THIN FILM ELECTROLUMINESCENT DEVICE

Abstract: A thin film DC operable electroluminescent device of increased life and improved properties is disclosed. The device employs a thin film (5) of a manganese activated zinc sulfoselenide luminescent material and a layer of yttrium oxide (7) positioned between the luminescent material (5) and a reflective electrode (11).

Description: Thin film electroluminescent device. The invention in this case relates to a novel direct current (DC) operated thin film electroluminescent device. Thin film electroluminescent devices find an increased number of applications. Among commercial uses of these devices are flat panel video displays, information displays, advertising displays and similar uses. In general, these electroluminescent displays are AC operated although operation by DC requires a simpler structure with reduced operating voltage. However, there are serious problems in the DC operated electroluminescent devices which are presently available. A major problem is that the DC operated devices exhibit a severe tendency to catastrophic degradation due to electrical breakdown in the phosphor layer of the device. Another problem with these DC operated devices is that many of them exhibit a non-uniformity of light emission. An example of such a DC operated electroluminescent device is the device as shown for example in British patent 1,300,548. As shown in Figure 5 of the patent such a device may comprise a phosphor layer sandwiched between a transparent electrode which may be for example a film of tin oxide or indium oxide and a nontransparent electrode such as copper or aluminium graphite. As the phosphor of this Brithish patent may employ manganese activated zinc sulphide, zinc selenide or a mixture of zinc sulphide and zinc selenide. Upon the application of DC to the electrodes an electroluminescence is produced which luminescence is visible through the transparent electrode. By proper design of the electrodes it is possible to produce luminescence of any desired configuration. While the output of the electroluminescent device of this Brithish patent has been found to be initially satisfactory, it has been found to deteriorate rapidly and to have an unsatisfactorily short life. Electroluminescent devices of improved life are those shown United patent 4,342,945. In the device of this patent, dielectric layers, for example 180 nm thick layers of yttrium oxide, are positioned between the electrodes and the luminescent film. While the yttrium oxide serves to prevent degradation of the luminescent film, the device is not operable by application of DC as the thickness of the layers does not permit carriers to tunnel through it. Further, the luminescence produced frequently is not of a sufficiently high level. It is a general object of this invention to provide an improved DC operable thin electroluminescent device. According to the invention a DO operable thin film electroluminescent device comprises a thin manganese activated zic sulpho-selenide phosphor of the formula ZnS Se sMn wherein .2(x.8 sandwiched between two 7-x x electrodes, one of the electrodes being transparent to luminescence from the phosphor film. The other electrode is reflective to luminescence from the luminescent film. Further acceding to the invention, between the reflective electrode and the luminescent film a thin film of yttrium oxide of a thickness of about 5-15nm is positioned. Operation of the electroluminescent device of the invention requires in general the application of DC voltage of about 30 to 100 volts across the electrodes. Luminance is produced which in general has great magnitude and a significantly higher spatial uniformity than that produced in DC electroluminescent devices of the prior art. In addition, it has been found that the useful life of the electroluminescent device of the invention significantly exceeds that of the prior art devices. For a more complete understanding, the invention will now be described in greater detail with reference to the drawing, the sole figure of which is a cross sectional view of a preferred electroluminescent device of the invention and he following example. A 160nm thick layer of indium oxide was thermally evaporated onto a glass substrate 1 to form a transparent electrode 3. A 600nm thick phosphor layer 5 of ZnS Se Mn was then deposited by thermal deposition 0.5 0.5 on the free surface of the indium oxide layer 3. A 7.5nm thick layer of yttrium oxide 7 was then deposited by electron beam evaporation on the free surface of the phosphor layer 5. In a similar manner, a further deposition of the yttrium oxide was then carried out on selected portions of the free surface of the 7.5nm thick yttrium oxide layer 7 to form 75nm thick electrical contact area 9. A 50Qnm thick reflective metal electrode layer 11 of aluminium was then deposited by thermal deposition on the free surfaces of the 7.5nm thick yttrium oxide layer 7 and the contact area 9. A DC voltage of 50-60 volts was then applied between the transparent electrode 3 and the metal electrode layer 11 with the metal electrode layer 11 functioning as the anode A. Driving of the device was carried out by use of unipolar pulses or continuous direct current. A yelloworange luminescence with a maximum at 583 nm was emitted from the device. Under these conditions the peak luminescen- ce was 1200fL (4110 nit) and the average luminescence was 600fL (2055 nit). The continuous, DC driven operating lifetime of the device was found to be in excess of 50 hours. While we have described the invention with reference to a particular example and drawing other modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

Claims: 1. A thin film (direct current) (DO) operable electroluminescent device characterized in that it comprises a thin luminescent film of manganese activated zinc sulfoselenide of the formula ZnS1 xSex:Mn wherein .21xfl.8 sandwiched between two electrodes, one of said electrodes being transparent to luminescence from said luminescent layer, and a layer of yttrium oxide having a thickness of 5-15 nm positioned between said luminscent film and other of said electrodes which other electrode is reflective to luminescence from said luminescent film. 2. The electroluminescent device of Claim 1,characterized in that the layer of yttrium oxide is disposed on the surface of said thin luminescent film opposing said electrode reflective to luminescence from said luminescent film. 3. The electroluminescent device of Claim 1 or 2, characterized in that X=0.5. 4. The electroluminescent device of Claim 1, 2 or 3, characterized in that the reflective electrode is an aluminium electrode. 5. The electroluminescent element of Claim 1,2, 3 or 4, characterized in that the transparent electrode is an indium sesquioxide electrode.

Inventors: BHARGAVA, RAMESHWAR NATH, CAMMACK, DAVID ALAN, DALBY, RICHARD JOHN, WALZ, DAVID GREGORY
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1985
Main classification: H05B33
IPC codes: H05B33, C09K11