TRANSPARENT DIFFUSIVE OLED SUBSTRATE AND METHOD FOR PRODUCING SUCH A SUBSTRATE

Abstract

A method for preparing a laminate substrate for a light emitting device includes providing a glass substrate having a refraction index, at 550 nm, of between 1.45 and 1.65, coating a glass frit having a refractive index, at 550 nm, of at least 1.7 onto the glass substrate, firing the resulting frit coated glass substrate at a temperature above the Littleton temperature of the glass frit thereby forming a first high index enamel layer, coating a metal oxide layer onto the first high index enamel layer, and firing the resulting coated glass substrate at a temperature above the Littleton temperature of the glass frit, thereby making react the metal oxide with the underlying first high index enamel layer and forming a second high index enamel layer with a plurality of spherical voids embedded in the upper section of the second high index enamel layer near the interface with air.

Claims

1. A method for preparing a laminate substrate for a light emitting device, comprising at least the following steps: (a) providing a glass substrate having a refraction index, at 550 nm, of between 1.45 and 1.65, (b) coating a glass frit having a refractive index, at 550 nm, of at least 1.7 onto said glass substrate, said glass frit comprising at least 30 weight % of Bi.sub.2O.sub.3, (c) firing the resulting frit coated glass substrate at a temperature above the Littleton temperature of the glass frit thereby forming a first high index enamel layer, (d) coating a metal oxide layer onto said first high index enamel layer, and (e) firing the resulting coated glass substrate at a temperature above the Littleton temperature of the glass frit, comprised between 530° C. and 620° C., thereby making react the metal oxide with the underlying first high index enamel layer and forming a second high index enamel layer with a plurality of spherical voids embedded in the upper section of the second high index enamel layer near the interface with air.

2. The method according to claim 1, wherein the metal oxide layer has a thickness of between 5 and 60 nm.

3. The method according to claim 1, wherein the metal oxide is selected from the group consisting of TiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2, Nb.sub.2O.sub.5, HfO.sub.2, Ta.sub.2O.sub.5, WO.sub.3, Ga.sub.2O.sub.3, In.sub.2O.sub.3 and SnO.sub.2, and mixtures thereof.

4. The method according to claim 1, wherein the refractive index of the glass frit is comprised between 1.70 and 2.20.

5. The method according to claim 1, wherein the glass frit comprises at least 50 weight % of Bi.sub.2O.sub.3.

6. The method according to claim 1, wherein the firing steps (c) and (e) are carried out at a temperature comprised between 540° C. and 600° C.

7. The method according to claim 1, further comprising (f) coating a transparent electro-conductive layer on the second high index enamel layer.

8. A laminate substrate obtainable by the method according to claim 1, comprising (i) a glass substrate having a refractive index of between 1.45 and 1.65, (ii) a high index glass enamel layer having a refractive index, at 550 nm, of at least 1.7, and comprising at least 30 weight % of Bi.sub.2O.sub.3, wherein a plurality of spherical voids are embedded in the high index enamel layer near the surface thereof, at least 95% of the spherical voids having a diameter significantly smaller than the half-thickness of the high index enamel layer and being located in the upper half of the high index enamel layer near the interface with air.

9. The laminate substrate according to claim 8, wherein the spherical voids have an average equivalent spherical diameter of between 0.2 μm and 8 μm.

10. The laminate substrate according to claim 8, wherein the thickness of the second high index enamel layer is comprised between 3 μm and 25 μm.

11. The laminate substrate according to claim 8, further comprising (iii) a transparent electro-conductive layer on the high index enamel layer.

12. The laminate substrate according to claim 8, wherein the spherical voids occupy at least 20% of the surface and at most 80% of the surface of the area previously covered by the metal oxide.

13. The method according to claim 1, wherein the metal oxide layer has a thickness of between 10 and 40 nm.

14. The method according to claim 1, wherein the metal oxide layer has a thickness of between 15 and 30 nm.

15. The method according to claim 1, wherein the refractive index of the glass frit is comprised between 1.80 and 2.10.

16. The method according to claim 1, wherein the glass frit comprises at least 60 weight % of Bi.sub.2O.sub.3.

17. The laminate substrate according to claim 8, wherein at least 99% of the spherical voids have a diameter significantly smaller than the half-thickness of the high index enamel layer.

18. The laminate substrate according to claim 8, wherein essentially all of the spherical voids have a diameter significantly smaller than the half-thickness of the high index enamel layer.

19. The laminate substrate according to claim 8, wherein the spherical voids have an average equivalent spherical diameter of between 0.5 μm and 3μm.

20. The laminate substrate according to claim 8, wherein the thickness of the second high index enamel layer is comprised between 5μm and 15 p.m.

Description

[0063] FIG. 1 is a flowchart showing the process of preparing the laminate substrate of the present invention

[0064] FIG. 2 is a Scanning Electronic Microscope (SEM) photograph showing a view of cross-section and top surface of the laminate substrate of the present invention after the firing step (e).

[0065] FIG. 3 is a Scanning Electronic Microscope (SEM) photograph showing a cross-sectional view of a laminate substrate according to the invention.

[0066] FIG. 4 shows four top views of laminate substrates with preexisting open-bubble surface defects, before and after curing by coating and firing of a thin metal oxide layer.

[0067] FIG. 5 shows a top view and cross-sectional view of a laminate substrate prepared according to the below Example.

[0068] In FIG. 1, a flat transparent glass substrate 1 is first provided in step (a). In step (b) a high index glass frit 3 is then coated onto one side of this substrate for example by screen printing a paste including the glass frit and an organic medium (polymer & organic solvent). In the next step (step (c)) the resulting glass frit-coated substrate is heated to a temperature sufficient to melt the frit into a continuous high index enamel layer 4. This first high is index enamel layer 4 is then coated, for example by magnetron sputtering, with a metal oxide 2.

[0069] The resulting substrate carrying the metal oxide layer 2 on the first high index enamel layer 4 is then submitted to a second firing step (step (e)). During this final heating step, spherical voids 6 form from reaction between the metal oxide and the first high index enamel layer 4, resulting in a second high index enamel layer 5 incorporating, below its interface 7 with atmosphere, a row of fine spherical voids 6. The spherical voids are rather close to the surface of the second high index enamel layer 5 but do not rise to the surface of this layer. A transparent electro-conducting layer 6 is then coated, in step (f), onto the perfectly smooth surface of the second high index enamel layer 5.

[0070] On the SEM photograph of FIG. 2 one can see both the row of small spherical voids below the surface of the enamel layer, and also the high surface quality of this enamel layer.

[0071] On the SEM photograph of FIG. 3 the black glass substrate is covered by a lighter grey layer of high index enamel, corresponding to the second high index layer 5 of FIG. 1. A monolayer of spherical voids of different sizes is completely embedded therein and located close to but clearly below the interface with air (upper black coloured zone).

[0072] The laminate substrate shown does not yet comprise a transparent electro-conducting layer. One can see that the surface of the high index enamel layer is perfectly smooth and free of crater-like surface irregularities.

[0073] At FIG. 4 one can see at the first line two surface defects resulting from solidified burst bubbles in the high index enamel layer after firing step (c) of the present invention. The same defects were then again photographed after coating and firing for 10 minutes at 570° C. of a thin TiO.sub.2 layer. Both defects are still slightly visible but the sharpness of their edges has completely disappeared. Healing of those surface defects took place.

EXAMPLE

[0074] A 0.7 mm soda lime glass sheet was screen printed with a paste is comprising 75 weight % of a high index glass frit (Bi.sub.2O.sub.3-B.sub.2O.sub.3-ZnO-Al.sub.2O.sub.3-SiO.sub.2) and 25 weight % of an organic medium (ethyl cellulose and organic solvent) and submitted to a drying step (10 minutes at 150° C.).

[0075] The substrate was then fired for about 10 minutes at 570° C. resulting in a high index enamel layer having a thickness of 12 μm.

[0076] The resulting enamel-coated glass sheet was then coated by sol-gel deposition with a TiO.sub.2 precursor and fired for about 10 minutes at 560° C., resulting in a high index enamel layer incorporating a plurality of spherical voids near the surface.

[0077] The mean size of the spherical voids and the coverage rate (percentage of area of TiO.sub.2 coated surface occupied by the spherical voids) were measured by image analysis. The below table shows the mean size of the spherical voids, the coverage rate and the haze ratio of the resulting substrate in comparison to an identical substrate submitted to the same treatment and analysis except for the TiO.sub.2 coating step.

TABLE-US-00001 TABLE 1 Negative control Example Thickness of the TiO.sub.2 layer 0 22 nm Mean size of spherical voids — 1.8 μm Coverage rate —   77% IEL haze ratio 12.9% 67.5%

[0078] The high index enamel layer of the negative control was free of spherical voids located at the top of the enamel layer.

[0079] These experimental data clearly show that the spherical voids at the top of the enamel layer result from the interaction of the metal oxide layer with the overlying high index glass frit.

[0080] FIG. 5 shows a top view (left) and cross-sectional view (right) of the sample corresponding to table 1.