Thin film electroluminescent element

Details Thin film electroluminescent element Thin film electroluminescent element

1984-01-26

1985-10-15

Lesmes, George F.

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

428690, 428917, H05B 3312

Patent

active

045477035

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a thin film luminescent element which produces luminescence under application of an electric field.


BACKGROUND OF THE INVENTION

In a thin film EL (electroluminescent) element which produces luminescence in response to the application of an electric field, increased brightness is attempted to be attained by sandwiching a phosphor thin film, onto which one or both surfaces thereof is deposited a dielectric thin film, between two electrode layers. The element for which the dielectric thin film is deposited on one surface of the phosphor thin film is characterized by a simplified structure and a low driving voltage. The element for which both surfaces of the phosphor thin film layer have dielectric thin films deposited thereon, respectively, is advantageous in that it is less easy for dielectric breakdown to occur and that brightness is significantly increased. It is known to use ZnS, ZnSe, ZnF.sub.2 or the like added with an activator for the phosphor material. In particular, in the case of an element employing phosphor which is composed of ZnS as the host material and contains Mn as the activator for light emission, brightness in the range of 3500 to 5000 cd/m.sup.2 at maximum is attained. As the typical dielectric material, Y.sub.2 O.sub.3, SiO, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5 and the like may be used. The layer of ZnS has a thickness in the range of 500 to 700 nm and a dielectric constant of about 9. The thickness of the dielectric film is in the range of 400 to 800 nm and its dielectric constant is in the range of 4 to 25.
When the element is driven by using an AC voltage, the voltage applied across the element is divided between the layer of ZnS and the dielectric thin film, wherein about 40% to 60% of the voltage applied across the electrodes is found in the layer of ZnS. The voltage required for producing brightness thus appears to be higher. In the case of the element having both surfaces provided with dielectric thin films, brightness is produced by applying a voltage of 200 V or greater at a frequency on the order of KHz at the present state of art. Such a high voltage imposes a great load on the driving circuit, requiring a special, expensive, integrated circuit (IC) capable of withstanding the high voltage.
In this connection, it is proposed to use as the dielectric thin film a thin film which contains TbTiO.sub.3, Pb(Ti.sub.1-x Zr.sub.x)O.sub.3 or the like as its main component and exhibits a high dielectric constant, for lowering the driving voltage. Although this type thin film has a dielectric constant (hereinafter represented by .epsilon..sub..gamma.) as high as 100 or more, electric field intensity at which the dielectric breakdown occurs (hereinafter represented by E.sub.b) is as low as 0.5 MV/cm, which means that the film thickness be significantly increased when compared with that of the heretofore used dielectric material. In the case of an element designed for high brightness, it is required that the thickness of the ZnS-layer be on the order of 0.6 .mu.m. Further, from the stand point of reliability of the element, the aforementioned dielectric thin film has to be formed in thickness not smaller than 1.5 .mu.m. When temperature of the substrate is high, increase in film thickness results in the growth of particles within the film. As the consequence, a film becomes turid and white, decreasing light transmission. In an EL element in which such white-turbid film is employed and which is arranged in an X-Y matrix configuration, even a non-selected pixel will scatter light emitted by other pixels, causing the troublesome problem of cross-talk.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides sectional and top views illustrating a self-healing type dielectric breakdown in a dielectric layer, and FIG. 2 provides sectitonal and top views illustrating a dielectric breakdown in a dielectric layer which is not of the self-healing nature.
FIG. 3 is a sectional view of a thin film electroluminescent element shown for the purpose of comparison w

REFERENCES:
patent: 3054919 (1962-09-01), Lehmann
patent: 3129108 (1964-04-01), Katona
patent: 4132919 (1979-01-01), Maple
patent: 4225653 (1980-09-01), Brixner
patent: 4387141 (1983-06-01), Patten
patent: 4416933 (1983-11-01), Antson et al.

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