KR20020065893A - Organic light emitting diode having spherical shaped patterns - Google Patents

Abstract

The light emitting device includes a transparent substrate having a first surface and a second surface, a conductive transparent layer on the first surface, an organic layer disposed on the conductive transparent layer, and an upper contact portion disposed on the organic layer, the second surface of the substrate The contour of has a non-planar shape.

Description

Organic light emitting diode having a spherical pattern {ORGANIC LIGHT EMITTING DIODE HAVING SPHERICAL SHAPED PATTERNS}

The need to download and view vast amounts of data at even higher speeds and access to the Internet, along with the frequent demand for portability and small footprint, place greater demands on the performance of display devices. The display device chosen for such applications is a flat-panel display, but current liquid crystal display (LCD) technology used by most flat-panel displays is limited in its ability to meet their increasing needs. However, new display technologies offer significant possibilities to overcome the limitations of LCD technology. The new technology is based on the application of organic light emitting diodes (OLEDs), using thin film materials that emit light when excited by electrical current.

Conventional OLEDs include flat glass substrates (t _sub-1 mm, n _sub = 1.51), layers of indium tin oxide (ITO) (t _ITO- 100 nm, n _ITO- 1.8), one or more organic layers (t _org- 0.1 nm, n _org = 1.6-1.8), and a reflective cathode (eg, Mg; Ag or Li; Al), where t refers to the layer thickness and n refers to the layer refractive index. For simplicity, the issue in the text will be based on a single organic layer where luminescence occurs. However, those skilled in the art will readily appreciate that the above points and the resulting analysis can be easily extended to more complex device structures.

An important illustration of the advantages for a display system is the conversion efficiency of the input power to light emission. For OLED displays, an important factor in determining this system efficiency is the coupling efficiency (η _ext ) at which the internally generated light is coupled from the device. In order to meet the expected needs of future display systems, there is a need to improve the coupling efficiency of OLEDs.

Summary and purpose of the invention

It is an object of the present invention to provide an increase in coupling efficiency for OLEDs. For that purpose, a method is provided for increasing the emission intensity of an organic light emitting diode (OLED), and the total external emission efficiency for an OLED at a normal viewing angle. By the method of the present invention, they provided a spherical pattern on the back of the device substrate, thereby increasing the factor of 9.6 and 3.0, respectively. The method of the invention captures previously lost light to guide the wavelength to the substrate, and previously lost light to guide the wavelength to the organic / anode layer by selection of the appropriate substrate. According to the method of the present invention, when compared to devices fabricated on conventional flat glass substrates, it is possible to at least double the radiation efficiency for OLEDs when glass substrates are used, and OLED emission efficiency when high refractive index plastic substrates are used. A surface texturing means is provided that can at least triple three times.

The present invention relates to U.S. Provisional Patent Application No. 60 / 162,552, filed October 29, 1999, entitled "Improvement of Output Coupling Efficiency of Organic Light Emitting Diodes by Backside Substrate Patterning". The provisional patent application is assigned to the same assignee and is incorporated herein by reference.

The present invention relates to the field of light emitting devices, and more particularly to organic light emitting devices (OLEDs) and their luminous efficiency.

1 shows ray diagrams for various light paths of planar OLEDs.

2 (a) and 2 (b) provide schematic diagrams of the use of spherical shapes to improve luminous efficiency for OLEDs according to the present invention.

FIG. 3 (a) shows the far field intensity distribution pattern measured for a flat glass substrate and the expected profile of the Lambertian emitter.

Figure 3 (b) shows the results of experiments on glass substrate devices with and without the spherical shape provided according to the present invention.

Figure 3 (c) shows the results of experiments on PC substrate devices with and without the spherical shape provided according to the present invention, along with the flat glass substrate results.

New means for significantly improving the emission efficiency of OLEDs are described in the text. In describing such means, first, an analytical construct is developed to provide an evaluation tool used to describe the radiation efficiency improvement of the present invention and to compare the results with the prior art. Appropriately, using the analytical construct, the means of the present invention for achieving improved OLED emission efficiency are described. Finally, a number of embodiments for the improved luminous efficiency means of the present invention are implemented by the inventors and such embodiments are described.

As described in the background section, the coupling efficiency (η _ext ) of OLEDs is an important factor in the determination of emission efficiency for OLED displays. By considering the refractive index associated with each layer, it is easy to analyze η _ext for the type of layer structure used in the OLED. As a description of the analysis, taking into account the diagram for the planar OLEDs shown in FIG. 1, the loss due to light trapping in the substrate layer (ray II) and in the organic / anode layer (ray III) Prove it. As shown by the ray I in the figure, only the light emitted at a sufficiently small angle exits.

Since the refractive index for the substrate is below the organic layer refractive index (ie, n _subs n _org), sin ^-1 (n there _subs / n _org) is a, the critical angle θ _org.c2 defined as can be obtained, wherein the organic layer is emitted in an angle greater than θ _org.c2 light of ITO and organic layers within the Wave-guided at. This light emission path is shown as ray III of FIG. Similarly, n _glass Since n _subs , the threshold θ _org.c1 , defined as sin ⁻¹ (n _air / n _org ), can be obtained, where an angle of θ _org.c1 or more, as shown by ray II of FIG. 1, is obtained. The light emitted to the organic layer is guided to the substrate. Since only light emitted at an angle of _θorg.c2 or less, shown by the line I in FIG. 1, is emitted from the device, all remaining waveguided light is effectively lost, indicating a decrease in η _ext . Applying ray optics and θ _Assuming a transfer coefficient of 1 and otherwise 0 for θ _org.c1 , T and isotropic emission from point sources of the organic layer, η _ext and η _subs can be calculated, the latter representing the ratio of radiated light that is guided to the substrate. .

[Angular dependence of the emission from a conjugated polymer light-emitting diode: implications for efficiency calculations, ”by N. Greenham, R. H. Friend, and D. C. Bradley. Mat. 6,491 (1994)]

Moreover, under the same assumption, the external brightness is given by

Ji, Gu, D.Jet, Gabouzov, P. B. Virous, S. Benkatesh, and S. R. Forest, "High-external-quantum-efficiency organic light-emitting devices", Opt. Lett. 22, 396 (1997)]

The luminous intensity is similar to the cosine luminous intensity profile of the Lambertian emitter. (It can be appreciated that both the above equations and modeling ignore well-known microcavity effects that complicate the model but do not alter the qualitative efficacy of the methodology and the results described in the text.)

θ _Note that the assumption that T = 1 for θ _org.c1 represents a simplification. In particular, it represents the boundary above the expected radiant intensity. The lower boundary (ignoring the microcavity effect) can be obtained by using the equation for T (θ) determined by applying the Fresnel equations to each interface. However, simplification can be seen to provide an adjacent approximation. θ In the determination of the factors represented by equations (1), (2) and (3) for the case of T = 1 for θ _org.c1 , re-allow any of the internally reflected light at the second approach (lower boundary) Although not assumed, it is implicitly assumed that all light reflected inwardly at this angle is gradually emitted. The results obtained for I _ext (θ _ff ) in both cases are shown in FIG. 3A along the side of the experimental results obtained for the OLED fabricated on the flat glass substrate, along with the cosine results obtained for the Lambertian emitter. have. The difference between the two refraction models (i.e., R = 1, which represents the case where all reflected light is gradually escaped) and R = 0, which represents the case where all the reflected light is lost, is small, so that either assumption It is generally valid to adopt. Since the equations obtained under the T = 1 assumption are simpler, they will be used for the remainder of this description.

For the expected range of refractive indices for the organic layer between 1.6 and 1.8, the corresponding range for the coupling efficiency, η _ext , will be between 0.20 and 0.15, indicating the significance of the coupling efficiency in decreasing system efficiency-ie internal Between 80 and 85 percent of the light generated is trapped in the device. External bonding efficiency was improved by an index of 1.9 ± 0.2 by etching grooves in the glass around the OLED to redirect light trapped in the substrate and organic / ITO layers (see G. Gu et al., Id.). However, this method does not, in itself, assist in the fabrication of device arrays in which metal lines and / or circuitry for passive or active matrix drivers must traverse deep grooves. This method also has the disadvantage of requiring etching of finely controlled, non-vertical profiles to the substrate, which greatly increases the complexity of the fabrication.

According to the method of the present invention, a solution to the light-trapping problem is provided by patterning the substrate backside into a sphere having its center of light source. Most of the light impinges perpendicularly on the air-substrate interface, thereby guiding it to the substrate, which significantly reduces the loss of light. Such spherical patterning of the substrate is schematically illustrated in FIG.

Referring to the figures, attaching a sphere to the backside of the substrate, or shaping the substrate into such a sphere, shown in Figure 2 (a), causes the light rays to exit the substrate at a greater angle. The maximum angle of the _outgoing light beam is from θ _org.c1 of the planar device θ _max = tan ⁻¹ (of the back-patterned device of the invention (shown in FIG. 2 (a)) _lens / t _subs ). To the extent that θ _max is determined by a substantial pattern and can be made large, the external coupling efficiency is increased to η _ext = 1-cosθ _max (which is reached by changing the upper limit of the integral of equation (1)). If the center of the spherical curved surface does not correspond directly to the OLED position, no one will be able to propagate through the lens / air interface (or the molded substrate / air interface) at normal incidence, so that no one can see the source field pattern due to the refractive effect ( far field pattern). Other methods of breaking the planarity of the substrate back side also achieve less total internal reflection and bus increased external efficiency.

Applicants show below a number of embodiments for the present invention in which they have realized and developed experimental results. Essential parameters for the implementation of these embodiments are summarized in Table 1 below. The three derived parameters shown in Table 1 can be best understood with reference to FIG. The first of these derived parameters is the substrate refractive index (n _subs ) which completely determines θ _org.c1 and θ _org.c2 and also completely determines η _ext and η _ext , where n _subs also replaces n _air with n _subs To obtain an intensity distribution profile inside the substrate layer, which can be obtained from equation (3).

I _subs (θ _subs ) is important because Equation (3) describes only the external intensity distribution when the substrate is planar. If the substrate forms a hemisphere with the device at its center, for example, I _ext (θ _ff ) will be equivalent to I _subs (θ _subs ). In fact, I _subs (θ _subs ) plays a direct role in determining I _ext (θ _ff ) in all but the specific cases of planar substrates. Subsequently, I Effect of _subs n _subs on is, to the point I _subs (θ _subs) is for reproducing the isotropic intensity distribution occurs initially in the organic layer, n _subs = n _org when, to concentrate the distribution of the n _subs increase will be.

The second of the derived parameters is the maximum angle (within the substrate) of light collected by the lens θ _subs.max . These parameters are expressed in the formula, θ _subs.max = tan ^-1 ( _lens radius (in _lens / t _subs ) _lens ) and the total substrate thickness (t _subs ). If θ _subs.max is sufficient to capture all the light transmitted out of the substrate without the lens (ie, θ _subs.max > sin ⁻¹ (n _air / n _subs )), the radiation is emitted at an angle greater than θ _subs.max. Any light will be guided and lost into the substrate. In accordance with the same analysis that leads to equation (1), the equation is _subject to the same assumptions about θ _subs.max for external coupling efficiency for OLEDs centered on a spherically shaped substrate. Can be obtained.

In order to explain the importance of _subs.max θ, θ = _subs.max 76˚ (i.e., _{If lens} = 4t _subs ) and n _subs = n _org , it can be considered that unbonded 14 ° represents 24 percent of the light transmitted to the substrate.

The third and final parameter is derived are vertically offset _(offset d) of the device from the surface of the lens. This parameter is important because it strongly affects the atomic distribution pattern. The equation for I _ext when d _offset ≠ 0 can be easily recognized in the art. For this issue, when the OLED is positioned too far from the lens (d _offset > 0), I _ext will be concentrated than the I _subs , and when the OLED is placed too close to the lens (d _offset <0), I _ext Fully indicate that it will be less concentrated than I _subs . However, because over a wide range of offset values, | d _offset |> 0, There is only a negative damping effect on _ext .

The ray used to define the circle-magnetic field angle θ _off of FIG. 2 (a) is shown in the diagram non-zero so that the offset, d, between the curved center of the lens and the OLED can be clearly identified, It can be appreciated that it is shown for the case of d = 0. It can be appreciated that the spherical features implemented as a plastic lens array laminated to a planar substrate are shown in detail in FIG. 2 (b).

Although it was recognized that the base substrate surface design was used before using conventional LEDs, the means were not pre-applied to improve the output coupling of the OLED. In addition, such OLED means will not be presented by LED applications. It can also be appreciated that the extremely high refractive index (eg, n> 4) found in the emissive materials of conventional LEDs, with some advantages excluded, can be obtained with OLEDs. In particular, by matching the refractive index of the substrate to the refractive index of the emissive material in addition to shaping the substrate, it is possible to potentially eliminate all external bond losses of the device, and a suitable refractive index range (i.e., n to 1.6-1.8) is readily available.

The inventors have implemented the means of the present invention for a number of embodiments which are further described below. OLEDs making up these various embodiments were fabricated on glass and polycarbonate (PC) substrates. The glass substrates consisted of 0.7 mm and 1.1 mm thick soda lime glass purchased from Applied Films Co. coated with ITO by the manufacturer. PC substrates consisted of 175 μm thick sheets purchased from Goodfellows Co. with a 100 nm ITO film disposed on it of an Edwards A306 RF magnetron sputter with 2 m Torr pure Ar gas at 150 W RF power at room temperature. The sputter target was 90% In ₂ O ₃ -10% SnO of 3 inches in diameter. The deposition rate was 33 nm / min. OLEDs are single poly- (N-vinylcarbazole) (PVK) / 2- (4-biphenyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (PBD) / It is prepared by spinning on a coumarin 6 (C6) layer and evaporating 100-200 nm Mg: Ag cathode [Wu]. The refractive index of the organic layer was measured to be 1.67 by ellipsometry at λ = 634 nm and λ = 830 nm. Typical device sizes and geometries consisted of 1.75 mm diameter circles.

The experiments were carried out with six different substrate structures, indicated in Tests 1-6 of Table 1. A diagram of the substrate setup (where all relevant parameters are identified) is shown in Figure 2 (a).

Test substrate material Lens material R _lens (mm) p _lens (mm) t _subs (mm) θ _max d (mm) I _normal / I ₀ F / F ₀ ± 0.1 ± 0.1 1 Glass (n = 1.51) N / AN / AN / A 0.7 N / AN / A 1.0 1.02 Glass (n = 1.51) Glass (n = 1.51) 3.4 3.4 0.7 78 ° +1.0 3.6 2.03 Glass (n = 1.51) Glass (n = 1.51) 3.4 3.4 2.0 60˚ +2.3 9.5 1.64 Glass (n = 1.51) Silicon (n = 1.41) 2.7 2.4 1.9 51˚ +0.6 2.1 1.65 PC (n = 1.61) N / AN / AN / A 1.0 N / AN / A 1.0 1.06 PC (n = 1.61) Epoxy (n = 1.60) 2.7 2.4 1.0 67˚ -0.3 1.6 3.0

Substrate and lens parameters (defined in FIG. 2A) for other external coupling experiments. I _normal / I ₀ and F / F ₀ represent the respective ratios of the vertical and total radiation intensities to the results obtained for the same devices fabricated on planar substrates of the same substrate material. Total radiation intensity measurements do not include edge radiation.

In test 1, unmodified flat glass substrates were used. In trials 2 and 3, glass substrates with glass condenser lenses purchased from Edmund Scientific Co. were used. In test 4, glass substrates with molded silicon lenses were used. In the latter three tests, lenses were attached to the substrate using an index matching gel purchased from FIS CO. If necessary, the substrate thickness was increased by gluing on the extra glass slides using the same refractive index matching gel. Silicon lenses were molded into Teflon blocks with ball-milled indentations using GE RTV615 silicon (n = 1.405). In tests 5 and 6, PC substrates were used. In test 6 molded epoxy lenses were attached to the PC substrate, but in test 5, a standard flat substrate was employed. Epoxy lenses were made using the same mold as described above with Master Bond EP42HT two-component epoxy (n = 1.61). The lenses were bonded to the substrate with an uncured epoxy. For each test, the field strength pattern was measured using a wide area (1 cm diameter) Si photodetector at a distance of 10 cm from the device with a 6 ° increase between 0 ° (vertical) and 90 °.

Vertical offset of the substrate refractive index (n _subs), up to the light unit from the center of each curved surface (θ _subs.max), and the combined lens in the lens (d _offset - and the FIG. 2 (a) the three derived parameters shown in )-All play an important role in determining the total radiant flux and the external intensity distribution of the experiment. Using these parameters under consideration, it is a simple matter to evaluate the experimental results. The vertical and total radiated fluxes measured for each test are listed in Table 1. A plot of the radiant flux measured at each angle for each test is provided in Figures 3B and 3C. Total radiated flux results are initially presented.

In tests using glass substrates (test 2-4), the total radiated flux was increased by a factor between 1.6 and 2.0 ± 0.1. These results are consistent with the analytical values obtained from Eqs. (5) and (1), in which test 3 recognizes that a large d _offset is expected to slightly reduce the effectiveness of the lens. (θ _{subs, max} Since sin ^-1 (n _silicone / n _subs ), the low exponent of silicon does not have a significant effect on the results. For experiments using a PC substrate and an epoxy lens (Experiment (6)), the total radiated flux is , A factor of 3.0 ± 0.1 over the flat PC substrate case (Experiment 5), which is again in agreement with the analytical values obtained using equations (5) and (1), was increased. If Equation (1) assumes that the flat cases for PC and glass substrates are the same, it can be observed in FIG. 3C that the two cases (Experiments 1 and 5) are effectively identical substantially within the experimental uncertainty of the experiment. It is recognized. This provides experimental justification to compare the results obtained for the PC substrates with the results obtained for the glass substrate.

The radiated flux distribution result is now described. For all results, the normal radiated flux can be correlated with the value for d _offset . Test 3 exhibits one extreme: d _offset = 2.3 mm and I _normal / I _o = 9.5. Test 6 shows different extremes: d _offset = -0.3 mm and I _normal / I _o = 1.6. Since different substrate and lens materials were used in each test, the value of I _normal is not determined solely by d _offset ; However, for the equation and results for I _ext , for a vertical 6 ° cone, d _offset is the dominant variable. In Figures 3b and 3c, the results of the fully radiated intensity distribution are provided. These results again demonstrate the effect of increasing d _offset . In addition, the results from Test 6 show how effective the regeneration of the isotropic intensity distribution generated in the organic layer using a lens with a high index substrate—the radiated flux effectively remains constant from 0 ° to 72 ° in this test. Prove it.

In Figures 3 (a) and 3 (b), an essentially perfect correlation can be seen between the data for the planar glass substrate (test 1) and the different refractive models. Moreover, the data deviate significantly from Lambertian distribution results, which further enhances the effectiveness of the refractive models provided herein.

As described in detail in the present invention, it should be understood that the above description is not intended to limit the scope and spirit thereof. What should be protected by the invention is set forth in the appended claims.

Claims (20)

In the light emitting device, A transparent substrate having a first surface and a second surface; A conductive transparent layer disposed on the first surface of the substrate; An organic layer disposed on the conductive oxide layer; And And an upper contact disposed on the organic layer. And the contour of the second surface of the substrate is non-planar. The light emitting device of claim 1, wherein the non-planar shape is spherical. 2. A light emitting device according to claim 1, wherein said second surface contour makes a non-planar shape by shaping said substrate surface. The method of claim 1, wherein the second surface contour is non-planar by laminating a first surface of a second transparent layer having a non-planar second surface to the second surface of the substrate. Light emitting device. A method for increasing light emissivity for an organic light emitting diode (OLED) disposed on a first surface of a transparent substrate, Causing the second surface of the substrate to take a non-planar shape. 6. The method of claim 5, wherein the non-planar shape is spherical. 6. The method of claim 5, wherein the second surface contour forms a non-planar shape by shaping the substrate surface. 6. The method of claim 5, wherein the second surface contour is non-planar by laminating a first surface of a second transparent layer having a non-planar second surface to the second surface of the substrate. How to. In the method for constructing a light emitting device, Providing a transparent substrate having a first and a second surface; Disposing a conductive transparent layer on the first surface of the substrate; Disposing an OLED layer on the conductive transparent layer; And Causing the contour of the second surface of the substrate to take a non-planar shape; Method comprising a. 10. The method of claim 9, wherein the non-planar shape is spherical. 10. The method of claim 9, wherein the second surface contour forms a non-planar shape by shaping the substrate surface. 10. The method of claim 9, wherein the second surface contour is non-planar by laminating a first surface of a second transparent layer having a non-planar second surface to the second surface of the substrate. How to. In the light emitting device, A transparent substrate having a first surface and a second surface; And An OLED layer disposed on the first surface of the substrate; And the second surface of the substrate is shaped for a non-planar shape. The light emitting device of claim 13, wherein the non-planar shape is spherical. In the light emitting device, A transparent substrate having a first surface and a second surface; And An OLED layer disposed on the first surface of the substrate; And the contour of the second surface is non-planar by laminating the first surface of the second transparent layer having the non-planar second surface to the second surface of the substrate. 16. The light emitting device according to claim 15, wherein the non-planar shape is spherical. A method for increasing light emissivity of a light emitting device comprising at least two OLEDs disposed on a first surface of a transparent substrate, Including contouring of selected portions of the second surface of the substrate to take a non-planar shape; And wherein said selected portions are selected to be optically aligned with light output from one of said at least two OLEDs. 18. The method of claim 17, wherein the non-planar shape is spherical. 18. The method of claim 17, wherein the selected second surface contours form a non-planar shape by shaping the substrate surface. 18. The method of claim 17, wherein the selected second surface contours take a non-planar shape by laminating a first surface of a second transparent layer having a non-planar second surface to the second surface of the substrate. How to.