U.S. patent application number 09/151346 was filed with the patent office on 2002-01-31 for organic electroluminescent apparatus.
Invention is credited to Lee, Hsing-Chung, Shi, Song Q., XU, JI-HAI.
Application Number | 20020011780 09/151346 |
Document ID | / |
Family ID | 22538349 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011780 |
Kind Code |
A1 |
XU, JI-HAI ; et al. |
January 31, 2002 |
ORGANIC ELECTROLUMINESCENT APPARATUS
Abstract
Organic electroluminescent apparatus including an organic
electroluminescent device for emitting blue-green light. A
microcavity structure receives the blue-green light and is tuned to
a resonance such that the blue-green light is enhanced to blue and
green light. A color converting medium receives and absorbs the
blue-green light and emits red light in response thereto.
Inventors: |
XU, JI-HAI; (GILBERT,
AZ) ; Lee, Hsing-Chung; (Calabasas, CA) ; Shi,
Song Q.; (Phoenix, AZ) |
Correspondence
Address: |
VINCENT B INGRASSIA
MOTOROLA INC
INTELLECTUAL PROPERTY DEPT
SUITE R3108 BOX 10219
SCOTTSDALE
AZ
852710219
|
Family ID: |
22538349 |
Appl. No.: |
09/151346 |
Filed: |
September 11, 1998 |
Current U.S.
Class: |
313/501 |
Current CPC
Class: |
H01L 51/5036 20130101;
H01L 27/322 20130101; H05B 33/20 20130101; C09K 11/06 20130101;
H01L 51/5265 20130101 |
Class at
Publication: |
313/501 |
International
Class: |
H01J 001/62; H01J
063/04 |
Claims
What is claimed is:
1. Organic electroluminescent apparatus comprising: an organic
electroluminescent device for emitting light having a broad
spectrum; a color converting medium for absorbing light coupled
thereto and emitting light in response to absorbed light, the color
converting medium having absorption peaks at a first wavelength and
a second wavelength and emitting light at a third wavelength
different than the first and second wavelengths; and a microcavity
structure coupling emitted light from the organic
electroluminescent device to the color converting medium, the
microcavity structure having a resonance such that light emitted
having a broad spectrum is enhanced by the microcavity structure to
light having a first resonant peak which substantially overlaps one
of the first and second absorption peaks and a second resonant peak
which substantially overlaps the other one of the first and second
absorption peaks.
2. Organic electroluminescent apparatus as claimed in claim 1
wherein the organic electroluminescent device is constructed to
emit light in a blue-green spectrum.
3. Organic electroluminescent apparatus as claimed in claim 2
wherein the color converting medium is constructed with the first
and second absorption peaks substantially in a blue-green
spectrum.
4. Organic electroluminescent apparatus as claimed in claim 3
wherein the color converting medium is constructed with the light
emitting wavelength substantially at a red spectrum.
5. Organic electroluminescent apparatus as claimed in claim 1
wherein the microcavity structure includes a resonance tuned near
the first and second absorption peaks of the color converting
medium.
6. Organic electroluminescent apparatus comprising: an organic
electroluminescent device for emitting blue-green light; a
microcavity structure positioned to receive the blue-green light
from the organic electroluminescent device, the microcavity
structure having a resonance such that the blue-green light is
enhanced by the microcavity structure to substantially blue and
green light; and a color converting medium coupled to receive the
substantially blue and green light from the microcavity structure
for absorbing the substantially blue and green light and emitting
substantially red light in response thereto.
7. A method of converting light having first and second wavelengths
to light having a third wavelength comprising the steps of:
providing an organic electroluminescent device for emitting light
having a broad spectrum and a color converting medium having a
first absorption peak at a first wavelength and a second absorption
peak at a second wavelength; enhancing the broad spectrum light
emitted from the organic electroluminescent device to light having
a first resonant peak which substantially overlaps the first
absorption peak and a second resonant peak which substantially
overlaps the second absorption peak; and applying the enhanced
light having the first and second resonant peaks to the color
converting medium, whereby the light at the first and second
resonant peaks is absorbed by the color converting medium and
emitted at a third wavelength different than the first and second
wavelengths.
8. A method as claimed in claim 7 wherein the step of enhancing the
broad spectrum light includes providing a microcavity structure
having a resonance such that the broad spectrum light is enhanced
by the microcavity structure to light having the first and second
resonant peaks which substantially overlap the first and second
absorption peaks.
9. A method as claimed in claim 8 wherein the step of providing the
organic electroluminescent device includes providing the device
constructed to emit light in a blue-green spectrum.
10. A method as claimed 9 wherein the step of providing the color
converting medium includes providing the color converting medium
constructed with the first absorption peak substantially in a blue
spectrum and the second absorption peak substantially in a green
spectrum.
11. A method as claimed in claim 10 wherein the step of providing
the color converting medium includes providing the color converting
medium constructed with the light emitting wavelength substantially
at a red spectrum.
12. A method as claimed in claim 8 wherein the step of providing
the microcavity structure includes providing the microcavity
structure with the resonance tuned near the first and second
absorption peaks of the color converting medium.
Description
FIELD OF THE INVENTION
[0001] The present invention pertains to organic electroluminescent
apparatus and more specifically to apparatus for enhancing
conversion of blue light to red light.
BACKGROUND OF THE INVENTION
[0002] Light emitting diode (LED) arrays are becoming more popular
as an image source in both direct view and virtual image displays.
One reason for this is the fact that LEDs are capable of generating
relatively high amounts of light (high luminance), which means that
displays incorporating LED arrays can be used in a greater variety
of ambient conditions. For example, reflective LCDs can only be
used in high ambient light conditions because they derive their
light from the ambient light, i.e. the ambient light is reflected
by the LCDs. Some transflective LCDs are designed to operate in a
transmissive mode and incorporate a backlighting arrangement for
use when ambient light is insufficient. In addition, transflective
displays have a certain visual aspect and some users prefer a
bright emissive display. However, these types of displays are
generally too large for practical use in very small devices such as
portable electronic devices.
[0003] Organic electroluminescent device (OED) arrays are emerging
as a potentially viable design choice for use in small products,
especially small portable electronic devices such as pagers,
cellular and portable telephones, two-way radios, data banks, etc.
OED arrays are capable of generating sufficient light for use in
displays under a variety of ambient light conditions (from little
or no ambient light to bright ambient light). Further, OEDs can be
fabricated relatively cheaply and in a variety of sizes from very
small (less than a tenth millimeter in diameter) to relatively
large (greater than an inch) so that OED arrays can be fabricated
in a variety of sizes. Also, OEDs have the added advantage that
their emissive operation provides a very wide viewing angle.
[0004] A problem in the use of OEDs in displays is the generation
of the colors necessary to achieve a full color display. Red, green
and blue OEDs can be fabricated but they require different organic
materials and, thus, each color must be fabricated separately.
Furthermore, the colors achieved are not a pure primary color, but
have a relatively broad spectrum. Emission of red light is very
difficult to achieve in OEDs however, it is known to convert other
colors, such as blue light, to red light. One such technique is
disclosed in Japanese Publication, Kokai Patent No. Hei 8-286033
entitled "Red Emitting Device Using Red Fluorescent Converting
Film", published 1 November 1996. While converting blue light to
red light, the efficiency of the conversion is unacceptably low,
and the red light contains unacceptable levels of blue green
components.
[0005] Accordingly, it is highly desirable to provide apparatus and
a method of converting broad spectrum light to red light.
[0006] It is a purpose of the present invention to provide a new
and improved apparatus and a method of generating red light.
[0007] It is a further purpose of the present invention to provide
apparatus and a method of efficiently generating red light.
SUMMARY OF THE INVENTION
[0008] The above problems and others are at least partially solved
and the above purposes and others are realized in organic
electroluminescent apparatus including an organic
electroluminescent device for emitting light having a broad
spectrum. A color converting medium absorbs light coupled thereto
and emits light in response to absorbed light. The color converting
medium has first and second absorption peaks at first and second
wavelengths and emits light at a third wavelength different than
the first and second wavelengths. A microcavity structure is used
to couple emitted light from the organic electroluminescent device
to the color converting medium. The microcavity structure has a
resonance such that the broad spectrum light received from the
organic electroluminescent device is enhanced by the microcavity
structure to enhanced light having first and second resonant peaks
which substantially overlap the first and second absorption peaks,
respectfully.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Referring to the drawings:
[0010] FIG. 1 is an enlarged and simplified sectional view of
organic electroluminescent apparatus in accordance with the present
invention;
[0011] FIG. 2 is a graphical spectrum representation illustrating a
broad spectrum light in relation to the absorption peaks of the
color converting medium;
[0012] FIG. 3 is a graphical spectrum representation illustrating
the enhancement of the broad spectrum light; and
[0013] FIG. 4 is a graphical spectrum representation illustrating
the enhanced broad spectrum light in relation to the absorption
peaks of the color converting medium.
[0014] DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] Turning now to the figures, FIG. 1 is an enlarged and
simplified sectional view of organic electroluminescent apparatus
10 in accordance with the present invention. Organic
electroluminescent apparatus 10 includes an organic
electroluminescent device (OED), generally designated 11 and a
microcavity 12 carried by a transparent substrate 13, such as
glass. Microcavity 12 is positioned in alignment with the light
output from OED 11 to enhance the light spectrum.
[0016] In this embodiment, OED 11 includes an upper metal
electrical contact 15, an electron transporting layer 16, a hole
transporting layer 17 and a lower electrical contact 18. Upper
electrical contact 15 essentially forms a reflective surface to
reflect all light generated within OED 11 downwardly. Lower
electrical contact 18 is formed of some electrically conductive
material which is transparent to light generated in OED 11, such as
indium-tin-oxide (ITO) or the like and communicate the OED light
output to the remainder of apparatus 10. Electron transporting
layer 16 and hole transporting layer 17 define an organic light
emitting zone with either or both layers 16 and 17 emitting light
in response to the recombination of holes and electrons therein. It
will of course be understood that OED 11 could include from one
organic layer to several, depending upon the material utilized.
[0017] Microcavity structure 12 is illustrated in FIG. 1 as
including OED 11 and a mirror stack 21. Mirror stack 21 includes a
plurality of layers of material having different indexes of
refraction. The plurality of layers is divided into pairs of
layers, one layer of each pair having a first index of refraction
and another layer of each pair having a second index of refraction
lower than the first index of refraction with each pair of layers
cooperating to form a partial mirror and to reflect light. The
plurality of layers can be formed from a variety of materials
including various semi-transparent metals and various dielectrics.
In a typical example, mirror stack 21 is preferably formed of, for
example, alternate layers of TiO.sub.2 and SiO.sub.2. Generally,
from 2 to 4 pairs of layers provides a reflectivity of
approximately 0.74, which is believed to be optimal for the present
purpose. As is understood by those skilled in the art, each pair of
layers of mirror stack 21 defines a partial mirror with an
operating thickness of an integer multiple of one half wavelength
of the emitted light so that all reflected light is in phase.
[0018] The combined thickness of organic layers 16 and 17 and lower
contact 18 is designed to position mirror stack 21 in spaced
relationship from reflective upper contact 15 and define an optical
length L1 of microcavity structure 12 in cooperation with the OED
11. Stack 21 further defines a light output for microcavity
structure 12, and the optical length L1 of microcavity structure 12
is generally designed such that light emitted from the light output
has resonant peaks as will be described presently.
[0019] A color converting medium (CCM) 23 is positioned on
substrate 13 to receive enhanced light from microcavity structure
12. In this embodiment, CCM 23 is positioned on a surface of
substrate 13 opposite microcavity structure 12. However, it will be
understood that CCM 23 can be positioned between microcavity
structure 12 and substrate 13. Recently it has been demonstrated
(see Japanese publication cited above) that efficient RGB light
emission can be achieved by combining an organic OED emitter with a
CCM device, such as CCM 23. CCM 23 is made up of organic
fluorescent media which change the color of emitted light from
blue-green to blue, green or red. To achieve an efficient CCM
device, a red fluorescent converting film is made by dispersing a
first fluorescent pigment including, for example, rhodamine, and a
second fluorescent pigment into a light transmitting medium. The
first pigment has an absorption range from 450-610 nm and emits red
light above 600 nm. The second pigment absorbs light in the blue
region under 520 nm. By combining the first and second pigments, an
efficient CCM film (CCM23) is provided which absorbs light as
illustrated by absorption waveform 25 in FIG. 2.
[0020] The light emitted from OED 11 is a broad spectrum light, for
example in a blue-green spectrum as illustrated by waveform 27 in
FIG. 2. It can be seen from FIG. 2, that the absorption of CCM 23
is not a uniform function of wavelengths, and there exists a valley
26 at approximately 490 nm. It should be noted that the blue-green
spectrum emitted by OED 11 has a substantial portion overlying
valley 26. As a result, without the inclusion of microcavity
structure 12, CCM 23 would have to be very thick to absorb the
light around 490 nm. Furthermore, if microcavity structure 12 were
not employed, a small blue light spectrum component would remain
around 490 nm even with a very thick CCM 23 reducing the quality of
the emitted red light.
[0021] By employing microcavity structure 12, the blue-green
spectrum emitted by OED 11 is enhanced to resonant peaks as
represented by waveforms 30A and 30B in FIG. 3, thus giving rise to
saturated blue and green colors. This leads to much brighter blue
and green colors (2 enhancement) when CCM 23 is used. The scale of
FIG. 3 has been expanded to accommodate waveforms 30A and 30B, and
waveform 27. As can be seen by comparing waveform 27 with waveforms
30A and 30B, the intensity of the emitted light represented by
waveform 27 has been greatly increased at approximately 450 nm and
540 nm, and the component at 490 nm has been greatly reduced. With
reference to FIG. 4, by tuning microcavity structure 12 resonance
to wavelengths near the absorption peaks (about 450 nm and about
540 nm) of CCM 23, the absorption of blue-green colors can be
maximized, which results in a maximum blue-green to red conversion
efficiency. The broad spectrum light represented by waveform 25 is
enhanced to resonant peaks represented by waveforms 30A and 30B
which overlap absorption peaks 25A and 25B of waveform 25 at
approximately 450 nm and 540 nm. Thus all of the enhanced light
from microcavity structure 12 is absorbed by CCM 23 and pure red
light is emitted. The component of the broad spectrum light emitted
by OED 11 corresponding to valley 26 has been shifted by
enhancement to an absorbable wavelength (e.g. 450 nm), increasing
the light absorbed by the CCM and eliminating small spectrum
components produced when microcavity structure 12 is absent. Thus,
efficiency is improved while improving the purity of the light
emitted, and by the combination microcavity 12 with CCM 23, it is
possible to achieve a higher color-conversion efficiency for
blue-green to red, and a brighter RGB display can be readily built
based on this approach.
[0022] While we have shown and described specific embodiments of
the present invention, further modifications and improvements will
occur to those skilled in the art.
[0023] We desire it to be understood, therefore, that this
invention is not limited to the particular forms shown and we
intend in the appended claims to cover all modifications that do
not depart from the spirit and scope of this invention.
* * * * *