US2991384A - Electroluminescent device - Google Patents

Description

July 4, 1961 P. GOLDBERG ELECTROLUMINESCENT DEVICE Filed July 16, 1957 1N VEN TOR.

PAUL 60L DBERG A TTORNE Y My invention is directed toward electroluminescent devices.

In the presence of an electric field, certain types of phosphors will luminesce, the intensity of the emitted light being a function of the electric field intensity. Consequently, films or layers containing such phosphors can be used to transform electrical energy to light energy. Phosphors of this type are said to be electroluminescent.

More particularly, one type of electroluminescent layer is formed from a suspension of electroluminescent powders in. dielectricmedia, as described, for example, in the copending patent application Serial No. 306,909, filed August 28, 1952, by Norman L. Harvey.

Conventionally an electroluminescent layer of this type is interposed and electrically connected between first and second electrically conductive films at least one of which is transparent, thus forming an electroluminescent device. A voltage is applied between the two films, and the device luminesces in accordance with the amplitude of an applied alternating voltage. Such devices have been used, for example, as lamps, information storage and display devices and the like.

In the present state of the art, the luminescence of any electroluminescent device utilizing an electroluminescent layer of fixed geometry and composition can only be varied by changing the ampliture or the frequency of the applied voltage. In contradistinction, I have discovered that the luminescence of such a device can be varied and indeed can be increased substantially over that hitherto obtainable without changing the frequency or amplitude of the applied voltage and further without changing the composition of the electroluminescent layer.

Accordingly, it is an object of the present invention to increase the luminescence of an electroluminescent device without changing the frequency or amplitude of the voltage applied thereto.

Another object is to provide a new and improved electroluminescent device of the character indicated.

Still another object is to increase the luminescence of an electroluminescent device for a given applied voltage by optimizing the size of the electroluminescent particles contained in the electroluminescent layer.

These and other objects of the invention will either be explained or will become apparent hereinafter.

The particles of electroluminescent phosphors conventionally used in electroluminescent devices vary in size over a range of about 2 microns to 40 microns. Surprisingly, I have discovered that when a voltage gradient having a predetermined value is established across an electroluminescent layer having a fixed composition and geometry, i.e. a fixed volume of phosphor in a predetermined volume of dielectric, the light emission from the layer is dependent upon the mean particle size of the electroluminescent layer. Further, I have discovered that this emission can be substantially increased over that hitherto obtainable by matching or optimizing the mean particle size to correspond to the predetermined value of the voltage gradient. Stated differently, the light emission from an electroluminescent device having an electroluminescent layer of given geometry and composition, for any given value of the voltage gradient established States Patent Patented July 4, 1961 therein, will vary as the mean size of the electroluminescent phosphor particles varies, and will attain maximum emission for one mean particle size corresponding to the given value of the voltage gradient. As the voltage gradi-.

ent increases, the mean size required for maximum emission decreases.

While it is not my intention to be bound by theory, I believe that this relationship between particle size and voltage gradient can be explained as follows. For a given concentration of phosphor in suspending dielectric and a given electroluminescent layer geometry, as the mean particle size increases, a larger voltage drop appears across each particle. Since the brightness of any electroluminescent phosphor particle increases as the voltage drop thereacross increases, each particle emits more light as the particle size increases. However, since the phosphor concentration is constant, as the particle size increases, the total number of particles decreases. These effects tend to oppose each other. Hence, the relationship between light emission and particle size for a given electroluminescent layer geometry and composition is a function both of the particle size and the voltage gradient, and maximum light emission is obtained when the particle size is matched with the voltage gradient in the manner indicated above.

An illustrative embodiment of my invention will be explained with reference to the accompanying figure."

EXAMPLE I The mean particle size of a green electroluminescent phosphor of the type described in an article entitled Electroluminescent Zinc Sulfide Phosphors, published in the Journal of the Electrochemical Society, vol. 100, pp. 566-57l (1953), (i.e. a zinc sulfide phosphor activated with copper, chloride and trace amounts of lead) was measured and found to be about 10 microns. A suspension of this phosphor in castor oil (phosphor concentration by volume of 25%) was prepared and placed in a cell having a gap width of 5 mils. An alternating voltage of fixed frequency (6000 c.p.s.) was applied across the cell causing the suspension to lurninesce. The light emitted from a fixed area of the phosphor layer passed through a transparent side of the cell and was measured photoelectrically. The amplitude of the alternating voltage was varied so as to respectively establish various voltage gradients across the layer, and the light emitted was measured for each gradient in turn.

This phosphor was fractionated by settling through various liquids to produce different samples having various mean particle sizes. Corresponding suspensions of these samples having the same phosphor concentration as the unfractionated phosphor were tested in a cell under the same conditions as above; i.e. the same gradients were established and the emitted light was measured in the same manner.

Illustrative results are tabulated in Table I below wherein the brightness gain is the ratio of the brightness of a cell utilizing a specified phosphor fraction to the brightness of a cell utilizing the unfractionated phosphor,

It will be apparent from a study of Table I that for each specified voltage gradient there is one mean particle size that is optimum and results in maximum brightness, and further that the optimum size decreases as the voltage gradient increases.

EXAMPLE II The procedure of Example I was repeated using a blue electroluminescent phosphor having a mean particle size of 15 microns, different gradients and different particle sizes being used. Illustrative results are tabulated in Table 11 below.

Table II Mean Optimum Particle Size Field Strength, Volts/ cm. Brightness Gain It will be seen that again for each specified voltage gradient there is an optimum particle Size and that the optimum size decreases as the voltage gradient increases.

Further tests indicated that the above indicated relationships between voltage gradient and particle size hold for all types of electroluminescent phosphor and were not dependent upon the frequency of the applied voltage.

While I have pointed out and shown my invention as applied above, it will be apparent to those skilled in the art, that many modifications can be made within the scope and sphere of my invention as defined in the claim which follows.

What is claimed is:

An electroluminescent device comprising an electroluminescent layer containing electroluminescent phosphor particles dispersed in dielectric media, opposite sides of said layer being coated with corresponding electrically conductive films, at least one of said films being light transparent, and an alternating voltage source coupled between said films to apply a voltage between said films to establish a voltage gradient of predetermined value in said layer, the mean particle size of said particles being matched with said value, said mean size falling within the approximate range 240 microns, the value of said gradient falling within the approximate range 12,000-200,000 volts per centimeter, the mean sizes of said matched particles decreasing in an approximately linear manner as the value of said gradient increases.

References Cited inthe file of this patent UNITED STATES PATENTS 2,566,349 Mager Sept. 4, 1951 2,660,566 Froelich Nov. 24, 1953 2,728,870 Gungle Dec. 27, 1955 2,819,420 Koller Jan. 7, 1958 2,824,992 Bouchard Feb. 25, 1958

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