WO2011077153A2

WO2011077153A2 - Deposition process

Info

Publication number: WO2011077153A2
Application number: PCT/GB2010/052189
Authority: WO
Inventors: Kevin David Sanderson; Troy Darrell Manning; Allan Stuart Dale; Simon James Hurst
Original assignee: Pilkington Group Ltd
Current assignee: Pilkington Group Ltd
Priority date: 2009-12-22
Filing date: 2010-12-22
Publication date: 2011-06-30
Anticipated expiration: 2012-06-22
Also published as: GB0922395D0; WO2011077153A3

Abstract

Flame deposition of Anti-Reflective coating at relatively low temperature are both durable and have excellent AR properties. Refractive index in the range 1.25 to 1.40. Coating mixture contains coating precursor, oxidant (e.g. air) and optionally comburant (hydrocarbon or hydrogen to generate higher temperature flame) and/or carrier gas in specified ratio.

Description

DEPOSITION PROCESS

This invention relates to processes for the deposition of an anti-reflection coating on a substrate and to coated substrates comprising an anti-reflection coating on at least one surface.

Reduced reflections are a desirable feature of many optical systems. Anti reflection coatings increase transmission of incident light (by reducing reflection) and are a potentially useful feature of devices such as the covers for photovoltaic panels and photovoltaic cells.

The degree of anti reflection which is provided by a coating comprising a single layer of material on the surface of a substrate is often highest if porous coatings are deposited. However, the deposition of such porous coatings is not straightforward.

Sol gel type deposition processes have been proposed in which a silica sol is coated on to the surface of a substrate and heated at elevated temperature so as to drive off organic material resulting in the production of the anti reflective silica coating. Processes of this type have been disclosed in EP 1429997, DE 10146687, EP1328483 and USP 6918957 and are in commercial use. However the process is time consuming and the cost of production is relatively high. Moreover the coatings produced may be insufficiently durable to resist secondary glass processing such as toughening and laminating processes without significant deterioration in their properties.

There exists a need in the art for a process for depositing an anti reflection coating that exhibits improved durability and that can be deposited rapidly onto a substrate thereby lowering the costs of producing the coated substrate.

USPA 2006/003108 discloses a process for depositing a reflection reducing coating on to the surface of a glass substrate at 80°C in which a silicon containing precursor is decomposed with a flame and the substrate is introduced into the flame so as to apply the precursor to the substrate directly from the gas phase as an SiO_x (OH)₄__x coating wherein 0 < x < 2. The processes are used to coat glass panes which are passed repeatedly through the flame in order to deposit a coating having the desired properties.

WO-A-2009/00745 discloses flame pyrolysis processes for deposition of anti reflective coatings upon the surface of a continuous glass substrate. Another process for the deposition of silica coatings having a refractive index of 1.45 or greater on to the glass substrate formed during a float glass production process is disclosed in WO 2005/023723.

It is an aim of the present invention to overcome problems associated with the prior art and to provide processes for deposition of anti-reflection coatings which are flexible enough to be used to deposit coatings before, during and/or after other coatings have been deposited, can be used both off-line and on-line (i.e. during the glass production process) yet are still able to provide durable and effective anti reflection coatings.

The present invention accordingly provides, in a first aspect, a process for the deposition of an anti-reflection coating on a substrate, the process comprising providing a substrate at a substrate temperature of 100°C to 350°C, passing a fluid mixture comprising a coating precursor through a flame, and contacting at least one surface of the substrate with the coating precursor during or after its passage through the flame.

Surprisingly, the inventors have found that coatings deposited at this relatively low temperature are both durable and have excellent anti reflection properties.

Preferably, the substrate temperature is 105 to 310°C, 105 to 300°C, 105 to 250°C, more preferably 105 to 200°C and most preferably 105 to 190°C. The most preferred temperature range is 160 to 190°C. Deposition on to substrates at a temperature of greater than 100°C is advantageous because it results in good coatings without requiring a subsequent heating step (which is time consuming and relatively inefficient in production).

The fluid mixture preferably further comprises a comburant and/or a carrier fluid (preferably a carrier gas). The fluid mixture preferably further comprises an oxidant which will usually be oxygen e.g. from air.

In a preferred embodiments of the present invention the amount of oxidant in the fluid mixture is such that the parameter λ _oxidant,

wherein A_oxidant is the amount of oxidant, Aoxcomburant is the amount of oxidant necessary to fully oxidise the comburant and Aa Ixprecursor is the amount of oxidant necessary to fully oxidise the coating precursor. When the amount of oxidant is in this range, it has been surprisingly discovered that the anti-reflection properties of the coated substrate are good especially at the relatively low temperature used in the process according to the first aspect of the invention. Previously (in for example WO-A-2009/00745) it was thought that the amount of oxidant (oxygen) should preferably be greater than 1.5.

It is preferred if _oxidaiit is less than 1.3, more preferably less than 1.2 and most preferably 1 or lower.

The present invention provides, in a second aspect, a process for the deposition of an anti reflection coating on a substrate, the process comprising providing a substrate, passing a fluid mixture through a flame, the fluid mixture comprising a coating precursor, an oxidant, and, optionally, a comburant, contacting at least one surface of the substrate with the fluid mixture during or after its passage through the flame, characterised in that the amount of oxidant in the fluid mixture is such that the parameter λ₀,^_αη:,

^oxidant Aoxidant — 1 · 5

wherein A_oxidant is the amount of oxidant, Aoxcomburant is the amount of oxidant needed for complete combustion of the comburant and Aoxprecursor is the amount of oxidant needed for complete combustion of the coating precursor.

In either aspect of the invention the refractive index of the anti reflection coating is preferably 1.25 to 1.4 to provide good anti-reflection properties. The effective refractive index varies with the porosity and the surface roughness of the deposited coating. These parameters are influenced by the temperature of the substrate, by the material from which the anti reflection layer is formed and the precursor of that material that is used and by the conditions under which the process is carried out.

Usually the anti reflection coating will comprise a porous portion which is advantageous because a porous portion of e.g. a silicon oxide coating generally has a lower refractive index.

Preferably, the fluid mixture comprises a silicon oxide, preferably silica, coating precursor and the anti reflection coating comprises a silicon oxide, preferably silica. Examples of precursors which may be used in the formation of silicon oxide, preferably silica, coatings include compounds having the general formula SiX₄ wherein the groups X, which may be the same or different, represent a halogen atom especially a chlorine atom or a bromine atom, a hydrogen atom, an alkoxy group having the formula - OR or an ester group having the formula -OOCR wherein R represents an alkyl group comprising from 1 to 4 carbon atoms. Particularly preferred precursors for use in the present invention include tetraethoxysilane (TEOS), hexamethyldisiloxane (HMDSO) and silane.

Preferably, the precursor is directed to the substrate surface at a flow rate of 2 to 25 m/min, more preferably 3 to 21 m/min and most preferably 4 to 16 m/min (where the units are derived from chemical flow (in m³min^_1) ÷ burner face area (in m²) to take account of flow rates in burners of different face areas.) These flow rates are advantageous because they result in excellent coatings with good properties.

Preferably, the anti reflection coating is deposited to a thickness of 10 to 500 nm. Preferably the thickness of the coating is greater than 25 nm, 40 nm, 50 nm, 80 nm or 100 nm and most preferably 105 to 500 nm. This is advantageous because it enables good anti- reflection properties to be provided. Very thin coatings are generally not significantly anti- reflective.

The thickness of the coating is preferably that which will result in destructive interference between the light reflected from the surface of the coating and the surface of the glass. For optimum destructive interference the length of the optical path in the coating should be equal to one half of the wavelength of the light. This thickness can be calculated from the equation: where t is the thickness of the coating, λ is the wavelength of the incident light and n is the refractive index of the coating.

In a preferred embodiment of the invention the substrate comprises glass, especially float glass (i.e. glass manufactured by the float glass process) or rolled glass. It is advantageous if the glass has an iron content of 0.015% by weight or lower because this tends to increase the visible light transmission. The process of the invention may be conducted on line or off-line. If conducted on line during the float glass production the substrate will generally be a ribbon of float glass.

Whether conducted on line or off line, the coating, if deposited on float glass, may be deposited on the gas side surface of float glass, but is preferably deposited on the tin side surface of float glass. This is advantageous because subsequent coating of the coated substrate may then be performed directly on the gas side surface of the glass.

Whichever surface of the glass is coated, the process may further comprise a step of depositing at least one further coating either on the same surface as the anti reflection coating or on another (or the other) surface. The, or each, further coating may be deposited before, during (if on the other surface), or after deposition of the anti reflection coating. One possible further coating is a coating of a transparent conductive oxide. Transparent conductive oxide coatings include coatings of doped tin oxide, doped zinc oxide or indium oxides (e.g. indium tin oxide, ITO).

It has surprisingly been discovered by the inventors of the present invention that the coatings deposited by the process are of much better quality if the deposition is conducted when the flame is substantially stable. Thus, it is preferred if the process is conducted with a substantially stable flame.

Flame deposition processes usually comprise the steps of forming a fluid mixture comprising a precursor of an oxide of a metal or a metalloid, an oxidant and, optionally, a comburant. This fluid mixture may then be ignited at a point which is adjacent to the surface of the substrate. The precursor for the oxide may be any compound of a metal or metalloid which may be dispersed in the fluid mixture and which will decompose to form an oxide when the mixture is ignited. Processes in which the precursor is in the vapour phase are commonly termed combustion chemical vapour deposition processes (hereinafter for convenience CCVD processes). In a preferred embodiment the processes of this invention are CCVD processes.

The burner used in the flame deposition process preferably extends across the full width of the substrate although a series of smaller burners may be used. The burner is preferably positioned above the substrate in close proximity to the surface of the glass substrate. The distance between the burner and the substrate will typically be in the range of from 2 to 20 mm and preferably in the range 5.0 to 15.0 mm. Such close proximity results in a coating having improved properties possibly because it minimises the amount of recombination between the species produced by burning the precursor before they are deposited upon the surface of the substrate. It may be necessary to adjust the distance between the burner and the surface of the glass substrate in order to optimise the properties of the desired coating. A plurality of burners positioned along the length of the substrate may be used in order to deposit a coating having the desired thickness.

The thermal output of the burners useful in the processes of this invention may be from 0.5 to 10 kW/10cm², preferably from 1 to 5 kW/10cm². The concentration of precursor in the fluid mixture which is delivered to the burner is typically from 0.05 to 25 vol%, preferably from 0.05 to 5 vol% gas phase concentration.

The burner is preferably associated with means for extracting the exhaust gases from the area adjacent to the surface of the substrate. In the preferred embodiments at least one means for extraction is positioned adjacent to each burner. The extraction means is typically a conduit associated with a fan which produces an updraft at the mouth of the conduit. Each extraction means is preferably provided with control means whereby the draft provided may be adjusted. In the preferred embodiments of the invention the extraction means are controlled so as to isolate the burner flames from each other, to control the direction of the flame so as to optimise the impingement of the flame over the surface of the substrate and to efficiently remove the by products which are generated by the combustion.

The inventors have discovered that the quality of the coating which is deposited can be improved by extracting the exhaust gases in a manner which causes the tail of the flame to be positioned above the surface of the substrate i.e. when the burner is located above the substrate surface the tail of the flame is also located above the substrate surface and when the burner is located below the substrate surface the tail of the flame is also below the substrate surface. Extracting the gases in this way has been found to reduce powder formation and to improve the uniformity of the coating. These are significant advantages, especially in an on line coating process where a high deposition speed is advantageous.

The temperature of the flame varies with the choice of comburant. Any gas which can be burnt to generate a sufficiently high flame temperature to decompose the precursor is potentially useful. Generally the comburant will be one which generates a flame temperature of at least 1700°C. The preferred comburants include hydrocarbons such as propane, butane, acetylene, methane and natural gas or hydrogen.

The process uses an oxidant which may comprise a source of oxygen. Typically the oxidant may be air. The ratio of precursor and/or comburant to oxidant (e.g. air) may be adjusted so that the flame is either oxygen rich or oxygen deficient. The use of an oxygen rich flame favours the production of a fully oxidised coating whereas the use of an oxygen deficient flame favours the production of a coating which is less than fully oxidised.

The present invention provides, in a third aspect, a substrate having an anti reflection coating obtainable by a process as discussed above in the first and second aspects.

Uses of coated substrates provided by the present invention include in photovoltaic modules to increase visible light transmission and thereby improve efficiency of the module.

The present invention is illustrated by the Figure which is a graph of light transmission (%) against wavelength for Example 10 (see below).

The invention is also illustrated by the following Examples in which a fluid mixture comprising propane, air and hexamethyldisiloxane (HMDSO) was fed to a burner for flame deposition of silica coatings. Six passes of the substrate under the burner were made. The substrate was reduced iron (<0.015 wt %) float glass. Deposition conditions for the silica coatings were as indicated in Table 1. The HMDSO precursor was introduced by syringe drive/evaporator (200 °C) directly into air flow.

Burner

Face gas

HMDSO flow Propane speed

Example (cc/min) (SLM) Air (SLM) 0₂ (SLM) ^oxidant (m/s)

1 0.2 3.0 75.0 4.1 1.30 2.90

2 0.73 1.8 56.5 0.8 1.30 2.09

3 0.73 3.6 77.0 0.0 0.85 2.85

4 0.36 2.8 68.0 0.0 0.99 2.51

5 0.36 2.3 57.0 0.0 1.00 2.10

6 0.2 2.8 68.0 0.0 1.00 2.50

7 0.36 3.0 74.0 4.5 1.30 2.88

8 0.36 3.6 67 0 0.76 2.50

9 0.2 3.3 56.0 0.0 0.70 2.10

10 0.73 3.6 66.0 0.0 0.73 2.46

11 0.73 2.2 57.0 0.0 1.00 2.10

Table 1.

Further details of the deposition conditions are described in Table 2 including substrate temperature at deposition.

Table 2

In Table 2, HMDSO flow is given in m/min (chemical flow ÷ burner face area) to allow comparison to burners of different size. Table 2 is ordered by temperature of base glass. T_vis of base glass (without AR coating) for the examples was 90.3%. T_vi_s values were calculated from the spectra of the samples to ISO9050 and EN410/673.

The optical properties of the Examples were analysed and the % improvement of weighted transmission values were calculated for standard photovoltaic cells of varying types. The results are as indicated in Table 3.

The silica coating significantly improves the transmission of light into PV cells with consequent improvement in efficiency. Surprisingly, the durability of the coatings did not appear to be affected by temperature of deposition and was good even at the low temperature (180°C ) deposition.

Table 3

In Table 3, c-Si refers to crystalline silicon PV cell, CdTe to a cadmium telluride PV cell, a-Si to amorphous silicon PV cell, uc-Si to microcrystalline silicon PV cell and CIGS to copper indium gallium selenide PV cell.

Results for Example 2 are not reproduced in Table 2 or 3 because it was observed that the flame was not stable during deposition resulting in poor and uneven coating quality.

Claims

1. A process for the deposition of an anti-reflection coating on a substrate, the process comprising

a) providing a substrate at a substrate temperature of 100°C to 350°C, b) passing a fluid mixture comprising a coating precursor through a flame, and

c) contacting at least one surface of the substrate with the coating precursor during or after its passage through the flame.

2. A process as claimed in claim 1, wherein the substrate temperature is 105 to 310°C, preferably 110 to 300°C.

3. A process as claimed in either claim 1 or claim 2, wherein the substrate temperature is 110 to 250°C, preferably 110 to 200°C.

4. A process as claimed in any one of the preceding claims wherein the fluid mixture further comprises a comburant.

5. A process as claimed in any one of the preceding claims wherein the fluid mixture further comprises a carrier fluid, preferably a carrier gas.

6. A process as claimed in any one of the preceding claims wherein the fluid mixture further comprises an oxidant.

7. A process as claimed in claim 6, wherein the amount of oxidant in the fluid mixture is such that the parameter λ _oxidant,

^oxidant Aidant 1.5

(Aoxcomburant^Aoxprecursor) wherein Aidant is the amount of oxidant, Aoxcomburant is the amount of oxidant needed for complete combustion of the comburant and Aoxprecursor is the amount of oxidant needed for complete combustion of the coating precursor.

8. A process for the deposition of an anti reflection coating on a substrate, the process comprising

a) providing a substrate,

b) passing a fluid mixture through a flame, the fluid mixture comprising a coating precursor, an oxidant, and, optionally, a comburant, c) contacting at least one surface of the substrate with the fluid mixture during or after its passage through the flame,

characterised in that the amount of oxidant in the fluid mixture is such that the parameter λ _oxidant,

^oxidant Aidant — 1.5

9. A process as claimed in any one of the preceding claims, wherein the refractive index of the anti reflection coating is 1.25 to 1.4.

10. A process as claimed in any one of the preceding claims wherein the anti reflection coating comprises a porous portion.

11. A process as claimed in any one of the preceding claims, wherein the fluid mixture comprises a silicon oxide coating precursor and the anti reflection coating comprises a silicon oxide.

12. A process as claimed in any one of the preceding claims, wherein the thickness of the anti reflection coating is 10 to 500 nm, preferably 80 to 200 nm more preferably 95 to 200 nm, most preferably 105 to 200 nm.

13. A process as claimed in any one of the preceding claims, wherein the substrate comprises glass.

14. A process as claimed in claim 13, wherein the glass is float glass or rolled glass.

15. A process as claimed in either claim 13 or claim 14, wherein the glass has an iron content of 0.015% by weight or lower.

16. A process as claimed in either claim 14 or claim 15, wherein the surface of the glass is the tin side surface.

17. A process as claimed in any one of the preceding claims wherein the flame is substantially stable.

18. A process as claimed in any one of the preceding claims further comprising a step of depositing at least one further coating.

19. A process as claimed in claim 18, wherein the further coating is deposited on the same surface as the anti reflection coating.

20. A process as claimed in claim 18, wherein the further coating is deposited on a different surface to the anti reflection coating.

21. A process as claimed in any one of claims 18 to 20, wherein the further coating is a coating of a transparent conductive oxide.

22. A coated substrate obtainable by a process as claimed in any one of the preceding claims.

23. A photovoltaic module comprising a coated substrate as claimed in claim 22.