WO 2009/001389 PCT/IT2007/000469 TITLE METHOD FOR THE FORMATION OF A NON-RECTIFYING BACK-CONTACT IN A CDTE/CDS THIN FILM SOLAR CELL DESCRIPTION 5 Field of the invention The present invention relates to the field of the solar cells technology and more particularly concerns a process for the large-scale production of CdTe/CdS thin film solar cells. In particular, the invention relates to 10 an improvement to this process relating to the formation of a non-rectifying back-contact. Even if in the present specification reference is made to "CdTe/CdS thin-film" solar cells for sake of simplicity, it is to be understood that this term includes all the salt mixtures comprised in 15 the formula ZnCd 1 .,S/CdTeyS 1 y wherein 0 x0.2 and 0.95 ysl. Background art of the invention As is known, a typical configuration of a CdTe/CdS 20 solar cell has a film sequence of the multi-layer arrangement comprising a transparent glass substrate carrying a transparent conductive oxide (TCO) film, a CdS film representing the n-semiconductor, a CdTe film representing the p-semiconductor and a metallic back 25 contact. -A solar cell with a layer arrangement and structure of this type is disclosed, for example, in US 5304499. The commercial float glass may be used as a transparent substrate, but, in spite of its low cost, 30 special glasses are often preferred to avoid drawbacks of the float glass, in particular Na diffusion into TCO film. The most common TCO is In 2 0 3 containing 10% of Sn WO 2009/001389 PCT/IT2007/000469 -2 (ITO). This material has a very low resistivity on the order of 3x10~ 4 O2cm and high transparency (>85%) in the visible region of the solar spectrum. However, this material is made by sputtering and the ITO target after 5 several runs forms some nodules which contain an In excess and a discharge between nodules can happen during sputtering which can damage the film. Another material which is commonly used is fluorine doped SnO2 which however exhibits a higher resistivity close to 10- Ocm 10 and as a consequence a 1 pm thick layer is needed in order for the sheet resistance to be around 10 0/square. A high TCO thickness decreases the transparency and then the photocurrent of the solar cell. The use of Cd 2 SnO 4 has also been proposed by the NREL group (X. Wu et al., Thin 15 Solid Films, 286 (1996) 274-276) . Also this material has some drawbacks since the target is made up of a mixture of CdO and SnO 2 and, being CdO highly hygroscopic, the stability of the target may result to be unsatisfactory. W003/032406, in the name of the same applicant, 20 discloses a process for large-scale production of CdTe/CdS thin-film solar cells in which the deposition of the TCO film is conducted in such a way that a film of very low resistivity can be deposited without formation of any metal nodules on the target and allowing the use of a 25 inexpensive substrate. To this end, the TCO layer is formed by sputtering in an inert gas athmosphere containing hydrogen, or an argon-hydrogen mixture, and a gaseous fluoralkyle compound, e.g. CHF 3 . In this way the TCO is doped with fluorine. 30 The CdS film or layer is deposited by sputtering or Close-Spaced Sublimation (CSS) from CdS granulate material. This last technique allows the preparation of thin films WO 2009/001389 PCT/IT2007/000469 -3 at a substrate temperature much higher than that used in simple vacuum evaporation or sputtering, because substrate and evaporation source are put very close to each other at a distance of 2-6 mm and the deposition is carried out in 5 the presence of an inert gas such as Ar, He or N 2 at a pressure of 10-3-100 mbar. A higher substrate temperature allows the growth of a better crystalline quality material. An important characteristic of the close-.spaced sublimation is a very high growth rate up to 10 pm/min, 10 which is suitable for large-scale production. CdTe film or layer is deposited on top of CdS film by close-spaced sublimation (CSS) at a substrate temperature of 480-520 0 C. CdTe granulate is generally used as a source of CdTe which is evaporated from an open 15 crucible. The electric back contact on the CdTe film is generally obtained by deposition of a film of a highly p dopant metal for CdTe such as copper, e.g. in graphite contacts, which, upon annealing, can diffuse in the CdTe 20 film. The use of a Sb 2 Te 3 film as a back-contact in a CdTe/CdS solar cell has been disclosed by the same inventors (N. Romeo et al., A highly efficient and stable CdTe/CdS thin film solar cell, Solar Energy Materials & Solar Cells, 58 (1999), 209-218). 25 The back-contact in the CdTe/Cd thin film solar cells plays a very important role in achieving their efficiency. A rectifying contact, i.e. a metal semiconductor contact which does not follow the Ohm law, that is to say there is no linear relationship between 30 voltage and current, gives rise to a "roll over" (intersection in the first quadrant of the dark condition/lighting condition J-V characteristic curves) in WO 2009/001389 PCT/IT2007/000469 -4 the J-V characteristic, i.e. in the diagram showing the behaviour of the current density as a function of the voltage, which considerably decreases the "Fill factor", and consequently the cell efficiency (D. Bonnet and P.V. 5 Meyers, J. Mater. Res. 13 (1998) 2740-2753)). Since CdTe has an high electronic affinity (Z) and an high prohibited band (1,5 eV), the majority of the metals forms a Schottky barrier limiting the hole transport in the p-type CdTe. When'using Cu to form the contact on the CdTe, before Cu 10 deposition a chemical etching is carried out in a phosphoric/nitric acid bath (the so called N-P etching) on CdTe to create a Te-rich surface forming the CufTe (1 X 2) compound with Cu. This compound, by interdiffusion, forms a low 15 resistance close contact with CdTe, but its stability is limited to the CuxTe phase in which 1 X1.4, whereas the Cu 2 Te phase is not a stable compound and therefore releases Cu which, being a fast diffusive element, penetrates the CdTe in particular through the grain edges, 20 this possibly resulting in the cell degradation. Since Cu is a positive ion, its diffusion within CdTe depends on the internal electric field of the junction which, in turn, depends on the fact that the cell is undergone to an external bias or illumination. The device degradation is 25 clearly faster when it is heated to a temperature higher than 60 0 C or is subjected to high lighting (>1 sun). In order to avoid or at least limit this drawback, the solar cells using this type of back-contact, for example the solar cells produced by First Solar Inc. (USA), use a 30 Cu thickness of 2 nm deposited after CdTe is subjected to a chemical etching (C. R. Corwine et al., Sites, Sol. Energy Mat. & Solar Cells 82 (2004) 481-489).
WO 2009/001389 PCT/IT2007/000469 5 To avoid any degradation of the device new back contact materials, namely Sb 2 Te 3 and As 2 Te 3 , are disclosed in W003/032406 patent application in the name of the same applicant as an alternative to the use of Cu. In 5 particular, Sb 2 Te 3 is a material with a low gap (0.3 eV), is of the p-type and has a resistivity close to 10-4 cm. When deposited at a substrate temperature of ~ 300'C, it forms a close contact with CdTe and can allow efficiencies close to 16%- to be reached. This type of contact has 10 proven very stable even with a device illumination 'of 10 20 suns and temperatures higher than 100 0 C. However, even if a good quality ohmic contact is formed in this way, under particular conditions of CdTe film growth, the presence of "roll-over" in the J-V characteristic curve 15 has been observed, this being an indication that some rectification, even if not very marked, is present in the back-contact. It is therefore a general object of the present invention to provide a method to form a ohmic contact for 20 a CdTe thin film which would be completely non-rectifying and ensure the film stability. A particular object of the present invention. is to provide a method to form an ohmic back-contact of CdS/CdTe thin film solar cells which allows the stability of the 25 cell to be ensured even under high illumination and temperature conditions and therefore to improve, or at least maintain unchanged, the cell efficiency with respect to the prior art. Another object of the present invention is to 30 provide a method to form a back-contact of thin film solar cells of the above mentioned type wherein, even if Cu is used in the formation of the back-contact, the control of WO 2009/001389 PCT/IT2007/000469 -6 the thickness of the deposited Cu film does not affect the cell stability in the same critical way as occurs in the process according to the prior art. A further object of the present invention is to 5 provide a method to form a thin film solar cell back contact of the above mentioned type wherein a treatment of chemical etching of the CdTe film is not necessary before the back-contact is formed. Still another object of the present invention is to 10 provide a thin film solar cell wherein the back-contact is completely not-rectifying in such a way to ensure an high stability even under high illumination and temperature conditions, and thus improve their efficiency or, at least, maintain it unchanged with respect to the known similar 15 solar cells. Summary of the Invention These objects are reached with the method to form a non-rectifying back-contact for a CdTe/CdS thin film solar cell and with the solar cell according this method whose 20 essential features are set forth in claims 1 and 14. According to an aspect of the invention, a method to form a ohmic contact is provided which maintains the photovoltaic device stable in the time without changing the way the CdTe film is treated with respect to the 25 process disclosed in WO 03/032406 and therefore without using any kind of etching of the CdTe film surface. This new way of contacting the p-type CdTe consists in the sequential deposition of, first, an As 2 Te 3 film and then a Cu film by sputtering, but the true contact is 30 provided neither by As 2 Te 3 nor by Cu, but through the CufTe (with 15x1.4) compound. It is this compound that ensures both the ohmic behaviour and the time stability of the WO 2009/001389 PCT/IT2007/000469 -7 contact and, therefore, of the solar cells. In other words, the method according to the invention provides a way to form a non-rectifying ohmic back-contact of the CdTe film consisting in forming a 5 Cu.Te (with 1!x1.4) thereon, which otherwise could not be formable due to the reactivity between Cu and Te. As a matter of fact, if a film containing Cu and Te would be deposited with any method, the final result will be, in any case, the separation of several phases, including the 10 Cu 2 Te phase that does not give an ohmic contact and is unstable as it releases Cu atoms. The stable phase between Cu and Te is that with a Cu content comprised between 1 and 1.4, i.e. the phase which, under energetically favorable conditions, is formed by sputtering deposition 15 of a Cu film on a As 2 Te 3 film, which in turn is deposited on the surface of a CdTe film as treated in the usual way. The maximum amount of Cu that it is useful to deposit on the As 2 Te 3 layer must ensure at the same time a good non-rectifying contact and a stable system and 20 therefore must allow the formation of CuTe (with 1!x1.4) either without leaving free Cu or avoiding the Cu 2 Te formation, which would cause the atomic Cu diffusion through the CdTe film and as a consequence the p-n function degradation. 25 In particular, the CuxTe (with 1 x:1.4) compound can be formed in a native way either directly, by carrying out the Cu film deposition on As 2 Te 3 at a temperature comprised between 150 0 C and 2500 C, or by depositing the As 2 Te 3 at low temperature (<100 0 C) and then heating the 30 layer assembly at a temperature comprised between 1500 and 2500 C. A particularly preferred temperature in both cases is at least 180 0 C. Even if it is not essential to the end WO 2009/001389 PCT/IT2007/000469 -8 of the CuTe (with 1!x51.4) compound formation, it can be helpful to maintain the thus formed back-contact at this temperature for at least 1 minute. In the formation of the back-contact according to the 5 present invention advantage is taken of the particular interaction between these materials during the sputtering deposition of the Cu film on As 2 Te 3 . In the sputtering technique the atoms reaching the substrate can have an energy of some tens of eV (with thermal evaporation it can 10 be as high as some tenths of eV) . At 200 0 C the As 2 Te 3 film surface starts to become thermally unstable (it starts to reevaporate at 2500C) . On the other side, the Cu atoms have a large energy excess that is partly lost through surface impacts and partly used to break the As 2 Te 3 15 molecule and take the place of the As to form a more stable compound (that is to say with a higher formation energy) at that temperature, i.e. CuxTe (with 1 x:1.4). The stechiometry can be variable (with X variable between 1 and 1.4), as hybridization of the chemical bonds may 20 occur and this may result in increasing formation energies passing from x=1.4 to x=1. As shown by the X-rays diffractograms, As 2 Te 3 blocks Cu, as it reacts with it and if the Cu film is kept at a value not higher than 20 nm, a stable material is formed, 25 i.e. CufTe with x comprised between 1 and 1.4, whidh form a non-rectifying contact with CdTe (see figures 3 and 4). It has been observed that the same result is not achieved if Sb 2 Te 3 is used in the place of As 2 Te 3 , as Sb 2 Te 3 is very stable and does not react with Cu, which may 30 therefore diffuse in the CdTe layer through the Sb 2 Te 3 film thus' damaging the device. Brief description of the drawings WO 2009/001389 PCT/IT2007/000469 -9 The invention will be now described in further detail with reference to the attached drawings, in which: figure 1 schematically shows the structure of a CdTe/CdS thin film solar cell with the back-contact 5 according to the present invention; figure 2 shows the J-V characteristic curve for two solar cells whose back-contact has been deposited according to the method of the invention, but at two different deposition temperatures (namely: ambient 10 temperature, curve a; 200 0 C, curve b); figure 3 is the X-ray analysis of an As 2 Te 3 film deposited on glass at a substrate of 200 0 C with (curve b) and without (curve a) a layer of 20 nm of Cu deposited thereon at the same temperature; 15 figure 4 is the X-ray analysis of an As 2 Te 3 film deposited on glass at a substrate temperature of 200*C with (curve b) and without (curve a) a Cu layer of 50 nm deposited thereon at the same temperature. Detailed description of the invention 20 The main steps featuring the production of CdTe/CdS tin film solar cells with the new As 2 Te 3 + Cu back-contact according to the method of the present invention are: a. Washing of the glass in such a way to remove any trace of organic residues (grease, solvents, etc.) and 25 microparticoles (powder dust with size greater that 1 pim). b. Deposition of the fore transparent contact by sputtering on the glass said contact comprising two layers: the first layer is ITO (indium-tin oxide) which ensures the condudibility and the second layer is ZnO 30 which operates as a buffer layer or as a barrier against the possible diffusion of impurities in the layers which will be deposited in the next steps. Both layers as a WO 2009/001389 PCT/IT2007/000469 - 10 whole must ensure a transparency not lower than 85% in the visible wave length region. c. Deposition of the CdS film by reactive sputtering (RF-magnetron) under Ar + %5 CHF3 environment, the CdS 5 being a n-type semiconductor providing the first part of the junction.. d. Deposition of the CdTe film by CSS (Close-Spaced Sublimation). The CdTe, being a p-type semiconductor, provides the second part of the junction and ensure the 10 complete absorption of the visible light. e. Thermal treatment at 4000C of the whole previously prepared assembly: the CdTe film surface is exposed in a Ar+Freon atmosphere for not more than 5 minutes and then, keeping the temperature at 400 0 C for other 5 minutes, 15 vacuum conditions are established thus allowing the volatile compounds, which could have been formed during the first part, to reevaporate from the CdTe film surface. f. Deposition of the back-contact by sputtering, said back-contact according to the invention comprising two 20 layers: the first one, As 2 Te 3 , and the second one, Cu:on the back-contact formed in this way a Mo film is then deposited to ensure a proper sheet resistance. The schematic structure of the solar cell thus produced is shown in figure 1. 25 The As 2 Te 3 layer is deposited directly on the CdTe surface, without subjecting the latter to any chemical etching, whereas the Cu layer is deposited at a substrate temperature of around 200 0 C , preferably 180 0 C. As 2 Te 3 is a p-type semiconductor with prohibited energy band of 0.6 30 eV and with a resistivity of around 10- Q cm. The As 2 Te 3 thickness can vary between 100 and 300 nm, whereas the Cu thickness can vary between 2 and 20 nm. In the WO 2009/001389 PCT/IT2007/000469 - 11 experimental tests both As 2 Te 3 and Cu are deposited by sputtering, the first one with a deposition velocity between -10 and 20 A/sec and the second one with a deposition velocity of 5 A/sec. 5 If As 2 Te 3 and Cu are both deposited at ambient temperature without any thermal treatment, the result is a rectifying contact as can be seen from figure 2, curve a, where a "roll-over" (bending of J-V curve) in the first quadrant of the J-V characteristic curve is visible. If Cu 10 is deposited at a substrate temperature of about 200 0 C the roll over disappears (curve b of figure 2) and the fill factor of the device is very higher in this case (0,7 instead of 0,57 in the first case). To understand the behaviour of this double layer of 15 As 2 Te 3 + Cu, some samples were prepared by depositing As 2 Te 3 + Cu directly on glass and Cu was deposited at a substrate temperature of about 200 0 C. Moreover, some samples were prepared by depositing a Cu thickness up to 20 nm on As 2 Te 3 , whereas others were prepared depositing a 20 Cu layer of about 50nm. These samples were x-rays analysed and compared with samples containing As 2 Te 3 only. It was observed that the samples containing Cu with a layer thickness not higher than 20 nm exhibited several CufTe phases with 1 X:1.4 (figure 3, curves a and b) , whereas 25 the samples containing Cu with a layer thickness of 50nm exhibited even the Cu 2 Te phase (figure 4, curves a and b) . The result of the above tests is that a layer of Cu up a 20 nm thickness can be deposited forming phases of CufTe (with 15X51.4) which form a stable non-rectifying contact 30 with CdTe. This is also confirmed by the J-V characteristic curve shown in figure 2, curve b, of a CdTe/CdS cell in which the back-contact has been made by WO 2009/001389 PCT/IT2007/000469 - 12 depositing in sequence, at a substrate temperature of about 2000C, 200 nm of As 2 Te 3 and 20 nm of Cu, without carrying out any etching on the CdTe surface. The fill factor of this cell is ~ 0.7. 5 From these data it can be concluded that As 2 Te 3 behaves as a barrier for Cu and that, when Cu is deposited at a lower temperature and then is brought at about 200 0 C after the deposition, a solid state reaction between As 2 Te 3 and Cu takes place in which Cu displaces As forming the CuxTe 10 phase. The way of forming a non-rectifying contact on p-type CdTe looks like to that commonly used in which a Te-rich surface is fist created by a chemical etching of CdTe and then Cu is deposited to form CufTe. However, the 15 substantial difference consists in that, in the method of the invention, any CdTe etching is not carried out and that an up to ten times higher amount of Cu can be used. This makes less critical the risk of formation of the rectifying contact thereby allowing a greater stability of 20 the contact. To the aim of assessing the performances and the photovoltaic parameters, several samples of solar cells were prepared following the method of the invention by depositing in sequence by sputtering different thicknesses 25 of As 2 Te 3 and Cu as set forth in the following table: As 2 Te 3 Cu substrate sample nm nm temperature, OC 1 100 20 200 2 300 5 200 3 200 10 <100 WO 2009/001389 PCT/IT2007/000469 - 13 In the case of the sample 3 the system formed by all the deposited layer was brought to a substrate temperature comprised between 1800C and 2500C in a Ar atmosphere at a pressure comprised between 100 mbar and 1 atm. In all the 5 samples the contact was completed by depositing a Mo layer of 150 nm on the surface of the As 2 Te 3 +Cu film. Physically relevant differences of the contact behaviour as a function of the deposition velocity were not observed (both for As 2 Te 3 and Cu) when the velocity 10 was comprised between few A/sec up to 50 A/sec and the substrate temperature varied from 1500C to 2500C. In all these cases the back-contact has proven to be a good contact for the CdTe/Cds thin solar film solar cell as shown by the J-V characteristic (figure 2, curve b). In 15 fact, in the positive part of the characteristic (10 quadrant), no bending is displayed, which demonstrates that the contact is non-rectifying and from the curve inclination and fill factor it can be deduced that there is not any series resistance effect. Therefore, the 20 contact is non -rectifying and is of low resistance. Stability tests were carried out by subjecting the device, in open circuit condition, to "light soaking", i.e. an exposition to an intense illumination, up to 10 suns and temperature up to 1000C for 8 hours without noting any 25 significant degradation of the photovoltaic parameters of the device. Even if the preferred deposition technique for both layers of As 2 Te 3 and Cu is by sputtering, they may be also deposited by thermal evaporation, by electronic gun 30 evaporation or electrodeposition. Variations and/or modifications may be brought to the method for forming a non-rectifying ohmic contact for WO 2009/001389 PCT/IT2007/000469 - 14 CdTe/Cds thin films and to the thin film solar cell according to the present invention without departing from the scope of the invention as seth forth in the following claims.