GB2518881A - Solar cell manufacturing method - Google Patents

Solar cell manufacturing method Download PDF

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GB2518881A
GB2518881A GB1317610.2A GB201317610A GB2518881A GB 2518881 A GB2518881 A GB 2518881A GB 201317610 A GB201317610 A GB 201317610A GB 2518881 A GB2518881 A GB 2518881A
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solar cell
cdte
cell stack
chloride source
mgci2
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Jonathan Major
Rob Treharne
Laurie Phillips
Ken Durose
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University of Liverpool
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University of Liverpool
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    • 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
    • H01L21/02551—Group 12/16 materials
    • H01L21/02562—Tellurides
    • 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
    • H01L21/02617—Deposition types
    • H01L21/02631—Physical deposition at reduced pressure, e.g. MBE, sputtering, evaporation
    • 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
    • H01L21/02656—Special treatments
    • H01L21/02664—Aftertreatments
    • 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
    • H01L31/0296—Inorganic materials including, apart from doping material or other impurities, only AIIBVI compounds, e.g. CdS, ZnS, HgCdTe
    • 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 at least one potential-jump barrier or surface barrier
    • 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 at least one potential-jump barrier or surface barrier the potential barriers being only of the PN heterojunction type
    • H01L31/073—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 at least one potential-jump barrier or surface barrier the potential barriers being only of the PN heterojunction type comprising only AIIBVI compound semiconductors, e.g. CdS/CdTe solar cells
    • 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
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1828—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIBVI compounds, e.g. CdS, ZnS, CdTe
    • 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
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/186—Particular post-treatment for the devices, e.g. annealing, impurity gettering, short-circuit elimination, recrystallisation
    • H01L31/1864—Annealing
    • 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
    • Y02E10/543—Solar cells from Group II-VI materials
    • 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

Abstract

A process for treating a CdTe solar cell comprising the steps of treating an exposed surface of a CdTe layer of a partially constructed solar cell stack with a chloride source, and then heating the partially constructed solar cell stack, wherein the chloride source is selected from a group consisting of MgCl2, NaCl, CaCl2, CuCl, KCl, SbCl3 and NH4Cl. The chloride source may be a solid, wherein the solid may be in pellet, granular, particulate or powder form. Wherein treating the surface may further comprise heating the chloride source under a vacuum in the presence of the solar cell stack, such that a film of evaporated chloride source is deposited on the exposed surface of the CdTe layer. The chloride source may also be dissolved in a solvent. The process provides a non-toxic and inexpensive alternative to currently-employed solar cell post-growth processing techniques.

Description

SOLAR CELL MANUFACTURING METHOD
iNTRODUCTION
[0001] The present invention relates to the manufacture of solar cells, particularly cadmium telluride (CdTe) solar cells. The invention also relates to a process for improving the performance of CdTe solar cells, and to the CdTe solar cells prepared by the methodology described herein.
BACKGROUND OF THE INVENTION
[0002] Solar cells (also termed photovoltaic cells) are electrical devices that are able to convert light energy directly into electrical current via the photovoltaic effect. A number of different materials are known to exhibit the photovoltaic effect and have therefore attracted interest in the field of solar cell manufacture. Currently, the semiconductor most widely used in photovoltaic devices is mono-crystalline silicon. However, due to the cost involved in producing bulk material of this type, devices made by this method are often prohibitively expensive for all but the smallest scale or most specialized applications.
Elsewhere, various thin film systems are demonstrating their worth as cost effective alternatives.
[0003] Thin film CdTe photovoltaics are still considered a promising area of thin-film solar cells despite recent commercial difficulties. The two key properties of this material which make it a better material for solar cells are its near ideal band gap for photovoltaic conversion efficiency of 1.45 eV, and its high optical absorption coefficient. Typically the structure of a CdTe/CdS solar cell is composed of multiple layers: a transparent layer of glass; a low resistivity oxide transparent conducting oxide (TCO) layer which acts as a front contact; a high resistivity transparent oxide coating which forms a "buffer" layer; a CdS film (n-doped), which is also called the window layer owing to its transparency down to wavelengths of around 515 nm; a CdTe film (p-doped) also known as the absorber layer, which has an energy gap of around 1.5 eV and is ideally suited to the solar spectrum; and a back metal contact adjacent the CdTe film.
[0004] Production of CdTe thin film materials requires a follow-on processing step to recrystallize its polycrystalline structure so that effective photovoltaic devices can be made from the film stack. This step has long since been accomplished by treating the CdTe film with cadmium chloride (CdCI2), which has been shown to improve the conversion efficiency of the resulting photovoltaic device to beyond 10%.
(0005] However, this follow-on processing step comes at a significant cost to the photovoltaic industry since CdCI2 cannot be sourced cheaply. Furthermore, CdCI2 is understood to be a highly toxic carcinogenic substance, an over-exposure to which can result in lung, kidney and liver damage, blood and prostate disorders, complications to the reproductive system, anemia, and a loss of sense of smell. In some scenarios, the exposure may even prove fatal. Elsewhere, CdCI2 and has been shown to exhibit harmful effects on the environment, including entering food chains, and pollution of air and water.
(0006] The present invention was devised with the foregoing in mind.
SUMMARY OF THE INVENTION
[0007] According to a first aspect of the present invention there is provided a process for treating an exposed surface of a CdTe layer during the manufacture of a CdTe solar cell stack, the process comprising: a) treating the exposed surface of the CdTe layer with a chloride source, and b) heating the solar cell stack, wherein the chloride source is selected from the group consisting of MgCI2, NaCI, CaCI2, CuCI, KCI, SbCI3 and NH4CI.
(0008] According to a second aspect of the present invention there is provided a process for improving the performance of a CdTe solar cell, the process comprising:.
a) treating an exposed surface of a CdTe layer in a partially constructed CdTe solar cell stack with a chloride source, and b) heating the solar cell stack, wherein the chloride source is selected from the group consisting of MgCI2, NaCI, CaCI2, CuCI, KCI, SbCI3 and NH4CI.
(0009] According to a third aspect of the present invention, there is provided a process for the manufacture of a CdTe solar cell, the process comprising: a) preparing a partially constructed CdTe solar cell stack having an exposed CdTe layer, b) treating the exposed CdTe layer accordance with the process as herein defined, and c) performing one or more subsequent manufacturing steps to complete the CdTe solar cell.
(0010] According to a fourth aspect of the present invention, there is provided a CdTe solar cell prepared by the process of the third aspect of the invention.
DETAILED DESCRIPTION OF THE INVENTION
(0011] Embodiments of the invention will now be described, by way of example only, for the purpose of reference and illustration.
(0012] As previously stated, the present invention provides a process for treating an exposed surface of a CdTe layer during the manufacture of a CdTe solar cell stack, the process comprising: a) treating the exposed surface of the CdTe layer with a chloride source, and b) heating the solar cell stack, wherein the chloride source is selected from the group consisting of MgCI2, NaCI, CaCI2, CuCI, KCI, SbCI3 and NH4CI.
(0013] The inventors have surprisingly found that by employing the treatment methodology of the present invention an alternative follow-on processing step for improving the performance of CdTe solar cells is possible without the need for expensive, toxic and environmentally harmful Odd2. In particular, the inventors have demonstrated that by employing the treatment method of the present invention, device efficiencies that are acceptable, and in some cases comparable to and even better than, CdCI2-doped solar cells, can be readily achieved. Moreover, given the similarities between the present treatment method and currently-employed CdCI2 doping techniques, the latter can be readily exchanged for the former with minimal disruption to solar cell manufacturing protocols.
(0014] Also provided is a process for improving the performance of a CdTe solar cell, the process comprising: a) treating an exposed surface of a CdTe layer in a partially constructed CdTe solar cell stack with a chloride source, and b) heating the solar cell stack, wherein the chloride source is selected from the group consisting of MgCI2, NaCI, CaCI2, CuCI, KCI, SbCI3 and NH4CI.
(0015] The inventors have surprisingly found that by employing the treatment method of the present invention an alternative follow-on processing step for improving the performance of CdTe solar cells is possible without the need for expensive, toxic and environmentally harmful CdCI2. In one embodiment, the performance of the CdTe solar cell will be understood to relate to the photovoltaic conversion efficiency of the completed CdTe solar cell device. The inventors have demonstrated that by employing the treatment method of the present invention, device efficiencies that are acceptable, and in some cases comparable to, or even better than, CdCI2-doped solar cells, can be readily achieved.
[0016] Suitably, the processes of the present invention are carried out in air.
[0017] Suitably, the chloride source is selected solely from the group consisting of MgCI2, NaCI, CaCI2, CuCI, KCI, SbCI3 and NH4CI, i.e. no other chlorides may present.
Most suitably, the chloride source does not comprise any CdCI2.
[0018] In an embodiment of the invention, the chloride source is selected from MgCI2 and NaCI. The use of MgCI2 and NaCI as part of the present treatment methods has been demonstrated to afford acceptable levels of photovoltaic efficiency when compared with currently-employed CdCI2 doping techniques. Moreover, such chloride sources do not pose the same health risks as CdCI2, and are available at only a fraction of the cost.
[0019] In a particular embodiment, the chloride source is MgCI2. The use of MgCI2 as part of the present treatment methods has been demonstrated to afford device efficiencies that are comparable, and even superior, to those observed when current CdCI2 doping techniques are used. Furthermore, in stark contrast to CdCI2, there are no notable health or environmental risks resulting from long-term or repeated exposure to MgCI2. MgCI2 is also markedly cheaper to source.
[0020] In an embodiment, the chloride source is a solid. In such embodiments, the chloride source is optionally in a pellet, granular, particulate or powder form.
[0021] Optionally, step a) of the processes defined above further comprises heating the chloride source under a vacuum in the presence of the solar cell stack, such that a film of evaporated chloride source is deposited on the exposed surface of the CdTe layer. The treatment methods of the present invention may therefore involve deposition of the chloride source onto the CdTe exposed surface by a thermal evaporation technique. As the chloride source evaporates, quantities of it strike the exposed surface of the CdTe and form a thin film. The thermal evaporation technique forming part of the present invention may be advantageous due to the similarities it shares with currently-employed industrial CdCI2 doping techniques. The present invention therefore serves as a convenient alternative to standard CdCI2 techniques, without the need for an overhaul of manufacturing protocols and equipment.
(0022] In an embodiment, the film deposited on the exposed surface of CdTe has a thickness of between 10 nm and 3 pm. Most suitably, the film has a thickness of between 300 and 850 nm.
[0023] In an embodiment, the chloride source is dissolved in a suitable solvent. Certain treatment methods of the present invention require the chloride source to be in a liquid state prior to deposition onto the exposed CdTe surface. A range of solvents are envisageable, depending on the chloride source being used. Suitably, the solvent is an alcohol, for example methanol. Suitably, an excess of chloride source is dissolved in the solvent, such that a saturated solution is achieved.
[0024] In a further embodiment, the dissolved chloride source is applied directly to the exposed surface of the CdTe layer. The treatment methods of the present invention may therefore involve deposition of the chloride source onto the CdTe exposed surface by a solution deposition technique. The solution treatment method may allow for a more accurate determination of the quantity of chloride source applied to the CdTe exposed surface. Moreover, the simplicity of the technique means that it may serve as a convenient alternative to currently-employed industrial CdCI2 doping techniques.
[0025] In an embodiment, the chloride source is applied to the exposed surface of the CdTe layer as a vapour during step b) of the processes defined above.
[0026] The treatment methods of the present invention may therefore involve deposition of the chloride source onto the CdTe exposed surface by a vapour deposition technique. The vapour deposition technique may involve dissolving an excess of a chloride source in a solvent. Suitably, once dissolved, a quantity of the chloride source solution is deposited onto the surface of a substrate. Following deposition onto the substrate, both the substrate and a partially constructed solar cell stack having a CdTe layer with an exposed surface are heated in accordance with step b), such that chloride vapour emanating from the substrate is deposited onto the exposed surface of the CdTe layer. There is no direct contact between the chloride source solution and the exposed surface of the CdTe layer. The vapour treatment method may allow for more uniform chloride treatment across the CdTe exposed surface. Moreover, the simplicity of the technique means that it may serve as a convenient alternative to currently-employed industrial CdCI2 doping techniques.
(0027] In an embodiment, step b) comprises heating the solar cell stack to a temperature between 300 and 550°C.
[0028] In a further embodiment, step b) comprises heating the solar cell stack to a temperature between 400 and 480°C.
(0029] In a further embodiment, step b) comprises heating the solar cell stack to between 400 and 450°C.
(0030] In an embodiment, step b) comprises heating the solar cell stack for between 1 and 300 minutes.
[0031] In a further embodiment, step b) comprises heating the solar cell stack for between 10 and 180 minutes.
(0032] In a further embodiment, step b) comprises heating the solar cell stack for between 10 and 120 minutes.
(0033] In a further embodiment, step b) comprises heating the solar cell stack for between 10 and 60 minutes.
(0034] In a further embodiment, step a) comprises treating the exposed CdTe surface using a solution deposition technique, and step b) comprises heating the solar cell stack at 300 and 550°C (e.g. between 400 and 450°C) for between 10 and 180 minutes.
(0035] In an embodiment, step a) comprises treating the exposed CdTe surface using a solution deposition technique, and step b) comprises heating the solar cell stack at 400 and 450°C (e.g. at 410°C) for between 10 and 120 minutes.
(0036] In a further embodiment, step a) comprises treating the exposed CdTe surface using a vapour deposition technique, and step b) comprises heating the solar cell stack at 300 and 550°C (e.g. 400 and 450°C) for between 10 and 180 minutes.
(0037] In an embodiment, step a) comprises treating the exposed CdTe surface using a a vapour deposition technique, and step b) comprises heating the solar cell stack at 400 and 450°C (e.g. at 410°C) for between 10 and 60 minutes.
(0038] In an embodiment, step a) of the treatment method involves depositing a chloride source film onto the CdTe exposed surface by thermal evaporation, said film having a thickness of 10 nm and 1 pm (or preferably 300 and 850 nm), and step b) involves heating the solar cell stack at 300 and 550°C (e.g. 400 and 450°C) for between 10 and minutes.
(0039] In an embodiment, step a) of the processes defined above are preceded by the step of etching the exposed CdTe surface. Etching the surface of the CdTe layer results in a Te-rich surface, which enhances the diffusion of chloride ions into the CdTe layer during the treatment methods of the present invention. Without wishing to be bound by theory, it is considered that the diffusion rate of chlorine ions into the CdTe layer has an impact on the effectiveness of the treatment methods.
(0040] In an embodiment, the CdTe surface is etched using an ion beam. Suitably, the CdTe surface is etched using an acid. Suitably, the acid is nitric-phosphoric acid.
(0041] According to a third aspect of the present invention, there is provided a process for the manufacture of a CdTe solar cell, the process comprising: a) preparing a partially constructed CdTe solar cell stack having an exposed CdTe layer, b) treating the exposed CdTe layer accordance with the process as herein defined, and c) performing one or more subsequent manufacturing steps to complete the CdTe solar cell.
(0042] Employing the treatment methods of the present invention as part of a solar cell manufacturing process allows for the production of solar cells having device efficiencies that are acceptable, and in some cases comparable to and even better than, CdCI2-doped solar cells, without the cost, health and environmental implications of the latter.
(0043] The step of preparing a partially constructed CdTe solar cell stack may comprise the steps of i) providing a front electrode; ii) applying a layer of an n-doped semiconductor above the front electrode; and iii) applying a layer of CdTe directly above the n-doped semiconductor layer.
(0044] The layer of glass or transparent plastic may be coated by a layer to form the front electrode. In an embodiment, the glass layer is a soda-lime glass.
(0045] In an embodiment, the front electrode is a layer of a transparent conductive oxide. Suitably the transparent conductive oxide is fluorine-doped tin oxide.
(0046] In an embodiment, the front oxide layer is cleaned prior to deposition of the buffer layer by i) ultrasonication (e.g. in de-ionised water); and/or ii) rinsing (e.g. with an alcohol, such as, for example, isopropyl alcohol).
(0047] The partially constructed solar cell stack may also comprise a buffer layer situated between the front electrode and the n-doped semiconductor layer. In an embodiment, the buffer layer is formed from zinc oxide film.
(0048] In an embodiment, the zinc oxide buffer layer films are deposited by reactively sputtering a zinc target under an oxygen atmosphere.
(0049] In an embodiment, the n-doped semiconductor layer is formed from cadmium sulphide (CdS). Suitably the CdS layer is deposited by radio frequency sputtering.
(0050] In an embodiment, the CdTe layer is deposited using close space sublimation.
(0051] In an embodiment, prior to treating in accordance with the present invention, the exposed surface of the deposited CdTe layer is subjected to an acid etch treatment.
Suitably the acid is nitric-phosphoric acid. The acid etch treatment is advantageous since it removes any Cd-rich surface layer that may exist on the deposited CdTe layer, thereby resulting in a Te-rich surface which aids chloride diffusion into the CdTe layer during the treatment processes of the present invention.
[0052] The step of performing one or more subsequent manufacturing steps to complete the CdTe solar cell may comprise applying a rear electrode to the chloride-treated surface of the CdTe layer.
[0053] In an embodiment, the rear electrode is formed from gold. Suitably, the rear electrode is deposited by thermal evaporation under vacuum.
[0054] The step of performing one or more subsequent manufacturing steps to complete the CdTe solar cell may also comprise subjecting the chloride-treated surface of the CdTe layer to an acid etch treatment prior to applying the rear electrode. Suitably the acid is nitric-phosphoric acid. Suitably the acid-etched chloride-treated CdTe layer is then rinsed with de-ionised water in preparation for applying the rear electrode.
[0055] Preferred, suitable, and optional features of any one particular aspect of the present invention are also preferred, suitable, and optional features of any other aspect.
BRIEF DESCRIPTION OF THE DIM WINGS
[0056] Figure 1 shows a basic CdTe solar cell structure Figure 2 is a graph demonstrating the effect of the vapour and solution treatment methods of the present invention on device efficiency as a function of annealing time at 410°C, when compared with standard CdCI2 treatment techniques.
Figure 3 is a current density vs voltage plot comparing the highest device efficiencies observed for each of the treatment methods compared in Figure 2.
Figure 4 is a current density vs voltage plot comparing the highest device efficiency observed for the thermal evaporation treatment method of the present invention, with that observed using a standard CdCI2 treatment technique.
Figure 5 compares carrier concentrations calculated from Mott-Schottky capacitance vs voltage plots for CdSICdTe solar cells treated in accordance with the solution and vapour treatment methods of the present invention with those treated using a standard CdCI2 technique.
Figure 6 compares X-Ray diffraction spectra of a CdTe solar cell treated in accordance with the solution treatment method of the present invention with a CdTe solar cell treated using a standard CdCI2 technique and an as-grown (i.e. untreated) CdTe solar cell.
Figure 7 compares SEM images of CdTe solar cells treated in accordance with the vapour and solution treatment methods of the present invention, with those treated using a standard CdCI2 technique.
Figures Ba, Sb and Sc show complete SIMS profiles CdTe solar cells treated in accordance with the vapour and solution treatment methods of the present invention, as well as those treated using a standard CdCI2 technique.
Figures 9a, 9b and 9c show SIMS profiles for oxygen, chlorine and magnesium in CdTe solar cells treated in accordance with the vapour and solution treatment methods of the present invention, as well as those treated using a standard CdCI2 technique.
Figure 10 compares EQE curves for the highest device efficiency observed using the thermal evaporation treatment method of the present invention, with that observed using a standard CdCI2 treatment technique.
EXAMPLES
Example 1
Device fabrication [0057] Referring to Fig 1, the CdTe solar cell stacks used in conjuction with the methods of the invention are produced by successive deposition of the component layers. Typically, a soda-lime glass superstrate having a fluorine-doped tin oxide transparent conducting oxide (TCO) coating (TECTM glass, obtained from Nippon Sheet Glass Co. Ltd) is cleaned via ultrasonication in de-ionised water, followed by rinsing with isopropyl alcohol. A zinc oxide buffer layer film is then deposited onto the surface of the ICO coating by reactively sputtering a zinc target under an oxygen atmosphere to a thickness of 100 nm. Subsequently, a film of CdS is deposited on the surface of the buffer layer by radio frequency sputtering from a CdS target under 5 mTorr of argon. A target power of 60 W, a growth temperature of 200CC, and a growth time of between 16 and 32 minutes were used in order to give a CdS film having a thickness in the region of 120-240 nm. A CdTe film is then deposited on the surface of the CdS film by close space sublimation using source and substrate temperatures of 605°C and 520°C respectively.
Growth was carried out under a 25 Torr nitrogen atmosphere for a growth time of 7 minutes, thereby yielding a film of between 4 and 6 p thick. Once deposited, the CdTe is subjected to a nitric-phospohoric acid etch for 30 seconds in order to remove any Cd-rich surface layer that may exist. The resulting Te-rich surface is then rinsed with de-ionised water.
(0058] The exposed surface of the CdTe layer is then treated in accordance with the methods of the present invention.
[0059] The treated CdTe layer is then subjected to second nitric-phosphoric acid etch to clean the surface in preparation for the application of the rear electrode. The etched surface is then rinsed with de-ionised water, and gold rear electrodes measuring 50 x 50 mm and having a thickness of approximately 60 nm are then deposited thereon by thermal evaporation under vacuum.
Example 2
Chloride treatment methods MgCI2 solution deposition [0060] An excess of MgCI2 is dissolved in methanol (10% by weight of MgCI2). Once dissolved, a few drops of the MgCI2 solution is deposited directly onto the exposed surface of the CdTe layer of a partially constructed solar cell stack. Following deposition, the chloride-treated solar cell stack is annealed at a temperature of 410°C for between 5 and 60 minutes.
MgC/2 vapour deposition [0061] An excess of MgCI2 is dissolved in methanol. Once dissolved, a few drops of the MgCI2 solution is deposited onto the surface of a glass slide. Following deposition, both the glass slide and a partially constructed solar cell stack having a CdTe layer with an exposed surface are annealed substantially adjacent one another at a temperature of 410°C for between 5 and 60 minutes, such that chloride vapour emanating from the substrate is deposited onto the exposed surface of the CdTe layer. There is no direct contact between the MgCI2 solution and the exposed surface of the CdTe layer.
MgCI2 Thermal evaporation [0062] A quantity of granular MgCI2 is heated under vacuum in the presence of a partially constructed solar cell stack having a CdTe layer with an exposed surface. As the MgCI2 evaporates, quantities of it strike the exposed surface of the CdTe and form a film having a thickness of between 40 and 800 nm. Following formation of the film, the solar cell stack is annealed at a temperature of between 410 and 450°C for between 20 and minutes.
Example 3
Current vs voltage analysis for MaCI2 solution and vapour treated devices [0063] Current vs voltage (J-V) analysis is the basic measurement used to evaluate solar cell performance. It is done under standard illumination conditions of AM1.5, and yields the solar conversion efficiency. Current vs voltage measurements were obtained for both MgCI2 solution and vapour-treated devices annealed at 410°C for varying time periods. Similarly, current vs voltage measurements were obtained for devices doped with CdCI2 according to a standard thermal evaporation protocol then annealed at 410°C for varying time periods. The best performing samples for each treatment were selected and the device scribed such that the front contacting was improved so as to minimise series resistance effects in each case. These devices were then re-measured, with the performance results being shown in Fig 2.
[0064] Fig 2 demonstrates that the optimal treatment time for both the CdCI2 treatment and the MgCI2 solution treatment is approximately 20 minutes. The process appears to be slower for the MgCI2 vapour treatment process, which requires approximately 60 minutes. This is perhaps not surprising since the vapour treatment process may have simply served to slow the in-diffusion rate of the chloride into the CdTe layer. Fig 2, when read in conjunction with Table 1 (below) show that the overall performance of devices is comparable between CdCI2 and MgCI2 treatment processes.
:HTtreatP1en*t* Peak FF(%i:: Peak hó OliN rn?). ..H. .(V) ::..
CdCI2 1303 7128 2230 083 MgCI2SoI 1269 6908 2241 082 13.46 70.24 23.26 0.82 Table 1: Performance parameters for highest efficiency MgCI2 solution and vapour treated devices [0065] Fig 3 shows J-V curves for the highest efficiency MgCI2 solution and vapour treated devices. So-called "rollover" of J-V curves at high forward bias implies that there may be some back contacting issues for the MgCI2 treated devices. It is suspected that this is due to the presence of surface residues, although it is clear that device performance has not been hindered.
Current vs voltage analysis for MgCI2 thermal evaporation treated devices [0066] Whilst the MgCI2 solution and vapour treatment methods have been clearly demonstrated to be effective post growth treatment steps, MgCI2 treatment via thermal evaporation may be considered the more attractive commercial embodiment of the invention due to the resemblance it bears to currently-employed CdCI2-doping techniques in industry. When interpreting the results reported below, it should be borne in mind that i) current vs performance measurements are not comparable to those observed for MgCI2 solution and vapour treated devices due to a change in the cell baseline process resulting in lower efficiencies for both MgCI2 thermal evaporation and CdCI2 treated devices (i.e. the lower efficiencies are not due to the change in MgCI2 treatment method), and U) the MgCI2 film thicknesses reported are approximate, due to the difficulty in calibrating the film thickness owing to its softness and hygroscopic nature.
[0067] A number of devices were made in order to establish the optimal processing conditions for devices using thermally evaporated MgCI2 compared to CdCI2. The CdCI2 treatment had been optimised previously with the highest efficiency devices produced for a film thickness of 200nm and annealing in air at 410°C for 30 minutes. For MgCI2, thermally evaporated films of approximately 40 nm, 200 nm, 400 nm and 800 nm were deposited. Annealing times varied between 20 and 120 minutes at temperatures of between 410 and 450°C. Thinner films (i.e. those having a thickness of less than approximately 200 nm), although showing improvements when compared with untreated devices, were observed to peak at a device efficiency of approximately 5%, irrespective of annealing time and temperature. Thicker films did not demonstrate this limitation, and the highest efficiency devices were found to be those annealed at 430°C. For devices having an approximately 400 nm thick MgCI2 film, a peak efficiency of 8.44% was achieved after 120 mm annealing. Further annealing may have led to higher efficiency but this was not attempted. For devices having an approximately 800 nm thick MgCI2 film, a peak efficiency of 9.41% was achieved after 30 minutes of annealing at 430 °C.
(0068] Tables 2 and 3 (below) respectively show the average and best device performance parameters for thermally evaporated MgCI2 and CdCI2 treated devices.
Ave Ave FF (%) Ave V0 (V) (mA/cm2) CdCI2 8.93 ± 0.73 59.74 ± 3.31 0.73 ± 0.01 20.41 ± 0.50 MgCI2 8.86 ± 0.46 57.03 ± 1.84 0.75 ± 0.02 20.68 ± 0.56 Table 2: Average device results for thermally evaporated MgCI2 and CdCI2 treated devices Peak n (%) Peak FF (%) Peak V0 (V) CdCI2 9.85 64.28 0.74 20.94 MgCI2 9.41 59.01 0.78 21.21 MgCI2 improved 10.07 64.93 0.78 19.88 front contact ____________ _____________ _____________ ____________ Table 3: Peak device results for highest performing thermally evaporated MgCI2 and CdCI2 treated devices (0069] Referring to Fig 4, the thermally evaporated MgCI2 treated device exhibiting the highest efficiency contact was found to show large series resistance in the forward bias section of the J-V curve owing to the contact being a the opposite side of the substrate ot the front contact. This contact was re-measured after the front contact had been improved. The average thermally evaporated MgCI2 treated device performance seen in Table 2 is seen to be almost identical to the CdCI2 treated device, and all performance parameters agree within the boundary of errors. Peak device performance values are also very similar, although the thermally evaporated MgCI2 treated device does show a significant improvement in the V0 of 40 my.
Example 3
Capacitance vs voltage analysis for MgCI2 solution treated devices (0070] Capacitance vs voltage (C-V) analysis, density-depth profiling, was used to compare the carrier concentration for MgCI2 solution and CdCI2 treated CdTe solar cell devices. The carrier concentration, extracted from the linear region at reverse bias of a Mott-Schottky C-V plot, of various samples is shown in Fig 5. In spite of there being a spread of values for carrier concentration in both MgCI2 solution and CdCI2 treated samples, the range of values is very similar in both cases. Moreover, the carrier concentration can be easily influenced by minor changes in processing conditions and it is therefore not surprising that a degree of variation is observed. The conclusion is that there is no systematic difference in the results produced by each technique.
Example 4
Ph vs/cal character/sat/on [0071] Fig 6 shows X-ray diffraction patterns determined for a MgCI2 solution treated and a CdCI2 treated CdTe layer. The diffraction pattern of an "as-grown" (no chloride treatment) CdTe layer is also shown. Table 4 (below) summarises the texture coefficient and preferred orientation values for each sample. Peaks not identified in Fig 6 are attributable to the Sn02 coated substrate. The as-grown sample shows the strongest preferred orientation, a=0.47, although this is still not highly orientated. Both treated samples show a reduction in the preferred orientation, more so in the case of the MgCI2 treated sample, indicating some recrystallization of the CdTe layer in both cases.
:..::JrtJent::::: ::Ci::..: As-grown 163 022 087 109 055 109 155 047 Cd012 142 025 100 109 057 117 150 042 :Mc!::Th: 1.56 0.33 0.93 1.12 0.61 1.09 1.36 0.39 Table 4: Values for texture coefficients, C,,kI, and degree of preferred orientation, a, for CdTe samples with various treatments.
[0072] Images from SEM analysis of the device back surface are shown in Fig 7. It can be seen from this that MgCI2 treated samples display a different morphology than for CdCI2 treatment, having a smaller grain size and less well defined crystal facets.
Example 5
Secondary ion mass sQectrometry analysis [0073] Secondary ion mass spectrometry (SIMS) analysis of devices with different chloride treatments was carried out. Three devices were compared i) optimised MgCI2 solution treated sample annealed for 20mm at 410°C, ii) optimised MgCI2 vapour treated sample annealed for 60mm at 410°C and iii) optimised Odd2 treated sample annealed for 20mm at 410°C (this sample was produced prior to commencement of MgCI2 activities so no contamination with MgCI2 was possible). All samples had a device efficiency of H3%.
[0074] Figs Ba (CdCI2 treated), Sb (MgCI2 solution treated) and Bc (MgCI2 vapour treated) show elemental profiles as a function of sputtering time which can be correlated to depth from the CdTe back surface. The various layers of the device can be distinguished from the drop or rise at certain points of specific elements. The CdS layer is taken to begin at the point at which the S profile shows an increase combine with a drop in the Te counts. The TCO layer is assumed to begin at the point the Sn profile reaches a peak and the S decreases. The glass superstrate is reached when the Si profile peaks. The approximate positions of the layers have been marked on Figs Ba, Bb and Sc. Unfortunately as can be seen from Figs Ba, Sb and Sc there was a significant variation in the CdTe thickness between samples measure. This is due to film non-uniformity resulting from CSS deposition and some variation in the source sublimation rate (the CdCI2 treated sample was produced 45 growth runs prior to the MgCI2 samples and the growth rate decreases with time).
[0075] In order to directly compare the chemical content of the CdTe layers a plot of the first 400s of sputter time for each sample is shown in Figs 9a (oxygen content), 9b (chlorine content) and 9c (magnesium content), which show the change in content to equivalent depths within each of the CdTe layers.
[0076] The data show a significant amount of magnesium present in all samples, including the CdCI2 treated sample. From the full cell plots (Figs Ba, Sb and Sc) it can be seen that the majority of the magnesium content arises via diffusion from the glass superstrate with the magnesium peak coinciding with the silicon peak in all cases. The presence of magnesium is not unexpected due to a non-trivial magnesium oxide content in commercial glass, however the level of magnesium within the Odd2 treated device is surprising. Comparing the magnesium content of the CdTe layers (Figs 9a, Yb and Yc) it can be seen that towards the CdTe back surface (0 s) the MgCI2 treated device show an increased magnesium content, particularly for the MgCI2 solution treated device. As the sputtering time increases the variation between samples is reduced. It therefore appears that the increased magnesium content resulting from the MgCI2 treatment is confined to the CdTe layer. Indeed in the CdS layer the magnesium content is significantly higher for the CdCI2 treated sample.
[0077] For the CdTe profiles (Figs 9a, 9b and 9c) it is observed that the in-diffusion of both chlorine and oxygen into the CdTe layer is increased for MgCI2 treatment compared to CdCI2 treatment. This is true irrespective of either vapour or solution processing.
Whilst there is little difference in the oxygen content between the two MgCI2 treatments, there is an increase in the chloride content for the vapour processed device. This may, however, be due to the longer processing required for optimisation allowing greater chloride in-diffusion to the CdTe layer.
[0078] Without wishing to be bound by theory, both oxygen and chlorine have been linked to enhanced p-type doping in CdTe and their in-diffusion is viewed as a key part of the post growth treatment step. The enhanced concentrations of these elements for MgCI2 treatment in comparison to CdCI2 treatment is therefore considered to be beneficial.
Example 6
External quantum efficiency analysis for MgCI, thermal evaporation treated devices [0079] External quantum efficiency (EQE) analysis (Fig 10) shows a cell's response to light as a function of incident wavelength, with or without a 1 sun intensity background white light bias. Both CdCI2 treated and MgCI2 thermal evaporated treated devices show a similar curve shape indicating the junction position is in the same position.
[0080] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claims (31)

  1. CLAIMS1. A process for treating an exposed surface of a CdTe layer during the manufacture of a CdTe solar cell stack, the process comprising: a) treating the exposed surface of the CdTe layer with a chloride source, and b) heating the solar cell stack, wherein the chloride source is selected from the group consisting of MgCI2, NaCI, CaCI2, CuCI, KCI, SbCI3 and NH4CI.
  2. 2. A process for improving the performance of a CdTe solar cell, the process comprising: a) treating an exposed surface of a CdTe layer in a partially constructed CdTe solar cell stack with a chloride source, and b) heating the solar cell stack, wherein the chloride source is selected from the group consisting of MgCI2, NaCI, Cad2, CuCI, KCI, SbCI3 and NH4CI.
  3. 3. The process of claim 1 or 2, wherein the chloride source is MgCI2 or NaCI.
  4. 4. The process of any of claims 1, 2 or 3, wherein the chloride source is MgCI2.
  5. 5. The process of claim 4, wherein the chloride source is a solid.
  6. 6. The process of claim 5, wherein the chloride source is in pellet, granular, particulate or powder form.
  7. 7. The process of claim 5 or 6, wherein step a) further comprises heating the chloride source under a vacuum in the presence of the solar cell stack, such that a film of evaporated chloride source is deposited on the exposed surface of the CdTe layer.
  8. 8. The process of claim 7, wherein the film has a thickness of between 10 nm and 3 pm.
  9. 9. The process of claim 8, wherein the film has a thickness of between 300 and 850 nm.
  10. 10. The process of any of claim 1 to 4, wherein the chloride source is dissolved in a solvent.
  11. 11. The process of claim 10, wherein the solvent is an alcohol, for example methanol.
  12. 12. The process of claim 10 or 11, wherein the solvent is methanol.
  13. 13. The process of any of claims 10, 11 or 12, wherein an excess of chloride source is used.
  14. 14. The process of any of claims 10 to 13, wherein the chloride source is applied directly to the exposed surface of the CdTe layer.
  15. 15. The process of any of claims 10 to 13, wherein the chloride source is applied to the exposed surface of the CdTe layer as a vapour during step b).
  16. 16. The process of any preceding claim, wherein step b) comprises heating the solar cell stack to between 300 and 550°C.
  17. 17. The process of any preceding claim, wherein step b) comprises heating the solar cell stack to between 400 and 460°C.
  18. 18. The process of any preceding claim, wherein step b) comprises heating the solar cell stack to between 400 and 450°C.
  19. 19. The process of any preceding claim, wherein step b) comprises heating the solar cell stack for between 1 and 300 minutes.
  20. 20. The process of any preceding claim, wherein step b) comprises heating the solar cell stack for between 1 and 180 minutes.
  21. 21. The process of any preceding claim, wherein step b) comprises heating the solar cell stack at 300 to 550°C for between 1 and 180 minutes.
  22. 22. The process of any of claims 10 to 14, wherein step b) comprises heating the solar cell stack at 400 and 460°C for 10 to 120 minutes.
  23. 23. The process of claim 15, wherein step b) comprises heating the solar cell stack at 300 to 550 °C for between 1 and 180 minutes.
  24. 24. The process of any of claims 1 to 9, wherein step a) comprises depositing a film of chloride having a thickness of 10 nm to 1 jim, and step b) comprises heating the solar cell stack at 300 to 550 C for between 1 and 180 minutes.
  25. 25. The process of any of claims 1 to 9, wherein step a) comprises depositing a film of chloride having a thickness of 30010 850 nm, and step b) comprises heating the solar cell stack at 400 to 450t for 10 to 180 minutes.
  26. 26. The process of any preceding claim, wherein step a) is preceded by the step of etching the exposed CdTe surface to remove a Cd-rich layer.
  27. 27. The process of claim 26, wherein the CdTe surface is etched using an ion beam.
  28. 28. The process of claim 26, wherein the CdTe surface is etched using an acid.
  29. 29. The process of claim 28, wherein the acid is nitric-phosphoric acid.
  30. 30. A process for the manufacture of a CdTe solar cell, the process comprising: a) preparing a partially constructed CdTe solar cell stack having an exposed CdTe layer, b) treating the exposed CdTe layer in accordance with the process of any preceding claim, and c) performing one or more subsequent manufacturing steps to complete the CdTe solar cell.
  31. 31. A CdTe solar cell obtained by the process of claim 30.
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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120003784A1 (en) * 2010-07-02 2012-01-05 Primestar Solar, Inc. Methods of forming a conductive transparent oxide film layer for use in a cadmium telluride based thin film photovoltaic device
WO2013177047A1 (en) * 2012-05-21 2013-11-28 First Solar, Inc. Apparatus and method for improving efficiency of thin-film photovoltaic devices
WO2013177048A2 (en) * 2012-05-21 2013-11-28 First Solar, Inc. Method of providing chloride treatment for a photovoltaic device and a chloride treated photovoltaic device

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120003784A1 (en) * 2010-07-02 2012-01-05 Primestar Solar, Inc. Methods of forming a conductive transparent oxide film layer for use in a cadmium telluride based thin film photovoltaic device
WO2013177047A1 (en) * 2012-05-21 2013-11-28 First Solar, Inc. Apparatus and method for improving efficiency of thin-film photovoltaic devices
WO2013177048A2 (en) * 2012-05-21 2013-11-28 First Solar, Inc. Method of providing chloride treatment for a photovoltaic device and a chloride treated photovoltaic device

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