OLED panel with broadened color spectral components

A method and device in which the light emitted from a color sub-pixel in an organic light emitted display panel can be the sum of two or more light beams of slightly different colors in the same wavelength range. The difference in color is the result of difference in the length of the resonant cavity within the same color sub-pixel. In the manufacturing process, the non-uniformity in the layer thickness can cause a shift in the color coordinates in the color sub-pixels. The color shift when the width of the color spectrum is narrow is more noticeable. By broadening the width of the color spectrum, the color shift would become less appreciable. Thus, broadening the width of the color spectrum would ease the strict requirements in manufacturing.

FIELD OF THE INVENTION

The present invention relates generally to an organic light-emitting device and, more particularly, to an active-matrix display panel having a plurality of color pixels composed of the organic light-emitting devices.

BACKGROUND OF THE INVENTION

An active matrix display device, such as an active-matrix organic light-emitting display panel, has a two-dimensional pixel array comprising a plurality of pixel rows. Each of the pixel rows has a plurality of pixels arranged in the x direction, as shown inFIG. 1. These pixel rows are arranged as lines in the y direction so that they can be sequentially driven by a plurality of scanning signals provided by the scanning lines in one or more scanning circuits. InFIG. 1, the display panel1has a plurality of color pixels10arranged in a two-dimensional array.

In a color organic light-emitting display panel, each color pixel10generally comprises three color sub-pixels R, G, B for emitting red, green and blue colors, respectively, as shown inFIG. 2. The color sub-pixels R, G, B comprise three different organic light-emitting devices24,25and26in a diode form. Organic light-emitting diodes (OLEDs) are known in the art. For example, the red organic light-emitting device24is an organic electroluminescent device having a plurality of organic layers disposed between a cathode layer150and an anode layer110formed on a substrate100, as shown inFIG. 3. The organic layers in the organic layer section consist of a hole injection layer (HIL)120, a hole transport layer (HTL)122, an emissive layer (EML)130, and an electron transport layer (ETL)140. The green and blue organic light-emitting devices25and26are structurally similar to the layer structure as shown inFIG. 3, except that each of the organic light-emitting devices24,25and26comprises a different light emitting material in the EML layer so that each device emits a different color spectrum. InFIG. 3, the thickness of the organic layer section, L, serves as a resonant cavity in an OLED.

Each of color spectra has a different spectral peak expressed by the coordinates in a CIE color space. For example, a red color spectrum can be represented by CIEx: 0.682 color coordinate. It is desirable to produce an organic light emitting display wherein the CIE coordinates can be controlled.

SUMMARY OF THE INVENTION

The present invention provides a method and device in which the light emitted from a color sub-pixel can be the sum of two or more light beams of slightly different colors in the same wavelength range. The difference in color is the result of difference in the length of the resonant cavity within the same color sub-pixel.

Thus, the first aspect of the present invention is a method for use in an organic light emitting display panel, the light emitting panel comprises a plurality of color sub-pixels, each color sub-pixel configured to emit light in a color spectrum of a first wavelength distribution within a wavelength range. The method comprises:

arranging at least some of the color sub-pixels to emit light in a color spectrum of a different second wavelength distribution within said wavelength range; and

combining the emitted light of the first wavelength distribution and the emitted light of the second wavelength distribution for providing combined emitted light.

According to various embodiments of the present invention, the color spectrum of the first wavelength distribution has a first spectral width and the color spectrum of the second wavelength has a second spectral width, wherein the combined emitted light has color spectrum with a third spectral width broader than the first spectral width and broader than the second spectral width.

According to the present invention, each of the color sub-pixels comprises a light emitting component, the light emitting component comprises a first organic layer section disposed between an anode layer and a cathode layer, the first organic section comprising a first thickness between the anode layer and the cathode layer for emitting light in the color spectrum of the first wavelength distribution and a second organic layer section disposed between an anode layer and a cathode layer, the first organic section comprising a first thickness between the anode layer and the cathode layer for emitting light in the color spectrum of the second wavelength distribution. Each of the first organic layer section and the second organic layer section comprises:

an emissive layer between the anode layer and the cathode layer;

a hole transport layer between the emissive layer and the anode layer; and

an electron transport layer between the emissive layer and the cathode layer.

Each of the first organic layer section and the second organic layer section may also comprises a hole injection layer between the hole transport layer and the anode layer.

The second aspect of the present invention is an organic light emitting device. The device comprises:

an anode layer;

a cathode layer;

a first organic layer section between the anode layer and the cathode layer, the first organic layer section comprising a first thickness arranged for emitting light in a color spectrum of a first wavelength distribution in a wavelength range, and a second organic layer section between the anode layer and the cathode layer, the second organic layer section comprising a second thickness different from the first thickness arranged for emitting light in a color spectrum of a second wavelength distribution in said wavelength range.

The third aspect of the present invention is an organic light emitting display panel. The display panel comprises:

a plurality of color sub-pixels, each sub-pixel comprising an anode layer; a cathode layer; a first organic layer section between the anode layer and the cathode layer, the first organic layer section comprising a first thickness arranged for emitting light in a color spectrum of a first wavelength distribution in a wavelength range, wherein at least some of the color sub-pixels comprise a second organic layer section between the anode layer and the cathode layer, the second organic layer section comprising a second thickness different from the first thickness arranged for emitting light in a color spectrum of a second wavelength distribution in said wavelength range.

According to various embodiments of the present invention, the color spectrum of the first wavelength distribution has a first spectral width and the color spectrum of the second wavelength distribution has a second spectral width, and wherein the combined emitted light has a color spectrum with a third spectral width broader than the first spectral width and broader than the second spectral width.

According to various embodiments of the present invention, each of the color sub-pixels comprises a light emitting component, the light emitting component comprises a first organic layer section disposed between an anode layer and a cathode layer, the first organic section comprising a first thickness between the anode layer and the cathode layer for emitting light in the color spectrum of the first wavelength distribution, and a second organic layer section disposed between the anode layer and the cathode layer, the second organic section comprising a second thickness between the anode layer and the cathode layer for emitting light in the color spectrum of the second wavelength distribution.

The difference between the first thickness and the second thickness can be in the range of 0.5% to 30%, dependent upon the resonance order of the cavity.

According to various embodiments of the present invention, each of the first organic layer section and the second organic layer section comprises:

an emissive layer between the anode layer and the cathode layer;

a hole transport layer between the emissive layer and the anode layer; and

an electron transport layer between the emissive layer and the cathode layer.

According to various embodiments of the present invention, each of the first organic layer section and the second organic layer section further comprises a hole injection layer between the hole transport layer and the anode layer.

According to various embodiments of the present invention, the plurality of color sub-pixels comprise a plurality of red sub-pixels having a wavelength range between 600 nm and 640 nm; a plurality of green sub-pixels having a wavelength range between 510 nm and 550 nm; and a plurality of blue sub-pixels with a wavelength range between 440 nm-480 nm.

The present invention will become apparent upon reading the description taken in conjunction withFIGS. 4-16.

DETAILED DESCRIPTION OF THE INVENTION

In an organic light-emitting diode (OLED) where the organic layers are placed in a resonant cavity, the coordinates of the spectrum in the color space is shifted if the length of the resonant cavity changes. The shifting of the color coordinates means a change in the color of the emitted light. The shifting of the color coordinates is dependent upon the width of the color spectrum. A spectrum having a broader spectral width, in general, will have a lesser effect in the changes of color. In an OLED, the spectral width can vary with the thickness of the cathode layer, for example. The width of a color spectrum can be increased over 100% if the thickness of the cathode layer reduces from 20 nm to 10 nm. An OLED with a thin cathode layer, however, has a shorter life and its light-emitting efficiency is inferior to an OLED with a thicker cathode layer.

The present invention provides a method for increasing the width of a color spectrum without appreciably affecting the useful life of an OLED. According to various embodiments of the present invention, the color spectrum of the OLED for each color sub-pixel is the sum of two or more color spectra of approximately the same color. For example, a spectrum for a red OLED can be the sum of two component spectra of CIEx: 0.657 and CIEx: 0682. If the spectral width of each of the component spectra is 23 nm, then the width of the resultant spectrum is about 35 nm. In order to produce such a resultant spectrum, each color sub-pixel has two or more pixel segments and each pixel segment emits a light beam of a different component spectrum. As shown inFIG. 4, the color pixel10has three color sub-pixels R, G and B. The color sub-pixel R comprises two pixel segments241and242, the color sub-pixel G comprises two pixel segments251and252, and the color sub-pixel B comprises two pixel segments261and262.

An exemplary cross section of the color sub-pixel R is shown inFIG. 5. As shown inFIG. 5, while the basic structure of the organic layer section is basically the same, the length L2of the resonant cavity in the pixel segment242is greater than the length L1of the resonant cavity in the pixel segment241. As such, the color spectrum of the light beam emitted from the pixel segment24is shifted toward the longer wavelength region, as shown inFIG. 8a.

The change in the resonant cavity can be achieved by increasing the thickness of one or more layers in the organic layer section. For example, as shown inFIG. 6a, only the thickness of the layer120(hole injection layer HIL) in the pixel segments242is different from the pixel segments241, whereas the thickness of each of the remaining layers is the same in both pixel segments241and242. Also, the thickness of the anode layer110and the cathode layer is substantially the same in both pixel segments241and242. In this example, the increase in the cavity length from L1and L2can be achieved by depositing additional HIL material on the pixel segment242.

In various embodiments of the present invention, the HIL can be made of poly(3,4-ethylenedioxythiophene) (PEDOT), 4,4′,4″-tris(3-methylphenyl-phenylamino)-triphenylamine (MTDATA) or copper phthalocyanine (CuPc), for example, and its thickness can be in the range of 1 nm to 300 nm. The HTL can be made of N-propyl bromide (NPB) or 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA), for example, and its thickness can be in the range of 1 nm to 300 nm. The EML can be made of a host material such as tris-(8-hydroxyquinoline)aluminum (Alq3) or TCTA, for example, and its thickness can be in the range of 1 nm to 100 nm. The dopant for the red color sub-pixel can be 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidy 1-9-enyl)-4H-pyran (DCJTB) or dicyanomethylene (DCM), for example. The dopant for the green color sub-pixel can be coumarin 545 tetramethyl (C545T) or tris(2-phenylpyridine)iridium (Irppy3), for example. The dopant for the blue color sub-pixel can be 9,10-di(2-naphthyl)anthracene (ADN) or Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (Firpic), for example. The ETL can be made of Alq3, 2,2′,2″-(1,3,5-benzenetryl)tris(1-phenyl)-1H-benzimidazol (TPBi), 4,7-Diphenyl-1,10-phenanthroline (Bphen), for example, and its thickness can be in the range of 1 nm to 300 nm. In some of the embodiments, an electron injection layer (EIL, not shown) can be disposed between the ETL and the cathode layer. The EIL can be made of LiF, Ca or Mg, for example, and its thickness can be in the range of 1 nm to 50 nm.

The cavity length, L, of an OLED, is generally determined by the wavelength λ of the emitted light, the refractive index n, and the resonance order m as follows:
L=m λ/2n

For a crude estimation of the cavity length, let us use λ=620 nm (nanometer) and n=1.7 for the red color sub-pixel. We have
L=m*182.5 nm,m=1,2,3, . . .
With m=2, for example, L is about 365 nm. If we increase the cavity length by 3%, the shift of the color spectrum would be about 20 nm. Since the useful wavelength ranges in a color OLED are: R: 600˜640 nm, G: 510˜550 nm and B: 440˜480 nm, the increase in the cavity length can be 0.5% to 30%, for example, depending on the resonance order m.

It should be noted that, any one of the organic layers120-142can be changed in order to change the length of the resonant cavity. As another example, the layer122(hole transport layer) in the pixel segments242is different from the pixel segments241and, whereas the thickness of each of the remaining layers is the same in both pixel segments241and242, as shown inFIG. 6b. When a voltage is applied across the cathode150and the anode110, the pixel segment241emits a light beam221and the pixel segment242emits a light beam222simultaneously, as shown inFIG. 7. Because of the difference in the length of the resonant cavity, the color coordinates of the color spectrum for the light beam221emitted from the pixel segment241and the color spectrum for the light beam222emitted from the pixel segment242are different. As shown inFIG. 8a, the color spectrum242for the light beam222emitted from the pixel segment242is shifted toward the longer wavelength region as compared to the color spectrum241for the light beam221emitted from the pixel segment241. As a result, the light emitting from both the pixel segments241and242has a broader color spectrum249, as shown inFIG. 8b. The resultant color spectrum249is the sum of the spectra241and242as shown inFIG. 8a. It is desirable that color shift between the two light beams221and222(FIG. 7) is not too far such that the color spectra241and242are significantly overlapped.

It is possible to produce a color sub-pixel having three or more pixel segments. As shown inFIG. 9, each of color sub-pixels R, G, B has three pixel segments. The R color sub-pixel has three pixel segments241,242and243. The G color sub-pixel has three pixel segments251,252and253. The B color sub-pixel has three pixel segments261,262and263. The cross section of the R color sub-pixel is shown inFIG. 10. As shown inFIG. 10, the organic layer section has three different thicknesses or cavity lengths L1, L2and L3. When a voltage is applied across the cathode150and the anode110, three light beams of different color spectra will be simultaneously emitted from the pixel segments241,242and243. The width of the resultant color spectrum can be further increased.

According to various embodiments of the present invention, each of the color sub-pixels can have two or more pixel segments with different cavity lengths. The proportion of the pixel segments in one color sub-pixel can be the same as that in the other two color sub-pixels, as shown inFIG. 4. The proportion can be different, as shown inFIG. 11. When the cavity length in two adjacent rows of color pixels is increased in the same amount, it is desirable to arrange such that the pixel segments with an increased cavity length in one row is adjacent to similar pixel segments in the other row, as shown inFIG. 12a. As such, it is possible to deposit an additional layer80on the relevant organic layer during manufacturing so that all six pixel segments with an increased cavity length can be achieved at the same time, as shown inFIG. 12b. Alternatively, each row of the color sub-pixels has a separate additional layer80, as shown inFIG. 12c. In a different embodiment of the present invention, the arrangement of R, G, B color sub-pixels is different from one row of color pixel to another, as shown inFIG. 13. In yet another embodiment of the present invention, the additional layer80can be broken up into a plurality of smaller areas, as shown inFIG. 14. Furthermore, the color sub-pixels R, G, B can be arranged such that one color sub-pixel in the same pixel is adjacent to the other two color sub-pixels, as shown inFIG. 15.

In the manufacturing process, the non-uniformity in the layer thickness can cause a shift in the color coordinates in the color sub-pixels. The color shift when the width of the color spectrum is narrow is more noticeable. By broadening the width of the color spectrum, the color shift would become less appreciable. Thus, broadening the width of the color spectrum would ease the strict requirements in manufacturing.

It should be noted that, the present invention is applicable to the top-emitting OLED, as shown inFIG. 7. It is also applicable to the bottom-emitting OLED, as shown inFIG. 16.

In summary, the present invention provides a method and device in which the light emitted from a color sub-pixel can be the sum of two or more light beams of slightly different colors in the same wavelength range. The difference in color is the result of difference in the length of the resonant cavity within the same color sub-pixel.

Although the present invention has been described with respect to one or more embodiments thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the scope of this invention.