High refractive index nanocomposite layer

Abstract

The present invention relates to an OLED internal light extraction scheme with graded-index layer and embedded scattering particles.

Claims

1. An organic light emitting diode (OLED) lighting device comprising an internal light extraction layer, wherein the internal light extraction layer comprises: a high refractive index nanocomposite comprising inorganic nanocrystals embedded in a polymer matrix, wherein the nanocomposite has a refractive index of 1.7 and higher at 400 nm in at least part of the nanocomposite, and wherein a 3-4 micron thick sample of the nanocomposite has an optical transmittance of at least 90% over a range of 440 nm to 800 nm.

2. The OLED lighting device of claim 1, wherein the nanocrystals comprise ZrO.sub.2 nanocrystals.

3. The OLED lighting device of claim 1, wherein the nanocrystals have a size smaller than 10 nm in at least one dimension.

4. The OLED lighting device of claim 1, wherein the polymer matrix comprises an acrylic polymer.

5. The OLED lighting device of claim 1, wherein the internal light extraction layer has an index gradient.

6. The OLED lighting device of claim 5, wherein the index gradient results at least in part from a concentration gradient of the inorganic nanocrystals.

7. The OLED lighting device of claim 1, wherein the internal light extraction layer further includes light scattering particles.

8. The OLED lighting device of claim 7, wherein the scattering particles comprise TiO.sub.2 particles.

9. The OLED lighting device of claim 7, wherein the scattering particles comprises polymer beads.

10. The OLED lighting device of claim 7, wherein the scattering particles comprise agglomerated ZrO.sub.2 nanocrystals.

11. The OLED lighting device of claim 1, wherein the inorganic nanocrystals are capped nanocrystals.

12. The OLED lighting device of claim 2, wherein the inorganic nanocrystals are capped nanocrystals.

13. An organic light emitting diode (OLED) lighting device comprising an internal light extraction layer comprising: a high refractive index nanocomposite comprising inorganic nanocrystals embedded in a polymer matrix in an amount of greater than 70% by weight of the nanocomposite, wherein the nanocomposite has a refractive index of 1.7 and higher at 400 nm in at least part of the nanocomposite, and wherein a 3-4 micron thick sample of the nanocomposite has an optical transmittance of at least 90% over the range of 440 nm to 800 nm.

14. The OLED lighting device of claim 13, wherein the nanocrystals comprise ZrO.sub.2 nanocrystals.

15. The OLED lighting device of claim 13, wherein the inorganic nanocrystals are capped nanocrystals.

16. The OLED lighting device of claim 14, wherein the inorganic nanocrystals are capped nanocrystals.

17. The OLED lighting device of claim 1, wherein at least a part of the nanocomposite has a refractive index of 1.8 and higher at 400 nm.

18. The OLED lighting device of claim 13, wherein at least a part of the nanocomposite has a refractive index of 1.8 and higher at 400 nm.

19. The OLED lighting device of claim 1, wherein at least a part of the nanocomposite has a refractive index of 1.9 at 400 nm.

20. The OLED lighting device of claim 13, wherein at least a part of the nanocomposite has a refractive index of 1.9 at 400 nm.

Description

LIST OF FIGURES AND TABLE

(1) FIG. 1A: An OLED device without light extraction scheme.

(2) FIG. 1B: Internal light extraction with scatterers or surface texture, with all light being scattered.

(3) FIG. 1C: The present invention, an ideal gradient-index layer with sparse light scatterers adjacent to the transparent electrode. The dashed rays represent the light travels within the cone of acceptance (the maximum solid angle where light can be coupled into the next layer) and the solid rays represent the light travels outside. The solid rays are refracted backward by the gradient-index profile, resulting in a much longer optical path and much higher scattering probability than the dashed rays, a more efficient mechanism compared to (b).

(4) FIG. 2A: The refractive index data of a photo-curable acrylic formulation, developed by Pixelligent, with different nanocrystal loadings.

(5) FIG. 2B: Transmission electron microscopy of the nanocrystal loaded polymer. The inset showcases the clarity of a 5 ?m thick nanocomposite film with 80 wt % ZrO.sub.2 nanocrystal loading.

(6) FIG. 3: An illustration of the process flow of the present invention to create a nanocomposite gradient index layer. 1) The first layer of nanocomposite is applied on the substrate using slot-die coating as an example, other film formation processes could also be used; 2) the applied layer is partially UV-cured (shown with four arrows above the films in (2) and (4), also other curing mechanism like thermal, electron exposure, etc.) to retain certain mechanical strength to with stand the next coating step; 3) a second nanocomposite layer with a different nanocrystal loading is again applied by slot-die coating; 4) the second layer is again partially cured; and step 3 and 4 may be repeated until the intended index profile is completed; 5) and then a final cure is applied to solidify the gradient index layer. After the film is solidified, the nanocrystals will no longer be able to diffuse. Note: For simplicity, the light scatterers are not shown in the figure.

(7) FIG. 4A: UV absorption spectrum of film from formulation (ZrO.sub.2-(2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid) in 70:30 BMA-TMPTA) after post bake at (101) 120 C for 3 minute in air, (102) thermal bake at 175 C for 1 hour under N.sub.2.

(8) FIG. 4B: UV transmission spectrum of film from formulation (ZrO.sub.2-(2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid) in 70:30 BMA-TMPTA) after post bake at (201) 120 C for 3 minute in air, (202) thermal bake at 175 C for 1 hour under N.sub.2.

(9) FIG. 5A: UV absorption spectrum of film from formulation (ZrO.sub.2-(2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid) in 70:30 BMA-TMPTA) after post bake at (101) 120 C for 3 minute in air, (103) thermal bake at 200 C for 1 hour under N.sub.2.

(10) FIG. 5B: UV transmission spectrum of film from formulation (ZrO.sub.2-(2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid) in 70:30 BMA-TMPTA) after post bake at (201) 120 C for 3 minute in air, (203) thermal bake at 200 C for 1 hour under N.sub.2.

(11) FIG. 6A: UV absorption spectrum of film from formulation (ZrO.sub.2-(2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid) in 70:30 BMA-TMPTA) after post bake at (101) 120 C for 3 minute in air, (104) thermal bake at 200 C for 2 hour under N.sub.2.

(12) FIG. 6B: UV transmission spectrum of film from formulation (ZrO.sub.2-(2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid) in 70:30 BMA-TMPTA) after post bake at (101) 120 C for 3 minute in air, (104) thermal bake at 200 C for 2 hour under N.sub.2.

(13) Table 1: Film results of capped ZrO.sub.2 nanocrystals in monomer mixture. Good indicates that the film does not yellow or crack when heated at those indicated temperatures. Cracked indicates that the film cracked during thermal baking. Disadvantage of this formulation is that it comprises of PGMEA to aid in the solubility.

(14) The present application provides an OLED lighting device containing an internal light extraction layer, the internal light extraction layer containing: a high refractive index nanocomposite containing inorganic nanocrystals embedded in a polymer matrix. The OLED of the presently disclosed technology may contain nanocrystals containing ZrO.sub.2 nanocrystals. The OLED of the present disclosure may contain nanocrystals having a size smaller than 10 nm in at least one dimension. The OLED of the present disclosure may contain a polymer matrix containing an acrylic polymer. The internal light extraction layer of OLEDs of the present disclosure may have a refractive index greater than 1.6 at visible wavelength in at least part of the layer. Further, the internal light extraction layer of OLEDs of the present disclosure may have an index gradient. The index gradient of OLEDs of the present disclosure may results at least in part from a concentration gradient of the inorganic nanocrystals. The internal light extraction layer of OLEDs of the present disclosure may further include light scattering particles. Scattering particles of the OLEDs of the present disclosure may include any one, or combinations of TiO.sub.2 particles, polymer beads, and/or agglomerated ZrO.sub.2 nanocrystals.

(15) The present disclosure further provide methods of manufacturing an internal light extraction layer for an OLED lighting device that includes applying a first nanocomposite layer comprising inorganic nanocrystals dispersed in a polymer matrix with one concentration on a substrate; at least partially curing the first nanocomposite layer; applying a second nanocomposite layer on at least part of the first nanocomposite layer, the second nanocomposite layer containing inorganic nanocrystals dispersed in a polymer matrix with a second concentration; optionally allowing the second nanocomposite layer to at least partially settle on said first nanocomposite layer for an intermediate period; and fully or further curing the first and second nanocomposite layers to form an internal light extraction layer of the present disclosure.

EXAMPLES

Example 1

(16) In one example of said exemplary non-limiting formulation, acrylic monomers, benzyl methacrylate (BMA) and trimethylolpropane triacrylate (TMPTA), was mixed in a mass ratio of 70-75 to 25-30. 1-5 wt % of benzophenone as photo initiator, was dissolved in the monomer mix either by stirring or vortexing at temperature of 20-30 C. The solution was then filtered to remove dusts and then added to dry ZrO.sub.2 nanocrystal and allowed to soak in the monomer blend until no ZrO.sub.2 powder was observed. In large scale, gently shaking the dried nanocrystals with the monomer blend is acceptable. Once all ZrO.sub.2 nanocrystals powder was completely dispersed in BMA-TMPTA, the viscous suspension was mixed for 10-15 hours. Finally, the viscous suspension was filtered before processing the film.

(17) The suspension was validated by coating films and characterizing the physical properties of the films such as thermal stability and transmittance.

(18) As a standard method, the suspension was coated on a 2 silicon wafer or fused silica wafer to inspect its quality. The wafers were cleaned before applying the film to remove contaminants and dusts. 3-4 micron thick film was spin coated on silicon wafer at 1000-4000 rpm for 1-5 minute.

(19) An optional pre-bake process at 90 C may be performed to remove the residual solvent if that is a concern. In these formulations the solvent is typically less than 10 wt %, more preferably less than 1 wt %. The film was inspected for defects from undispersed particles or air bubbles. If no defects were observed, its surface roughness is measured using a surface profilometer.

(20) The film coated on glass slide or fused silica wafer was cured by UV exposure for 60-200 seconds using a Dymax EC-5000 system with a mercury H bulb and then post-baked for 2-5 minutes at 120-150 C under air. Further, the thermal stability of the film was tested by heating the film at a temperature of 175 C or above, more preferably 200 C, under nitrogen atmosphere for 1-2 hours. A crack free, colorless film is desirable and indicates a good formulation.

(21) These film demonstrate a refractive index of 1.80 or greater at 400 nm and transmittance>89% at 400 nm.

(22) The refractive index is measured with a Woollam M-2000 spectroscopic ellipsometer in the spectral range from 350 nm to 1700 nm and the transmittance was measured using a Perkin Elmer Lambda 850 Spectrophotometer.

(23) This example formulation with 65-75:25-35 mass ratio of BMA to TMPTA with nanocrystal loading of 50 wt % and above produced films that are UV curable and can withstand a thermal baking at 200 C for 1-2 hour under nitrogen, as shown in Table 1.

Example 2

(24) Films spin coated from formulation containing zirconium oxide nanocrystals capped with 2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid at 50-80 wt % loading in the BMA-TMPTA (65-75:25-35 mass ratio) were stable and did not crack when heated at temperatures up to 200 C. However, films from formulation containing zirconium oxide nanocrystals capped with 2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid at 82-85 wt % loading in the BMA-TMPTA (65-75:25-35 mass ratio) were stable only at temperatures below 120 C, as shown in Table 1. Also, zirconium oxide nanocrystals modified with other capping agents such as methoxy(triethyleneoxy) propyltrimethoxysilane and/or 3-methacryloyloxypropyltrimethoxysilane and/or n-octyl trimethoxysilane and/or dodecyltrimethoxysilane and/or m,p-ethylphenethyl trimethoxysilane formed good dispersions in BMA-TMPTA mixture, as well as good films, but was only stable up to 120 C.

(25) One advantage of this exemplary non-limiting embodiment is that both monomers are in liquid form at room temperature so no solvent is necessary at room temperature and the film is UV curable. Surface modified ZrO.sub.2 nanocrystals are dispersed directly in the monomer. Such a direct dispersion eliminates, for example, the need to remove the solvent at a later step.

(26) Nanocrystals of the exemplified embodiments of the present disclosure have been surface modified with various capping agents such as 2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid and/or methoxy(triethyleneoxy) propyltrimethoxysilane and/or 3-methacryloyloxypropyltrimethoxysilane and or n-octyl trimethoxysilane and/or dodecyltrimethoxysilane and/or m,p-ethylphenethyl trimethoxysilane. In an exemplified method of producing the capped nanocrystals of the present disclosure, the as-synthesized nanocrystals are allowed to settle for at least 12 hours after synthesis. Since the nanocrystals are surface modified in a solvent other than the synthesis solvent, the nanocrystals are separated from the reaction liquid by decanting off the reaction liquid and rinsing the nanocrystals with the capping solvent. The rinsing solvent is decanted off to obtain a wet cake of uncapped nanocrystals.

(27) For the surface modification of the nanocrystals with 2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid, the nanocrystals are suspended in the capping solvent, for example, toluene for 2-[2-(2-9-methoxyethoxy)ethoxy]acetic acid modification, at a loading of 10 wt % or greater, or 20 wt % or greater, or 30 wt % or greater, calculated based on the weight of the wet nanocrystal cake. While the suspension is stirred, the capping agent is added to it slowly. The amount of capping agent used is in the presently exemplified embodiment 8-60 wt % to the weight of the wet nanocrystal cake. The suspension is allowed to stir at 20-27? C. for 10-30 minutes and then refluxed at the boiling point of the capping solvent for 30-60 minutes. After refluxing, the clear solution is cooled to 50-60? C. slowly. This suspension is then filtered to remove dusts and aggregates bigger than 200 nm sizes.

(28) The capped nanocrystals are then precipitated out from the capping solvent using heptane (2-4 times the mass of the capped solution). The precipitated nanocrystals are collected by centrifugation. The nanocrystal thus collected is dispersed in tetrahydrofuran (THF) and again re-precipitated using heptane. This process is repeated twice. The wet cake of nanocrystals collected in the final step is dried under vacuum for at least 12 hours.

(29) TABLE-US-00001 TABLE 1 Content of ZrO.sub.2 Post baked at Post baked at Post baked at Monomer mix to monomer Capping agent 120 C./60/air 175 C./60/N.sub.2 200 C./N2/60 min 2-10 wt % Bisphenol 50-80 wt % 2-[2-(2-9- good cracked A diglycerolate methoxyethoxy) dimethacrylate in ethoxy]acetic acid BMA 2-25 wt % TMPTA 50-80 wt % methoxy(triethyleneoxy) good cracked in BMA propyltrimethoxysilane and 3- methacryloyloxypropyl trimethoxysilane 25-30 wt % TMPTA 50-80 wt % methoxy(triethyleneoxy) good good cracked in BMA propyltrimethoxysilane and 3- methacryloyloxypropyl trimethoxysilane 20-30 wt %T MPTA 50-80 wt % 2-[2-(2-9- good good good in BMA methoxyethoxy) ethoxy]acetic acid 25-30 wt % TMPTA 50-80 wt % methoxy(triethyleneoxy) good cracked cracked in BMA propyltrimethoxysilane 25-30 wt % TMPTA 82-86 wt % 2-[2-(2-9- good cracked cracked in BMA methoxyethoxy) ethoxy]acetic acid