ITFI20120090A1 - PROCESS FOR THE PRODUCTION OF SOLAR FILMS WITH THIN FILMS - Google Patents

Description

PROCESS FOR THE PRODUCTION OF THIN FILM SOLAR CELLS

DESCRIPTION

The present invention relates in general to the sector of the production of solar cells, and more particularly of thin-film solar cells. More specifically, the invention relates to a process for the production of thin-film solar cells in which the absorber layer consists of a thin film of Cu (ln, Ga) Se2.

CdS thin-film solar cells with a light-absorbing layer consisting of Cu (ln, Ga) Se2 on soda-lime glass substrates are already known. These solar cells have achieved, at least on small areas, an efficiency greater than 20%, a value far greater than the efficiency values obtainable with any other thin-film cell, and comparable only with the performance of the best monocrystalline Si solar cells. .

However, this efficiency was achieved by preparing the Cu (ln, Ga) Se2 absorber layer with a process that works well on a laboratory scale, but which can hardly be used on an industrial scale. In fact, a three-stage process is used: in the first stage, In, Ga, and Se are deposited simultaneously by co-evaporation from crossed beams obtained from separate crucibles on a soda-lime glass substrate covered by a layer of Mo with a thickness equal to about 1 pm; in the second stage Cu and Se are deposited and finally in the third stage In, Ga and Se are deposited again. The substrate temperature is kept low during the first stage and then raised to about 550 ° C during the second and third stages. The main difficulty in bringing this three-stage process to industrial level is that, using large area substrates, as required in an industrial process, the use of cross beams entails the possibility of a variable composition of the material from zone to zone. . The non-uniformity of the composition of the material represents, as is known, a serious drawback because it negatively affects the efficiency of the device, not allowing to achieve high efficiencies. In fact, since Cu (ln, Ga) Se2 is a quaternary material, in the areas where there is an excess of Cu it is easy to form the binary phase Cu2Se which, being unstable, releases Cu which short-circuits the device.

An alternative method consists of the selenization and / or sulfurization of layers of elements deposited one on top of the other. These layers are generally deposited by sputtering, electroplating or electron gun. However, considering that In and Ga have rather low melting points (In melts at 156.6 ° C while Ga melts at 29.8 ° C), the preparation of the precursors, which consists in the homogeneous mixing of the layers, is rather complicated. and requires long annealing times. Alternatively, a fast heating process (Rapid Thermal annealing, RTP) is used which manages to bring the substrate to the right process temperature (500-550 ° C) in a few minutes, to which the layers are exposed to a gas containing H2Se or H2S for selenization or sulfuration (Y. Chiba et al., 35 <th> IEEE Photovoltaic Specialists Conference, Honolulu, Hawaii, USA, June 20-25, 2010, pp.

164-168). In this case H2Se and H2S have been used instead of Se or S since they exhibit a higher reactivity which seems to be necessary in case selenization or sulfurization is done by a fast heating process. However, it must be considered that both H2Se and H2S are very poisonous gases and therefore should not be used in industrial production, if not using very stringent precautions and procedures for use, which would in any case increase the production cost of the finished module.

Therefore, the need is still felt to have a process for the production of thin-film solar cells with a Cu (ln, Ga) absorber layer. 'H2Se nor of low-melting elements.

The object of the present invention is therefore to have a solar cell production process that overcomes the disadvantages highlighted above for the processes of the known art.

This object and others, which will become clear from the detailed description that follows, have been achieved by the Applicant who has surprisingly found a new process that is scalable at industrial level, in which the deposition of In and Ga is carried out by sputtering InSe, GaSe or them. mixed compounds, selenization can be carried out using Se instead of H2Se. This process has been shown to be able to provide Cu (ln, Ga) Se2 selenized precursors which, used as absorbers on soda-lime glass substrates and followed by layers of CdS, ZnO and ITO, have allowed the Applicant to realize solar cells with a efficiency around 17%. Furthermore, by carrying out the deposition of each layer with sputtering techniques and using only selenization in addition to sputtering, the process is highly reproducible and easily scalable even over large areas for industrial-level production.

Therefore, the object of the present invention is a process for the production of thin-film solar cells in which the light-absorbing layer is constituted by Cu (ln, Ga) Se2, and the use of targets of InSe, GaSe or their mixed compounds, for the preparation of a Cu (ln, Ga) Se2 absorber layer for thin-film solar cells, the essential characteristics of which are defined in the independent claims annexed hereto.

Further important features are contained in the dependent claims. The present invention is illustrated in detail in the following description made by way of non-limiting example, also with the aid of the following annexed drawings in which:

Figure 1 schematically illustrates the sequence of deposition on the substrate of the compounds that make up the absorber layer from Mo to Cu, according to a first embodiment of the invention;

Figure 2 shows the XRD spectrum (grazing angle, 10 °) of the absorber layer obtained, in which the orientations of CulnSe2, InSe and GaSe are highlighted, while for simplicity of reading the orientations relative to the Mo peaks have been omitted;

Figure 3 shows a scanning electron microscope photograph of the film of Figure 1 after selenization;

Figure 4 shows an X-ray analysis on the film of Figure 3; Figure 5 is a schematic illustration of the sequence of layers in a solar cell obtained by depositing on the film of Figure 1 layers of CdS, ZnO and ITO;

Figure 6 schematically illustrates the deposition sequence on the substrate of the compounds that make up the absorber layer, according to a second embodiment of the invention in which a final layer of GaSe is added to the film of Figure 1;

- Figure 7 shows the current-voltage characteristic curve i-V for a Cu (ln, Ga) Se2 / CdS solar cell obtained using the film of Figure 6 as the absorber layer.

The process of the invention concerns the manufacture of solar cells of the Cu (ln, Ga) Se2 / CdS type, based on the preparation of the Cu (ln, Ga) Se2 absorber layer by sputtering using Indium and Gallium selenide targets instead of In and Ga respectively, followed by selenization with Se instead of H2Se. In particular, the process involves the deposition by sputtering, in a vacuum system, of Mo, InSe, GaSe (or a mixed compound thereof as better specified below) and Cu, and subsequent selenization with Se. The deposition of the aforesaid products is carried out in sequence on a suitable substrate on which the Mo is deposited first, then the other products in succession, in the sequence indicated above. Mo and Cu are preferably deposited by pulsed DC magnetron sputtering, while the In and Ga selenides are deposited by radiofrequency magnetron sputtering.

A soda-lime glass substrate can be used as the substrate, for example 1 square inch in area and 4 mm thick. These dimensions are exemplary and do not affect the present solar cell manufacturing process. Alternatively, the present process has also been applied directly on ceramic, in particular on ceramic substrates with a surface of 1 square inch, specially produced by Panaria SpA. Ceramic substrates such as those usually used for buildings can be used as substrates for the purposes of the present invention, to realize photovoltaic modules directly on the ceramic roofing of buildings, having performances completely similar to those of the same modules made with traditional soda-lime glass supports. The surface of the ceramic substrate is typically prepared by depositing a vitreous enamel with chemical-physical characteristics similar to those of soda-lime glass substrates.

Mo is deposited on this substrate at room temperature in an Argon flow, preferably in a double layer, a rather thin first layer, with a thickness between 20 and 60 nm, preferably equal to 30 nm, and a second layer with a thickness between 300 nm. and 1000 nm, preferably equal to 500 nm. For the deposition of the first layer, the flow of Ar is between 30 and 60 sccm (standard cubic centimeter per minute), which corresponds to a pressure between 5x10 <'3> and 10 <' 2> mbar, and preferably is equal to 45 sccm which corresponds to a pressure of 7.5x10 <'3> mbar. For the deposition of the second layer, the flow of Ar is then decreased until it is included in the interval between 5 and 20 sccm (corresponding to a pressure in the interval between 0.8 x 10 <'3> and 3.3 x10 < '3> mbar, and preferably it is brought to 15 sccm which corresponds to a pressure of 2.5 x 10 <' 3> mbar. The deposition of Mo in double layer as described above is a preferred condition of the present process, since it allows to remove the stress that the Mo presents when it is deposited in particular on glass, and consequently allows to have a good adhesion of the Mo film.

While the Mo bilayer is deposited at room temperature, the subsequent selenides of In and Ga, InSe and GaSe or a mixed compound thereof InxG1- - with 0 <x <1, are preferably deposited at a temperature ranging from 370 and 450 ° C, and more preferably at about 400 ° C, thus preventing the relative layers from growing with an excess of Se. The products marketed by Sematrade Technologies and Solutions, which have a high density of the order of 99%, can be used as InSe and GaSe targets. For the deposition of InSe, a sputtering power between 100 and 200 W can be used, preferably equal to 150 W, which corresponds to a deposition speed between 8 and 24 À / s, and respectively equal to about 15 À / s ; while for the deposition of GaSe the power used can be in the range between 80 and 150 W, and preferably it is equal to 100, which corresponds to a deposition rate between 6.5 and 15 À / s and respectively equal to 9 À / s. In this way, a thickness of the InSe layer between 1 and 2 μm is obtained, and preferably equal to about 1.5 μm while for that of GaSe the thickness obtained is between 0.2 and 1 μm, and is equal to about 0.5 μm.

According to a preferred embodiment of the present process, the deposition of the Cu layer is also carried out at a temperature between 370 and 450 ° C, and more preferably at 400 ° C, to obtain a layer with a thickness of between 0.1 and 0. , 6

μm, preferably equal to about 0.35 μm.

Figure 1 shows a schematic illustration of the layer deposition sequence, carried out according to a first embodiment of the process of the invention, in which 100 is the substrate, 101 a and 101 b are the two Mo layers, 102 is the layer of InSe, 103 is GaSe and 104 is Cu. In an alternative embodiment to that shown in Figure 1, the layers 102 and 103 can be replaced by a single layer of a mixed In and Ga selenide as defined above, having for example a thickness of between 1.5 and 2.5 μm , and preferably equal to 2 μm.

Under the conditions described above for the present process, the subsequent depositions generate a mixing of the preceding layers, without the need for further annealing. From an X-ray analysis carried out after the deposition of the Cu layer, and shown in Figure 2, it can be seen that at this stage Cu (ln, Ga) Se2 has already formed, with only residues of InSe and GaSe.

At this point the process of the invention provides for a selenization stage with Se, which can be carried out in a vacuum chamber, where pure Se is evaporated from a graphite crucible. This selenization procedure is very fast, being able to last from 5 to 15 minutes overall, for example 7 minutes, of which from 3 to 10 minutes, for example 5 minutes, to bring the temperature in the range 500-550 ° C, preferably 530 ° C, and from 1 to 5 minutes, for example 2 minutes, to keep the material at that temperature. Figure 3 shows the morphology of the Cu (ln, Ga) Se2 absorber layer once selenized, obtained by electron microscopy: the film obtained is well crystallized and, as can be seen in the figure, the triangular structures typical of the chalcopyrite phase of the Cu (ln, Ga) Se2.

Figure 4 also shows an X-ray analysis on the film of Figure 3, which shows how the main species formed are Culnoi7Gaoi3Se2 and Culnoi4Gaoi6Se2. This absorber layer has a thickness comprised for example between 1 and 3 μm.

With the absorber layer prepared and subjected to selenization as described above, thin-film solar cells of the Cu (ln, Ga) Se2 / CdS type were manufactured according to procedures known and commonly used in the sector, for the sequential application on the absorber layer. of CdS, ZnO and ITO (Indium Tin Oxide) layers.

A CdS layer with a thickness of for example between 50 and 120 nm, and preferably equal to 80 nm, can be deposited by means of radiofrequency magnetron sputtering at a substrate temperature between 150 and 250 ° C, and preferably equal to 200 ° C , in an atmosphere of Ar with R23 (CHF3), added to an extent of 1-4% by volume with respect to Ar, and preferably to an extent of 3% (it is also possible to use alternatively other hydrofluorocarbons of the R23 family such as for example R134a, or C2H2F4). Thanks to the presence of the hydrofluorocarbon, the presence of F <-> ions in the sputtering discharge causes the stoichiometric CdS to grow without any excess of either Cd or S, since there is a bombardment of the surface by the F <-> ions during the growth of the film; however, the amount of added hydrofluorocarbon must be limited, as specified above, since an excessive amount of hydrofluorocarbon could inhibit the growth of CdS.

A layer of intrinsic ZnO, with a thickness for example between 60 and 150 nm, and preferably equal to 100 nm, is then deposited by reactive sputtering (magnetron-radiofrequency) using a metal target of pure Zn in an atmosphere of Ar + 02 [P02 = 30-60% of PAr + 02] Finally, an ITO layer with a thickness for example between 300 and 700 nm, and preferably equal to 500 nm, is deposited by pulsed DC sputtering in an atmosphere of Ar + 02 [P02 = 2-10% of PAr + 02] · To complete the cell, a grid contact is formed over the ITO layer by continuous sputtering.

Figure 5 shows a schematic illustration of the sequence of layers in the Cu (ln, Ga) Se2 / CdS solar cell where 100 is the substrate, 101 a and 101 b are the two Mo layers, 105 is Cu (ln, Ga) If2 corresponding to layers 102 to 104 in Figure 1 after selenization, 106 is the CdS layer, 107 is the ZnO layer and 108 is the ITO layer. Also in Figure 5, 109 represents the lower contact created with special adhesive strips of tinned copper of a commercial type, and with 110 the upper grid contact accessible from the outside using for example the same strips of tinned copper used for the lower contact .

Solar cells manufactured with the present process using the absorber layer shown in Figure 1 have a photovoltaic conversion efficiency around 12% with a Voc, i.e. an open circuit potential, which never exceeds 500 mV, with a current density of short circuit Jsc around 40 mA / cm <2> and a “wire factor”, or filling factor, of 0.62 - 0.64. The characteristics of the solar cells were measured using a solar simulator from Oriel Corporation under a light of 100 mW / cm <2> in AM 1.5 at a temperature of 24 ° C.

Solar cells have been manufactured with the present process also starting from an absorber layer like the one shown in Figure 1, to which a layer of GaSe is added after the deposition of Cu, as shown in Figure 6, which is then subjected to selenization. The further layer of GaSe, for example with a thickness of between 0.1 and 0.5 μm, is indicated in Figure 6 with 103bis. The Cu (ln, Ga) Se2 / CdS solar cells manufactured starting from this type of absorber layer having a different concentration profile, with a higher concentration of Ga on the surface, showed even better characteristics than the previous ones: the Vocdi these cells is around 600 mV and the "wire factor" around 0.72 while the short-circuit current density Jsc is around 37 mA / cm <2>. The efficiency is approximately 16-17%. The current-voltage characteristic curve obtained for one of these cells with an area of 0.5 cm <2>, is shown in Figure 7; the photovoltaic parameters detected in this case are: Voc = 576.1 mV, “wires factor” = 0.74, Jsc38.13 mA / cm <2>, efficiency η = 16.2%. The efficiency measured on other areas of the substrate also provided similar values, thus indicating a good uniformity of cell performance over the entire area of the substrate.

A large number of solar cells, about 60 samples, were prepared with the present process described above and all showed a conversion efficiency between 13% and 17% with a reproducibility peak between 16% and 17%. , thus indicating a good reproducibility of the process.

The present invention has been described up to now with reference to a preferred embodiment thereof. It is to be understood that there may be other embodiments that refer to the same inventive core, all falling within the scope of the claims set out below.

Claims (9)

CLAIMS 1. A process for the production of thin-film solar cells comprising an absorber layer consisting of Cu (ln, Ga) Se2, characterized in that said absorber layer is prepared by sputtering deposition of InSe and GaSe, or of a compound thereof mixed InxG1- with 0 <x <1, on a substrate coated with Mo, followed by deposition by sputtering of Cu and selenization with Se. The process according to claim 1, wherein said substrate is made of building ceramic. 3. The process according to claim 1, wherein said deposition by sputtering of InSe, GaSe, or of a mixed compound thereof, and of Cu, is carried out at a temperature comprised between 370 and 450 ° C. 4. The process according to claim 3, wherein said deposition by sputtering of InSe, GaSe, or of a mixed compound thereof, and of Cu, is carried out at a temperature of about 400 ° C. 5. The process according to claim 1, wherein said selenization is carried out by exposing the layers deposited by sputtering to a pure Se vapor for 5-15 minutes at a temperature of 500-550 ° C in a vacuum chamber. The process according to claim 1, further comprising the sputtering deposition of a further GaSe layer prior to selenization. The process according to any one of the preceding claims, further comprising the deposition by sputtering of a CdS layer on said absorber layer. The process according to claim 7, in which the deposition by sputtering of a layer of ZnO and of a layer of ITO is also carried out on said CdS layer. 9. Use of InSe, GaSe or a mixed compound thereof InxG1- with 0 <x <1, as a target for the preparation by sputtering deposition of an absorber layer of Cu (ln, Ga) Se2 for thin film solar cells.