This invention relates to coated substrates comprising a coating on at least one surface with a transparent conductive coating on at least one other surface. The invention also relates to processes for the production of such coated substrates.

It is known to deposit coating on substrates for various purposes. For example, sol gel type deposition processes have been proposed in EP 1429997, DE 10146687, EP 1328483 and USP 6918957 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 a silica coating.

Other types of deposition processes include chemical vapour deposition where a vapour of a precursor is directed at the substrate surface, often at elevated temperature. Deposition processes may involve directing the precursor through a flame on to the substrate surface. USPA 2006/003108 discloses a process for depositing a coating on to the surface of a glass substrate 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. WO-A-2009/00745 also discloses flame pyrolysis processes for deposition of coatings upon the surface of a continuous glass ribbon.

Deposition processes for deposition of transparent conductive coatings include those processes used to deposit conductive oxides such as indium tin oxide, doped tin oxide, doped zinc oxide and doped cadmium oxide. These processes may include chemical vapour deposition, flame pyrolysis, sputtering (or other types of physical vapour deposition) and other processes. One of the uses of transparent conductive coatings is in photovoltaic (PV) modules.

In photovoltaic (PV) modules, substrates may be coated with a transparent conductive coating and then further layers, as components of the PV cells, may be deposited on the conductive coating. In use, however, the transparent conductive coatings may occasionally delaminate from the substrate surface which causes failure of PV cells and may cause the whole module to fail.

It is an aim of the present invention to address the delamination problem. The present invention accordingly provides, in a first aspect, a coated substrate comprising a substrate having a first surface and a second surface, a transparent conductive coating on the first surface and a second coating on the second surface.

This is advantageous because, surprisingly, the presence of the second coating on the second surface reduces the potential for delamination of the conductive coating on the first surface.

It is preferred if the second coating is an antireflection coating, preferably, with a refractive index of 1.25 to 1.4. This is particularly advantageous when the substrate is a transparent (or substantially transparent or translucent) substrate because it increases transmission of light. If the coated substrate is to be used in PV modules this can be a very significant improvement because an increase in transmission of just 1 to 3% can have a beneficial effect on the efficiency of a PV module.

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 (if the substrate is 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.

The thickness of the second coating is preferably in the range 10 to 1100 nm. It is preferred if the thickness of the second coating is 25 nm or greater, 40 nm or greater, 50 nm or greater, 80 nm or greater or 100 nm or greater. The more preferred thickness is 105 to 500 nm, most preferably 105 to 200 nm.

The second coating may have a porous portion and/or a dense portion. If a dense portion is present it is preferably in contact with the substrate surface. The advantage of the porous portion of the second coating is that is tends to reduce the refractive index of the second coating. At least a part of the dense portion preferably has a thickness in the range 10 to 150 nm, more preferably 10 to 95 nm, most preferably 15 to 80 nm. At least a part of the porous portion preferably has a thickness in the range 50 to 1000 nm, more preferably 50 to 600 nm, most preferably 60 to 250 nm. Preferably, the second coating comprises a silicon oxide, for example silicon oxynitride (SiNO), silicon oxycarbide (SiCO) or silicon dioxide (silica). The preferred material of the second coating is silicon dioxide.

The substrate will usually comprise glass, preferably float glass or rolled glass. The glass may be a soda lime float glass, a low iron float glass or a body tinted float glass comprising a higher proportion of iron, cobalt or selenium which may have a green, grey or blue colouration. In a preferred embodiment the glass has an iron content of 0.015% by weight or lower.

The glass substrate may have a thickness of from 0.5 mm to 25 mm preferably of from 2 mm to 20 mm and a visible light transmission of from 10.0% to 90.0%.

The coated glass, where the second coating is an antireflection coating, may have a visible light transmission which is from 1% to 3.5% greater than the glass before the coating was applied.

If the glass is float glass it is preferred that the second surface of the glass is the tin side surface and the first surface of the glass is the gas side surface. This is advantageous because subsequent processing (including of deposition of other layers on the conductive coating) is usually preferred by PV producers to be on the gas side surface (also known as the air side surface).

The transparent conductive coating is preferably a transparent conductive oxide coating. Preferred oxides are selected from tin oxide, zinc oxide, copper oxide, indium oxide, a mixed oxide and/or mixtures thereof. The most preferred oxide is doped tin oxide (in particular fluorine doped tin oxide).

The coated substrate may comprise one or more further coatings which may be deposited under or above the second coating and/or under or above the transparent conductive coating. Typical further coating include coatings comprising one or more layers of a metal oxide or a silicon oxide.

In a second aspect the present invention provides a process for the deposition of a second coating on a substrate, the process comprising providing a substrate having a transparent conductive coating on a first surface, passing a fluid mixture comprising a coating precursor through a flame, and contacting at least a second surface of the substrate with the coating precursor during or after its passage through the flame, thereby depositing the second coating. Flame pyrolysis 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 (often known as CCVD processes). In a preferred embodiment the processes of this aspect of the invention are CCVD processes.

Examples of precursors which may be used in the formation of silica coatings include compounds having the general formula SiX ₄ 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.

The thermal output of the burners useful in the processes of this invention may be from 0.5 to 10 kW/10cm ² , preferably from 1 to 5 kW/10cm ² . 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 process may be carried out by passing the fluid mixture to a burner which is positioned above or below the surface of the substrate. One burner or a series of smaller burners may be used to coat the substrate evenly. The burner is preferably positioned by the substrate in close proximity to the second surface. The distance between the burner and the surface 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 in order to optimise the properties of the desired coating. The process, whether off line or on line, may be carried out at a range of relative speeds of the substrate (relative to the burner or burners). Typically relative speeds are 1 to 25 m/min, preferably 2 to 20 m/min.

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 (if there are a plurality of burners) from each other, to control the direction of the flame so as to optimise the impingement of the flame over the surface and to efficiently remove the by-products which are generated by the combustion. Where a single conduit is associated with a burner it is preferably positioned upstream of the conduit but in the preferred embodiments exhaust conduits are provided both upstream and downstream of each burner head.

The Applicants 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 surface the tail of the flame is also located above the surface and when the burner is located below the surface the tail of the flame is also below the 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, acetylene, methane and natural gas or hydrogen.

The temperature of deposition may be at a substrate temperature of 20 to 650°C, preferably 100 to 450°C, more preferably 100 to 300°C and most preferably 100 to 250°C. The fluid mixture preferably further comprises a carrier fluid, preferably a carrier gas, and/or an oxidant and optionally a comburant. The comburant may be burnt in any gas which comprises a source of oxygen. Typically the comburant will be mixed with and burnt in air. The ratio of comburant to 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.

It is preferred if the amount of oxidant in the fluid mixture is such that the parameter λ _0X idant,

wherein A _ox idant 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 such that ·5.

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

Values of _ox idaiit in this range are advantageous because they result in improved anti reflection properties of the second coating, even at relatively low deposition temperatures.

Coated substrates according to the first aspect of the invention may be used in many areas of industry. In particular coated substrates according to the invention may be used as substrates in photovoltaic modules.

Thus, in a third aspect the present invention provides a photovoltaic module comprising a coated substrate according to the first aspect of the invention. The invention is illustrated by the following Figures in which:

Figure 1 is a scanning electron micrograph of a second coating according to the invention.

Figure 2 is a graph of transmission (%) against wavelength for Example 5 (higher transmission curve) compared to the TCO coated substrate (lower transmission curve) without the second coating.

Figure 3 is a graph of transmission (%) against wavelength for Example 1 (higher transmission curve) compared to the TCO coated substrate (lower transmission curve) without the second coating.

Figure 1 illustrates a silica coating deposited on the tin side of TCO coated float glass. The silica coating is deposited as described in the Examples with 6 passes under the coater. The coating comprises two portions: a relatively dense portion against the substrate surface approximately 100 nm thick, and a porous portion above the dense portion approximately 540 nm thick. SIMS analysis indicates that the dense portion is silica, relatively high in sodium.

Examples

The invention is further illustrated by the following Examples.

Coatings comprising silica were deposited on the tin side of float glass with a transparent conductive oxide (TCO) coating deposited by on line CVD on the air side. The TCO coating were deposited on the glass during the float glass production process in the tin bath and/or lehr gap.

The layers and thicknesses of the TCO coating for each Example were as indicated in table 1.

Example Substrates: nature of TCO Coating

1 Glass/Sn0 ₂ (25 nm)/Si0 ₂ (25 nm)/SnO ₂ :F(340 nm)

2 Glass/SnO ₂ (60nm)/SiO ₂ (25 nm)/SnO ₂ :F(630 nm)

3 Glass/Sn0 ₂ (25 nm)/Si0 ₂ (25 nm)/SnO ₂ :F(420

nm)/Sn0 ₂ (75 nm)

4 Low iron glass (600 ppm)/Sn0 ₂ (25 nm)/Si0 ₂ (25

nm)/SnO ₂ :F(420 nm)/Sn0 ₂ (75 nm)

5 Glass/Sn0 ₂ (40-80nm)/SiO ₂ (10-20nm)/SnO ₂ :F(700- 850nm)

Table 1

A fluid mixture comprising propane, air and hexamethyldisiloxane (HMDSO) was fed to a burner for flame deposition of silica coatings. The deposition conditions for the silica coatings were: glass temperature 180 °C, propane flow rate 3.5 standard litres/minute, air 75 standard litres/minute. Six coating passes were made for each coating. In Example 1 the flow rate of HMDSO (liquid) was 3.3 cm ³ /hour, for Examples 2 to 5 the flow rate of HMDSO (liquid) was 12 cm ³ /hour. The silica coatings were deposited across about 85 mm width of the substrate. The substrate was moved at about 3 m/min relative to the burner.

The optical properties of the Examples were analysed. T _v i _s values were calculated from the spectra of the samples (according to ISO9050 and EN410/673). The values are given in Table 2 for the substrates (carrying a TCO coating on the first surface) and the Examples (i.e. substrates after coating with the second coating on the second surface). Figures 2 and 3 illustrate the % transmission against wavelength for Examples 5 and 1 compared to TCO coated substrates without the tin side (second) coating. The optical properties of the other Examples are similar. In each case the second coatings significantly increase T _v i _s . The durability of the coatings was also good.

The % improvement of weighted transmission values were calculated for standard photovoltaic cells of varying types. The results of this calculation are described in Table 2. The silica coatings of Examples 1 to 5 significantly improve the transmission of light into PV cells with consequent improvement in efficiency.

Table 2.

In Table 2, 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.

The effect of the second (tin) side coatings on delamination of the TCO coatings were investigated using the Electro-chemical delamination (ECDL) test. ECDL is an accelerated screening test using a hot plate and voltage source to rapidly drive Na+ ion from the glass substrate to the TCO layer to generate stress, which eventually results in cracking and peeling of the coatings. In the test, the glass side of the sample is positively biased under 100 V, at 185°C for 15 minutes. The sample is then removed from the heat and bias, and kept within a humidity chamber (50%HR) to allow water vapour to diffuse to the interface and initiate the delamination process. After 5 min of storage in the humidity ambient air, the TCO is scratched with a razor blade. The scratching generates a path for water vapor to reach the TCO-glass interface, and it also creates a mechanical defect that helps to initiate the delamination in a damaged coating. If no delamination occurs within 15 min after the scratching is applied, it suggests the TCO coating has a good adhesion to the glass substrate, indicating less risk of delamination in use. The results of the ECDL test for Example 3 are described in Table 3 and compared to the results for the same TCO coated glass without the tin side coating (Comparative Example).

ECDL current (μΑ)

Result Time to t=0 t=3 t=6 t=9 t=12 t=15

(#pass at 15 failure min min min min min min

min / total

tested)

Comparative 2/2 Approx. 82.7 69.9 66 62.2 59.5 56.9

Example 18 min

Example 3 2/2 Approx. 67.9 43.9 39.1 35.2 32.3 30.6

18

hours

Table 3

During the manufacture of float glass it is inherent in the production process that some of the molten tin which the glass is floated on diffuses into the bottom surface of the glass. This is known to have a detrimental impact on the optical quality of the glass by increasing the reflection from this bottom surface and thus resulting in reduced transmission.

The inventors have discovered that the dense silica layer or dense silica layer with a porous top coat significantly reduces / eliminates this increased reflection / loss of transmission. This effect can be seen when coating on the bottom surface of float glass but is of a further benefit when a TCO is coated on the top surface.

In this context, dense silica layers are regarded as layers having a density of the same order of magnitude as the glass substrate and porous top coats are regarded as coats which substantially allow for fluid communication between opposing surfaces or interfaces due to porosity.

A coating having a dense portion and a porous portion refers to any coating having two portions, one of which has a greater density than the other.