Wavelength compensation for resonant cavity electroluminescent devices

Disclosed is a microcavity organic light emitter having reduced variation in emission spectra per change in viewing angle. In an illustrative embodiment, a microcavity EL device comprises a microcavity layer structure stacked on a symmetric, non-planar surface of a substrate. The microcavity layer structure includes at least a first reflective layer on the non-planar substrate surface, a second reflective layer and an active layer having organic material capable of electroluminescence between the first and second reflective layers. The non-planar surface may be a shallow cone, frustum, a dome-like surface, or a combination thereof.

FIELD OF THE INVENTION 
The present invention relates to resonant cavity, organic 
electroluminescent devices. 
BACKGROUND OF THE INVENTION 
Resonant cavity electroluminescent devices (also called resonant cavity 
light emitting devices or RCLEDs), and, specifically, microcavity organic 
light emitters, are known in the art. An RCLED is referred to as "organic" 
when the electroluminescent ("EL") material used therein is organic. As 
its name implies, a microcavity structure has a cavity platform on the 
order of micrometers. 
Generally, the fluorescence spectrum of some electroluminescent organic 
materials are broad, in some cases covering the entire visible region. 
Manipulation of the spontaneous emission rates and profiles of luminescent 
systems can be accomplished by incorporating them into structures, such as 
microcavity structures, that alter their nominal free space density and 
photon states. A planar microcavity structure can tailor the spontaneous 
emission of organic thin films. Thus, a single emissive layer of organic 
material in a planar microcavity structure, for example, can be used to 
construct red, green or blue light emitters. This is known in the art and 
described in further detail in A. Dodabalapur, et al., "Microcavity 
Effects In Organic Semiconductors," 64(19) Appl. Phys. Lett. 2486 (May 9, 
1994) ("Dodabalapur I"); A. Dodabalapur, et al., "Electroluminescence From 
Organic Semiconductors In Patterned Microcavities," 30 Elect. Lett. 1000 
(1994) ("Dodabalapur II"); A. Dodabalapur, et al., "Color Variation With 
Electroluminescent Organic Semiconductors In Multimode Resonant Cavities," 
65(18) Appl. Phys. Lett. 2308 (Oct. 31, 1994) ("Dodabalapur III") and U.S. 
Pat. No. 5,405,710 (the "Dodabalapur patent"), all of which are 
incorporated herein by reference. 
In brief, narrowing of the bandwidth of the emitted light to a "single" 
color in a planar microcavity structure, for example, is due to the 
enhancement by the reflective layers incorporated into the device. This is 
described in Nakayama, et al., "Organic Photo- And Electroluminescent 
Devices With Double Mirrors," 63(5) Appl. Phys Letter 594 (Aug. 2, 1993), 
which is also incorporated herein by reference. The wavelength of the 
emitted light is further determined by the optical thickness (also called 
optical length) of the cavity, which can be manipulated by changing the 
thickness of the layers comprising the cavity. Other optical properties 
may be changed to create this effect, such as the index of refraction of 
the layers, or the center wavelength of the stop band of a quarter wave 
stack used as a reflective surface. As disclosed in the Dodabalapur 
patent, a filler layer of appropriate thickness may be incorporated into 
the microcavity structure to control the emitted wavelength. By using 
different thicknesses for the filler layer in distinct regions of a planar 
microcavity structure, a single emissive layer of organic material can be 
used to construct red, green or blue light emitting elements in the 
distinct regions. 
Consequently, microcavity organic light emitting devices (LEDs) employing a 
variable filler layer are advantageous since they can be used to create a 
full color display without the need to combine different emissive 
materials. Once it is decided what layer or property is to be varied to 
achieve the particular color of the particular light emitter, only that 
layer or property need be varied to obtain the desired color. All other 
layers comprising the particular microcavity, including the organic EL 
layer, will remain constant for the different color emitters. 
SUMMARY OF THE INVENTION 
The origins of the present invention stem from a recognition that, in 
general, resonant cavity electroluminescent devices and, specifically, 
microcavity organic light emitters, have an emission spectrum that 
undesirably varies as a function of the viewing angle from the device. 
That is, a blue shift in the emitted wavelength (i.e., a shift towards 
shorter wavelengths) occurs with an increase in the viewing angle from the 
normal to the emitting surface of the device. In microcavity devices, the 
distance between standing wave nodes of incident and reflected waves 
decrease with an increase in viewing angle. Thus, to match the 
characteristic dimension of the cavity requires shorter wavelengths. 
Accordingly, the peak wavelength of a typical microcavity organic light 
emitter may decrease by about 25 to 50 nm with a 45.degree. shift in 
viewing angle from the normal to the plane of light emission. The blue 
shift limits the use of microcavity LEDs in a number of important 
applications, such as displays, where visual perception and impressions 
are important. 
Accordingly, the present invention overcomes the above-noted problems of 
prior art microcavity LEDs by providing a microcavity organic light 
emitter that reduces or minimizes the variation in the wavelength of light 
emitted per change in viewing angle. In an illustrative embodiment, a 
microcavity electroluminescent device comprises a substrate having a 
surface with a multiplicity of predetermined regions with at least one of 
the regions comprising a non-planar surface feature, and a microcavity 
layer structure stacked on the non-planar surface feature. The microcavity 
layer structure includes at least a first reflective layer on the 
non-planar substrate surface feature, a second reflective layer and an 
active layer having organic material capable of electroluminescence 
between the first and second reflective layers. The substrate may have a 
planar surface opposite its symmetric, non-planar surface feature, and the 
symmetric, non-planar surface of the substrate may be a cone extending 
away from the opposite planar surface. The cone may be a right circular 
cone with wedge angle between 8.degree. and 15.degree.. Alternatively, the 
non-planar surface feature may be a frustum, a dome-like surface, or a 
combination thereof. The electroluminescence from opposing regions of the 
non-planar surface add in the far field in a manner such as to reduce the 
wavelength variation of the emitted light with viewing angle, as compared 
to planar microcavity LEDs. 
A display can be fabricated with a plurality of microcavity LEDs in 
accordance with the present invention fabricated on a common substrate. 
The substrate has a planar surface on one side and an opposing surface 
having a plurality of projecting non-planar surface features. The LEDs are 
fabricated on the projecting non-planar surface features, such that each 
forms a sub-pixel. Each pixel of the display comprises, e.g., at least 
three adjoining sub-pixels of different colors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 presents a representative perspective view of a typical microcavity 
organic light emitter 10 as known in the art. Emitter 10 has a generally 
solid rectangular geometry and can be used as a sub-pixel or pixel of a 
flat display. Emitters such as this are also described in Dodabalapur 
I-III and the Dodabalapur patent referred to above. Descriptions of how to 
construct the emitters through various deposition, spin coating and 
masking, which is known in the art, is likewise described in these 
references. Mathematical modeling of the layer system, allowing 
construction of emitters of a chosen wavelength, is also described. 
Light emitter 10 is comprised of an organic microcavity layer structure 15 
formed on a first planar surface 17 of a substrate 12. The substrate 12 
has a second planar surface 13 opposite planar surface 17, through which 
light generated by the microcavity structure 15 is emitted. A significant 
difference between illustrative embodiments of the present invention to be 
described below and light emitter 10 is that substrate 12 is replaced by a 
substrate having a non-planar surface upon which the microcavity structure 
is formed. In these embodiments, the microcavity layer structure is of a 
generally uniform thickness along the entire non-planar surface of the 
substrate. As such, the following discussion of the layer composition of 
the microcavity structure 15 is applicable to both the prior art planar 
light emitter 10 and to the light emitters of the present invention to be 
described below. 
At a minimum, microcavity structure 15 is comprised of a bottom mirror 
layer 14 which is a multilayer dielectric stack, an organic 
electroluminescent (EL) layer 22 (the "active" layer), a top metallic 
mirror 24 and some means for facilitating application of an electric field 
across EL layer 22 to cause it to emit light through the bottom mirror 14. 
Optionally, however, the active layer can comprise, in addition to the 
organic EL layer 22, one or more additional layers, e.g., a hole transport 
layer 20 and/or an electron/hole blocking layer (not shown) between EL 
layer 22 and top mirror 24. The EL material of layer 22 can be single 
layer, or it can be two or more layers that differ in their emission 
characteristics. Each of the EL material layers can be doped or un-doped. 
The substrate 12, as well as the substrates having non-planar surfaces 
employed in embodiments of the invention described below, is substantially 
transparent to radiation of the relevant wavelength. By "substantially 
transparent" we mean herein that, over the relevant distance, the 
attenuation of radiation of the relevant wavelength is typically not more 
than 25%. Exemplary substrate materials are fused silica, glass, sapphire, 
quartz, or transparent plastic such as poly(ethylsulfone). 
The multilayer mirror 14 consists of alternating layers of substantially 
non-absorbing materials of appropriately chosen thickness (typically 
.lambda./4). Such mirrors are well known. Its reflectivity depends in a 
known way on the number of layer pairs and the refractive index of the 
materials used. Exemplary material pairs are SiO.sub.2 and Si.sub.x 
N.sub.y ; SiO.sub.2 and SiN.sub.x ; and SiO.sub.2 and TiO.sub.2. In FIG. 
1, mirror 14 is shown by way of example to consist of alternating 
SiO.sub.2 and Si.sub.x N.sub.y layers 14a-14f, i.e., three layer pairs. It 
is understood that more or fewer pairs can be used. 
The organic EL layer 22 is the source of the light emission. Exemplary EL 
materials are tris(8-hydroxyquinoline)aluminum(Alq), perylene derivatives, 
anthracene, poly(phenylene vinylenes), oxadiazole and stilbene 
derivatives. EL materials optionally can be doped, exemplary with 
coumarine, a DCM, or a rhodamine derivative, in order to tailor the EL 
spectrum of the material and/or enhance the efficiency of the device. The 
EL material can consist of multiple layers including some which are doped 
as described in Jordan et al., Appl. Phys. Lett. 68, 1192 (1996). 
Optionally, hole transport layer 20 is included, which can be any 
substantially transparent material that can facilitate the transport of 
holes to EL layer 22, where electron-hole recombination takes place. 
Examples of suitable materials are diamine (e.g., triphenyl diamine or 
TAD) and poly(thienylene vinylene). 
An (optional) electron transport layer (not shown) may be employed between 
EL layer 22 and top mirror 24. The electron transport layer can be any 
substantially transparent material that can facilitate electron transport 
from the top mirror to the EL layer. Exemplary of such materials are 
2-(4-Biphenyl)-5-phenyl-1, 3, 4-oxadiazole (PBD), butyl PBD, or either of 
these doped in an inert polymer such as poly(methyl methacrylate) (PMMA) 
or a poly(carbonate). 
The top metallic mirror layer 24 injects electrons into the adjacent layer. 
Exemplary reflective materials are Al, Ag or Au, or alloys such as Mg/Al, 
Mg/Ag, or Li/Al. Mirror layer 24 is patterned in display applications, 
both in the prior art and in the embodiments herein, in order to separate 
adjacent LEDs from one another. As such, voltages can be selectively 
applied to the top metallic mirrors of the individual LEDs to produce 
electroluminescence. 
Appropriate choice of EL material can make possible elimination of one 
(possibly both) of the hole transport layer and the electron transport 
layer. For instance, Alq can function both as EL material and electron 
transport medium, and poly(phenylene vinylene) can function both as EL 
material and hole transport medium. 
An (optional) filler layer 16 can be any substantially transparent material 
that is chemically stable under the manufacturing and operating conditions 
that can be patterned by an appropriate technique. Exemplary filler 
materials are transparent polymers (e.g., polyimide) or transparent 
dielectrics (e.g., Si.sub.x N.sub.y or SiO.sub.2). 
Preferably, a transparent (or semitransparent) electrode layer 18 is 
employed as the means for facilitating application of an electric field 
across the active layer or layers. Exemplary choices for electrode layer 
18 are: indium tin oxide (ITO); another conducting oxide such as 
GaInO.sub.3 or Zn.sub.1.2 In.sub.1.9 Sn.sub.0.1 O.sub.x ; a conducting 
polymer such as polyaniline; or, a thin layer (e.g., about 10 angstroms) 
of metal (e.g., Au or Al). 
The electric field across the EL layer, which causes electroluminescence, 
is preferably created by applying a voltage between the top mirror layer 
24 and the electrode layer 18. Electroluminescence is observed when 
approximately 10 volts is applied between these layers. The device 
typically operates at about 10% internal quantum efficiency (i.e., photons 
per injected electron). 
To facilitate the manufacturing of a large number of light emitters of 
several, (e.g., three) selective colors on a common substrate, such as in 
display applications, the filler layer 16 is preferably used as the layer 
which controls the color emitted. As such, the same organic EL material 
can be used throughout the display to produce the different colors. This 
technique was described in the Dodabalapur patent and is also applicable 
to the light emitters of the present invention described below. Basically, 
the thickness of the filler layer is used to manipulate the total optical 
length of the cavity and, thus, the principal emission wavelengths. (As 
noted in the Dodabalapur patent, for example, the thicknesses and 
refractive indices of the other layers of microcavity layer structure 15 
may also be tailored to set the total optical length of the cavity. The 
"cavity" itself is defined by its optical length). 
FIG. 2 tabulates exemplary thicknesses and materials which can be used for 
the various layers of the emitter 10 of FIG. 1. A typical index of 
refraction for each layer is also given. This particular configuration 
results in a yellow emission normal to the planar bottom surface 13 of 
substrate 12 (i.e., along the z axis of FIG. 1). 
Shown in FIG. 3 are curves of the measured intensity of the 
electroluminescence (EL) emitted by the planar microcavity emitter 10 of 
FIG. 1 having the layer characteristics given in FIG. 2. 
Electroluminescence is plotted versus wavelength for various viewing 
angles .theta., where .theta. represents the far field angle from the 
normal axis z to the substrate 12 (see FIG. 1). In the normal direction 
(.theta.=0.degree.), a narrow peak is observed at approximately 590 nm, 
with a spectral width of approximately 25 nm. The peak emission wavelength 
becomes shorter as the viewing angle .theta. increases. Thus, in the 
example, the peak emission wavelength shifts from about 590 nm at 
.theta.=0.degree. to 565 nm at .theta.=45.degree., a "blue shift" of 25 
nm. Also, the bandwidth of the wavelength increases, and the peak 
intensity decreases, with increasing viewing angle. The peak EL intensity 
at .theta.=45.degree. is approximately one third of the value observed at 
.theta.=0.degree.. It is noted that an approximation for the EL intensity 
and emission wavelengths can be obtained analytically, with close 
agreement with the measured results of FIG. 3. This analysis will be 
discussed in detail below. 
In accordance with the present invention, emission wavelength variation 
with viewing angle is reduced by providing a microcavity structure on a 
symmetric, non-planar surface of a substrate. The non-planar surface is 
selected to reduce or minimize such emission wavelength variation. 
In a first embodiment, this non-planar surface is conical. FIG. 4A is a 
perspective view of a conical type microcavity light emitter 40 in 
accordance with the present invention. A substrate 42, e.g., fused silica, 
has a solid rectangular base portion unitary with a conical portion. A 
uniform microcavity layer structure 15 of light emitting device layers is 
formed on the conical portion of substrate 42. The conical portion is 
preferably a right circular cone. As such, the emitter 40 is symmetric 
about a Z axis running through the apex of the cone. As shown in FIG. 4B, 
which is the view AA of FIG. 4A, the base portions and conical portions of 
substrate 42 are designated as 42a and 42b, respectively. Light is emitted 
through a planar bottom surface 43 of base portion 42a. Conical portion 
42b has a shallow wedge angle .psi. which may be in the range of 
8.degree.-15.degree.. Viewing angle .theta.' from the z axis (normal to 
surface 43) is analogous to viewing angle .theta. of the planar LED 10 
discussed above (FIG. 1). 
Layered structure 15 is preferably formed on the conical portion 42b by 
means of evaporation sublimation of the successive layers. This process 
allows the layers to be formed with substantially uniform thickness over 
the entire conical surface. (It is noted that it may be possible to form 
layered structure 15 on conical portion 42b by spin casting from a 
solution). Layered structure 15 is thus analogous to that used for the 
planar LED of FIG. 1, and forms a microcavity structure, with the 
thickness and index of refraction of each layer influencing the emission 
spectrum normal to each region of the microcavity structure 15. 
Wavelength variation of emitted light as a function of viewing angle 
.theta.' is reduced with light emitter 40 as compared to the variation 
exhibited by planar light emitters such as that shown in FIG. 1. This 
wavelength variation reduction is a result of the addition, in the far 
field, of the light spectra originating from the various regions of the 
conical structure. Referring to FIG. 4B, the far field light emission at 
each angle .theta.' in the x-z plane can be roughly approximated as an 
addition of the light emitted by opposing halves 15a and 15b of the LED 
layer structure 15. Thus, for example, sections 15a and 15b can be 
envisioned as two planar microcavity LEDs squinted towards one another, 
each having emission characteristics similar to that shown in FIG. 3. As 
such, one can ascertain from the geometry that the addition of the spectra 
will provide diminished wavelength variation with viewing angle. Because 
of the symmetry of the device, the far field spectrum in all planes (as 
well as the x-z plane) will be essentially the same. The conical case is a 
relatively easy one to model analytically, since the far field pattern can 
essentially be obtained by averaging the contributions from both halves of 
the cone in each plane. 
Referring to FIG. 5A, computed EL intensity of the conical light emitter 40 
vs. wavelength is plotted for viewing angles .theta.' of 0.degree., 
15.8.degree., 25.2.degree. and 45.degree., for the case of light emitter 
40 having analogous layer characteristics to those of FIG. 2. Thus, a 90 
nm thick Alq layer is used for EL layer 22, and so forth. The substrate 42 
is assumed to be fused silica with an index of refraction of 1.5 and a 
base portion 42a thickness of 1-10 millimeters. The wedge angle .psi. of 
the conical portion is 12.degree. for this case. The curves indicate that 
the emission wavelength variation with viewing angle is reduced as 
compared to the prior art light emitter 10 of FIG. 1. The principal 
emission wavelength at .theta.'=0.degree. is approximately 588 nm--this 
shifts slightly higher to about 594 nm as .theta.' reaches 25.2.degree., 
and then lower to about 583 nm for a .theta.' of 45.degree.. Hence, the 
LED 40 exhibits a peak emission wavelength shift of +6/-5 nm between a 
viewing angle .theta.' of 0.degree. and 45.degree., as compared to a -25 
nm shift for the prior art case of FIGS. 1-3. As shown in FIG. 5B, the 
peak EL intensity variation with viewing angle is also reduced with the 
conical LED 40, at the expense of lower EL intensity normal to the device. 
Curve 54 is a plot of peak EL intensity vs. viewing angle for the LED 10 
of FIGS. 1-3, while curve 52 is for the conical LED 40 with the same 
microcavity layer structure 15. 
FIG. 5C shows a CIE chromaticity plot of angular dependence for various 
LEDs disclosed herein. The accepted system for quantifying color 
perception is the CIE representation in terms of chromaticity coordinates 
X, Y and Z which are designed to approximate human visual pigment 
responses. See, e.g., Colorimetry, 2nd edition, CIE Publication 15.2, 
Vienna, Austria (1986). The response to any spectrum can be reduced to 
these coordinates and the similarity between two sets of spectra can be 
measured. 
In the CIE plot of FIG. 5C, normalized x and y coordinates are plotted with 
z determined by the normalization x+y+z=1. Pure monochromatic colors fall 
along the plotted locus 59 and superpositions therefore fall inside the 
bordered area 54 defined by the locus. The spectra of the planar LED of 
FIGS. 1-3 are indicated by the filled squares 55. The dispersion reflects 
the perceived color variation with angle. It is noted that points for an 
analogous green Alq microcavity device with identical structure other than 
filler layer thickness would be more closely spaced, indicating that angle 
variation is less of a problem in this case. This is because the human eye 
is less sensitive to wavelength changes in the upper part of the CIE phase 
space, as documented by human perception experiments. See, e.g., D. L. 
MacAdam, J. Opt. Soc. Am. 32, 247 (1942). 
Also plotted in FIG. 5C are the computed results (data points 56) in CIE 
space for the conical LED 40, with 12 degree wedge angle. The same 
microcavity layer structure 15 as the planar LED 10 was used, except that 
an extra 3 nm filler layer thickness was added to compensate for the 
slight blue shift at 0.degree. viewing angle associated with the cone 
structure. The plotted data points for both cases are for viewing angles 
.theta.(or .theta.' for the conical case) of 0, 15.8, 25.2 and 45 degrees. 
FIG. 6A shows a portion of a display 60, which is comprised of LED 
microcavity structures 15.sub.1 -15.sub.4 fabricated on a common substrate 
62, e.g., fused silica. Substrate 62 has a solid rectangular base 62a and 
a plurality of conical portions atop the base 62a. Each LED structure 
15.sub.i overlays an associated one of these conical portions. This is 
shown more clearly in the cross-sectional view BB of FIG. 6B (bisecting 
the cones associated with multicavity structures 15.sub.2 and 15.sub.3), 
where conical portions 62b.sub.2 and 62b.sub.3 of substrate 62 are shown. 
The substrate 62 can be fabricated with such conical structures by 
stamping the silica from a mold. The stamping mold can be made, for 
example, by etching the reverse conical structures in a semiconductor and 
then metallizing it. Alternatively, for relative large conical or other 
non-planar structures, the substrate may be machined directly to form the 
structures. Microlens arrays may be appropriate substrates and are 
available commercially. 
With continuing reference to FIGS. 6A and 6B, each LED structure 15.sub.i 
is grown on an associated one of the substrate's conical portions 
62b.sub.i to form a sub-pixel. Each LED structure 15.sub.i is designed to 
emit a specific color, which may be accomplished by utilizing a different 
Si.sub.x N.sub.y filler layer 16.sub.i for each LED. Varying only the 
filler layer to realize the different color sub-pixels has manufacturing 
advantages in that the same organic active layer can be used for each 
sub-pixel. For example, three clustered LEDs such as 15.sub.1 -15.sub.3 
can together comprise a pixel, with each sub-pixel designed to emit one of 
the primary colors red, green or blue. As such, any color can be generated 
by the overall pixel with appropriate biasing and superposition of the 
three primary colors. It is noted that the wedge angle .psi..sub.i used 
for each sub-pixel type (each type being associated with a given color) 
may be different. The extent of the emission wavelength variation with 
viewing angle is generally different for each color in planar microcavity 
LEDs and thus, the wedge angles can be tailored to minimize the variation 
for each sub-pixel type. Each sub-pixel may be defined by either the 
circular base of the associated conical portion 15.sub.i or by a square 
platform with sides S (as defined by dotted lines 65) including one 
conical portion 15.sub.i. Each side S may be on the order of 100 
micrometers long, for example. A typical display comprises thousands of 
pixels with each pixel comprised of three or four sub-pixels. 
The diameter of each conical portion 62.sub.i of the substrate is typically 
slightly less than the length of the associated sides S. In the regions 67 
between the conical LEDs, at least the top mirror layer 24 is absent, such 
that the individual microcavity structures 15.sub.i are electrically 
separated. This is accomplished by patterning the mirror layer 24, e.g., 
by masking the regions 67 prior to the layer 24 deposition or by 
photolithography and etching of layer 24. 
Preferably, at least the transparent electrode layer 18 remains in the 
regions 67 to facilitate the biasing of the individual sub-pixels. Thus, a 
large sheet of ITO can be used in the area comprising either the entire 
display or large strips of the display. This sheet or set of strips would 
then comprise the ITO layer 18 for hundreds or thousands of sub-pixels, 
and is maintained at a constant reference potential to facilitate biasing. 
For ease of manufacturing, the other layers (aside from the top mirror) of 
LED structures 15.sub.i are also preferably present in the regions 67 and 
are deposited as large sheets of material. Such is the configuration shown 
in FIG. 6B, where it is seen that only the top mirror layers 24.sub.2 and 
24.sub.3 are discontinued in region 67 between the microcavities 15.sub.2 
and 15.sub.3. Also, in this example, filler layers 16.sub.2 and 16.sub.3 
are of different thicknesses. As such, with the other layers being of 
essentially the same material and thicknesses, the two microcavities 
produce different colors. (It is noted that in FIG. 6B, the thicknesses of 
the layers of microcavity structures 15.sub.2 and 15.sub.3 are exaggerated 
with respect to the size of the conical portions, for clarity). 
The thickness of the filler layer typically is in the range of 50-2000 nm. 
The filler layer may actually be absent in one of the LED microcavities 
15.sub.i of each pixel (i.e., the filler layer thickness may be zero in 
one of the microcavities). Typically, a filler layer of essentially 
constant thickness is formed on the bottom mirror 14, e.g., by spin 
coating and baking of polyimide, followed by patterning by appropriate 
means, e.g., photolithography and etching. The purpose of the patterning 
is to provide optical cavities differing in their optical length such that 
different colors can be produced. 
The sub-pixels can be biased by a conventional means in which the electrode 
layer 18 is held to a common reference potential and voltages are 
selectively applied to the top mirror layer 24.sub.i of each microcavity 
structure 15.sub.i. The sub-pixels can thus be excited at appropriate 
times to create any desired image on the display. Any appropriate 
circuitry can be used to drive the sub-pixels. See, for instance, K. 
Murata, Display Devices, pp. 47-50, 1992, incorporated by reference. At 
page 49, FIG. 9a of this reference, a matrix driving circuit is disclosed 
which could be used in a display according to the present invention. 
Referring to FIG. 7, an alternate embodiment of the present invention is 
the light emitter 70, shown in a cross-sectional view. A substrate 72 is 
fabricated with a solid rectangular base portion 72a and a top portion 72b 
in the shape of a truncated cone (frustum). LED layer structure 15 is then 
formed on the top portion 72b to provide an LED in the shape of a frustum 
with a flat top 74. The LED layers of layer structure 15 are essentially 
the same as those used for the conical light emitter 40. The wedge angle 
.psi. of the frustum may be in the range of 8.degree.-15.degree.. As in 
the conical case, the emission wavelength variation with viewing angle 
.theta.' is reduced as compared to the planar light emitter of FIG. 1. The 
truncation point on the frustum, which defines the area of the flat 
surface 74, can be optimized empirically or analytically. 
FIG. 8 shows another light emitter 80 in accordance with the present 
invention, which also affords improved emission wavelength variation with 
viewing angle. Light emitter 80 is similar to the frustum type emitter 70, 
except that a dome-shaped top portion 84 replaces the flat top 74. A 
substrate 82 is comprised of a base portion 82a, a frustum portion 82b and 
a dome portion 82c atop the frustum portion. The dome portion 82c is 
preferably spherical; however, it is understood that other symmetric 
shapes are possible. LED layer structure 15 is grown on the frustum and 
dome portions of the substrate. The truncation point of the frustum 
portion 82b, the shape of the dome portion 82c, and the frustum wedge 
angle .psi. (typically between 8.degree.-15.degree.) can be varied 
empirically or analytically to optimize the emission wavelength with 
viewing angle for a given LED layer structure 15. 
Referring to FIG. 9, another alternative embodiment of the present 
invention is the light emitter 90. A substrate 92 is fabricated with a 
solid rectangular base portion 92a and a dome portion 92b which may be a 
shallow arc of a sphere. For example, a height H of 20 microns may be used 
in conjunction with a width W of 100 microns for the dome 92b. LED layer 
structure 15 is uniformly grown on the dome. For given LED layer 
compositions and thicknesses, the shape of the dome 92b can be varied to 
desensitize the emission wavelength with viewing angle as compared to the 
planar LED case. 
It is understood that a plurality of any of the light emitters 70, 80 and 
90 of FIGS. 7-9 can be fabricated on a common substrate to form a display 
in an analogous fashion as was described in reference to the display of 
FIGS. 6A-6B. It is also possible to form each pixel with differently 
shaped sub-pixels. For example, light emitter 80 may be used for one color 
while light emitter 70 may be used for another color, such that each pixel 
will have at least one sub-pixel as emitter 70 and at least one sub-pixel 
as emitter 80. 
Computational Procedure 
The EL intensity and spectrum of light emitted from an organic microcavity 
structure as a function of viewing angle from the structure, can be 
computed based on the factors set forth below. The computation is for a 
microcavity structure disposed on a planar substrate such as that shown in 
FIG. 1. The EL intensity and spectrum for an LED having the same 
microcavity structure disposed on a symmetric, non-planar substrate, such 
as in the embodiments of FIGS. 4-9, can then be determined in any plane by 
adding the computed contributions from both halves of the symmetrical 
structure. For example, the result shown in FIGS. 5(A-C) for the 
conical-type LED case was obtained based on this approach. 
FACTOR I: molecular emission versus wavelength. This is measured by 
photoluminescence or by measuring the emission that the organic layers 
would provide in a non-cavity LED. In general, the molecular emission is a 
very broad distribution as a function of wavelength. (For organic 
materials that have narrow molecular emission, e.g., &lt;.about.10 nm, the 
angular remediation scheme herein would not be as useful, since the color 
cannot change much with viewing angle in this case anyway). 
FACTOR II: enhancement of the density of states in the cavity relative to 
free space. This is described by the magnitude and frequency dependence of 
the cavity finesse, which is defined as free spectral range (mode spacing) 
divided by cavity mode width. For example, the finesse at a viewing angle 
of zero degrees (perpendicular to the device layers) is calculated from 
the formula: 
##EQU1## 
where the cavity mirror reflectivities R.sub.1 and R.sub.2 are determined 
using the Fresnel equations as given in M. Born and E. Wolf, "Principles 
of Optics", Pergamon Press, Norwich 1975 (5th Edition) pps. 40-49, 55-70. 
The frequency dependence is Lorentzian with a spectral width and center 
frequency given by the cavity mode width and resonance position. These are 
calculated by applying a transfer matrix formalism to the entire LED 
multilayer structure. A transfer matrix formalism is described in G. Bjork 
and O. Nilsson, "New Matrix Theory of Complicated Laser Structures", Jnl. 
of Lightwave Technology, Vol. LT-5, No. 1, January 1987, pp. 143-146. It 
is noted that the density of states enhancement depends not only on 
wavelength but on the direction in which the light is emitted. 
FACTOR III: the position of the emissive region relative to the peaks and 
troughs of the field in the cavity mode. The field pattern at a given 
wavelength and for a given emission direction is also calculated from the 
transfer matrix formalism. The value at the emissive layer relative to the 
antinode is easily determined. 
FACTOR IV: the fraction of the emitted light which actually escapes the LED 
(i.e., which travels through the dielectric stack mirror). This is given 
by 
##EQU2## 
an expression which also includes the finesse factor II. See E. F. 
Schubert et al., infra. 
FACTOR V: the molecular lifetime reduction caused by the cavity. This has 
been found to be fairly small experimentally. The result can vary slightly 
if the molecules in the sample have varying emission spectra. The 
calculation can be performed by averaging the molecular emission rate over 
all directions including those where the light is totally internally 
reflected and does not escape the device. (See, e.g., Vredenberg et al., 
Phy. Rev. Lett. 71, 517 (1993).) Computations based on the above five 
factors of EL intensity and spectrum as a function of viewing angle from a 
planar microcavity device have been found to be in close agreement with 
measured results. For example, FIG. 10 shows computed EL intensity and 
spectrum for the LED 10 of FIG. 1 having the layer characteristics given 
in FIG. 2. The reflectivity of the Al layer 24 was assumed to be 
0.826+i1.5. The results show close agreement with the measured results 
shown in FIG. 3. As noted, computed results for microcavity LEDs 
fabricated on non-planar substrates in accordance with the present 
invention can be obtained by adding the results from various regions of 
the device. The conical case is the easiest to analyze in that the two 
halves of the case can be approximated as planar devices in any plane. The 
results presented above in FIG. 5 were based on this approach. Microcavity 
structures fabricated on dome-like substrate surfaces can be analyzed by 
integration in any plane of the far field contributions of each region of 
the curved dome-like surface. 
Although microcavity LEDs in accordance with the present invention are 
particularly useful in color displays, they are also useful in other 
applications. For example, the LEDs can be used in a transmitter in 
optical interconnect means or in optical fiber communication means, or in 
a print head in an LED printer means. Such means will differ from the 
corresponding prior art means substantially only with respect to the light 
sources. 
It will be understood that various modifications can be made to the various 
embodiments of the present invention herein disclosed without departing 
from its spirit and scope. As noted above, for example, various 
geometrical configurations of a surface of the substrate are contemplated 
in order to minimize the blue shift of the microcavity electroluminescent 
device. Also, there are a wide range of choices of the number of layers, 
materials and characteristics of the material that can make up a 
microcavity electroluminescent device. All combinations would fall within 
the scope of the present invention. Similarly, various modifications may 
be made to the above-described invention in method without departing from 
its spirit and scope. As noted, a mathematical model of the device may be 
used to refine and/or verify a design of a microcavity EL device that is 
intended to minimize the blue shift, or it may be used to create the 
design itself. Therefore the above description should not be construed as 
limiting the invention but merely as presenting preferred embodiments of 
the invention. Those skilled in the art will envision other modifications 
within the spirit and scope of the present invention as defined by the 
claims presented below.