Transparent diffusive OLED substrate and method for producing such a substrate
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.
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 2 O 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 2 , Al 2 O 3 , ZrO 2 , Nb 2 O 5 , HfO 2 , Ta 2 O 5 , WO 3 , Ga 2 O 3 , In 2 O 3 and SnO 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 2 O 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. The method according to claim 1 , wherein the metal oxide layer has a thickness of between 10 and 40 nm.
9. The method according to claim 1 , wherein the metal oxide layer has a thickness of between 15 and 30 nm.
10. The method according to claim 1 , wherein the refractive index of the glass frit is comprised between 1.80 and 2.10.
11. The method according to claim 1 , wherein the glass frit comprises at least 60 weight % of Bi 2 O 3 .
12. 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 2 O 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.
13. The laminate substrate according to claim 12 , wherein the spherical voids have an average equivalent spherical diameter of between 0.2 μm and 8 μm.
14. The laminate substrate according to claim 12 , wherein the thickness of the second high index enamel layer is comprised between 3 μm and 25 μm.
15. The laminate substrate according to claim 12 , further comprising (iii) a transparent electro-conductive layer on the high index enamel layer.
16. The laminate substrate according to claim 12 , 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.
17. The laminate substrate according to claim 12 , 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 12 , 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 12 , wherein the spherical voids have an average equivalent spherical diameter of between 0.5 μm and 3 μm.
20. The laminate substrate according to claim 12 , wherein the thickness of the second high index enamel layer is comprised between 5 μm and 15 μm.