JPS6010629B2 - Driving method of thin film EL element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a new method for driving a three-layer thin film EL device.

In particular, the present invention provides a drive circuit that has a small number of circuit elements such as a power supply and transistors, and is easy to adjust. The three-layer thin film EL device used in this invention has a structure as shown in FIG.

A transparent electrode 2 of IQ03 is provided on a glass substrate 1.

On top of this is a dielectric material layer 3 of, for example, Y203, Sj3N4, Ti02, etc., and further on this is a Z layer doped with, for example, Mm.
The fluorescent layer 4 of ships etc. has a film thickness of 500 to 10000A,
Further, the same dielectric material 3' as described above is deposited thereon by vapor deposition, sputtering, etc. to form a three-layer structure, and a back electrode 5 made of AI or the like is disposed on top of this. The electrodes 2 and 5 are formed into strip-shaped electrodes that are perpendicular to each other to perform matrix display. ELPs with this structure have superior characteristics in terms of brightness, lifespan, and stability compared to conventional dispersion-type EL elements. A hysteresis phenomenon as shown in FIG. 2b is shown between the brightness and the applied voltage.

To explain this characteristic according to Fig. 2, when a pulse with a voltage amplitude of V is first applied as shown in Fig. 2 a, the luminance increases as shown in Fig. 2 b and c.
It is at level B as shown in . Here, V is V, ZVth, where Vth is the luminous threshold voltage Vth. When the write voltage V2 is applied to this, the brightness increases all at once to &, and even after the voltage value is returned to the sustaining voltage V, the brightness settles to B, which is larger than B2. When the erase voltage V3 is applied to this, the brightness level decreases rapidly, and when it is returned to the sustaining voltage y, the brightness settles to B. These temporal relationships are indicated by the symbol t.・Ra...Ra, are shown by corresponding to the positions of the same symbols in Figure c. This hysteresis phenomenon can take any small loop depending on the amplitude and pulse width (not shown) of the write voltage, as shown by the thin line in FIG. 2b. That is, it is also possible to display halftones. A major feature of the ELP, which other display elements do not have, is that once a write voltage is applied, the ELP continues to emit light without losing the gradation given by the sustain pulse. The above voltages vary greatly depending on the composition, film thickness, and applied waveform, but in the prototype example, V ratio = 200V, V, = 210V, V2 = 210 ~ 2
80V, V3=190V. In ELP, writing and erasing can be performed not only by the pulse amplitude as described above, but also by the pulse width and number of pulses.

Furthermore, writing and erasing using light is also possible. Next, regarding writing and erasing using pulse width with constant amplitude, Figure 3 a and b
Explain using.

There is also a hysteresis phenomenon in the pulse width-luminance characteristics as shown in FIG. 3a.

That is, as shown in FIG. 3b, in the application state of the sustaining pulse Ps up to the time oscillation 2, the luminance of the thin film EL element is Bs2.
It is. Next, when a write pulse Pw with a large pulse width is applied at time t, high-intensity light is emitted, and when a sustain pulse Ps is applied at the next time, time 3, the emission brightness shifts to Bw2. The light emission at the brightness Bw2 continues as long as there is a sustain pulse Ps applied at an appropriate cycle (write memory state).
After that, an erase pulse Pe with a narrow pulse width is added for a certain period of time,
When the sustain pulse Ps is returned again at time t25, the luminance becomes B.
It transitions to near e2 or Be2, and this state is connected by a sustain pulse (erased memory state). This hysteresis phenomenon changes depending on the magnitude of the pulse width, and it is possible to display halftones. As described above, writing and erasing can be performed by modulating the values of the pulse amplitude and pulse width during writing and erasing.

This is considered to be due to the following reasons. That is, even if the electric field applied as a writing means is removed, ZnS/Mn
The conduction electrons swept near the interface between the layer and the dielectric film are captured by the interface state near the interface, and when the next sustain pulse is applied, they escape from the interface state to the conduction band and travel through the ZnS:Nh layer to the other side. reaches the interface. At this time, for conduction electrons passing near the original electron trap in the ZnS/Mn layer, the probability of being swept to the other interface is higher than the probability of being re-trapped to the original trap level. This is because the conduction electrons have a high velocity due to the electric field caused by the sustain pulse. For this reason, the luminance does not return to its original state and exhibits a memory phenomenon. Next, when an erase voltage is applied, the bridging speed of conduction electrons passing near the electron trap level becomes lower, and the probability that the original electrons in the ZnS:Mn layer are drawn to the other interface by the applied discharge field is lower. The probability of being re-trapped at the trap level increases. Therefore, memory erasure will be performed. Incidentally, writing and erasing using the pulse amplitude has the advantage of easily increasing brightness and contrast ratio, but has the disadvantage of requiring a large number of power supplies and circuit elements such as transistors.

On the other hand, writing and erasing using pulse width has the advantage of requiring fewer power supplies and fewer circuit elements, but when the number of picture elements in a matrix display is large, transistors and thin film EL
It has the disadvantage that it is easily affected by variations in the wire resistance of the electrodes of the element and is difficult to adjust. In view of the above points, the present invention is a compromise between the two and uses amplitude modulation and pulse width modulation simultaneously to perform writing and erasing. For example, in the first embodiment, writing is performed by amplitude modulation and erasing is performed by pulse width modulation. Further, in the second embodiment, writing is performed by pulse width modulation, and erasing is performed by amplitude modulation. As an embodiment of the present invention, a method of writing by amplitude modulation and erasing by pulse width modulation will be described below with reference to the drawings.

As shown in FIG. 4, from the application state of the sustain pulse Ps with a voltage of 230 V and a pulse width of 250 seconds until time t3, the voltage is greater than the sustain pulse of time song 32.

After applying the write pulse Pw having an amplitude of 280 V, the sustain pulse Ps is applied again until time t33. Next, at time t34, only the pulse width was narrowed, that is, the voltage was 2
Erasing is performed by applying an erasing pulse Pe of 30 V and a pulse width of 20 seconds, and this is continued after time t35 by a sustain pulse. FIG. 5 shows that what was written by amplitude modulation can be erased by pulse width modulation.

In Figure 5, the axis represents voltage, and the vertical axis represents brightness.The solid line in the figure represents a pulse width of 250 msec, the dotted line represents a pulse width of 50 msec, and the dashed line represents a pulse width of 20 msec.
The case of c is shown. As can be seen from this figure, when the pulse width is 20 seconds, the write brightness at point A is 170 seconds, and the erase voltage is 170 seconds.
Even if the voltage exceeds V, the brightness becomes low and the erased state is reached. But 50
With a pulse of 1 sec, 250 rsec, the erase brightness is reached at around 200 V, and again at 230 V, light is emitted near the write brightness, and the erased state cannot be achieved. Contrary to the above embodiment, writing is performed using a write pulse with a larger pulse width.
We have also confirmed through experiments that erasing can be done by using an erasing pulse with a reduced amplitude.As mentioned above, the present invention requires only two power supplies, reduces the number of circuits such as transistors, and has two types of pulse width adjustment. It becomes easy because it is only

[Brief explanation of drawings]

FIG. 1a is a partially cutaway perspective view of a three-layer thin film EL device, FIG. 1b is a side view of the device, and FIG. 2a is an applied voltage waveform diagram.
Fig. 2b is a voltage vs. luminance luminance characteristic, Fig. 2c is a diagram showing luminance luminance changes, Fig. 3a is a luminance luminance characteristic diagram with respect to pulse width changes, Fig. 3b is a pulse waveform diagram of pulse width modulation, Fourth
The figure shows a waveform diagram of an applied pulse wave according to an embodiment of the present invention, and FIG. 5 shows an erasure characteristic diagram according to the above embodiment of the present invention, where the horizontal axis represents voltage and the vertical axis represents luminance. Ps is a sustain pulse, Pw is a write pulse, and Pe is an erase pulse. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. In a method for driving a thin film EL element having a thin film EL layer sandwiched between electrodes by a thin film dielectric layer and exhibiting a hysteresis phenomenon, the amplitude in the loop of the hysteresis characteristic in a range of a predetermined voltage value or higher is provided. If the difference between the minimum luminance when the amplitude increases and the maximum luminance when the amplitude decreases is sufficiently large and is within the predetermined pulse width, the maximum luminance when the pulse width decreases in the loop with the above hysteresis characteristic is the minimum when the pulse width increases. A sustain pulse train is selected by selecting an amplitude and a pulse width that have a sufficiently large difference from the emission brightness. Writing is performed using a write pulse with the same pulse width and a larger amplitude than the above sustain pulse, and erasing is performed using the above sustain pulse with the same amplitude. A method for driving a thin film EL element characterized by using an erase pulse with a shorter pulse width. 2. In a method for driving a thin film EL element having a thin film EL layer sandwiched between electrodes by a thin film dielectric layer and exhibiting a hysteresis phenomenon, the minimum light emission when the pulse width increases in the loop of the hysteresis characteristic in a range of a predetermined pulse width or more. The difference between the maximum luminance when the amplitude decreases and the minimum luminance when the amplitude increases in the loop with the hysteresis characteristic is within the range where the difference between the luminance and the maximum luminance when the pulse width decreases is sufficiently large and the amplitude is less than the predetermined voltage value. A sustain pulse train is selected with a pulse width and amplitude that have a sufficiently large difference.Writing is performed using a write pulse with the same voltage value and a pulse width longer than the above sustain pulse, and erasing is performed using a write pulse with the same voltage value and a pulse width longer than the above sustain pulse. A method for driving a thin film EL element, characterized in that the method is performed using a small erasing pulse.