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 (a) providing a glass substrate having a refraction index of between 1.45 and 1.65, (b) coating a metal oxide layer onto one side of the glass substrate, (c) coating a glass frit having a refractive index of at least 1.7 onto the metal oxide layer, the glass frit including at least 30 weight % of Bi.sub.2O.sub.3, (d) firing the thus coated glass substrate at a temperature comprised between 530 C. and 620 C. thereby making react the metal oxide with the melting glass frit and forming a high index enamel layer with a plurality of spherical voids embedded in the lower section of the enamel layer near the interface with the glass substrate.

Claims

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

2. The method according to claim 1, wherein the metal oxide layer has a thickness of between 5 and 80 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 of the high index glass frit is carried out at a temperature comprised between 540 C. and 600 C.

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

8. A laminate substrate obtainable by the method of 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 comprising at least 30 weight % of Bi.sub.2O.sub.3 and having a refractive index, at 550 nm, of at least 1.7, wherein a plurality of spherical voids are embedded in the high index enamel layer near an interface of the high index enamel layer with the underlying glass substrate, at least 95% of the spherical voids having a diameter significantly smaller than a half-thickness of the high index enamel layer and being located in a lower half of the high index enamel layer near the interface with the underlying glass substrate.

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

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

11. The laminate substrate according to claim 8, wherein the spherical voids are in contact with the underlying glass substrate.

12. The laminate substrate according to claim 8, wherein the spherical voids form a monolayer of individual voids in contact with the underlying glass substrate.

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

14. The method according to claim 2, wherein the metal oxide layer has a thickness of between 10 and 40 nm.

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

16. The method according to claim 4, wherein the refractive index of the glass frit is comprised between 1.80 and 2.10.

17. The method according to claim 5, wherein the glass frit comprises at least 60 weight % of Bi.sub.2O.sub.3.

18. 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.

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

20. The laminate substrate according to claim 9, wherein the spherical voids have an average equivalent spherical diameter of between 0.4 m and 4 m.

21. The laminate substrate according to claim 20, wherein the spherical voids have an average equivalent spherical diameter of between 0.5 m and 3 m.

22. The laminate substrate according to claim 10, wherein the thickness of the high index enamel layer is comprised between 4 m and 20 m.

23. The laminate substrate according to claim 22, wherein the thickness of the high index enamel layer is comprised between 5 m and 15 m.

Description

(1) In a preferred embodiment, the method of the present invention therefore further comprises an additional step of coating a transparent electro-conductive layer (TCL) on the high index enamel layer. This layer preferably is a transparent conductive oxide such as ITO (indium tin oxide). Formation of such a TCL may be carried out according to conventional methods familiar to the skilled person, such as magnetron sputtering.

(2) FIG. 1 is a flowchart showing the process of preparing the laminate substrate of the present invention

(3) FIG. 2 is a Scanning Electronic Microscope (SEM) photograph showing a cross-sectional view of a laminate substrate according to the invention

(4) In FIG. 1, a flat transparent glass substrate 1 is first provided in step (a). In step (b) a metal oxide layer 2 is then coated onto one side of this substrate by magnetron sputtering. In the next step (step (c)) a layer 3 of a high index glass frit is applied for example by screen printing a paste including the glass frit and an organic medium (polymer & organic solvent).

(5) The resulting substrate carrying the metal oxide layer 2 and glass paste layer 3 is then submitted in step (d) to a stepwise heating to first evaporate the organic solvent, then burn out the organic polymer and eventually fuse the glass frit powder to obtain a high index enamel layer 4. During this final heating step, spherical voids 5 form at the bottom of the glass frit layer from reaction between the metal oxide and the glass frit. The spherical voids stick to the interface of the high index enamel 4 and do not rise to the surface of the enamel layer. A transparent electro-conducting layer 6 is then coated, in step (e), onto the smooth surface of the high index enamel 4.

(6) On the SEM photograph of FIG. 2 the dark grey glass substrate is covered by a lighter grey layer of high index enamel. A monolayer of spherical voids is completely embedded therein and located in contact with the interface between the glass substrate and the overlying enamel. 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.

EXAMPLE

(7) A 0.7 mm soda lime glass sheet was spin coated with a solution of a TiO.sub.2 precursor. The coated glass sheet was then submitted for 10 minutes to a temperature of 150 C. for solvent evaporation and then for about 1 hour to a temperature of 400 C. to effect densification of the TiO.sub.2 layer.

(8) The resulting TiO.sub.2-coated glass sheet was screen printed with a paste comprising 75 wt % of a high index glass frit (Bi.sub.2O.sub.3B.sub.2O.sub.3ZnOSiO.sub.2) and 25 wt % of an organic medium (ethyl cellulose and organic solvent) and submitted to a drying step (10 minutes at 150 C.).

(9) The substrate was then fired for about 10 minutes at 570 C. resulting in a high index enamel layer (12 m) containing a plurality of spherical voids.

(10) The mean size of the spherical voids and the coverage rate (area of the TiO.sub.2-coated surface occupied by the spherical voids) were measured by image analysis on three different samples with increasing TiO.sub.2 layer thickness.

(11) The below table shows the mean size of the spherical voids, the coverage rate, and the haze ratio of the resulting substrate for increasing amounts of TiO.sub.2 in comparison with a negative control made by coating of the high index glass frit directly on the soda lime glass.

(12) TABLE-US-00001 Negative control Example 1 Example 2 Example 3 Thickness of the TiO.sub.2 layer 0 26 nm 32 nm 39 nm after densification Mean size of spherical 1.1 m 1.4 m 3.6 m voids Coverage rate 49.9% 65% 73.1% IEL haze ratio 12.9% 56.0% 74.1% 75.2

(13) The high index enamel layer of the negative control was free of spherical voids located at the bottom of the enamel layer.

(14) Increasing the amount of metal oxide resulted in an increase of the mean size of the spherical voids formed at the glass/enamel interface, of the area occupied by the voids and of the haze ratio of the resulting IEL layer.

(15) These experimental data clearly show that the spherical voids at the bottom of the enamel layer result from the interaction of the metal oxide layer with the overlying high index glass frit.

(16) FIG. 3 shows a projection view (left) and a cross-sectional view (right) for each of the above examples 1, 2 and 3 according to the present invention.