JPH063692A - Photo-conductive liquid crystal light bulb - Google Patents

Abstract

PURPOSE:To provide a photoconductive liquid crystal light bulb which can hold long the response of the read light output to the write light and is free from drop of the photo-sensitivity. CONSTITUTION:A photoconductive liquid crystal light bulb is structured so that a photoconductive layer 5 and a liquid crystal layer 1 are put one over the other and a pair of transparent electrodes 8, 9 are arranged outside of them, wherein the photoconductive layer 5 consists of amorphous silicon containing nitrogen. Because the photoconductive layer 5 where nitrogen is included in amorphous silicon, allows the impressed voltage required of the liquid crystal 1 to last for a certain while even after the write pulse light goes out, a good response of the read light level to the write light can be obtained, which along with small secular change of the layer sensitivity, generates always good projective images.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoconductive liquid crystal light valve used in a projection type liquid crystal display device.

[0002]

2. Description of the Related Art Generally, photoconductive liquid crystal light valves are
An image is drawn on the photoconductive layer by the writing light, and a voltage change corresponding to the photosensitive reaction corresponding to the image of the photoconductive layer is applied to the liquid crystal layer, and a read reflected light amount corresponding to the change in the orientation of the liquid crystal molecules is obtained. Project the image on the screen. When this photoconductive liquid crystal light valve is applied to a projection display by, for example, CRT optical writing, the writing surface of the photoconductive layer is scanned with writing light having an afterglow time of about 1 msec. In this case, when attention is paid to one pixel, as shown in FIG. 1A, the pixel is written by pulsed light having a relatively short irradiation time. Then, when the light valve is used for a television monitor, in order to make the liquid crystal layer have a sufficient response, it is necessary to use the light bulb shown in FIG.
As described above, it is usually necessary to apply a voltage of about a dozen msec, that is, about one field time (16.67 msec).

Optical writing characteristics when a conventional hydrogenated amorphous silicon (hereinafter referred to as a-Si: H) film is used as a photoconductive layer for changing the voltage applied to the liquid crystal layer according to the writing light. As shown in FIG. In the figure (a),
-Si: H film is a writing pulse light to be radiated, and when this is incident from the writing side of the light valve, the impedance change of the a-Si: H film changes with respect to the writing pulse light as shown in FIG. It reacts sensitively, and the impedance of the a-Si: H film instantly returns to a steady value almost at the same time as the fall of the pulsed light. Therefore, as shown in FIG. 7C, the voltage applied to the liquid crystal layer also immediately falls to the reference voltage at the fall, and the liquid crystal layer also responds to this and is reflected through this liquid crystal layer. As shown in FIG. 6D, the luminance signal in the read light is
There is a problem that the value of the dark level is instantly returned and the liquid crystal layer cannot respond for one field equivalent time.

In order to solve this problem, B is added to the a-Si: H film.
A photoconductive liquid crystal light valve having a photoconductive film in which (boron) is mixed is already disclosed in Japanese Patent Application No. 2-239 by the present applicant.
No. 654 is proposed. According to this, since the applied voltage required for the liquid crystal layer is maintained for a while even after the writing pulse light is extinguished, a sufficient response characteristic of the reading light level with respect to the writing light can be obtained, and the characteristic becomes as shown in FIG. . That is, similar to FIG. 2, when the pulsed light as shown in FIG. 2A enters the liquid crystal layer 1 from the writing side, the impedance of the photoconductive film 9 becomes as shown in FIG. 2B. After disappearance of the pulsed light, that is, at the trailing edge of the pulsed light, it rises to a steady value more slowly than when boron is not added. Therefore, as shown in FIG. 7C, the voltage applied to the liquid crystal layer 1 also slowly drops to the reference voltage value accordingly, and the response time of the liquid crystal layer 1 also becomes longer. As in the case of d), the brightness signal of the read light can also maintain the bright level for a long time.

However, boron-added aS
The i: H film has a problem that its photosensitivity gradually decreases as shown in FIG. 4 when the irradiation of the writing light is continued. FIG. 4 shows the static light sensitivity characteristics,
For an a-Si: H film added with boron at ppm, a transparent electrode is sandwiched from both sides of the film and a frequency of 10K is applied between the electrodes.
While applying a square wave voltage of Hz, the wavelength of 660
a when the writing light of nm is continuously irradiated
It shows the change over time in the photosensitivity of the Si: H film. Horizontal axis L. S. T. (Light Soaking Time) [Hr] is the elapsed time from the start of measurement, and the vertical axis I.D. D. R. Is the impedance reduction rate (Inpeadance Decrease Ratio), which is the ratio of the impedance | Zd | in the dark of the writing light and the impedance | Zp | in the irradiation | Zd | / | Zp
Represents | as the photosensitivity to the continuous incident writing light. In addition, as the measurement points in FIG.
The □, ▲, ■, ●, and ○ marks indicate the irradiation levels of 2.5 mW / cm ² , 480 μW / cm ² , and 130 μW /, respectively.
cm ² , 20 μW / cm ² , 6 μW / cm ² , 0 μW / c
This is a case where the writing light of m ² is continuously incident for a predetermined time, and the irradiation level is 10 between each measurement point.
The writing light of mW / cm ² is irradiated.

As is clear from this characteristic diagram, the sensitivity of the a-Si: H film doped with boron decreases with time when writing light is continuously incident, and particularly 130 μW / cm.
It can be seen that when the writing light of ² is incident, it is significantly reduced. Further, FIG. 5 shows the pulsed light sensitivity characteristics. For a photoconductive liquid crystal light valve including an a-Si: H film added with boron similar to the above, a rectangular wave having a frequency of 10 KHz between electrodes sandwiching a liquid crystal layer. When a voltage is applied, the wavelength from the LED is 660 nm, the average incident level is 180 μW / cm ² ,
It shows the change over time in the pulse light sensitivity of the light valve when writing pulse light with an afterglow time of 1 msec is incident. Horizontal axis L. S. T. (Light Soaking Tim
e) [Hr] is the elapsed time from the start of measurement, and the vertical axis [%] is the ratio (I / Imax) of the output brightness level I to the maximum brightness level Imax of the light valve when the reading light of a predetermined level is incident. ) × 100 is represented as a response level to the write pulse light. It should be noted that writing light having an irradiation level of 10 mW / cm ² was irradiated between each measurement point.

As is clear from this characteristic diagram, the photosensitivity of the a-Si: H film added with boron decreases with time, and the sensitivity becomes 0 after 10 hours.
You can see that As described above, when the photosensitivity of the photoconductive film is lowered, the projected image of the photoconductive liquid crystal light valve cannot be maintained well.

[0008]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a photoconductive liquid crystal light valve which can maintain the response of read light output to write light for a long time and does not deteriorate the photosensitivity. is there.

[0009]

A 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, and a pair of transparent electrodes is arranged outside the photoconductive layer and the liquid crystal layer. A photoconductive liquid crystal light valve, wherein the photoconductive layer is made of amorphous silicon to which nitrogen is added.

[0010]

In the photoconductive type liquid crystal light valve according to the present invention, the photoconductive layer containing nitrogen in amorphous silicon has an optical writing characteristic that maintains a sufficient response time for scanning the screen and has a small change over time. It has optical writing characteristics.

[0011]

The present invention will be described in detail below with reference to the drawings. FIG. 6 is a block diagram showing an embodiment of the present invention.
In the figure, a spacer 2 is provided around the liquid crystal layer 1,
Liquid crystal alignment films 3 and 4 are disposed on both surfaces of the liquid crystal layer 1.
The liquid crystal layer 1 and an a-Si: H film 5 which is a photoconductive film are laminated with a light reflection film (dielectric mirror) 6 and a light absorption film (light blocking film) 7 interposed therebetween. The light reflecting film 6 is for reflecting the projection light incident from the reading side, and the light absorbing film 7 is for absorbing the leaked light from the light reflecting film 6. Transparent conductive films 8 and 9 as transparent electrodes are arranged outside the liquid crystal layer 1 and the photoconductive film 5, all of which are formed on the glass substrate 1.
It is sealed by 0 and 11. An AC voltage is applied between the transparent conductive films 8 and 9 by the driving power supply 12.

The photoconductive film 5 is a hydrogenated amorphous silicon.
(A-Si: H) with N (nitrogen) added.
It The photoconductive film 5 is formed by NH₃Gas and SiH_FourWith gas
To NH ₃/ SiH_FourGas flow ratio (volume ratio) of
Are mixed at 0.01-1 and decomposed by plasma
Done by Writing incident on the light valve
The visible light passes through the glass substrate 11 and the transparent conductive film 9,
An image is drawn on the writing surface of the conductive film 5 for irradiation. Photoconductive film 5
Of the writing surface of the part illuminated by the writing light
Minute conductivity decreases depending on the light output intensity of the writing light
Therefore, the voltage from the driving power supply 12 is applied to the liquid crystal layer 1.
Change the orientation of liquid crystal molecules.

On the other hand, the reading light incident from the polarizing prism 13 is formed by the liquid crystal molecules whose arrangement changes according to the writing light, and the light reflecting film 6 arranged on the liquid crystal layer and the side surface thereof.
Is reflected or absorbed by. The reflected light reflected is transmitted through the liquid crystal alignment film 3, the transparent conductive film 8, the glass substrate 10 and the polarization prism 13, and then projected onto a screen (not shown).

The optical writing response characteristic of the photoconductive liquid crystal light valve constituted by the a-Si: H film containing N is as follows. 7 to 12 show a photoconductive liquid crystal light valve with a frequency of 10K between transparent electrodes.
While applying a rectangular wave voltage of Hz, the reading side irradiates a predetermined reading light, the writing side emits a writing pulse light having a wavelength of 660 nm, an average irradiation level of 180 μW / cm ² , and an afterglow of 1 msec. It shows the pulsed light writing response characteristic in the case where irradiation is performed 16 times continuously at an interval of 16.67 msec. The content of N is NH ₃ / SiH ₄
The gas flow ratio is 0 in FIG. 7, 0.0 in FIG. 8 and 0.0 in FIG.
5, Fig. 10 is 0.1, Fig. 11 is 0.2, and Fig. 12 is 0.5.
2 shows the characteristics of a photoconductive liquid crystal light valve composed of the a-Si: H film. The horizontal axis [ms] is the elapsed time from the start of writing pulse light incidence, and the vertical axis [%] is the ratio of the output brightness level I to the maximum brightness level Imax of the light valve when a predetermined level of read light is input. (I / Imax) × 100 is shown as the response level to the write pulse light.

From these characteristic diagrams, it is understood that the brightness level generated as an output also changes depending on the N content of the a-Si: H film. Each crest of each characteristic curve shows the output characteristic for each of the incident write pulse lights, and the output brightness for the pulse light incident in the first half of the 16 write pulse lights. Although the level also rises slowly and its peak level stays at a relatively small level, the output brightness level rises quickly and the peak level becomes a relatively large level for the pulsed light incident in the latter half. It can be seen that the output tends to be stable. Regarding the response time, for example, focusing on the output luminance characteristic (indicated by diagonal lines) with respect to the write pulse light that is incident last in FIG. 12, the peak of such characteristic has a peak at an output level of about 80%, From there, the output level is gradually reduced, and approximately 60 msec from the occurrence of mountains.
After that, the output of the peak level is attenuated to 10% (in this case, the output level is 8%) of the peak level which is considered to have disappeared. According to this characteristic, it is understood that the response time (one field time) required for one pixel as shown in FIG. 1 is sufficiently satisfied. In particular, a-S of 0.02 to 0.5 N content in NH ₃ / SiH ₄ ratio
The i: H film has good response characteristics.

FIG. 13 shows the static photosensitivity characteristics of an a-Si: H film in which the N content is 0.05 in the NH ₃ / SiH ₄ gas flow rate ratio, and is written under the same conditions as in FIG. It shows the change with time of the photosensitivity of the a-Si: H film when the incident light is continuously irradiated. Horizontal axis L. S. T. And the vertical axis I.D. D. R. , And each measurement point is the same as in FIG.

As is clear from this characteristic diagram, the sensitivity of the a-Si: H film added with N is higher than that of the a-Si: H film added with boron even when writing light is continuously incident. It can be seen that it shows a flat characteristic without decreasing so much with time. Further, FIG. 14 shows the pulsed light sensitivity characteristics, and is similar to the above in a photoconductive liquid crystal light valve including an a-Si: H film having an N content of 0.05 at a H ₃ / SiH ₄ gas flow rate ratio. In contrast, FIG. 6 shows a temporal change in pulse light sensitivity of the light valve when writing light is incident under the same conditions as in FIG. Horizontal axis L. S. T. Also, the vertical axis [%] is the same as in FIG. At each measurement point, for example, the average value of the output luminance levels (corresponding to the shaded area in FIG. 8) in response to the three writing pulse lights that finally entered in FIG. 9 is defined as the output luminance level I. It is a thing.

Then, this characteristic diagram and a with boron added
As is clear from comparison with FIG. 5 showing the pulsed light writing characteristics of the photoconductive type liquid crystal light valve including the —Si: H film, the a-Si: H film with N added shows the write pulse. It can be seen that the temporal change in photosensitivity due to light becomes gentle. Note that, in addition to the above-described embodiment, N is added to a tetrahedral amorphous alloy (a-SiC, a-SiGe, a-SiSu).
Is effective even if added.

[0019]

As described above, in the photoconductive liquid crystal light valve of the present invention, the photoconductive layer obtained by mixing nitrogen in amorphous silicon is applied to the liquid crystal layer even after the writing pulse light is extinguished. Since the voltage is maintained for a while, a sufficient response of the reading light level to the writing light can be obtained. Moreover, since such a photoconductive layer has a small change in photosensitivity with time, it is possible to always obtain a good projection image.

[Brief description of drawings]

FIG. 1 is a diagram showing an incident level of writing light when focusing on one pixel and a response characteristic of liquid crystal to the incident level.

FIG. 2 is a diagram showing optical writing characteristics in a conventional photoconductive type liquid crystal light valve when boron is not added to a-Si.

FIG. 3 is a diagram showing optical writing characteristics in a conventional photoconductive type liquid crystal light valve when boron is added to a-Si.

FIG. 4 is a diagram showing static photosensitivity characteristics in a conventional photoconductive liquid crystal light valve when boron is added to a-Si.

FIG. 5 is a diagram showing a photosensitivity characteristic by pulsed light writing in a conventional photoconductive liquid crystal light valve when boron is added to a-Si.

FIG. 6 is a diagram showing the structure of a photoconductive liquid crystal light valve according to an embodiment of the present invention.

FIG. 7 is a diagram showing optical writing response characteristics of the photoconductive liquid crystal light valve of the present invention when the N content of the photoconductive film is set to 0 in the H ₃ / SiH ₄ gas flow rate ratio.

FIG. 8 is a diagram showing optical writing response characteristics of the photoconductive liquid crystal light valve of the present invention when the N content of the photoconductive film is 0.02 in H ₃ / SiH ₄ gas flow rate ratio.

FIG. 9 is a diagram showing the optical writing response characteristics of the photoconductive liquid crystal light valve of the present invention when the N content of the photoconductive film is set to a H ₃ / SiH ₄ gas flow rate ratio of 0.05.

FIG. 10 is a diagram showing the optical writing response characteristics of the photoconductive liquid crystal light valve of the present invention when the N content of the photoconductive film is 0.1 at the H ₃ / SiH ₄ gas flow rate ratio.

FIG. 11 is a diagram showing the optical writing response characteristics of the photoconductive liquid crystal light valve of the present invention when the N content of the photoconductive film is 0.2 at the H ₃ / SiH ₄ gas flow rate ratio.

FIG. 12 is a diagram showing the optical writing response characteristics of the photoconductive liquid crystal light valve of the present invention when the N content of the photoconductive film is 0.5 at the H ₃ / SiH ₄ gas flow rate ratio.

FIG. 13 is a diagram showing static photosensitivity characteristics of the photoconductive liquid crystal light valve of the present invention.

FIG. 14 is a diagram showing photosensitivity characteristics by pulsed light writing in the photoconductive type liquid crystal light valve of the present invention.

[Explanation of symbols]

 1 Liquid crystal layer 2 Spacer 3,4 Alignment film 5 Photoconductive film 6 Light reflection film 7 Light absorption film 8,9 Transparent conductive film 10,11 Glass substrate 12 Driving power supply 13 Polarizing prism

Claims (2)

[Claims] 1. A photoconductive liquid crystal light valve having a structure in which a photoconductive layer and a liquid crystal layer are laminated, and a pair of transparent electrodes is arranged outside the photoconductive layer and the liquid crystal layer, wherein the photoconductive layer is , A photoconductive liquid crystal light valve, which is made of amorphous silicon containing nitrogen. 2. The photoconductive liquid crystal light valve according to claim 1, wherein a nitrogen content of the amorphous silicon is 0.01 to 1 in a NH ₃ / SiH ₄ gas flow rate ratio.