U.S. patent number 3,854,070 [Application Number 05/319,046] was granted by the patent office on 1974-12-10 for electroluminescent device with variable emission.
Invention is credited to Zhanetta Alexandrovna Pukhly, Natalya Andreevna Vlasenko, Stepan Andreevich Zynio.
United States Patent |
3,854,070 |
Vlasenko , et al. |
December 10, 1974 |
ELECTROLUMINESCENT DEVICE WITH VARIABLE EMISSION
Abstract
An electroluminescent device with variable emission colors,
comprises at least five layers applied onto a substrate in the
following order: a first electrode; a protective layer made from a
material transparent for emission, the protective layer having an
index of refraction equal or close to that of the
electroluminescent substance and being resistant to chemical
reaction with the substances of the adjoining layers; a layer of
the electroluminescent substance; an insulating layer and a second
electrode; one of the electrodes being partially transparent for
luminescent emission; both electrodes having a high reflection
coefficient and making up a Fabry-Perot cavity tunes in resonance
to a prescribed emission wavelength; the thickness of the
protective layer, the coefficient of diffusion of its material into
the layer of the electroluminescent substance, and the coefficient
of diffusion of the material of the first electrode into the
protective layer being selected so that during thermal annealing of
the electroluminescent layer there is no diffusion into the latter
of either the substance of the first electrode or the substance of
the protective layer.
Inventors: |
Vlasenko; Natalya Andreevna
(Kiev, SU), Zynio; Stepan Andreevich (Kiev,
SU), Pukhly; Zhanetta Alexandrovna (Kiev,
SU) |
Family
ID: |
23240647 |
Appl.
No.: |
05/319,046 |
Filed: |
December 27, 1972 |
Current U.S.
Class: |
313/503; 313/509;
313/112 |
Current CPC
Class: |
H05B
33/22 (20130101); H05B 33/26 (20130101) |
Current International
Class: |
H05B
33/26 (20060101); H05B 33/22 (20060101); H05b
033/22 (); H05b 033/26 () |
Field of
Search: |
;313/18A,18D,112
;331/94.5H |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Electroluminescence and Semiconductor Lasers," by Henry F. Ivey,
IEEE Journal of Quantum Electronics, Vol. QE-2, No. 11, November
1966, pages 713-726..
|
Primary Examiner: Demeo; Palmer C.
Attorney, Agent or Firm: Holman & Stern
Claims
What we claim is:
1. An electroluminescent device having variable emission colors,
said device comprising:
a substrate; and
at least five layers comprising, and being disposed onto said
substrate in the following order:
a. a first electrode;
b. a protective layer being formed from a material which is
transparent to luminescent emission;
c. a layer of electroluminescent substance, said protective layer
having an index of refraction which is substantially equal to that
of said electroluminescent substance and being resistant to
chemical reaction with the substances of the adjoining layers;
d. an insulating layer; and
e. a second electrode, one of said electrodes being partially
transparent for luminescent emission, said first and second
electrodes having a high reflection coefficient and forming a
Fabry-Perot cavity tuned in resonance to a prescribed emission
wavelength; the thickness of said protective layer, the coefficient
of diffusion of the material of said protective layer into said
layer of said electroluminescent substance and the coefficient of
diffusion of the material of said first electrode into said
protective layer being selected so that at the temperature of
thermal annealing of said layer of said electroluminescent
substances there is no diffusion into said layer of said
electroluminescent substance from either the material of said first
electrode or the material of said protective layer.
2. An electroluminescent device as claimed in claim 1, wherein said
layer of said electroluminescent substance comprises a film of
ZnS:Mn applied in a vacuum, said first electrode comprises a film
of gold, said second electrode comprises a film of aluminum, said
protective layer comprises a film of undoped ZnS subjected to
crystallization, and said insulating layer comprises a film of SiO.
Description
BACKGROUND OF THE INVENTION
The invention relates generally to electroluminescent devices and
more particularly, it relates to electroluminescent devices having
variable emission colors and used, for instance, as light sources,
electroluminescent condensers, etc.
Known in the art is an electroluminescent device with variable
emission colors wherein a layer of sublimed electroluminescent
substance is disposed between two electrodes, one of which is
partially transparent and the other is opaque for the
electroluminescent emission. The electrodes are made of a highly
reflective material and form a Fabry-Perot cavity while the
thickness of the electroluminescent layer is multiple to the
wavelength of emission with a given color, with the phase jump at
the metal-dielectric junction being taken into account.
A disadvantage of the known device is that in the course of thermal
annealing of the electroluminescent layer during the manufacture
the substance of the partially transparent electrode diffuses into
the luminophor layer applied directly onto this electrode.
The result of the diffusion is two-fold: first, it reduces the
reflection index of the electrode and hence, spoils the Q-factor of
the cavity; second, it lowers the efficiency of the output
emission.
SUMMARY OF THE INVENTION
The object of the present invention is to modify the design of an
electroluminescent device with variable emission color so as to
prevent the substance of the electrode from diffusing into the
electroluminescent layer in the course of manufacture. The object
is achieved by providing at least five layers applied onto a
substrate in the following order: a first electrode; a protective
layer made from a material transparent for emission, the protective
layer having an index of refraction equal or close to that of the
electroluminescent substance and being resistant to chemical
reaction with the substances of the adjoining layers; a layer of
the electroluminescent substance; an insulating layer and a second
electrode; one of the electrodes being partially transparent for
luminescent emission; both electrodes having a high reflection
coefficient and making up a Fabry-Perot cavity tuned in resonance
to a prescribed emission wavelength; the thickness of the
protective layer, the coefficient of diffusion of its material into
the layer of the electroluminescent substance, and the coefficient
of diffusion of the material of the first electrode into the
protective layer being selected so that during thermal annealing of
the electroluminescent layer there is no diffusion into the latter
of either the substance of the first electrode or the substance of
the protective layer.
It is also possible to provide an electroluminescent device,
wherein the electroluminescent layer is a film of ZnS: Mn applied
in vacuum, the partially transparent first electrode is a film of
gold, the non-transparent second electrode is a film of aluminum,
the protective layer is a film of unalloyed ZnS subjected to
crystallization, and the insulating layer is a film of SiO.
The electroluminescent device designed according to the present
invention is simple in production, and the thermal annealing of the
electroluminescent layer involves no diffusion of the electrode
substance into the luminophor layer which makes it possible to
obtain high quality devices that exhibit distinctly variable
emission colors having their absolute brightness sufficiently high
for practical applications.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described by way of example with
reference to the accompanying drawing which shows a cross section
of an electroluminescent device with variable emission colors
according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
The device shown in the drawing comprises a dielectric substrate 1
(glass, quartz, etc.).
In case the emission is brought out through a substrate 1, the
latter should be transparent for the emission. Applied onto the
substrate 1 by any known procedure (e.g., by vacuum evaporation,
cathode spraying or chemical deposition) is an electrode 2 having a
high reflective index (R.gtoreq.65 percent) made of, say, Au, Ag,
Mn. Applied directly onto an electrode 2 is a protective layer 3
made of a substance which is optically transparent for emission and
characterized by a refraction index whose value approaches that of
the luminophor. The diffusion properties and the thickness of the
layer 3 are selected so that no diffusion occurs into the
luminophor layer of either the substance of the electrode 2 or the
material of the protective layer 3 in the course of thermal
treatment. The substance of the layer 3 is resistant to chemical
reactions with the materials of the adjacent layers; the
coefficient of diffusion of the electrode substance into the layer
3 and the coefficient of diffusion of the layer's components into
the electroluminescent layer are low.
Preferably the layer 3 is made as a film of insulating oxides, such
as Ta.sub.2 O.sub.5, SiO.
It is more preferable that the protective layer 3 be made of a
substance used as the basis of the luminophor but without doping
impurities, for instance ZnS. The indices of refraction of such
substances are equal to that of the luminophor, thus making them
optically homogeneous with respect to multi-beam interference.
To reduce the coefficient of diffusion, the substance of the
protective layer 3 is subjected to crystallization.
This layer is made as thin as possible so as to reduce the voltage
drop across it and to keep the electrical loss in the
electroluminescent device to the minimum.
Then, using any of the known procedures, the layer 3 is coated with
an electroluminescent layer 4 the optical thickness of which
together with that of the protective layer 3 satisfy the
interference relationship for the maximum emission at a given
wavelength and a given viewing angle with the phase jump at the
metal-dielectric junction being taken into account. Placed onto the
layer 4 is an insulator layer 5 which is 20-25 nm thick and which
protects the device from short-circuits and plays but a minor role
in the multi-beam interference process due to its negligible
thickness. Applied onto the layer 5 is a second electrode 6 having
a high index of reflection (R.gtoreq.65 percent). The electrodes 2
and 6 form a Fabry-Perot cavity, one of the electrodes being
partially transparent for emission.
The device operates as follows.
A voltage applied to the electrodes 2 and 6 produces an electric
field in the layer 4 with an intensity of 10.sup.5 to 10.sup.6 V/dm
which makes the layer luminescent. The color of emission is
determined by the thickness of the layer 4.
In order to facilitate the understanding of the essence of the
present invention, the following examples of its embodiments are
given together with a description of the technological procedure
employed in the manufacture of the devices given by way of
examples.
EXAMPLE 1
The device comprises the following elements arranged in series: a
glass substrate 1, an electrode 2 made as an Au-film 50 to 60 nm
thick, an undoped ZnS-layer 3 which is from 120 to 180 nm thick, an
electroluminescent layer 4 made of a ZnS:Mn-film the thickness of
which together with that of the protective layer 3 satisfy the
interference relationship, an insulation layer 5 made of SiO 20 to
35 nm thick and an opaque electrode 6 made of an Al-film.
The interference relationship is given by the expression
2d.mu.cos.beta. = [m + (.phi..sub.1 + .phi..sub.2 /2.pi.)]
.lambda.,
where:
d is the total thickness of two films (the electroluminescent and
the protective layers),
.mu. is the refraction index,
.beta. is the viewing angle of the output emission,
m is the interference order,
.lambda. is the emission wavelength,
.phi..sub.1, .phi..sub.2 are the phase jumps occuring when the
emission is reflected from the electrodes.
By varying the thickness of the ZnS:Mn electroluminescent films, it
is possible to obtain devices having a green glow (.lambda. = 550
nm), orange (.lambda. = 585 nm) and red (.lambda. = 640 nm).
EXAMPLE 2
The device comprises the following elements arranged in series: a
glass substrate 1, an electrode 2 made as an Au-film 50 to 60 nm
thick, an undoped ZnS layer 3 which is from 120 to 180 nm thick, an
electroluminescent layer 4 made of a ZnS:Er-film the thickness of
which satisfies the interference relationship, a SiO-insulation
layer 5 which is from 20 to 35 nm thick and an opaque electrode 6
made of an Al-film.
EXAMPLE 3
The device comprises the following elements arranged in series: a
glass substrate 1, an electrode 2 made of 14 Au-film 50 to 60 nm
thick, an undoped ZnSe protection layer 3 which is from 120 to 180
nm thick, an electroluminescent layer 4 made of a Zn Se:Mn-film the
thickness of which satisfies the interference relationship, an
insulation SiO layer 5 which is from 20-35 nm thick and an opaque
electrode 6 made of an Al-film.
The technological procedure involved in the manufacture of these
devices is as follows.
A layer of gold 50 to 60 nm thick is evaporated under a vacuum of
1.10.sup..sup.-5 to 2.10.sup..sup.-5 torr onto a glass substrate 1
which has been cleaned beforehand. Then a film of undoped ZnS 120
to 180 nm thick is applied onto the gold layer.
After that the samples are annealed in vacuum at 550.degree. to
600.degree.C for 5 to 15 minutes. This results in a crystallization
of the undoped ZnS-film. Then a two-step spraying procedure is used
to obtain a film of the electroluminescent material of the required
thickness.
The two-step procedure is as follows.
The undoped electroluminescent material is sprayed onto a cold
substrate by means of evaporation in a vacuum of 1.10.sup..sup.-5
to 2.10.sup..sup.-5 torr. Then, an activator, for example, Mn, is
introduced into the electroluminescent material by means of vacuum
evaporation, the activator material being evaporated in the form of
a chemically pure metal of a predetermined weight calculated on the
basis of the required activator concentration, for instance, 1 to 5
percent in the case of Mn. Then, another layer of undoped
electroluminescent substance (ZnS) is sprayed and the device is
subjected to a thermal annealing for example at 650.degree. to
700.degree.C during 5-15 minutes in the case of ZnS:Mn. Under these
conditions the electroluminescent film is crystallized and
simultaneously the activator diffuses into it, the diffusion being
uniform in thickness.
Applied onto the electroluminescent film by means of vacuum
evaporation are the SiO insulation layer and the opaque electrode
of an Al-film.
* * * * *