JPH0115876B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an electroluminescent (hereinafter abbreviated as EL) display device, particularly for light emission control of a multicolor EL display device that is effective for use in various monitor displays and keyboard displays in computer terminal devices. It is about the method.

When displaying information processed by a computer or the like, it is desired to display it using several colors instead of a single color. In order to realize such a multicolor display device using an EL display device, it is necessary to use EL display devices that emit light of different colors.
One possible method is to arrange a large number of EL elements made of metal in a plane, but this method has many problems, such as reduced resolution and complexity of wiring for each EL element. There are problems and it has not been put into practical use.

The present invention was made in view of the above, and aims to provide a light emission control method including a multicolor EL display device with a simple structure. Between a pair of electrodes at least one side of which is transparent or semitransparent, two or more types of light emitting light of different colors are
For a multilayer EL element having a structure in which an EL layer and a non-luminous high resistance layer are sequentially laminated, a driving voltage source is connected between one electrode and the other electrode of the element. An electrode for output voltage,
By controlling at least one of the pulse width, the number of pulses, and the amplitude, the emitted light color can be changed depending on the time-divisionally emitted color of each EL layer. Such a unique light-emitting multilayer film
According to the EL element structure, asymmetry appears in the emitted light color depending on the applied polarity of the driving voltage, so it is possible to easily achieve multicolor by utilizing this phenomenon.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view of a main part showing the basic structure of an embodiment of an EL display device according to the present invention, in which 1 is an insulating substrate made of, for example, transparent glass, and indium oxide is disposed on the insulating substrate 1. _{( In2O3} )
On a transparent electrode 2 such as A high-resistance layer 4 mainly made of zinc sulfide (ZnS) and zinc sulfide (ZnS:
Second EL layer 5 that emits green light and is made of TbF ₃ )
are sequentially laminated, and further the second EL layer 5
A back electrode 6 made of aluminum (Al), for example, is disposed thereon. A driving voltage source DS is connected between the transparent electrode 2 and the back electrode 6.

Here, the high-resistance layer 4 interposed between the first and second different-color emitting EL layers functions as a separation layer that prevents mutual interference of the emission centers between the two EL layers. In other words, when luminescent centers that require different excitation energies are adjacent to each other, luminescent centers with lower excitation energy are more easily excited due to the relationship of the mean free path of electrons, and luminescent centers that require higher excitation energy are less likely to be excited. Control of emitted light color becomes virtually impossible. Furthermore, in a thin film EL element in which two types of luminescent centers, Mn and TbF ₃ are added to the EL layer, it is not possible to obtain mixed color luminescence of orange and green, which are the respective luminescent colors, and instead, the device emits red luminescence due to MnF. However, by providing the high-resistance layer 4 according to the present invention, the above-mentioned problems can be effectively solved, the excitation efficiency can be improved, and a predetermined emission color can be obtained. Note that this high-resistance layer 4 must not be a completely insulating base to allow the drive current to pass through, but on the other hand, if the resistance is too small, electrical isolation between adjacent pixels will not be possible in a matrix address configuration. Since individual addressing becomes impossible, it is preferable to form the layer as a high resistance layer of about 10 ⁶ Ωcm because it has weak conductivity. Such a high-resistance layer can also be formed of a ceramic material such as cermet, but instead of a luminescent center,
Forming the EL base material using an EL base material such as ZnS doped with Ga or the like is advantageous from the viewpoint of the process of forming a multilayer film using a vapor deposition method.

In the above configuration, when a DC voltage is applied between the transparent electrode 2 and the back electrode 6, the emission spectrum exhibits asymmetry depending on the electrode of the applied voltage. The results of observing the asymmetry of the emission spectrum with respect to the polarity of the applied voltage are shown in the second section.
As shown in FIG. FIG. 2 shows the emission spectrum when the back electrode 6 shown in FIG. 1 is set at a negative potential with respect to the transparent electrode 2, and FIG. In both figures, the horizontal axis indicates the emission wavelength, and the vertical axis indicates the relative value of the amount of light. As is clear from these figures, when the back electrode 6 is set to a negative potential with respect to the transparent electrode 2, the second EL layer 5 emits light in color (that is, green) as shown in FIG.
becomes dominant, and when the transparent electrode 2 is made negative potential with respect to the back electrode 6, the first
The emission color of the EL layer 3 (that is, orange) is predominant. In other words, the side closer to the electrode at negative potential
The emission spectrum of the EL layer becomes dominant.

When voltage is applied to multilayer EL layers that emit light of different colors in this way, the emission spectrum exhibits asymmetry depending on the polarity of the applied voltage, and the color perceived by the human eye is determined by the spectrum from the light emitting source within a certain period of time. By using the fact that the time integral of is determined,
As mentioned above, each of the two types of EL layers 3 and 5 can emit light by controlling the polarity, application time ratio, or amplitude of the voltage applied between the electrodes 2 and 6 using an EL display device with a multilayer film configuration. Any intermediate color can be obtained.

Next, a light emission control method for obtaining multicolor light emission using the EL display device described above, in which the polarity and number of pulses of the applied voltage are controlled, will be described with reference to FIGS. 4a to 4e. 4A to 4E show driving voltage waveforms, the horizontal axis shows time and the vertical axis shows voltage, the upper side in each figure is on the transparent electrode 2 side shown in FIG. Similarly, voltage waveforms applied to the back electrode 6 side are shown.

First, in FIG. 4a, there is no pulse applied to the transparent electrode 2 side, and a positive pulse voltage is applied only to the back electrode 6, so that the transparent electrode 2 has a negative potential relative to the back electrode 6. Therefore, the emission color is close to the emission color of the first EL layer 3, that is, orange. In addition, in Figure b, there is no pulse applied to the back electrode 6 side, and a positive pulse voltage is applied only to the transparent electrode 2, and the polarity is opposite to that in Figure A, so the emitted light color is the second. The color of the light emitted from the EL layer 5 is close to green. Further, as shown in Figure c, the ratio of the number of pulses applied to the back electrode 6 and the number of pulses applied to the transparent electrode 2 is set to 1:3. In this case, the ratio of the number of times of orange light emission, which corresponds to the waveform shown in FIG. As a result, an intermediate color close to green can be obtained even if it is an intermediate color between green and orange. Similarly, in the waveform shown in figure d, the ratio of the number of pulses applied to the back electrode 6 and the transparent electrode 2 is 3:1.
As a result, a luminescent color close to orange, which is between green and orange, is obtained. In addition, in the waveform shown in figure e, the ratio of the number of pulses applied to the back electrode 6 and the number of pulses applied to the transparent electrode 2 is set to be 1:1, and as a result, green and orange A luminescent color close to the middle can be obtained.

Note that the cycle of the applied pulses should be 30 milliseconds (msec) or less to eliminate flickering on the display, but if a neutral color is produced by the ratio of the number of pulses as shown in Figure 4 c to e, In order to suppress color flickering, it is desirable that the period T of one cycle, which includes pulses sufficient to determine the color, be 30 msec or less.

Next, a method for controlling light emission of the EL display device shown in FIG. 1 will be described with reference to FIG. 5, in which the polarity and pulse width of the applied voltage are controlled. Figure 5a
-e are diagrams showing drive voltage waveforms, where the horizontal axis represents time and the vertical axis represents voltage. First, in the waveform shown in Figure a, a positive pulse with a pulse width τa6 is applied to the back electrode 6, and the transparent electrode 2 is kept at zero potential. As a result, the emission color of the first EL layer 3 (orange) becomes dominant. In addition, in the waveform shown in Figure b, the back electrode 6 is kept at zero potential and a positive pulse with a pulse width τb2 is applied to the transparent electrode 2. As a result, the emitted light color is the same as that of the second EL layer 5 (green). becomes dominant. Next, in Figure c, a narrow pulse with a pulse width τc6 is applied to the back electrode 6, and then
<τc2 As a result of applying a wide pulse with a pulse width τc2 to the transparent electrode 2, the EL layer 5 and the EL layer 3
A light emission color close to the light emission color (green) of the EL layer 5 can be obtained even with an intermediate color of Since a narrow pulse with a pulse width τd2 such that τd6>τd2 is applied to the transparent electrode 2, a luminescent color close to the luminescent color (orange) of the first EL layer 3 is obtained even though the intermediate color of the EL layers 5 and 3 is It turns out. Further, FIG. 5e shows a case where pulses τe2 and τe6 having the same pulse width are alternately applied to the transparent electrode 2 and the back electrode 6, and a luminescent color intermediate between the first and second EL layers 3 and 5 is obtained.

Furthermore, regarding the voltage/luminance characteristics of an EL element, it usually starts emitting light at a certain applied voltage, and the luminance increases as the applied voltage increases. Therefore, it is also possible to control the color by changing the amplitude of the applied voltage, and an example thereof is shown in FIG. 1, and a case where it is applied to an EL display device will be described below.

FIG. 6 is a diagram showing an example of drive voltage waveforms a to e, where the ratio of the number of pulses and the pulse width applied to the transparent electrode 2 and the back electrode 6 are the same. In Figure a, the peak value Va ₂ of the pulse applied to the transparent electrode 2 is set lower than the emission threshold of the second EL layer 5, and the back electrode 6 is set such that the first EL layer 3 emits light. A pulse having a voltage peak value Va ₆ is applied, and the emission color (orange) of the first EL layer 3 becomes dominant.

In addition, in Figure b, the peak value Vb ₆ of the pulse applied to the back electrode 6 is set lower than the emission threshold of the first EL layer 3, and the voltage applied to the transparent electrode 2 is such that the second EL layer 5 emits light. A pulse having a peak value Vd ₂ is applied, and the emission color (green) of the second EL layer 5 becomes dominant. In addition, in Figure c, the voltages Vc ₂ and Vc ₆ are set so that the luminance of the first EL layer 3 is lower than the luminance of the second EL layer 5, and as a result, the luminescent color is green even though it is an intermediate color between orange and green. You can get something close to. On the other hand, in d of the same figure, the voltages Vd ₂ and Vd ₆ are set so that the luminance of the first EL layer 3 is higher than the luminance of the second EL layer 5, and an intermediate color close to orange is obtained as the luminescent color. Furthermore, in Figure e, the peak values Ve ₂ and Ve ₆ of the pulses applied to the transparent electrode 2 and the back electrode 6, respectively, are assumed to be such that the luminances of the EL layers 3 and 5 are approximately the same. As a result, an intermediate emission color is obtained after the first and second EL layers.

Note that the driving method is not limited to the method of applying a driving voltage as a positive pulse voltage to each of the transparent electrode 2 and the back electrode 6 as shown in FIGS. 4 to 6; It is also possible to apply a combination of positive and negative pulse voltages to the electrodes on the other side using one side as a reference, or to control the polarity of the applied voltage by arbitrarily combining the number of pulses, pulse width, or pulse height value. Of course, this is also possible.

Next, another embodiment of the EL display device according to the present invention will be described with reference to FIG. FIG. 7 is a cross-sectional view of a main part schematically showing an EL display device having a structure in which a carrier injection electrode is provided between the multilayer EL layer and each electrode. 7 is an insulating substrate made of, for example, transparent glass, and on the insulating substrate 7, for example,
A transparent electrode 8 made of In ₂ O ₃ or the like is arranged, on which a first carrier injection electrode 9 made of, for example, yttrium oxide (Y ₂ O ₃ ) having weak conductivity is formed. On top of that, for example, there is a first EL layer 10 that emits orange light and is made of ZnS:Mn, a high resistance layer 11 that has weak conductivity and is made of an EL base material of ZnS to which no luminescent center is added, and ZnS:
Second EL layer 12 that emits green light and is made of TbF ₃
are sequentially laminated, and a back electrode 14 made of Al is further disposed thereon via a second carrier injection electrode 13. Here, the carrier injection electrodes 9 and 1
3 is a base material of the EL layers 10 and 12 such as Y ₂ O ₃ with weak conductivity and cadmium sulfide (Cds).
Made of ZnS and a semiconductor layer with a different bandgap width, the EL
It is provided to form a heterojunction with layers 10 and 12. Thus, by this carrier injection electrode, carriers (electrons) injected from the transparent electrode 8 or the back electrode 14 are accelerated and efficiently injected into the EL layer, thereby improving luminous efficiency.

Even in such an EL display device, it is possible to change the emitted light color by controlling the polarity of the driving voltage, the number of pulses, the pulse width, and the pulse height value in the same way as in the EL display device shown in Figure 1. . In addition, in a display device having such a structure, in order to obtain sufficient light emission, the inter-electrode capacitance C ₁ to _{C 3} and the DC resistance component between the electrodes R ₁ to _{R 3} in the equivalent circuit of FIG. It goes without saying that the period of the drive pulse should be longer than the determined time constant.

Further, the semiconductor carrier injection electrodes 9 and 13 having a heterojunction configuration in the embodiment shown in FIG.
It may be replaced with a dielectric layer such as Si ₃ N ₄ or Y ₂ O ₃ , and in this case it will operate as a complete AC drive type device, but the method of controlling the emitted color is the fourth method.
This is the same as in the other embodiments shown in FIGS.

FIG. 9 is a chromaticity diagram plotting each emission color when the EL display device of each of the embodiments as described above is driven with the waveforms a to e in FIGS. 4 to 6. Each point a' to e' is the point a to e shown in Figures 4 to 6.
This corresponds to the emitted light color in the drive waveform of e.

By controlling the waveform of the voltage applied between the transparent electrodes 2 and 8 and the back electrodes 6 and 14 in this way, the first EL layers 3 and 10 and the second EL layer
Emission colors O (orange) and G (green) of layers 5 and 13
Any intermediate color on the line segment connecting the two points can be obtained.

The above is an explanation of a configuration example in which EL layers of two colors are laminated, but it is also possible to widen the range of change in emitted color by laminating EL layers of multiple colors through high-resistance layers. .

As is clear from the above description, in short, the present invention constructs a multilayer EL device by sequentially laminating two or more types of EL layers that emit light of different colors and a non-luminous high resistance layer. It is characterized by the fact that each EL layer is driven in a time-divisional manner using polarity dependence to emit multicolor light. There is an advantage that an EL display device can be obtained at low cost.

[Brief explanation of drawings]

1 and 7 are sectional views schematically showing essential parts of an embodiment of an EL display device applied to the present invention, and FIGS. 2 and 3 are sectional views of the EL display device shown in FIGS. 1 and 7. A diagram showing the emission spectra measured by changing the polarity of the DC applied voltage in the display device, Figures 4 to 6 are drive voltage waveform diagrams, and Figure 8 is the electrical equivalent circuit of the EL display device shown in Figure 7. Figure, No. 9
The figure is a diagram showing the emission colors of the EL display devices shown in FIGS. 1 and 7 plotted on a chromaticity diagram. 1, 7: insulating substrate, 2, 8: transparent electrode, 3,
5, 10, 12: EL layers with different emission colors, 4, 1
1: Non-luminescent high resistance layer, 6, 14: Back electrode,
9, 13: Carrier injection electrode, DS is driving voltage source.

Claims (1)

[Claims] 1. Between a pair of electrodes, at least one side of which is transparent or translucent, consisting of one electrode 2 and the other electrode 6 disposed on an insulating substrate 1, the electrodes emit light of different colors. For a multilayer EL element having a structure in which two or more types of EL layers 3 and 5 and a non-luminous high resistance layer 4 are sequentially laminated, a driving voltage source DS is applied between one electrode and the other electrode of the element. By connecting the drive voltage source and controlling the polarity and at least one of pulse width, number of pulses, and amplitude of the voltage output from this drive voltage source, light is emitted depending on the time-divisionally emitted color of each EL layer. A light emission control method for an EL display device, characterized in that the color is changed. 2 Between the multilayer EL layers 10, 12 and at least one side of the electrodes 8, 14, carrier injection electrodes 9, 13 formed by heterojunction of semiconductor layers having different forbidden band widths from the base material layer of the EL are provided. 2. The light emission control method for an EL display device according to claim 1, further comprising: providing a light emission control method for an EL display device. 3. The light emission control method for an EL display device according to claim 1, wherein dielectric layers 9 and 13 are provided between the multilayer EL layers 10 and 12 and at least one of the electrodes 8 and 14. 4. The light emission control method for an EL display device according to claim 1, wherein the non-luminescent high resistance layer 4 is mainly composed of an EL base material to which no luminescent center is added.