GB2100512A - Electroluminescence element - Google Patents

Abstract

An organic dispersion type electroluminescence element using a mixture of a field-luminescent powder (2) e.g. ZnS-Al and an organic binder (1) as luminous layer, said organic binder (1) being composed of a mixture of a high polymeric organic material e.g. cyanoethyl cellulose and a fine powder of a ferroelectric substance (1a) e.g. BaTiO3 or PZT. The luminous layer is capable of high luminescence at a low driving voltage. <IMAGE>

Description

SPECIFICATION Electroluminescence element This invention relates to an electroluminescence element having a luminous layer capable of producing high luminescence at a low driving voltage.

The hitherto known electroluminescence elements may be classified structurally into the following three types: the enameled type, the thin-film type and the organic dispersion type. The enameled type element is prepared by melting a field-luminescent powder into an inorganic glass material which is also melted, applying the melt on the surface of a plate such as a metal plate to form a coating thereon and then sintering and solidifying the coating. The thin-film type element is prepared by first making a field-luminescent material itself into a thin film by a known vacuum evaporation technique and then sandwiching said thin film between the insulating films.The organic dispersion type electroluminescence element is prepared by first mixing a finely divided field-luminescent powder with an organic binder such as epoxy resin, printing or coating a substrate (which also acts as an electrode) with said mixture and then baking same. Among these three types of electro-luminescence elements, the last-said organic dispersion type element is particularly noted for its potentialities for a wider scope of utilization, such as application to a lumineous layer having curved surfaces, because this type of element can be prepared as a flexible element in which case a flexible material is used as the substrate although a certain thickness must be reserved for the luminous layer itself, and thus growing interest is being shown recently in this type of electroluminescence element.

In the case of the enameled type element, a fairly large thickness is required for the luminant itself and the weight of the entire element including the substrate inevitably increases. Also, since the luminant is of course enameled, the finished element can not be fabricated into other desired shapes by an operation such as bending, so that the scope of use of this type of element is limited. In the case of the thin-film type element, although some significant results of studeis on this type of element have been published recently, it still involves serious problems such as rather low dielectric strength of the element itself and a very limited degree of freedom for the selection of the driving voltage waveform or driving frequency. Also, the reliability of the element itself is still unsatisfactory.On the other hand, the organic dispersion type electroluminescence element has many advantages over the said enameled type and thin-film type elements, but this type of element also involves various problems. For instance, it is impossible to reduce the thickness of the luminant layer itself beyond a certain limit because a luminous substance is mixed in a high polymeric organic binder and such mixture is applied on a substrate. Also, when using a high polymeric organic substance with good temperature stability, the specific dielectric constant of such material is limited to a value of several to ten odd at the most.Therefore, when it is desired to obtain a high luminance by applying a strong electric field to the luminant, the driving voltage applied to the element must be necessarily increased. (Usually, in the case of an organic dispersion type electroluminescence element, it is required to apply a driving voltage of about one hundred to one hundred and some tens of volts at lowest). Further, even when such high level of driving voltage is applied, the resultantly produced luminescence amounts only to several tens of ft-L at the outside.

According to the present invention there is provided an organic dispersion type electroluminescence element comprising a mixture of a field-luminescent powder and an organic binder as the luminous layer source, said organic binder being composed of a mixture of a high polymeric organic material and a finely powdered ferroelectric substance.

An electroluminescence element according to the present invention is capable of emitting luminescence of high luminance with a low driving voltage without compromising the advantages of the organic dispersion type electroluminescence element.

An embodiment of the invention will now be described, by way of an example, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of an electroluminescence element according to the present invention, Figure2 is a curve showing the relation between the volume ratio of PZTto cyanoethyl cellulose in which PZT has been dispersed and the specific dielectric constant of the obtained organic binder, Figure 3 is a curve showing the relation between the dielectric constant of the organic binder and the luminance, Figure 4 is a curve showing a comparison between dielectric constant of the organic binder using cyanoethyl cellulose alone and those of the organic binders containing BaTiOs or PZT in addition to said cyanoethyl cellulose, and Figure 5 is a characteristic curve showing the relation between the ratio of dielectric substance to cyanoethyl cellulose in which said dielectric substance has been dispersed and the dielectric constant of the obtained organic binder.

In Figure 1 of the drawings, reference numeral 1 indicates a high polymeric organic material la indicates the particles of a finely powdered ferroelectric substance, 2 indicates the field-luminescent powder particles, 3 indicates a transparent conductive film, 4 indicates a glass substrate, and 5 indicates a back electrode designed to double as a reflector.

The present invention features use of an organic binder 1 having a high specific dielectric constant that can never be obtained with the ordinary high polymeric organic materials when the used singly, said organic binder 1 being prepared by mixing fine powder of the ferroelectric substance la such as PZT (a sintered mixture of PbO, ZrO2 and TiO2) in an organic high polymeric material, and according to this invention a finely pulverized field-luminescent powder 2 is mixed with said organic binder 1 to form a luminous layer. It is thereby possible to obtain an electroluminescence element which, although being an organic dispersion type, has a capacity to luminescence with a luminance of above 100 ft-L by application of a very low effective driving voltage, thus, less than several tens of volts.

The present invention is described in further detail hereinbelow by way of an embodiment thereof.

Example Referring to Figure 1, there is shown a sectional view of an electroluminescence element according to this invention. This element is prepared from the following process. First, cyanoethyl cellulose (a ferroelectric high polymeric organic material) is dissolved in dimethylformamide to prepare about 25 wt% cellulose solution, and a PZTferroelectric powder is mixed therein. Said PZTferroelectric powder may be prepared in the following way.Powders of PbO, ZrO2 and TiO2 having an average particle size of about 0.5 - 1.0 micron are weighed and mixed so that the ratio by weight of PbO : ZrO2 : TO2 will be 2:1:1, and the mixture is kneaded with water and then fired at 1 ,0000C. The fired product is again pulverized to particle size of 1 urn by a pot mill. Figure 2 shows the change of dielectric constant of the high polymeric material film according to the mixing ratio (by volume) of PZTto cyanoethyl cellulose. When PZT is used as ferroelectric substance in the form of powder, and in case it is mixed with cyanoethyl cellulose in a ratio by volume of about 4: 6, the product shows the highest dielectric constant, which is almost six times as high as that of the products not containing said ferroelectric substance.Into this organic binder 1 is then mixed a ZnS-Al type field-luminiferous powder 2, and this mixture is coated on a glass substrate 4 covered with a transparent conductive film 3. After drying the coating at about 1 50 C, a silver electrode 5 is printed to let it serve as an opposite electrode which doubles as a reflector, and the whole mass is then fired to form an electroluminescence element.

Here, it the average dielectric constant given in case of mixing a powdered dielectric substance with a dielectric constant of Ep in a medium with a dielectric constant of E is expressed by Em, then such average dielectric constant Em can be given from the following formula when it is analyzed according to the theory of dielectrics:

(wherein, P = 4na3N/3, where a is particle size of the dielectric powder and N is the density thereof). Thus, it is understood that the average dielectric constant Em of the mixture changes successively from the dielectric constant E of the medium to the dielectric constant Ep of the dielectric powder in accordance with the amount of the dielectric powder mixed.

Accordingly, by mixing a ferroelectric powder such as sintered PZT powder as in the herein described embodiment of this invention, it is possible to obtain an organic binder having such a high dielectric constant as will never be obtainable with the ordinary high polymeric organic materials. On the other hand, the intensity of the electric field applied to the field-luminescent powder mixed in the dielectric increases almost proportionally to the value of the dielectric constant of the dielectric, or of said organic binder, when the external voltage applied to said element is constant. Also, the luminous intensity from the luminant increases in proportion to the intensity of the electric field applied to said luminous layer.Therefore, in the case of an electroluminescence element using an organic binder having a high effective dielectric constant such as provided in this invention, the driving voltage required to be applied for obtaining a certain given luminance can be reduced and it is possible to obtain luminescence of a higher luminance than obtainable with the conventional organic dispersion type elements under a given driving voltage.

In the foregoing embodiment of the invention, cyanoethyl cellulose was used as the high polymeric organic material forming the matrix of the organic binder, but the effect of this invention would not be impaired when using other types of high polymeric organic materials than said cellulose system.

Also, in the above-described embodiment, fine powder of sintered PZT was used as ferroelectric component, but other types of ferroelectric materials such as BaTiO3 may be also safely used to attain the described effect of this invention.

As described above it is possible according to this invention to obtain an organic dispersion type electroluminescence element having a high effective dielectric constant, that can never be obtained from use of the ordinary high polymeric organic substances, by mixing and dispersing fine powder of a ferroelectric substance in a specific organic binder. Accordingly, when the driving voltage applied to the element is constant, the intensity of the electric field applied to the field-luminescent powder in the organic binder is elevated to make it possible to obtain luminescence of a high luminance. Also, it is made possible by this invention to markedly lower the driving voltage to be applied externally for obtaining a required luminance.

Claims (6)

1. An organic dispersion type electroluminescence element comprising a mixture of a field-luminescent powder and an organic binder as the luminous layer source, said organic binder being composed of a mixture of a high polymeric organic material and a finely powdered ferroelectric substance.

2. An electroluminescence element as claimed in claim 1, in which said high polymeric organic material is cyanoethyl cellulose.

3. An electroluminescence element as claimed in claim 1 or claim 2, in which said finely powdered ferroelectric substance (1 a) is a fine powder of PZT (sintered mixture of PbO, ZrO2 and TiO2).

4. An electroluminescence element as claimed in claim 1, in which said finely powdered ferroelectric substance (1a) is BaTiO3.

5. An electroluminescence element as claimed in claim 3, in which the mixing ratio of PbO : ZrO2 : TiO2 in PZT used as said ferroelectric powder (1 a) is 2 :1:1 (by weight).

6. An electroluminescence element substantially as hereinbefore described with reference to the accompanying drawings.