JPS5929027B2 - Image display device with memory function - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image display device using an element having a history characteristic or memory function in the relationship between applied voltage and brightness, particularly an electroluminescent element (hereinafter referred to as EL). .

Generally, when EL is used as a video display element for TV etc.
A so-called matrix electrode arrangement, as seen in FIG. 1, is usually employed. That is, in FIG. 1, transparent electrodes 2 are arranged on a glass plate 1 in a grid pattern. For example, mainly Zn
On top of the EL light emitting layer 3 made of S or the like, metal electrodes 4 intersecting the electrodes 2 at right angles are arranged in a grid pattern. With this structure, one of the first electrodes 2 and the second electrode 4
When an alternating current voltage is applied to one of the electrodes, only a small area sandwiched between the two electrodes will emit light, which corresponds to one pixel on the screen. Therefore, when displaying an image on a TV or the like with the structure shown in Figure 1, the first electrode group 2 and the second electrode group 4 in Figure 1 are scanned at an appropriate period to sample the video signal in a certain time series. The light emission intensity of each picture element is changed according to the level determined, thereby obtaining a desired screen. Now, when a bipolar pulse as shown in Fig. 2a is applied to the EL element, the emission waveform corresponding to the applied pulse is shown in the same figure, and the relationship between the amplitude, width, frequency, and brightness of the pulse is shown in Fig. 2c,
It is shown in d.

'When a pulse voltage is applied to an EL element in an initial state where no voltage is applied, free electrons are swept toward the interface of the EL light emitting layer 3 by the electric field induced in the EL light emitting layer 3, and in this process, EL light emission occurs. radiated. The free electrons swept to the interface are captured and accumulated by the interface states, forming internal polarization. This internal polarization is captured and held for a while even after the external pulse voltage is removed, forming a polarization electric field. Therefore, when a pulse of the same polarity is subsequently applied, the electric field within the EL light emitting layer 3 becomes E because the external voltage is weakened by the polarization electric field.
Although the L emission brightness decreases, when a reverse polarity pulse is applied,
The electric field in the EL light emitting layer 3 becomes a high voltage due to the superimposition of the polarization electric field on the external voltage, and the EL light emission brightness becomes high. Afterwards, when a reverse polarity pulse train is applied, this state of high light emission brightness is maintained. Figures 2A and 2b show the relationship between the application of this reverse polarity pulse and the emission brightness; when a pulse voltage is applied to the EL element in the initial state, EL emission is emitted, and then when a second pulse voltage is applied. When applied with the same amplitude and opposite polarity, EL light emission with much higher brightness than before was obtained. Thereafter, by alternately applying pulse voltages of the same amplitude, this high brightness EL
Luminescence is emitted repeatedly. These relationships are characteristics that are also observed in ordinary EL elements, regardless of the presence or absence of a memory function, which will be described later. FIG. 3 shows a basic configuration block diagram of a conventional image display device such as T using the EL element 5 having the above-mentioned [Mxn] picture elements, Hl, H2...,
Hm, vl, v2..., Vn is a group of vertically and horizontally arranged electrodes 06. A composite video signal input from 06 is separated into a video signal 8 and a synchronization signal 9 by a signal separation circuit 7. I can be separated.

The video signal 8 is further temporally extracted by a signal processing circuit 10 using sampling pulses, and is then extracted by a holding circuit 11.
is held for a certain period of time, and the electrode group Hl is output from the output drive circuit 12.
, H2, . electrode groups Vl, v2,
..., Vn is also supplied with voltage from the output drive circuit 15 under the control of the timing pulse generator 14 O conventional TV
It has been said for a long time that all-solid-state display elements should be used, and although EL is seen as the most promising display element in this case, the reason why it has not become widespread is that its life stability and brightness are insufficient.

Vapor-deposited thin film EL with a three-layer structure in which a ZnS layer doped with an impurity such as Mn is sandwiched between layers of a dielectric material such as Y2O3 has far superior characteristics in terms of lifetime stability and brightness compared to conventional EL elements. At the same time, it was newly discovered that a hysteresis phenomenon as shown in Fig. 4b is exhibited between the luminance and the applied voltage. To explain this characteristic according to Fig. 4, first, as shown in Fig. 4a, the voltage amplitude V1 is When the pulse is applied, the brightness is at the level B1, as shown in B and c of the figure. The aforementioned polarization electric field is generated in the EL element, and the brightness B1 is maintained by applying the reverse polarity pulse train. It is desirable to set this brightness B1 to such a low brightness value that it is hardly visible even though it is in a light emitting state, in terms of contrast with brightness B2, which will be described later. When a write voltage V2 is applied to this, the amount of internal polarization of the EL element, which has been in an operating state due to the application of an alternating pulse train of amplitude 1, increases to an amount corresponding to voltage 2, and the brightness accordingly increases to B3. The brightness increases sharply towards the end, and after that, even if the voltage value is returned to the maintenance voltage V1 again, the brightness settles at B2, which is higher than B1. This is because the amount of internal polarization increased by the application of the write voltage V2 does not disappear immediately even if the applied voltage is returned to V1, but remains in an increased state than before the application of the write voltage 2, and the brightness B2 at this time is 0, which is a sufficiently visible value, and the erase voltage V
When 3 is applied, the brightness level decreases rapidly and the internal polarization disappears due to recombination, etc. 0 In this state, the maintenance voltage V is applied again.
When the brightness is returned to 1, the brightness goes through the process shown in FIG. 2b and finally settles down to B1. These temporal relationships are shown by making the numbers 1, 3, ..., 21 attached to Figure 4 a correspond to the positions of the same numbers in Figure 4 c. When the amplitude of the applied voltage is changed, the amount of internal polarization of the EL element in the operating state does not follow the change in amplitude and undergo a reversible change process, and the amount of internal polarization does not change reversibly to the same amplitude value during the step-up and step-down steps of the amplitude. Since the amount of polarization differs depending on the amount of polarization, the luminance of light emission corresponding to the amount of polarization shows a history characteristic with respect to the applied voltage. The present invention, which can take any small loop, aims to obtain a screen with even higher brightness by actively utilizing this memory characteristic. The block diagram is shown in Figure 5.

Components having the same functions as those in the conventional device block diagram FIG. 3 are designated by the same numbers. In FIG.
8 and 19 are gates, and by newly adding these gates, a high-brightness screen can be obtained as described in detail later. Below, for ease of understanding, we will use a (3×3) matrix. The method of the present invention will be explained based on the assumption that one frame (field) is composed of three horizontal scanning periods without considering interlacing, and that each horizontal scanning period is sampled at three points in time. As shown in Figure 6, H is the horizontal scanning period, F is the frame (field), W is the writing period,
S is a light emission sustaining period, and E is an erasing period.

That is, the signal level sampled at each point during the horizontal scanning period is held in a holding circuit, and the signal level may be held at the end of the horizontal scanning period or partially at the next horizontal scanning period, depending on the signal level at each sampling point. When a pulse of the opposite polarity is applied after a 0 write pulse belonging to the so-called line sequential scanning method in which each pixel is simultaneously written and emitted, the pixel emits light; If only one shot per period is used, the light emission will attenuate.Generally, the time constant of this attenuation is much smaller than the frame (field) period.The method according to the present invention applies a write pulse and then applies a sustain pulse with the opposite polarity. Since the signals are applied alternately, the light emission continues until the next picture element signal arrives.Of course, this light emission must be erased by applying an erase pulse before writing the picture element signal of the next frame (field) of the same picture element. In this type of display device, each pixel emits light for most of the frame (field) period, so for the person viewing the screen, compared to the conventional method, It is easy to understand that the luminance is several times higher. For simplicity, assume m=n=3, that is, a (3×3) matrix in FIG. The time chart is shown in Figure 7.

Vhl, Vh2, and Vh3 are the voltages applied to the vertical electrodes H1, H2, and H3 in FIG. 5, respectively;
Similarly, VV3 is the voltage applied to the horizontal electrodes Vl, V2, and V3. o The voltage Vij applied to the picture elements 1 and j where the vertical electrode Hi and the horizontal electrode j intersect is Vij=Hi-
Therefore, the voltage waveform applied to picture elements 1 and 1 is Vvj.
In the voltage waveform applied to 0 picture elements 1 and 1, which becomes hl-V1 as shown in Figure 7, 1 samples the video human input signal shown in the top row of Figure 7 and changes the amplitude accordingly. It is a write pulse, and the pulse amplitude of Vhl is VS/2+
VW (!.Vs which synthesized the pulse amplitude Vs/2 of VVl
It has an amplitude of +W and a sufficient pulse width (pulse application time).

◎ is a sustain pulse written based on the hysteresis characteristics in Fig. 4 to sustain high-intensity light emission of the picture element, and the pulse amplitude Vs/2 of h1 and pulse amplitude s of VVl that appear periodically following the above /2 has an amplitude of Vs. ◎ is an erase pulse train that erases a picture element in a high-intensity light emitting state, and has an amplitude of VE which is a combination of the pulse amplitude VE/2 of Vhl and the pulse amplitude VE/2 of VVl. Erasing pulse train 0 is configured such that a plurality of pulses are applied successively to completely erase picture elements. VV,,V
The pulses @, @′ appearing at V2 and VV3 are the electrodes V,
, V2, and V3 for writing, and has an amplitude of Vs/2 and forms the write pulse 4 described above. In the electrode line to which this pulse is applied, the pulse with an amplitude of s/2 that should have formed the sustaining pulse immediately before it has disappeared.
When writing according to a video input signal, first write picture elements 1, 1, 2, 1, 3, 1 on electrode V1 at the same time, then write picture elements 1, 2, 2, 2, 3, 2 on electrode 2 at the same time. Write at the same time. Finally, after simultaneously writing the picture elements 1, 3, 2, 3, 3, and 3 on the electrode V3, writing starts again from the picture elements on the electrode V. This method of simultaneously writing picture elements on each electrode line is called the line sequential scanning method.
In the figure, (d), 0f, and ox r are the selection pulses for the write electrode line, and by applying these pulses, the electrodes Vl, 2, and V
3 are sequentially scanned one by one.If the amplitude Vs of the emission sustaining pulse ◎ is set to a voltage value slightly higher than the emission threshold voltage Th in Fig. 4b, and the amplitude E of the erasing pulse O is set to a voltage value slightly lower than Vth. , the light emitting sustaining drive and erasing drive of the picture element are performed according to the history characteristics, and the pulse ho or E/2 of the amplitude Vs/2 derived from the voltage waveform applied to the picture elements 1, 1
Pulse 2 (f) hardly contributes to light emission or erasing and can be ignored. In addition, if the erasing voltage is Vth
If the voltage is too low, a sufficient electric field for moving the free electrons forming internal polarization will not be obtained, making it impossible to perform erasing drive. According to the video input signal, 3 points Sl in the H period
1, Sl2, Sl3 are sampled and held, and pulse components Wll, Wl2, W, 3 for writing are changed to Hl, Vh2 with Vw (Vw x s/2) changed according to each signal level.
, h3.

Since the selection pulse (D) is applied to the electrode V1 at the same time, these pulses are combined and the selection pulse (D) is applied to the electrode V1. ．． E picture element 1,1
, 2, 1, 3, 1 are written. Writing is not performed on other electrode lines to which the 0 selection pulse (d) is not applied. Similarly, in the 0th H period, the video input signal is S2l, S
22, S23 are sampled and held, and pulse components W2l, VV22, W23 are formed.
A selection pulse @5 is applied to the electrode line, and picture elements 1, 2, 2, 2, 3, and 2 on this electrode line are written. Such writing and driving are performed sequentially to write all picture elements of the matrix. The light emission waveform of picture elements 1 and 1 is
It is defined by pulses 4, ◎, and ◎ applied to this.

When the write pulse 4 is applied, the picture element is in the erased state with no internal polarization due to the application of the erase pulse ◎ just before that, so the internal polarization is newly generated as explained in Fig. 2A and b. However, high-intensity light emission corresponding to the brightness B3 in FIG. 4 is not obtained, and a low-intensity light emission state corresponding to the first pulse application as shown in FIG. 2b occurs. By applying the next sustain pulse ⊚ of opposite polarity, the internal polarization generated by applying write pulse 4 is superimposed thereon, so that high-intensity light emission corresponding to the brightness B2 in FIG. 4 is obtained. Thereafter, this light emitting state of luminance B2 is maintained by sustain pulses ⊚ which are periodically applied with opposite polarity, and this is maintained until near the same point in time of the next field. When newly writing to picture elements 1 and 1 in the next field, the previously written luminance is once erased to form an initial state in which internal polarization is completely extinguished.
New information is written. Therefore, before the write pulse 4 is applied, the erase pulse O is applied and the picture elements 1 and 1 are erased. Thereafter, the above driving is repeated for picture elements 1 and 1. In this example, the brightness modulation by pulse amplitude was mainly described, but at our place, the above-mentioned three-layer structure thin film E
In L, it has been confirmed that luminance has a memory effect on pulse width and frequency as well.

Therefore, in writing, by applying brightness modulation based on the pulse width and number of pulses, a high brightness screen can be obtained by applying sustain pulses in the same way as the above-mentioned amplitude modulation.

[Brief explanation of drawings]

Figure 1 shows an EL with a matrix structure. Figure 2 shows voltage application waveforms, light emission waveforms, and EL characteristics. Figure 3 shows a configuration diagram of a conventional matrix panel display device. Figure 4 shows an EL with hysteresis characteristics. A diagram showing the relationship between applied voltage and brightness,
FIG. 5 is a block diagram of a matrix type panel display device according to an embodiment of the present invention, and FIGS. 6 and 7 are time charts for explaining the method of the present invention.
...Light emission sustaining pulse〇

Claims (1)

[Claims] 1. An EL light-emitting layer that emits EL light upon application of a voltage is interposed as a light-emitting element corresponding to a position of intersection between matrix electrodes consisting of first and second electrode groups arranged in a direction crossing each other, and the matrix electrode is In an image display device having a matrix memory panel provided with a memory function based on the history characteristics between the applied voltage applied through the EL light emitting layer and the luminance of the EL light emitting layer, After applying a pulse-modulated write voltage to the light emitting element determined by the selective combination of the first and second electrode groups to bring the light emitting element into a light emitting state defined by the hysteresis characteristic, An image display device characterized in that a drive circuit section is connected to the matrix electrode for applying a sustaining pulse voltage to the light emitting elements until around the same point in the field.