WO2013168135A1 - A process for the manufacture of thin film solar cells - Google Patents

A process for the manufacture of thin film solar cells Download PDF

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Publication number
WO2013168135A1
WO2013168135A1 PCT/IB2013/053804 IB2013053804W WO2013168135A1 WO 2013168135 A1 WO2013168135 A1 WO 2013168135A1 IB 2013053804 W IB2013053804 W IB 2013053804W WO 2013168135 A1 WO2013168135 A1 WO 2013168135A1
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layer
solar cells
gase
light
absorbing layer
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PCT/IB2013/053804
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French (fr)
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Nicola Romeo
Alessandro Romeo
Alessio Bosio
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Advanced Research On Pv-Tech S.R.L.
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/032—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0322—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising only AIBIIICVI chalcopyrite compounds, e.g. Cu In Se2, Cu Ga Se2, Cu In Ga Se2
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02422—Non-crystalline insulating materials, e.g. glass, polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02436—Intermediate layers between substrates and deposited layers
    • H01L21/02439—Materials
    • H01L21/02491—Conductive materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02568—Chalcogenide semiconducting materials not being oxides, e.g. ternary compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02614—Transformation of metal, e.g. oxidation, nitridation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers
    • H01L31/072—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers the potential barriers being only of the PN heterojunction type
    • H01L31/0749—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers the potential barriers being only of the PN heterojunction type including a AIBIIICVI compound, e.g. CdS/CulnSe2 [CIS] heterojunction solar cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/541—CuInSe2 material PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention refers in general to the field of the manufacture of solar cells, and more particularly to thin film solar cells. More specifically, the invention relates to a process for the manufacture of thin film solar cells wherein the light- absorbing layer is formed by a thin layer of Cu(ln,Ga)Se 2 .
  • the main problem in scaling up this three-steps process consists in that, using substrates of large areas, as requested in an industrial process, the use of crossed-beams may bring to a variable composition from an area to the other. It is known that the non-uniform composition of the material is a serious problem because it negatively influences the efficiency of the device, preventing to get high efficiency values. Indeed, Cu(ln,Ga)Se 2 is a quaternary material and, in areas where an excess of Cu is present, it is easy the formation of the binary phase Cu 2 Se which is unstable, freeing Cu and sending short-circuit to the device.
  • An alternative method consists of selenization and/or sulphurization of layers of elements deposited one over the other. These layers are generally deposited by sputtering, electrodeposition or electron gun. However, since In and Ga have rather low melting points (In melts at 156, 6°C and Ga melts at 29,8°C), the preparation of the precursors, consisting in the homogeneous mixture of the layers, is rather complicated and requires long annealing time. Alternatively, a process of rapid warming-up is used (Rapid Thermal annealing, RTP) that brings in few minutes the substrate to the right temperature of process (500-550°C), at which the layers are exposed to a gas containing H 2 Se or H 2 S for the selenization or sulphurization (Y.
  • RTP Rapid Thermal annealing
  • H 2 Se and H 2 S have been used instead of Se or S because they have a greater reactivity which is apparently necessary when the selenization or sulphurization are made by a process of rapid warming-up.
  • H 2 Se as well as H 2 S are very poisonous gases and therefore they should not be used in an industrial production, except when using precautions and very stringent procedures of use, that would however increase the manufacture costs of the final device.
  • a purpose of the present invention is therefore to provide a process for the manufacture of solar cells which is able to overcome the drawbacks highlighted above for the prior art processes.
  • Figure 1 schematically shows the deposition sequence on the substrate of the compounds in the light-absorbing layer from Mo to Cu, according to a first embodiment of the invention
  • Figure 2 shows the XRD spectrum (glancing angle, 10°) of the obtained light-absorbing layer, where the orientation of CulnSe 2 , InSe and GaSe are highlighted, whereas for an easier reading, the orientation related to the Mo peaks are omitted;
  • Figure 3 shows an image from scanning electron microscope of the film of Figure 1 after the selenization
  • Figure 4 shows an X-rays analysis on the film of Figure 3;
  • Figure 5 is a schematic illustration of the sequence of layers in a solar cell obtained by deposing on the film of Figure 1 layers of CdS, ZnO and ITO;
  • Figure 6 schematically illustrates the sequence of deposition on the substrate of the compounds in the light-absorbing layer, according to a second embodiment of the invention wherein 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 solar cell Cu(ln,Ga)Se 2 /CdS obtained using the film of Figure 6 as light-absorbing layer.
  • the process of the invention relates to the manufacture of solar cells of the Cu(ln,Ga)Se 2 /CdS type, based on the preparation of the light-absorbing layer of Cu(ln,Ga)Se 2 by sputtering using targets of Indium and Gallium selenides to respectively replace In and Ga, followed by selenization with Se instead of with H 2 Se.
  • the process comprises the deposition by sputtering, in a vacuum system, of Mo, InSe, GaSe (or of a mixed compound thereof as better specified below) and Cu, and subsequent selenization with Se.
  • the deposition of the above said products is carried out sequentially on a suitable substrate on which is first deposited Mo, then the other products in succession, in the sequence indicated above.
  • Mo and Cu are preferably deposited by pulsed magnetron DC sputtering, whereas the In and Ga selenides are deposited by radio frequency magnetron sputtering.
  • a soda-lime glass substrate may be used, for instance of 1 square inch of surface and 4 mm of thickness. These dimensions are given as an example and do not affect the present process of manufacture of solar cells.
  • the present process was also applied directly on ceramics, in particular on ceramic substrates having a surface of 1 square inch, produced on purpose by Panaria SpA. Ceramic substrates as those generally used in buildings may be used as substrates for the purposes of the present invention, to prepare photovoltaic modules directly on the ceramic covering of buildings, having performances analogues to those of the same modules made with the traditional supports of soda-lime glass.
  • the surface of the ceramic substrate is typically prepared by deposition of a vitreous enamel having chemical-physical characteristics analogues to those of the soda-lime glass substrates.
  • a first layer which is rather thin, of thickness between 20 and 60 nm, preferably of 30 nm, and a second layer of thickness between 300 and 1000 nm, preferably of 500 nm.
  • the Argon flow is comprised between 30 and 60 seem (standard cubic centimetre per minute), with a corresponding pressure comprised between 5x10 "3 and 10 "2 mbar, and preferably is of 45 seem corresponding to a pressure of 7.5x10 "3 mbar.
  • the Argon flow is then lowered so as to be comprised in the range between 5 and 20 seem (corresponding to a pressure in the range between 0.8 x 10 "3 and 3.3 x10 "3 mbar, and preferably it is brought to 15 seem corresponding to a pressure of 2.5 x 10 "3 mbar.
  • the deposition of Mo in a double layer as described above is a preferred condition of the present process, because it allows the removal of the stress of Mo when it is deposited particularly on glass, and consequently allows a good adhesion of the Mo film.
  • the subsequent selenides of In and Ga, InSe and GaSe or a mixed compound thereof In x Ga ⁇ x Se with 0 ⁇ x ⁇ 1 are preferably deposited at a temperature comprised in the range between 370 and 450°C, and more preferably at about 400°C, thus avoiding that the related layers grow with Se in excess.
  • the products sold by Sematrade Technologies and Solutions may be used, having a high density of the order of 99%.
  • a sputtering power between 100 and 200 W may be used, preferably of 150 W, corresponding to a deposition speed between 8 and 24 A/s, and respectively of approximately 15 A/s; whereas for the deposition of GaSe the power used may be in the range between 80 and 150 W, and preferably is of 100, corresponding to a deposition speed comprised between 6.5 and 15 A/s and respectively equal to 9 A/s.
  • a thickness of the InSe layer between 1 and 2 ⁇ is obtained, and preferably is of approximately 1.5 ⁇ , while for the GaSe layer the thickness obtained is comprised between 0.2 and 1 ⁇ , and it is of about 0.5 ⁇ .
  • the deposition of the Cu layer is carried out at temperature ranging between 370 and 450°C, and more preferably at 400°C, in order to obtain a layer of thickness between 0.1 and 0.6 ⁇ , preferably of about 0.35 ⁇ .
  • Figure 1 a schematic illustration is shown of the sequence of deposition of the layers, carried out according to a first embodiment of the process of the invention, wherein 100 is the substrate, 101 a and 101 b are the two layers of Mo, 102 is the layer of InSe, 103 is GaSe and 104 is Cu.
  • the layers 102 and 103 may be replaced by a single layer of a mixed selenide of In and Ga as defined above, having for instance a thickness comprised between 1.5 and 2.5 ⁇ , and preferably equal to 2 ⁇ .
  • the subsequent deposition steps produce a mixing of the preceding layers, with no need of further annealing.
  • the process of the invention comprises a selenization step with Se, that may be carried out in a vacuum room, where pure Se is evaporated from a graphite crucible.
  • Such selenization procedure is very fast, overall it may last from 5 to 15 minutes, for instance 7 minutes, of which from 3 to 10 minutes, for instance 5 minutes, to bring the temperature in the range 500-550°C, preferably 530°C, and from 1 to 5 minutes, for instance 2 minutes, to maintain the material at this temperature.
  • Figure 3 shows the morphology of the absorbing layer Cu(ln,Ga)Se 2 once selenized, obtained by electron microscopy: the so obtained film is well crystallized and, as it may be seen in the figure, the triangular structures are visible, that are typical of the chalcopyrite phase of Cu(ln,Ga)Se 2 .
  • FIG 4 is furthermore shown the X-rays analysis of the film of Figure 3, where it can be seen how the main species formed are Culno ,7 Gao ,3 Se 2 and Culn 0,4 Ga 0,6 Se 2 .
  • Such absorbing layer has a thickness comprised for instance between 1 and 3 ⁇ .
  • thin film solar cells of the Cu(ln,Ga)Se 2 /CdS type are manufactured according to known procedures that are commonly used in this field, for the application in sequence on the absorbing layer of layers of CdS, ZnO and ITO (Indium Tin Oxide).
  • a layer of CdS of thickness comprised for instance between 50 and 120 nm, and preferably equal to 80 nm, may be deposited by radio frequency magnetron sputtering at a temperature of the substrate comprised between 150 and 250°C, and preferably of 200°C, under atmosphere of Ar with R23 (CHF 3 ), added in amount of 1-4% by volume with respect to Ar, and preferably in amount of 3% (other hydrofluorocarbons of the same family of R23, such as R134a, or C 2 H 2 F 4, may be also used in the alternative).
  • the hydrofluorocarbon Thanks to the hydrofluorocarbon, the presence of F " ions in the sputtering discharge makes the stoichiometric CdS grow without any excess of Cd or of S, because the F " ions bombard the surface during the growth of the film; the amount of the hydrofluorocarbon added has however to be a limited amount, as specified above, because an excessive amount of hydrofluorocarbon could inhibit the growth of CdS.
  • FIG. 5 a schematic illustration is shown of the sequence of layers in the solar cell Cu(ln,Ga)Se 2 /CdS wherein 100 is the substrate, 101a and 101 b are two layers of Mo, 105 is Cu(ln,Ga)Se 2 corresponding to layers from 102 to 104 in Figure 1 after selenization, 106 is the layer of CdS, 107 is the layer of ZnO and 108 is the layer of ITO.
  • 109 the lower contact is indicated which is created with special adhesive strips of tinned copper of the commercial type, and with 1 10 the upper grid contact accessible from the outside for instance by using the same strips of tinned copper used for the lower contact.
  • Solar cells manufactures by the present process using the light-absorbing layer shown in Figure 1 have a photovoltaic conversion efficiency of around 12% with a V oc , that is the open circuit voltage, never exceeding 500 mV, with a current density of short circuit Jsc of around 40 mA/cm 2 and a fill factor of 0.62 - 0.64.
  • the characteristics of the solar cells are measured with a solar simulator of 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 by using the present process also starting from a light-absorbing layer as that shown in Figure 1 , to which is tough also added a layer of GaSe after the deposition of Cu, as shown in Figure 6, subjected then to selenization.
  • the further layer of GaSe having for instance a thickness ranging between 0.1 and 0.5 ⁇ , is indicated in Figure 6 as 103bis.
  • a great number of solar cells approximately 60 samples, have been prepared by the present process as described above and they all showed an efficiency of conversion ranging between 13% and 17% with a reproducibility pick between 16% and 17%, thus showing a good reproducibility of the process.

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Abstract

The present invention refers to an improved process for the manufacture of thin film solar cells wherein the light-absorbing layer is formed by CulnGaSe2, which allows to prepare a homogeneous light-absorbing layer, of uniform stoichiometry also on wide surfaces, under conditions of higher safety, speed and readiness, which render the whole manufacture process more efficient and cost-effective.

Description

A PROCESS FOR THE MANUFACTURE OF THIN FILM SOLAR CELLS
DESCRIPTION
The present invention refers in general to the field of the manufacture of solar cells, and more particularly to thin film solar cells. More specifically, the invention relates to a process for the manufacture of thin film solar cells wherein the light- absorbing layer is formed by a thin layer of Cu(ln,Ga)Se2.
Already known are thin film solar cells of CdS with a light-absorbing layer consisting of Cu(ln,Ga)Se2 on soda-lime glass substrates. Such solar cells have reached, at least on small areas, an efficiency higher than 20%, a far higher value than the efficiency values obtainable with any other thin film cell, and comparable only with the performance of the best solar cells made of monocrystalline Si.
However this efficiency is reached by preparing a light-absorbing layer of Cu(ln,Ga)Se2 with a process that works well in laboratory but cannot be easily scaled up. Indeed a three-steps process is used: in the first step In, Ga, and Se are simultaneously deposited by co-evaporation through crossed-beams obtained from separate crucibles on a soda-lime glass substrate covered by a Mo layer having a thickness of approximately 1 μηι; in the second step Cu and Se are deposited and finally in the third step In, Ga and Se are deposited again. The temperature of the substrate is maintained at low levels during the first step and then it is increased up to 550°C during the second and the third steps. The main problem in scaling up this three-steps process consists in that, using substrates of large areas, as requested in an industrial process, the use of crossed-beams may bring to a variable composition from an area to the other. It is known that the non-uniform composition of the material is a serious problem because it negatively influences the efficiency of the device, preventing to get high efficiency values. Indeed, Cu(ln,Ga)Se2 is a quaternary material and, in areas where an excess of Cu is present, it is easy the formation of the binary phase Cu2Se which is unstable, freeing Cu and sending short-circuit to the device.
An alternative method consists of selenization and/or sulphurization of layers of elements deposited one over the other. These layers are generally deposited by sputtering, electrodeposition or electron gun. However, since In and Ga have rather low melting points (In melts at 156, 6°C and Ga melts at 29,8°C), the preparation of the precursors, consisting in the homogeneous mixture of the layers, is rather complicated and requires long annealing time. Alternatively, a process of rapid warming-up is used (Rapid Thermal annealing, RTP) that brings in few minutes the substrate to the right temperature of process (500-550°C), at which the layers are exposed to a gas containing H2Se or H2S for the selenization or sulphurization (Y. Chiba et al., 35th 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 because they have a greater reactivity which is apparently necessary when the selenization or sulphurization are made by a process of rapid warming-up. However H2Se as well as H2S are very poisonous gases and therefore they should not be used in an industrial production, except when using precautions and very stringent procedures of use, that would however increase the manufacture costs of the final device.
It is therefore still felt the need of a process for the manufacture of thin film solar cells with a light-absorbing layer of Cu(ln,Ga)Se2 which may be scaled up, does not require the use of a poisonous gas as H2Se or of low-melting elements.
A purpose of the present invention is therefore to provide a process for the manufacture of solar cells which is able to overcome the drawbacks highlighted above for the prior art processes.
This purpose and further purposes, which will become clearer in the following detailed description, were achieved by the Applicant who surprisingly found a novel scalable process, wherein the deposition of In and Ga is carried out by sputtering of InSe, GaSe or mixed compounds thereof, and the selenization may be carried out using Se instead of H2Se. It has been shown that this process is able to provide selenized precursors Cu(ln,Ga)Se2 that, when used as light-absorbing on soda-lime glass substrates and followed by layers of CdS, ZnO and ITO, allowed the Applicant to produce solar cells having an efficiency of approximately 17%. Furthermore, by carrying out the deposition of each layer by sputtering techniques and using, besides sputtering, just selenization procedures, the process is highly reproducible and easily scalable even on wide areas for an industrial production.
It is therefore a subject of the present invention a process for the manufacture of thin film solar cells wherein the light-absorbing layer consists of Cu(ln,Ga)Se2, and the use of targets of InSe, GaSe or mixed compounds thereof, for the preparation of a light-absorbing layer of Cu(ln,Ga)Se2 for thin film solar cells, whose essential features are as defined in the enclosed independent claims.
Further important features are included in the dependent claims.
The present invention is illustrated in details in the following description made as an example and not for limiting purposes with reference to the attached drawings, in which:
Figure 1 schematically shows the deposition sequence on the substrate of the compounds in the light-absorbing layer from Mo to Cu, according to a first embodiment of the invention;
Figure 2 shows the XRD spectrum (glancing angle, 10°) of the obtained light-absorbing layer, where the orientation of CulnSe2, InSe and GaSe are highlighted, whereas for an easier reading, the orientation related to the Mo peaks are omitted;
Figure 3 shows an image from scanning electron microscope of the film of Figure 1 after the selenization;
Figure 4 shows an X-rays analysis on the film of Figure 3;
Figure 5 is a schematic illustration of the sequence of layers in a solar cell obtained by deposing on the film of Figure 1 layers of CdS, ZnO and ITO;
Figure 6 schematically illustrates the sequence of deposition on the substrate of the compounds in the light-absorbing layer, according to a second embodiment of the invention wherein 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 solar cell Cu(ln,Ga)Se2/CdS obtained using the film of Figure 6 as light-absorbing layer.
The process of the invention relates to the manufacture of solar cells of the Cu(ln,Ga)Se2/CdS type, based on the preparation of the light-absorbing layer of Cu(ln,Ga)Se2 by sputtering using targets of Indium and Gallium selenides to respectively replace In and Ga, followed by selenization with Se instead of with H2Se. In particular, the process comprises the deposition by sputtering, in a vacuum system, of Mo, InSe, GaSe (or of a mixed compound thereof as better specified below) and Cu, and subsequent selenization with Se. The deposition of the above said products is carried out sequentially on a suitable substrate on which is first deposited Mo, then the other products in succession, in the sequence indicated above. Mo and Cu are preferably deposited by pulsed magnetron DC sputtering, whereas the In and Ga selenides are deposited by radio frequency magnetron sputtering.
As the substrate a soda-lime glass substrate may be used, for instance of 1 square inch of surface and 4 mm of thickness. These dimensions are given as an example and do not affect the present process of manufacture of solar cells. Alternatively, the present process was also applied directly on ceramics, in particular on ceramic substrates having a surface of 1 square inch, produced on purpose by Panaria SpA. Ceramic substrates as those generally used in buildings may be used as substrates for the purposes of the present invention, to prepare photovoltaic modules directly on the ceramic covering of buildings, having performances analogues to those of the same modules made with the traditional supports of soda-lime glass. The surface of the ceramic substrate is typically prepared by deposition of a vitreous enamel having chemical-physical characteristics analogues to those of the soda-lime glass substrates.
On such substrate Mo is deposited at room temperature under Argon flow, preferably in a double layer, a first layer which is rather thin, of thickness between 20 and 60 nm, preferably of 30 nm, and a second layer of thickness between 300 and 1000 nm, preferably of 500 nm. For the deposition of the first layer the Argon flow is comprised between 30 and 60 seem (standard cubic centimetre per minute), with a corresponding pressure comprised between 5x10"3 and 10"2 mbar, and preferably is of 45 seem corresponding to a pressure of 7.5x10"3 mbar. For the deposition of the second layer the Argon flow is then lowered so as to be comprised in the range between 5 and 20 seem (corresponding to a pressure in the range between 0.8 x 10"3 and 3.3 x10"3 mbar, and preferably it is brought to 15 seem corresponding to a pressure of 2.5 x 10"3 mbar. The deposition of Mo in a double layer as described above is a preferred condition of the present process, because it allows the removal of the stress of Mo when it is deposited particularly on glass, and consequently allows a good adhesion of the Mo film. Whereas the double layer of Mo is deposited at room temperature, the subsequent selenides of In and Ga, InSe and GaSe or a mixed compound thereof InxGa^xSe with 0 < x < 1 , are preferably deposited at a temperature comprised in the range between 370 and 450°C, and more preferably at about 400°C, thus avoiding that the related layers grow with Se in excess. As targets of InSe and GaSe the products sold by Sematrade Technologies and Solutions may be used, having a high density of the order of 99%. For the deposition of InSe a sputtering power between 100 and 200 W may be used, preferably of 150 W, corresponding to a deposition speed between 8 and 24 A/s, and respectively of approximately 15 A/s; whereas for the deposition of GaSe the power used may be in the range between 80 and 150 W, and preferably is of 100, corresponding to a deposition speed comprised between 6.5 and 15 A/s and respectively equal to 9 A/s. In this way a thickness of the InSe layer between 1 and 2 μηι is obtained, and preferably is of approximately 1.5 μηι, while for the GaSe layer the thickness obtained is comprised between 0.2 and 1 μηι, and it is of about 0.5 μηι.
According to a preferred embodiment of the present process, also the deposition of the Cu layer is carried out at temperature ranging between 370 and 450°C, and more preferably at 400°C, in order to obtain a layer of thickness between 0.1 and 0.6 μηι, preferably of about 0.35 μηι.
In Figure 1 a schematic illustration is shown of the sequence of deposition of the layers, carried out according to a first embodiment of the process of the invention, wherein 100 is the substrate, 101 a and 101 b are the two layers of Mo, 102 is the layer of InSe, 103 is GaSe and 104 is Cu. In an embodiment alternative to that shown in Figure 1 , the layers 102 and 103 may be replaced by a single layer of a mixed selenide of In and Ga as defined above, having for instance a thickness comprised between 1.5 and 2.5 μηι, and preferably equal to 2 μηι.
Under the conditions described above for the present process, the subsequent deposition steps produce a mixing of the preceding layers, with no need of further annealing. From X-rays analysis carried out after the deposition of the Cu layer, and illustrated in Figure 2, it can be seen that at this point Cu(ln,Ga)Se2 was already formed, with just residues of InSe and GaSe. Now the process of the invention comprises a selenization step with Se, that may be carried out in a vacuum room, where pure Se is evaporated from a graphite crucible. Such selenization procedure is very fast, overall it may last from 5 to 15 minutes, for instance 7 minutes, of which from 3 to 10 minutes, for instance 5 minutes, to bring the temperature in the range 500-550°C, preferably 530°C, and from 1 to 5 minutes, for instance 2 minutes, to maintain the material at this temperature. Figure 3 shows the morphology of the absorbing layer Cu(ln,Ga)Se2 once selenized, obtained by electron microscopy: the so obtained film is well crystallized and, as it may be seen in the figure, the triangular structures are visible, that are typical of the chalcopyrite phase of Cu(ln,Ga)Se2.
In Figure 4 is furthermore shown the X-rays analysis of the film of Figure 3, where it can be seen how the main species formed are Culno,7Gao,3Se2 and Culn0,4Ga0,6Se2. Such absorbing layer has a thickness comprised for instance between 1 and 3 μηι.
With the absorbing layer prepared and subjected to selenization as described above, thin film solar cells of the Cu(ln,Ga)Se2/CdS type are manufactured according to known procedures that are commonly used in this field, for the application in sequence on the absorbing layer of layers of CdS, ZnO and ITO (Indium Tin Oxide).
A layer of CdS of thickness comprised for instance between 50 and 120 nm, and preferably equal to 80 nm, may be deposited by radio frequency magnetron sputtering at a temperature of the substrate comprised between 150 and 250°C, and preferably of 200°C, under atmosphere of Ar with R23 (CHF3), added in amount of 1-4% by volume with respect to Ar, and preferably in amount of 3% (other hydrofluorocarbons of the same family of R23, such as R134a, or C2H2F4, may be also used in the alternative). Thanks to the hydrofluorocarbon, the presence of F" ions in the sputtering discharge makes the stoichiometric CdS grow without any excess of Cd or of S, because the F" ions bombard the surface during the growth of the film; the amount of the hydrofluorocarbon added has however to be a limited amount, as specified above, because an excessive amount of hydrofluorocarbon could inhibit the growth of CdS.
A layer of intrinsic ZnO, having a thickness ranging for instance from 60 to 150 nm, and preferably of 100 nm, is then deposited by reactive sputtering (radio frequency magnetron) by using a metallic target of pure Zn under atmosphere of Ar+02 [P02 = 30- 60% of ΡΑΓ+02]- Finally, a layer of ITO for instance of thickness comprised between 300 and 700 nm, and preferably of 500 nm, is deposited by pulsed DC sputtering under atmosphere of Ar+02 [P02 = 2-10% of ΡΑΓ+Ο2]- TO finish the cell, over the layer of ITO a grid contact is formed by continuous sputtering.
In the Figure 5 a schematic illustration is shown of the sequence of layers in the solar cell Cu(ln,Ga)Se2/CdS wherein 100 is the substrate, 101a and 101 b are two layers of Mo, 105 is Cu(ln,Ga)Se2 corresponding to layers from 102 to 104 in Figure 1 after selenization, 106 is the layer of CdS, 107 is the layer of ZnO and 108 is the layer of ITO. Always in Figure 5, with 109 the lower contact is indicated which is created with special adhesive strips of tinned copper of the commercial type, and with 1 10 the upper grid contact accessible from the outside for instance by using the same strips of tinned copper used for the lower contact.
Solar cells manufactures by the present process using the light-absorbing layer shown in Figure 1 have a photovoltaic conversion efficiency of around 12% with a Voc, that is the open circuit voltage, never exceeding 500 mV, with a current density of short circuit Jsc of around 40 mA/cm2 and a fill factor of 0.62 - 0.64. The characteristics of the solar cells are measured with a solar simulator of Oriel Corporation under a light of 100 mW/cm2 in AM 1.5 at a temperature of 24°C.
Solar cells have been manufactured by using the present process also starting from a light-absorbing layer as that shown in Figure 1 , to which is tough also added a layer of GaSe after the deposition of Cu, as shown in Figure 6, subjected then to selenization. The further layer of GaSe, having for instance a thickness ranging between 0.1 and 0.5 μηι, is indicated in Figure 6 as 103bis. The solar cells of Cu(ln,Ga)Se2/CdS type manufactured starting from this kind of light-absorbing layer having a different concentration profile, with a higher concentration of Ga on the surface, showed even better features of the previous ones: the Voc value of these cells is around 600 mV and the fill factor is around 0.72 whereas the current density of short circuit Jsc is around 37 mA/cm2. The efficiency is approximately 16-17%. The characteristic curve current-voltage obtained for one of these cells with a surface of 0.5 cm2, is shown in Figure 7; the photovoltaic parameters measured in this case are: Voc = 576.1 mV, fill factor = 0.74, Jsc 38.13 mA/cm2, efficiency η = 16.2%. Also the efficiency measured in other areas of the substrate provided analogues values, thus indicating a good uniformity in the performances of the cell on the whole area of the substrate.
A great number of solar cells, approximately 60 samples, have been prepared by the present process as described above and they all showed an efficiency of conversion ranging between 13% and 17% with a reproducibility pick between 16% and 17%, thus showing a good reproducibility of the process.
The present invention has been described here with reference to a preferred embodiment thereof. It should be understood that there can be other embodiments deriving from the same inventive core, all of which are covered by the scope of protection of the following claims.

Claims

A process for the manufacture of thin film solar cells comprising a light- absorbing layer of Cu(ln,Ga)Se2, characterised in that said light-absorbing layer is prepared by sputtering deposition of InSe and GaSe, or of a mixed compound thereof InxGa^Se wherein 0 < x < 1 , on a Mo-coated substrate, followed by sputtering deposition of Cu and selenization with Se.
The process according to claim 1 , wherein said substrate is made of ceramics. The process according to claim 1 , wherein said sputtering deposition of InSe, GaSe, or of a mixed compound thereof, and of Cu, is carried out at temperature ranging between 370 and 450°C.
The process according to claim 3, wherein said sputtering deposition of InSe, GaSe, or of a mixed compound thereof, and of Cu, is carried out at temperature of approximately 400°C.
The process according to claim 1 , wherein said selenization is carried out by exposing the deposed layers to pure Se vapour for 5-15 minutes at temperature of 500-550°C in a vacuum chamber.
The process according to claim 1 , further comprising the sputtering deposition of an additional layer of GaSe before selenization.
The process according to any of the preceding claims, further comprising the sputtering deposition of a CdS layer on said light-absorbing layer.
The process according to claim 7, wherein on said CdS layer is additionally carried out the sputtering deposition of a ZnO layer and of a ITO layer.
Use of InSe, GaSe or of a mixed compound thereof InxGa!-xSe wherein 0 < x <
1 , as targets for the preparation, by sputtering deposition, of a light-absorbing layer of Cu(ln,Ga)Se2 for thin film solar cells.
PCT/IB2013/053804 2012-05-10 2013-05-10 A process for the manufacture of thin film solar cells WO2013168135A1 (en)

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