the structure of a photoconductive liquid crystal light valve according to the present invention, a spacer 1 is provided around a liquid crystal layer 11.
2 are arranged, and alignment films 13 and 14 are provided on both sides of the liquid crystal layer 11. This liquid crystal layer 11 and a photoconductive film 15 of about 1 μm are laminated with a light reflecting film 16 and a light absorbing film 17 in between.

The photoconductive film 15 is made of an n-type semiconductor having a hole trap level, and for example, a CdSe alloy doped with Cu is used as the n-type semiconductor. The light reflecting film 16 is for reflecting the projected light incident from the reading side, and the light absorbing film 17 is for absorbing the leaking light from the light reflecting film 16. A transparent conductive film 18 as a transparent electrode is provided on the outside of the liquid crystal layer 11 and the photoconductive film 15.
19 are arranged, and all of these are glass substrates 20 and 21.
is sealed by. An alternating current voltage is applied between the transparent conductive films 18 and 19.

In the photoconductive liquid crystal light valve having such a configuration, since the photoconductive film 15 is composed of an n-type semiconductor having a hole trap level, the lifetime of electrons generated by light irradiation to the photoconductive film 15 is increased. Due to the effect of the hole trap, the charge accumulation time is extended. In other words, if there is no hole trap, the electrons that absorb light and are excited from the valence band to the conduction band are in an unstable state, as shown in 2E (a), and the electrons from the valence band are in an unstable state. The lifetime of the electron is short because it immediately recombines with the hole. However, when there is a hole trap, as shown in Figure 2(b), the hole trap captures the hole from the valence band, prolonging the time it takes for the electron to recombine with the hole. Therefore, the lifetime of the electron will be extended by that amount.

Due to the charge accumulation effect of the photoconductive film 15, the liquid crystal layer 11
This enables high-speed optical scanning because it has a voltage holding function and has the property of holding afterimages.
This means that it can support high-speed optical writing. Furthermore, the high-definition writing performance, which is a feature of photoconductive liquid crystal light valves, is not impaired. Note that appropriate voltage holding characteristics can be obtained by controlling the hole trap level of the n-type semiconductor constituting the photoconductive film 15.

As described in detail of the invention, in the photoconductive liquid crystal light valve according to the present invention, since the photoconductive layer is composed of an n-type semiconductor having a hole trap level, the features of the photoconductive liquid crystal light valve are The voltage holding effect on the liquid crystal layer can be obtained without impairing the high-definition writing performance, which means that high-speed optical writing can be supported.

[Brief explanation of the drawing]

Fig. 1 is a structural diagram showing an example of a photoconductive liquid crystal light valve according to the present invention, Fig. 2 is a diagram showing the recombination state of excited electrons, and (a) is a diagram showing the state of recombination of excited electrons. ,(
b) shows the case where there is a hole trap,
FIG. 3 is a block diagram of a projection type liquid crystal display device using an optical fiber tube as an image writing means. Explanation of symbols of main parts 1...Photoconductive liquid crystal light valve 5...
・Condenser lens 6...Polarizing beam splitter 8...Screen 11...Liquid crystal layer 15...Photoconductive film 16...Light reflecting film 17...
...Light absorption film 18.19...Transparent conductive film

Claims (2)

[Claims] (1) Laminating a photoconductive layer and a liquid crystal layer with a light reflective layer in between,
A photoconductive liquid crystal light valve having a configuration in which a pair of transparent electrodes are arranged outside the photoconductive layer and the liquid crystal layer, wherein the photoconductive layer is composed of an n-type semiconductor having a hole trap level. A photoconductive liquid crystal light valve featuring: (2) The photoconductive liquid crystal light valve according to claim 1, wherein the n-type semiconductor is a CdSe alloy doped with Cu.