Pub. No.: WO/2018/083480 International Application No.: PCT/GB2017/053305
Publication Date: 11.05.2018 International Filing Date: 02.11.2017
IPC:
H01L 31/18 (2006.01), H01L 31/073 (2012.01)
Applicants: LOUGHBOROUGH UNIVERSITY [GB/GB]; Ashby Road Loughborough Leicestershire LE11 3TU (GB)
Inventors: ABBAS, Ali; (GB).
BITTAU, Francesco; (GB).
GREENHALGH, Rachael; (GB).
KAMINSKI, Piotr; (GB).
LISCO, Fabiana; (GB).
WALLS, John Michael; (GB).
YILMAZ, Sibel; (GB)
Agent: BAILEY WALSH & CO LLP; 1 York Place Leeds Yorkshire LS1 2DR (GB)
Priority Data:
1618474.9 02.11.2016 GB
Title (EN) IMPROVEMENTS TO THE DEPOSITION AND FORMATION OF COATINGS FOR PHOTOVOLTAIC CELLS
(FR) PERFECTIONNEMENTS APPORTÉS AU DÉPÔT ET À LA FORMATION DE REVÊTEMENTS POUR CELLULES PHOTOVOLTAÏQUES
Abstract: front page image

(EN)The invention relates to the formation of a photovoltaic or solar cell device and in particular to a coating formed on a surface of the same. The device incorporates a substrate formed by a glass base on which there is provided a transparent conductive coating (TCO). Onto a surface of the substrate or this coating there is, in accordance with the invention in one embodiment, sputter deposited a layer of CdS followed by the sputter deposition of a layer including CdTe. A back contact layer can be applied as a final layer. The CdS layer acts as the n-type layer while the CdTe containing layer acts as the p-type layer of the device. The cross section of the photovoltaic cell, and the layers provided, can be altered to suit specific requirements. For example, a buffer layer of Sn02 or ZnO can be applied onto the TCO prior to the CdS coating to improve device performance.
Improvements to the Deposition and formation of coatings for photovoltaic cells

The invention to which this application relates is the provision of one or more coating layers onto a substrate and/or previously applied coating layer in order to form an article or cell for use, in one embodiment, and typically in conjunction with further cells, as part of apparatus for use in the generation of power from Solar energy.

It is known to provide cells which can be used to generate solar power and one particular form of cell, to which the invention particularly, although not necessarily exclusively, relates is the form of thin film solar cells which include at least one layer of Cadmium telluride (CdTe), which is a stable crystalline compound formed from cadmium and tellurium typically provided as a thin semiconductor layer designed to absorb and convert sunlight into electricity. As CdTe solar cells are regarded as the only thin film technology with lower costs than conventional solar cells made of crystalline silicon in multi-kilowatt systems this makes this type of solar cell of considerable commercial importance. Furthermore, on a lifecycle basis, it is believed that CdTe cells have the smallest carbon footprint, lowest water use and shortest energy payback time of the many different solar technologies which further adds to the commercial importance of this category. The manufacture of this form of cell is increasing in terms of planned production.

However the manufacture of this form of solar cell is not without significant problems. Conventionally the application of the CdTe layer is achieved using a known process called Vapour Transport Deposition (VTD) as this process has a relatively high deposition rate and therefore the manufacturing rate is relatively high. However, the uniformity of the coating layer which is applied is relatively poor and this means that conventionally, in order to attempt to mitigate this problem, it is necessary to deposit a relatively thick layer of CdTe such as at a thickness of ~3um even though it is believed that a thickness of CdTe of 1.5um would be sufficient to allow the complete light absorption to be achieved but due to the tolerance required for the lack of uniformity the greater thickness has to be applied to ensure that the coating is at least of a thickness of 1.5um at all parts of the coating.

Due to the problems of the uniformity of the CdTe layer, attempts have been made to use alternative forms of application and one of these is the use of Sputter Deposition coating in which there is provided a chamber in which the substrates to which the coating is to be applied are located. The substrates are located on a holder in the chamber in which there are provided a plurality of magnetrons, with the magnetrons provided with targets of the material which is to be sputter deposited therefrom, such as, in this case, a compound target of Cadmium Telluride. It has been found that this process significandy increases the uniformity of the layer of CdTe which is applied with uniformity typically of +/- 2%. This results in the ability to apply thinner coatings and hence significantly reduce the materials cost. The improvement in uniformity also allows the use of the coating for thin film applications such as in the use of a thin layer for semi-transparent power-producing windows where aesthetic considerations need to be taken into account.

However, despite these advantages, due to the perceived need to use RF power supplies to operate the magnetrons due to the high insulation of the targets of the material which is to be deposited, the rate of coating application which can be achieved is relatively slow. Also, the matching circuits used to deliver RF power are complex and inconvenient to use in many instances. It is also necessary to treat the applied coating by heating the same to approximately 400 degrees Celsius and applying a chloride containing solution which, in one embodiment, can be deposited in vacuum by evaporation or applied in solution with methanol in atmosphere, and then heated to recrystallize the CdTe coating layer and assist in removing defects and passivating grain boundaries in the deposited coating layer. It has been observed that sputtered CdTe often delaminates during this activation process and most typically the delamination occurs at the interface between the previously applied layer of CdS on the substrate and the newly applied CdTe.

An aim of the present invention is therefore to allow the application of the CdTe coating layer to a substrate or a coating layer applied thereto to be achieved in an efficient manner with a relatively high application rate. A further aim is to provide a process for the application of the CdTe coating layer to a substrate, or a coating applied thereto to be achieved which has reduced number of defects and/ or risk of failure during subsequent processing.

In a first aspect of the invention there is provided a method for the application of a coating including at least one layer including CdTe onto a surface, said at least one layer applied using a sputter coating process in which the CdTe material is sputtered from one or more targets of material and wherein the power supply used to cause and control the sputtering is a pulsed DC power supply.

The use of the pulsed DC power supply allows the method to control the operation of the sputter apparatus to deposit material at relatively high deposition rates in comparison to the use of an RF power supply.

Typically the surface to which the coating is applied and the material targets are located in a coating chamber during the sputter deposition of the material and formation of the coating.

Typically the sputter deposition of the material occurs in the presence of at least one working gas. Typically the said at least one working gas is introduced into the coating chamber in a controlled manner. In one embodiment the material is sputtered from

the targets of material in the presence of selected working gases in the chamber in which the sputtering of the material occurs.

In one embodiment the frequency of DC pulsing is > 150kHz. In one embodiment the working gas pressure is in the range of 2 and l^ba . In one embodiment a plurality of working gases are selectively used and, in one embodiment there is variation in the pressures at which the different working gases are used and in one example, the material is deposited in the presence of Xenon at a pressure ~3μbar, and the material is deposited in the presence of Argon at a pressure ~7^b and the substrate temperature is >250C.

In one embodiment the thickness of the target of material is in the region of 4mm which is significandy thinner than the conventional targets which have a thickness in the region of 10mm.

In one embodiment a deposition rate of 0.5um/minute is achieved using a 2kW pulsed DC power supply.

The magnetron sputtering process creates ions in a gas plasma created in the chamber in which the substrates are located and the ions bombard the negatively biased target of the material so as to cause the removal (sputtering) of atoms /molecules of the material from the target which then deposit on the substrate.

In one embodiment, a gas is present in the chamber during the sputtering of the CdTe material to form the CdTe coating layer. In one embodiment, a gas, different from the first, is present in the chamber during the deposition, typically also by sputtering, of CdS material to form a coating layer of Cadmium Sulphide (CdS). Typically the coating layer of CdS is applied to the substrate before the layer of CdTe.

In one embodiment the said gases are both Noble Gases and in one embodiment the gas present during the coating of the CdTe has an atomic mass which is the same or greater than the atomic mass of the Cd or Te material. Typically the diameter of the atom of the gas is greater than that of the Cd or Te material. In one embodiment the gas is Xenon.

In one embodiment the gas present in the chamber during the application of the CdS coating is Argon.

In one embodiment an annealing process is performed on the coating once applied. In one embodiment in addition the coating is activated by the application of Cadmium Chloride.

In one embodiment the material which is sputtered forms a coating of an alloy of CdTe.

In one embodiment the band gap of the ternary alloys of CdTe can be tuned by controlling the quantity of additional elements such as Se, Zn, Mg or Mn in Cd(l-x) Elementx Te alloy.

In one embodiment the addition of Se is especially beneficial.

In one embodiment the pulsed dc supply is used to control the sputtering of material from targets of alloy materials, including CdSeTe, to provide precisely graded structures.

In one embodiment the co-sputtering of material from CdTe targets along with material from other material targets such as CdSe targets is performed to precisely control the composition of the material of the coating which is performed.

In one embodiment the CdTe targets of material are prepared in an environment from which low mass inert gases are excluded. In one embodiment the method includes the step of excluding Argon from the environment in which the targets are formed.

In one embodiment the device with the said surface to which the coating is to be applied is located in a chamber with which there is provided a plurality of magnetrons, at least one plasma source and a DC pulsed power supply to cause and control the selective sputter deposition of material from targets of material provided with said magnetrons, the material of said targets selected, and the operation of the magnetrons controlled, to allow the deposition of a first coating layer and at least one CdTe containing layer and wherein first and second working gas supplies are provided and controlling the gas supplies to allow the introduction of the first gas into the chamber during the formation of the first coating layer and the second gas, without the first gas, into the chamber during the formation of the CdTe containing layer.

In a further aspect of the invention there is provided a method for the application of a material to form a layer of a coating of a solar cell device, wherein said method includes the sputter deposition of materials required to form the coating layer onto a substrate, or a previously applied coating layer, in the presence of a gas which has an atomic mass which is the same or greater than that of the constituents of the coating layer material.

In one embodiment the gas is Xenon and the coating layer is CdTe.

In one embodiment a further coating layer is applied in the presence of a different gas. In one embodiment the different gas is argon. In one embodiment this coating layer is first applied to the substrate and the coating layer is CdS or Magnesium (Mg) doped Zinc Oxide (ZnO).

In a further aspect of the invention there is provided a method for the application of a coating onto a substrate to form a photovoltaic cell, said coating including a first layer of a material and a further coating layer of a different material, said first coating layer applied using a sputter coating process in the presence of a first gas and the further coating layer applied using a sputter coating process in the presence of a second gas.

In one embodiment the material used to form the first coating layer is CdS. Typically the material used to form the said further coating layer is CdTe.

In one embodiment the first gas is argon and the second gas is xenon.

In a further aspect of the invention there is provided a method for the application of a coating including CdTe onto a substrate or to a coating previously applied to the substrate, said coating applied using a sputter coating process in which the CdTe material is sputtered from one or more targets of the material and wherein the CdTe is applied in the presence of xenon gas.

In one embodiment a layer of CdS material is also sputter deposited, typically prior to the application of the CdTe coating layer and this is deposited in the presence of a gas other than Xenon. In one embodiment the said gas is Argon.

In one embodiment the power supply used to activate the sputtering apparatus is a pulsed DC power supply or an RF power supply.

In a further aspect of the invention there is provided a solar cell device including a coating layer of CdTe formed in accordance with the method as herein described.

In a further aspect of the invention there is provided apparatus for the application of coatings to a substrate to form a photovoltaic cell, said substrates located in a chamber in which there is provided a plurality of magnetrons, at least one plasma source and a DC pulsed power supply to cause and control the selective sputter deposition of material from targets provided with said magnetrons, the material of said targets selected, and the operation of the magnetrons controlled, to allow the deposition of a first coating layer and a CdTe coating layer and wherein first and second gas supplies are provided and control means allow the introduction of the first gas into the chamber during the formation of the first coating layer and the second gas, without the first gas, into the chamber during the formation of the CdTe coating layer.

In one embodiment the first gas is Argon and the second gas is Xenon.

In one embodiment the targets of the said apparatus include no low mass inert gas trapped therein.

Specific embodiments of the invention are now described; wherein

Figure 1 illustrates a cross sectional view of CdTe containing thin film photovoltaic cell of a type formed in accordance with the invention;

Figure 2 illustrates a form of sputtering apparatus which can be used to form the coating in accordance with the invention;

Figure 3a-d illustrate sputtering deposition rates which can be achieved in accordance with the invention and a comparison between RF and DC power supplies and the pulsed DC magnetron;

Figure 4 illustrates apparatus used to anneal the CdTe coating once applied.

Figures 5 and 6 illustrate visual images of the CdTe and CdS coating after annealing has been performed;

Figure 7 illustrates a TEM cross-section of the sputter deposited CdTe and CdS coatings following the Cadmium Chloride activation process.

Figures 8a-c illustrate further views of the CdTe and CdS coatings after the Cadmium Chloride Activation Process; and

Figures 9a-c illustrate cross-section views of CdS/CdTe coatings deposited using a method in accordance with embodiments of the invention.

Referring firstly to Figure 1 there is illustrated a cross sectional view of the layers of one embodiment of a photovoltaic cell device 2 formed in accordance with one embodiment of the invention. The device incorporates a substrate formed by a glass base 4 on which there is provided a transparent conductive coating (TCO) 6. Onto a surface, in this embodiment the surface of the coating 6, there is, in accordance with the invention in this embodiment, sputter deposited a layer of CdS 8 followed by the sputter deposition of a layer including CdTe 10. The layer 10 may be formed of CdTe entirely or may be formed as a CdTe alloy and the particular composition is selected by the selective provision of targets of material which are available to be sputtered and then the controlled and selective deposition of material from said targets during the formation of the layer and the coating in general. A back contact layer 12 is then provided as the final layer. The CdS layer 8 acts as the n-type layer while the CdTe containing layer 10 acts as the p-type layer of the device. The cross section of the photovoltaic cell, and the layers provided, can be altered to suit specific requirements. For example, a buffer layer of Sn02 or ZnO can be applied onto the TCO prior to the CdS coating to improve device performance. In addition, a coating of Mg doped ZnO can be applied instead of the coating of CdS and in this case the TCO becomes the n-type layer. In one embodiment Copper can be used to dope the back contact by sputter depositing Cu doped ZnTe. Also, as already stated, targets of further material, such as Se, can be provided and selectively sputtered and/ or an alloy material target such as CdTeSe can be provided and then selectively operated to allow the selection of the make up of the layer 10 which includes CdTe.

Figure 2 illustrates a load locked magnetron sputtering apparatus 14 with pulsed DC power supply which can be used to form the coating layers in accordance with the invention. In this arrangement, the apparatus includes a holder on which the substrates to be coated are held which is located within a chamber 16 in which a vacuum is created and the substrates are moved, in this case rotated, with respect to a number of magnetrons 18, mounted to be selectively controlled to sputter material from targets located therewith into the chamber 16 in which the devices are located on the holder. In an alternative arrangement, perhaps better suited to larger volume manufacture, an “in line” coating apparatus is used in which the substrates are placed on a flat holder and moved linearly with respect to rotating magnetrons which are provided with the appropriate targets of material to be sputter deposited onto the substrates and this arrangement provides improved target utilisation The substrates can be heated to > 400°C when static or > 250°C when rotating. A flexible control

system for rapid process development is provided and a plasma source allows for the provision of pre-treatment and preparation of the substrates prior to the application of the layers including CdTe and CdS.

The substrates may be formed as shown in Figure 1 with the glass layer 4 with a TCO layer 6 already provided and it is the surface of the TCO layer which is exposed so as to have the further coating layers sputter deposited thereon in accordance with the invention.

In a first embodiment of the invention, the magnetron sputtering of the coating layers 8,10 of CdS and CdTe in accordance with the invention, is performed using a pulsed DC power supply and hence the pulsed deposition of material from the targets is achieved. It is found that by controlling the deposition in this method it is possible to achieve relatively high sputter deposition rates in the region of ~100nm/min at 500W and with a coating uniformity of +/-2%, and so full absorption is possible at 1.5um as shown in Figure 3a. This is achieved with the apparatus controlled to operate with a pulsed DC power supply of 150kHz, at a pressure of 7.5ubar, and 2.5usec reverse time. If this is factored up to use with a commercial power supply of, for example, 2kW, it will provide a deposition rate in the region of 0.5um/min with a coating grain size in the range of 200nm to 300nm as shown in Figure 3b. These parameters and results are significantly greater and improved on those that could be achieved using the prior art method of RF power supply sputtering of the type illustrated graphically in Figure 3c which shows an RF prior art power supply waveform operating at 16.34MHz and Figure 3d illustrates an example of a pulsed DC power supply operating at up to 300kHz in accordance with the invention and in one embodiment, and as shown in Figure 3d, with a pulsed DC waveform for 150kHz and 2.5μ8 reverse time. The use of the pulsed DC power supply allows a much greater duty cycle than the RF power supply to be achieved, which results in higher deposition rates of the coating materials, improved coating quality due to the ability to use advanced arcing suppression technology and improved compatibility with both resistive and dielectric targets. It is thus found that the deposited thin film CdTe coating layer using the pulsed DC sputtering method produces improved uniform films with a dense columnar structure.

An activation process step is then typically performed which includes the application of the Cadmium Chloride (CdC12), typically in solution and/ or by evaporation to the coating.

Annealing can also be performed which, for test purposes, was performed using a rapid thermal processing unit 20 of the type shown in Figure 4.

It has been found in tests described hereonin that the CdTe coating layer 10 can include relatively large voids which, it is believed, cause delamination of the CdTe coating layer from the CdS coating layer. Further analysis indicates that while the large voids were formed during the activation step, the cause of the problem could be traced back to the application step of the CdTe coating layer in which small ‘void like’ defects 24 are observed, particularly along the interface 22 between the CdS coating layer 8 and the CdTe coating layer 10 and at the CdTe grain boundaries as illustrated in Figure 5 and in Figure 6. The use of High Resolution Transmission Electron Microscopy reveals that the defects are approximately spherical in shape and ~5nm in diameter. Subsequent EDX analysis showed a high concentration of argon and the void like defects 24 are, in effect, argon bubbles caused by using argon as the working gas in the chamber 16 of the coating apparatus during the deposition of the CdTe coating layer 10. During the subsequent activation process in which CdC12 is applied to the coating layer 10 the argon bubbles coalesce as shown in Figure 7 which is a TEM cross-section of the sputtered thin film coating layers of CdTe 10 and CdS 8 following the CdC12 treatment. Catastrophic void formation 26 is shown to have occurred and these include voids 26′ formed along the CdS/CdTe interface 22 and these can eventually rupture causing the catastrophic failure observed as illustrated in Figures 8a-c which show the CdTe coating layer 10 and CdS coating layer 8 and interface 22 condition after the activation process.

It is therefore believed that in the sputter process and subsequent activation stage the argon atoms are incorporated, perhaps by substitution, into the film during deposition and as the temperature of the coating increases the argon diffuses and forms small bubbles which agglomerate and then can cause failure and/or blisters and perforated blisters 28 can appear on the surface of the CdTe coating layer 10 as illustrated in Figure 8a. Furthermore, exfoliation occurs at some of the surface blisters, causing a short when the back contact layer 12 is deposited as illustrated in Figure 8b and this is in addition to the delamination 30 which is shown in Figure 8c to be occurring at the CdS / CdTe coating layers interface 22.

In accordance with a preferred embodiment of the invention, the problem of substitutional gas absorption into the CdTe layer 10 is prevented by the use of a gas other than argon during the application of the CdTe coating layer 10 and, in particular, the use of a gas that has an atomic mass greater than that of the CdTe material. The use of this type of gas, such as Xenon as the working gas, is found to prevent the absorption of the working gas into the CdTe coating layer 10 from occurring during the sputter deposition of the CdTe coating layer.

Figures 9a and b illustrate a cross-section of CdS/CdTe coating layers applied as part of a photovoltaic cell device in which xenon has been used as the working gas during the sputter deposition of the CdTe coating layer 10 and argon was used as the working gas during the sputter deposition of the CdS coating layer 8. No voids are visible in the CdTe coating layer 10 after the CdC12 treatment as a result of using Xenon as the working gas, as no, or at least substantially less, bubbles of gas are formed in the CdTe coating layer 10 or at the interface 22 with the CdS coating layer 8 and, as a result, catastrophic failure of the CdTe/CdS coating layers, and hence the photovoltaic cell structure, does not occur. Figure 9a illustrates a bright field STEM image and Figure 9b illustrates a high angle annual dark field STEM image, both views showing the CdTe coating layer 10 which is sputter deposited in the presence of Xenon as the working gas in the chamber of the apparatus during the sputter deposition of that coating layer, whilst argon is the working gas during the deposition of the CdS coating layer 8 in the coating chamber 16 of the apparatus. The images are taken after the post coating heating and Cadmium Chloride activation step has been performed. It can be seen that no gas bubbles or voids are present in the CdTe coating layer 10 or at the interface 22 with the CdS coating layer 8, and so the integrity of the overall coating is achieved and the risk of delamination or other catastrophic failure is avoided. Thus, while it would be more straightforward in terms of control and for improved manufacturing speed, for the same working gas to be used during the deposition of both of the CdTe and CdS coating layers 10,8, it is identified in this invention that a significantly improved coating can be formed by the selective use of a first gas, such as argon, as the working gas during the deposition of the CdS coating layer 8 and a second gas, such as xenon, as the working gas during the deposition of the CdTe coating layer 10. It should also be noted that this advantage and solution to the problem of the CdTe coating layer and delamination can be achieved when using any sputtering technique, whether using a DC pulsed power supply or an RF power supply, with the difference between the two types of power supply illustrated in Figure 3c, which shows the RF power supply, and Figures 3d which illustrates the pulsed DC power supply preferably used in accordance with one embodiment of the invention. Figure 9c illustrates a cross sectional image of a blister and free cadmium telluride solar cell after Cadmium Chloride activation using Xenon as the working gas. The CdS/ CdTe device material is deposited using Xenon as the working gas for the duration of deposition of the CdTe layer and Argon for the duration of the deposition of the CdS layer. No voids are visible in the CdTe layer after the CdC12 treatment.

There is therefore provided in accordance with the invention as herein described, an effective method, and coatings formed in a manner which is particularly effective for solar cell usage.

Claims

1. A method for the application of a coating including at least one layer including CdTe onto a surface, said at least one layer applied using a sputter coating process in which the CdTe material is sputtered from one or more targets of material and wherein the power supply used to cause and control the sputtering is a pulsed DC power supply.

2. A method according to claim 1 wherein the pulsed DC power supply is used to control the operation of the sputter apparatus to deposit material to form the coating at relatively high deposition rates in comparison to the use of an RF power supply.

3. A method according to claim 2 wherein the frequency of DC pulsing is > 150kHz.

4. A method according to any of the preceding claims wherein at least one working gas is present during the sputter deposition of the material.

5. A method according to claim 4 wherein the pressure of the working gas is in the range of 2 and l^bar.

6 A method according to claim 4 wherein different working gases are selectively used and the different working gases are used at different pressures.

7 A method according to claim 6 wherein material is deposited in the presence of Xenon at a pressure which is less than the pressure of Argon when material is deposited in the presence of that gas.

8 A method according to any of the preceding claims wherein the substrate temperature is >250C.

9. A method according to any of the preceding claims wherein the target has a thickness in the region of 2-8mm.

10 A method according to claim 3 wherein the deposition rate of material is 0.5um/minute using a 2kW pulsed DC power supply.

11 A method according to claim 1 wherein the material is sputtered and the at least one working gas is present in a coating chamber in which the said surface is located during the deposition of material to form the at least . one layer including CdTe.

12 A method according to claim 11 wherein the gas present during the formation of the at least one layer including CdTe has an atomic mass which is the same or greater than the atomic mass of the Cd or Te material.

13 A method according to claim 11 or 12 wherein the diameter of the atom of the gas is greater than that of the Cd or Te material.

14 A method according to any of claims 11-13 wherein the gas is Xenon.

15 A method according to any of claims 11-14 wherein a gas, different from the gas present during the deposition of the CdTe, is present in the chamber during the deposition of CdS material to form a layer of the said coating.

16. A method according to claim 15 wherein the layer of CdS is applied to the said surface before the at least one layer including CdTe.

17. A method according to claim 15 wherein the gas present in the chamber during the application of the CdS coating is Argon.

18 A method according to any of the preceding claims wherein an annealing process is performed on the coating once applied.

19 A method according to any of the preceding claims wherein the coating is activated by the application of Cadmium Chloride.

20.A method according to any of the preceding claims wherein material is selectively sputtered to form the at least one layer including CdTe as an alloy of CdTe.

21 A method according to any of the preceding claims wherein the band gap of the ternary alloys of CdTe is selected by controlling the quantity of additional elements which are deposited.

22 A method according to claim 21 wherein the said elements are any or any combination of Se, Zn, Mg or Mn in Cd(l-x) Element x Te alloy.

23 A method according to claim 20 wherein the pulsed dc supply is used to control the sputtering of material from targets of alloy materials.

24 A method according to claim 23 wherein the co-sputtering of material from at least one target of CdTe material and material from targets of other material is performed to control the composition of the material of the at least one layer.

25 A method according to any of the preceding claims wherein targets of CdTe material are prepared in an environment from which low mass inert gases are excluded.

26 A method according to any of the preceding claims wherein the said surface is provided on a solar cell device.

27 A method according to claim 1 wherein the device with the said surface to which the coating is to be applied is located in a chamber with which there is provided a plurality of magnetrons, at least one plasma source and a DC pulsed power supply to cause and control the selective sputter deposition of material from targets of material provided with said magnetrons, the material of said targets selected, and the operation of the magnetrons controlled, to allow the deposition of a first coating layer and at least one CdTe containing layer and wherein first and second working gas supplies are provided and con tolling the gas supplies to allow the introduction of the first gas into the chamber during the formation of the first coating layer and the second gas,

without the first gas, into the chamber during the formation of the CdTe containing layer.

28. A method for the application of a material to form a layer of a coating on at least one surface of a solar cell device, wherein said method includes the sputter deposition of materials required to form the coating layer onto a substrate, or a previously applied coating layer, in the presence of a gas which has an atomic mass which is the same or greater than that of the constituents of the coating layer material.

29 A method according to claim 28 wherein the gas is Xenon and the coating layer is formed of CdTe material.

30. A method according to claim 28 wherein a further coating layer is applied in the presence of a different gas.

31 A method according to claim 30 wherein the different gas is argon.

32. A method according to claim 30 wherein the further coating layer is first applied to the substrate and the coating layer is CdS or Mg doped ZnO.

33. A method for the application of a coating onto a substrate to form a photovoltaic cell, said coating including a first layer of a material and a further coating layer of a different material, said first coating layer applied using a sputter coating process in the presence of a first gas and the further coating layer applied using a sputter coating process in the presence of a second gas.

34 A method according to claim 33 wherein the material used to form the first coating layer is CdS and the material used to form the said further coating layer is CdTe.

35. A method according to claim 33 wherein the first gas is argon and the second gas is xenon.

36. A method for the application of a coating including CdTe onto a substrate or to a coating previously applied to the substrate, said coating applied using a sputter coating process in which the CdTe material is sputtered from one or more targets of the material and wherein the CdTe is applied in the presence of Xenon gas.

37 A method according to claim 36 wherein a layer of CdS material is also sputter deposited, typically prior to the application of the CdTe coating layer and this is deposited in the presence of a gas other than Xenon.

38 A method according to claim 37 wherein the power supply used to activate the sputtering apparatus is a pulsed DC power supply or an F power supply.

39. A solar cell device with a coating including at least one layer of CdTe formed in accordance with the method as herein described.

40 Apparatus for the application of coatings to a substrate to form a photovoltaic cell, said substrates located in a chamber in which there is provided a plurality of magnetrons, at least one plasma source and a DC pulsed power supply to cause and

control the selective sputter deposition of material from targets provided with said magnetrons, the material of said targets selected, and the operation of the magnetrons controlled, to allow the deposition of a first coating layer and a CdTe coating layer and wherein first and second working gas supplies are provided and control means allow the introduction of the first gas into the chamber during the formation of the first coating layer and the second gas, without the first gas, into the chamber during the formation of the CdTe coating layer.

41. Apparatus according to claim 40 wherein there is provided a supply of Argon gas to allow the same to be the said first gas and a supply of Xenon gas to allow the same to be the said second gas.

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