WO2011014068A1 - Photovoltaic cell with a selective emitter and method for making the same - Google Patents

Photovoltaic cell with a selective emitter and method for making the same Download PDF

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Publication number
WO2011014068A1
WO2011014068A1 PCT/NL2010/050489 NL2010050489W WO2011014068A1 WO 2011014068 A1 WO2011014068 A1 WO 2011014068A1 NL 2010050489 W NL2010050489 W NL 2010050489W WO 2011014068 A1 WO2011014068 A1 WO 2011014068A1
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Prior art keywords
pattern
substrate
dopant source
nozzles
acid solution
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PCT/NL2010/050489
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French (fr)
Inventor
Woutherus Johannes Maria Brok
Emerentius Maria Josephus Antonius Van Dijk
Franciscus Cornelius Dings
Wouterus Johannes Paulus Carolus Van Vugt
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Otb Solar B.V.
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Priority to CN2010800440586A priority Critical patent/CN102549776A/en
Priority to EP10740419A priority patent/EP2460188A1/en
Publication of WO2011014068A1 publication Critical patent/WO2011014068A1/en

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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
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic System
    • 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
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/22—Diffusion of impurity materials, e.g. doping materials, electrode materials, into or out of a semiconductor body, or between semiconductor regions; Interactions between two or more impurities; Redistribution of impurities
    • H01L21/225—Diffusion of impurity materials, e.g. doping materials, electrode materials, into or out of a semiconductor body, or between semiconductor regions; Interactions between two or more impurities; Redistribution of impurities using diffusion into or out of a solid from or into a solid phase, e.g. a doped oxide layer
    • H01L21/2251—Diffusion into or out of group IV semiconductors
    • H01L21/2254—Diffusion into or out of group IV semiconductors from or through or into an applied layer, e.g. photoresist, nitrides
    • H01L21/2255—Diffusion into or out of group IV semiconductors from or through or into an applied layer, e.g. photoresist, nitrides the applied layer comprising oxides only, e.g. P2O5, PSG, H3BO3, doped oxides
    • 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
    • H01L31/068—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 homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction 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
    • Y02E10/547—Monocrystalline silicon 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 relates to a photovoltaic cell, also referred to as a solar cell, having a selective emitter, and to a method for manufacturing such a photovoltaic cell employing only one high-temperature diffusion step.
  • a selective emitter comprises two kinds of doped regions of the same conductivity (i.e. n-type or p-type) that differ mutually in doping level. One kind is heavily doped, whereas the other is doped only lightly. The heavily doped regions are disposed below a metallization pattern of electrodes provided on a major, light receiving surface of the cell, and enable the electrodes to form ohmic contacts to the underlying substrate. The lightly doped regions, on the other hand, extend between the electrodes and promote light collection and conversion.
  • the selective emitter thus provides a solution to conflicting electrical and optical requirements on the emitter of a photovoltaic cell.
  • the use of a selective emitter may improve the efficiency of a photovoltaic cell by approximately 0.5%- 1%.
  • the formation of differently doped regions implies additional process steps, e.g. more relatively lengthy high-temperature diffusion processes, that increase the cost of production. As this is clearly undesirable for a product that is to be mass-produced, several methods have been suggested to decrease the manufacturing costs, thereby focusing in particular on the creation of a selective emitter in one high temperature diffusion step.
  • United States patent 6,825,104 describes a method of manufacturing a photovoltaic cell having two or more selectively diffused regions in a single diffusion step.
  • the disclosed method includes the steps of (i) selectively applying a pattern of a solids-based dopant source to a first major surface of a semiconductor substrate, in particular by means of screen printing, and (ii) diffusing the dopant atoms from the solids-based dopant source into the substrate by a controlled heat treatment step in a gaseous environment surrounding the substrate.
  • the dopant from the solids-based dopant source diffuses directly into the substrate to form first diffusion regions while, at the same time, the dopant from the solids-based dopant source diffuses indirectly via the gaseous environment into said substrate to form second diffusion regions in at least some areas of said substrate not covered by the aforementioned pattern.
  • a drawback associated with the process described in US'104 is that most currently available screen-printable solids-based pastes suitable for creating a selective emitter are relatively expensive, either due to proprietary formulas or to relatively expensive chemical constituents.
  • a method of manufacturing a semiconductor device in particular a photovoltaic cell, is provided.
  • the device comprises a substantially flat semiconductor substrate.
  • the method includes the following three process steps. Step 1: applying a dopant source by selectively inkjetting a first pattern of a phosphoric acid or boric acid solution onto a main surface of said semiconductor substrate. Step 2: heating the substrate so as to diffuse phosphorus or boron atoms from said dopant source into said substrate, thereby forming first diffusion regions
  • the method according to the present invention employs common and inexpensive acid solutions for creating the selective emitter.
  • This is a phosphoric acid solution for an n-type emitter (typically in combination with a p-type substrate), and a boric acid solution for a p-type emitter (typically in combination with an n-type substrate).
  • the term 'phosphoric acid may be construed broadly, and includes both orthophosphoric acid (H3PO4) and polyphosphoric acids (such as diphosphoric acid (H4P2O7)) insofar as these acids may serve as phosphorus dopant source.
  • the term Tioric acid is intended to include both orthoboric acid (H3BO3), metaboric acid and
  • polyboric acids insofar as these acids may serve as boron dopant source.
  • Both types of acid, in particular the ortho-variants, are amply available and inexpensive, and therefore very suitable for the mass production of
  • a respective acid is selectively applied in solution to a substrate by means of inkjetting, a technique that allows for the production of fine structures having accuracies on the order of tens of micrometers.
  • inkjetting a technique that allows for the production of fine structures having accuracies on the order of tens of micrometers.
  • the use of phosphoric or boric acid in combination with the method of inkjetting this source material to a substrate enables the highly economical and reasonably accurate formation of a selective emitter in a solar cell. It is noted that both aspects provide an improvement over US'104: the phosphoric acid solution is less expensive than the phosphorus containing paste proposed by US'104, while inkjetting in itself is a more economical method than screen printing as it reduces the amount of waste material. Furthermore inkjet printing is a contactless method that significantly diminishes the risk of substrate fracture.
  • Step 1 is concerned with the selective application of a phosphoric or boric acid solution to a major surface of the preferably silicon substrate in a first pattern.
  • This first pattern corresponds to the metal contact pattern of electrodes that is applied to the substrate in step 3.
  • both phosphoric acid and boric acid are solids.
  • a carrier liquid such as, for example, water (H2O) or ethanol (C2H6O), both of which are inexpensive solvents.
  • H2O water
  • C2H6O ethanol
  • a mixture of water and ethanol may also be used.
  • the solution may further comprise surfactants in the form of, inter alia, diethylene glycol, glycerin, isopropyl alcohol (IPA), and polyethylene glycol (PEG).
  • surfactants in the form of, inter alia, diethylene glycol, glycerin, isopropyl alcohol (IPA), and polyethylene glycol (PEG).
  • IPA isopropyl alcohol
  • PEG polyethylene glycol
  • An inkjetable solution may preferably have a viscosity in the range of 0.1-1 cPa-s (centi-Pascal second; said range corresponding to 1-10 cP (centi-Poise)).
  • Step 2 entails the single high-temperature diffusion step of the manufacturing process.
  • the substrate is heated to a temperature of several hundreds of degrees Celcius, preferably to a temperature in the range of 800 - 1100 °C so as to enable swift diffusion.
  • the substrate is preferably kept at its final temperature for about 4 - 120 minutes, depending, inter alia, on the diffusion rate of the employed dopant source (boron generally has a lower diffusion rate than phosphorus, so that achieving a desirable boron doping level may take longer).
  • the chemistry involved in the diffusion process is known, and mentioned here only by way of example.
  • H3PO4 orthophosphoric acid
  • the gradual heating of the substrate causes at least part of the phosphoric acid (H3PO4) deposited thereon to decompose into phosphorus penta oxide (P2O5) and water (H2O).
  • the phosphorus penta oxide subsequently reacts with the silicon substrate to form silicon dioxide (SiCh) and metallic phosphorus (P), whereby the latter in turn diffuses into the substrate.
  • H3BO3 orthoboric acid
  • B2O3 boron trioxide
  • H2O water
  • the former will subsequently react with the silicon substrate to form a mixture of silica (SiCh) and boron (B) atoms, which latter atoms may diffuse into the substrate.
  • SiCh silica
  • B boron
  • the phosphorus or boron atoms diffuse into the substrate in agreement with the first pattern in which the phosphoric or boric acid solution was inkjetted onto the substrate, thereby forming first, highly doped regions immediately beneath said pattern. Regions of the substrate whose surface area was not treated with phosphoric or boric acid solution during step 1 do not undergo doping.
  • lightly doped regions may be created in between the highly doped regions, and these will be elaborated upon below.
  • Step 3 involves the formation of a metal contact pattern
  • a pattern of electrodes is applied to the main surface of the substrate, substantially overlaying the first, highly doped diffusion regions.
  • contacts may be fired to create ohmic connections between the metallization pattern and the first, highly doped regions in the underlying substrate.
  • the metal contact pattern typically takes the form of elongate bus bars and fingers, though other shapes are possible. See for example the SunWeb solar cell- design by Solland Solar Cells B. V., The Netherlands, which features a both functional and decorative flowerish metal contact pattern.
  • step 1 further comprises heating said main surface of the substrate at a temperature substantially equal to or greater than the boiling point of a solvent of the phosphoric acid or boric acid solution while inkjetting said solution onto the substrate.
  • Heating the main surface of the substrate that is being inkjetted on at a temperature substantially equal to or somewhat above the boiling point of a solvent of the acid solution effects the evaporation of the solvent.
  • the solvent used as a carrier to enable the inkjetting of the phosphoric or boric acid in the desired concentration, effectively becomes superfluous once the respective acid is deposited and may preferably be removed quickly thereafter to prevent it from spreading out the acid over the substrate's surface beyond the targeted first pattern.
  • the substrate's main surface is preferably heated at about 90-110 °C, whereas a temperature of about 75-90 °C may be used when ethanol is the solvent.
  • a print head used to jet the phosphoric or boric acid solution onto the substrate is, at least in part, made of a material that is chemically inert or resistant to the acid solution, such as PEEK.
  • Suitable print heads include the PL128L print head marketed by PixDro B. V., Eindhoven (The Netherlands).
  • first, highly doped regions in the substrate was described. Attention is now invited to ways in which second, lightly doped regions may be formed in between the first, highly doped regions so as to complete the selective emitter.
  • step 1 further comprises selectively inkjetting a second pattern of said phosphoric acid or boric solution onto said main surface of said semiconductor substrate, said second pattern having a lower concentration of acid solution per unit of substrate area than said first pattern.
  • step 2 further comprises forming second diffusion regions
  • This first manner thus involves applying both a first and a second pattern of the phosphoric or boric acid solution by means of inkjetting, yet at different surface concentrations for the respective patterns.
  • the first pattern from which the highly doped regions associated with the electrodes are to be formed, is provided with a higher surface concentration of acid solution than the second pattern.
  • Such a differentiation in surface concentration may be advantageously achieved in a single inkjetting pass using a single phosphoric or boric acid solution and a single print head capable of jetting the solution at different resolutions.
  • the phosphoric or boric acid solution may, for example, be selectively inkjetted onto the main surface of the semiconductor substrate using a print head comprising a plurality of inkjet nozzles, whereby each of the nozzles can be activated independently of the other nozzles to produce a droplet of said acid solution.
  • the nozzles of the print head producing droplets for the first pattern may then be activated at a greater frequency than the nozzles producing droplets for the second pattern.
  • the single phosphoric or boric acid solution may have a concentration of 0-20% of phosphoric or boric acid in ethanol, say 5%.
  • This solution may be inkjetted onto the substrate in droplets of several picoliters, e.g. 10-20 pi, at resolutions of about 800-1200 and 400-800 droplets per inch (dpi) for the first and second pattern respectively, so as to obtain patterns having a different surface concentration of phosphoric or boric acid.
  • these numbers are merely ball park figures that do not only exhibit a mutual dependency, but also depend on other factors such as the substrate
  • the following high-temperature diffusion step may preferably be performed in a belt diffusion furnace, thereby enabling a continuous throughput that is desirable for the large-scale production of solar cells.
  • the main surface of the substrate is subjected to a gaseous dopant atmosphere comprising phosphorus or a phosphorus compound (e.g. POCI3), respectively boron or a boron compound (e.g. H2B), such that, while forming said first diffusion regions, second diffusion regions are formed through diffusion of phosphorus or boron from said gaseous atmosphere into said substrate via one or more areas of said main surface not covered by the first pattern.
  • a gaseous dopant atmosphere comprising phosphorus or a phosphorus compound (e.g. POCI3), respectively boron or a boron compound (e.g. H2B)
  • Gaseous dopant atmospheres are preferably used in combination with batch furnaces, such as, for example, a POCl3-closed tube diffusion furnace in the case of phosphorus doping.
  • a phosphorus or boron comprising atmosphere may be effected by spraying/atomizing phosphoric acid or boric solution into the heated atmosphere to which the substrate is subjected.
  • the concentration of dopant source material in the atmosphere may be determined independently of the concentration of the acid solution that is inkjetted onto the substrate, allowing the final volume concentrations of phosphorus or boron in the first and second diffusion regions to be set independently.
  • the invention further provides a method of manufacturing a semiconductor device including a substantially flat semiconductor substrate, said method comprising the following steps.
  • Step 1 applying a dopant source by selectively inkjetting a first pattern and a second pattern of a same dopant source solution onto a main surface of said semiconductor substrate, said second pattern having a lower dopant source concentration per unit of substrate area than said first pattern.
  • Step 2 heating the substrate so as to diffuse dopant atoms from said dopant source into said substrate, thereby forming first diffusion regions immediately beneath the first pattern and second diffusion regions immediately beneath the second pattern; and step 3: forming a metal contact pattern substantially in alignment with said first diffusion regions.
  • the dopant source may be selectively inkjetted onto the main surface of the semiconductor substrate using a print head, said print head comprising a plurality of inkjet nozzles, each of which nozzles can be activated independently of the other nozzles to produce a droplet of dopant source solution.
  • the nozzles of the print head producing droplets for the first pattern are activated at a greater frequency than the nozzles producing droplets for the second pattern.
  • Fig. 1 schematically shows a top view of an exemplary photovoltaic cell having a selective emitter
  • Fig. 2 schematically shows a cross- sectional side view of the photovoltaic cell shown in Fig. 1;
  • FIG. 3 schematically shows a perspective view of a production line for carrying out the method according to the present invention.
  • Fig. 1 schematically shows a top view of the photovoltaic cell
  • Fig. 2 shows the same photovoltaic cell in a cross-sectional side view.
  • the exemplary photovoltaic cell 1 is of a conventional design and based on a semiconductor substrate 2, more particularly a silicon wafer of p- type conductivity.
  • the substrate 2 is successively provided with a selective emitter 3, an anti-reflection coating 8, and a metallization pattern 10.
  • the selective emitter 3 includes both relatively deep, heavily doped regions 4 and relatively shallow, lightly doped regions 6 of n-type conductivity.
  • the regions 4 and 6 are doped with phosphorus. Volume concentrations of phosphorus atoms in the highly doped regions 4 may typically be on the order of 10 20 -10 23 atoms/cm 3 , whereas those for the lightly doped regions may be on the order of 10 18 -10 21 atoms/cm 3 .
  • the ratio of the doping levels in the regions 4 and 6 is generally at least 2.
  • On top of the selective emitter 3 lies a preferably passivating anti-reflection coating 8, which itself is partially topped with the metallization pattern 10.
  • the metallization pattern 10 includes relatively wide bus bars 11 and relatively narrow fingers 12, and extends across the top side 2a of the substrate 2, overlaying the heavily doped regions 4.
  • the bus bars 11 and fingers 12 preferably include ohmic contacts 13 to these heavily doped regions 4.
  • the substrate 2 is provided with a back side
  • the latter pattern 14 is typically made of aluminium and substantially uniform across the back side 2b of the substrate 2.
  • the photovoltaic cell 1 of Figs. 1 and 2 is exemplary only. Some cell designs may include features not discussed above, such as, for example, separate passivating layers or back surface fields adjacent the back side metal contact 14 (e.g. a layer of p+-type conductivity added to reduce local electron-hole recombination in order to increase the cell's 1 efficiency), while others may not possess one or more of the described features, such as, for example, the anti-reflection coating 8. It is therefore explicitly noted that insofar as the present invention is concerned, all photovoltaic cell designs incorporating a selective emitter 3 that is based on phosphorus or boron doping are intended to fall within the scope of the appended claims. In fact, the method according to the present invention is not limited to the manufacture of photovoltaic cells; it is contemplated that it can be employed in other micro-electronic production processes as well.
  • FIG. 3 schematically shows an exemplary production line 30 for carrying out this method to produce the solar cell depicted in Figs. 1 and 2.
  • the production line 30 comprises several serially linked processing stations 34-46.
  • a silicon substrate 2 is supplied to the production line 30 at a wafer entry point 32. From there it is conveyed to a station 34 for saw damage removal and texturization.
  • the texturization of a substrate's surface which serves to increase absorption of incident solar radiation, may be performed in any suitable manner, such as, for example, through laser- texturing or chemical etching.
  • the substrate 2 is then transported on to inkjet station 36, where step 1 of the method according to the invention is performed.
  • the inkjet station 36 applies a dopant source to the front main surface 2a of the substrate 2 by selectively inkjetting a first pattern, corresponding to the metal contact pattern 10 of the front surface electrodes 11, 12 to be applied at a later stage, of an phosphoric acid or boric acid solution onto this surface.
  • the substrate 2 may move relative to a print head, or vice versa.
  • the dimensions of the print head substantially correspond to those of one or more substrates, such that the dopant source can be applied to the substrate in one pass.
  • the substrate 2 is advanced through a high- temperature diffusion belt furnace 38, wherein the substrate is heated to a temperature in the range of 800-1100 °C.
  • step 2 of the method according to the present invention executes step 2 of the method according to the present invention.
  • the heating of the substrate 2 causes phosphorus atoms to diffuse from the applied dopant source into the substrate, thereby forming the first, heavily doped diffusion regions 4 immediately beneath the first pattern 10.
  • the second, lightly doped diffusion regions 6 are also formed in the belt furnace 38.
  • the phosphorus dopant source may either have been applied by the inkjet station 36, as discussed above, or be added through spraying a phosphoric acid solution onto the substrate inside the belt furnace 38.
  • any remaining phosphosilicate glass (PSG, SiCh comprising P2O5) on the substrate's surface may be removed using a chemical etching solution, e.g.
  • station 42 may apply a passivating anti-reflection coating, preferably a hydrogenated silicon nitride (SiN x :H) layer which may be applied by means of plasma enhanced chemical vapor deposition (PECVD) techniques.
  • PECVD plasma enhanced chemical vapor deposition
  • other anti- reflection coatings e.g. made of TiCh, may be applied, or an anti-reflection coating may be omitted altogether.
  • the metallization pattern 10 typically of silver
  • the metallization pattern 14 typically of aluminium
  • the metallization patterns 10, 14 are preferably applied through screen printing and subsequent drying, though other thick film deposition techniques may be used as well.
  • metal paste is selectively applied to the front side of the substrate 2 in alignment with the heavily doped regions 4. Whether the back side metallization pattern 14 involves selective or non- selective application of a metal paste depends on the design of the particular photovoltaic cell 1.
  • contacts may be fired through the anti-reflection coating 8 at station 46. The applied metal thereby penetrates the SiN x :H-coating 8 to form low-resistance ohmic contacts, while hydrogen from the coating diffuses into the bulk of the cell 1 to passivate impurities and defects.
  • the contacts may be fired by means of a variety of techniques, such as laser-firing or conveying the substrate 2 through an infra-red conveyor belt furnace.
  • the cell 1 may be tested before it exits the production line 1 at 48. It is understood that although the above description of the production line 30 is phrased in terms of a p-type silicon substrate being doped with phosphorus, the example is, mutatis mutandis, equally applicable to, inter alia, an n-type silicon substrate being doped with boron atoms.

Abstract

Method of manufacturing a semiconductor device (1) including a substantially flat semiconductor substrate (2). The method comprises three steps. Step 1: applying a dopant source by selectively inkjetting a first pattern of a phosphoric acid or boric acid solution onto a main surface (2a) of said semiconductor substrate. Step 2: heating the substrate so as to diffuse phosphorus or boron atoms from said dopant source into said substrate, thereby forming first diffusion regions (4) immediately beneath the first pattern; and step 3: forming a metal contact pattern (10, 11, 12) substantially in alignment with said first diffusion regions. A photovoltaic cell manufactured through a method is also provided for.

Description

Title: Photovoltaic cell with a selective emitter and method for making the same
Field of the Invention
The present invention relates to a photovoltaic cell, also referred to as a solar cell, having a selective emitter, and to a method for manufacturing such a photovoltaic cell employing only one high-temperature diffusion step.
Background
To improve the efficiency of a photovoltaic cell, it may be fitted with a selective instead of a homogeneous emitter. A selective emitter comprises two kinds of doped regions of the same conductivity (i.e. n-type or p-type) that differ mutually in doping level. One kind is heavily doped, whereas the other is doped only lightly. The heavily doped regions are disposed below a metallization pattern of electrodes provided on a major, light receiving surface of the cell, and enable the electrodes to form ohmic contacts to the underlying substrate. The lightly doped regions, on the other hand, extend between the electrodes and promote light collection and conversion. The selective emitter thus provides a solution to conflicting electrical and optical requirements on the emitter of a photovoltaic cell. In general, the use of a selective emitter may improve the efficiency of a photovoltaic cell by approximately 0.5%- 1%. Unfortunately however, the formation of differently doped regions implies additional process steps, e.g. more relatively lengthy high-temperature diffusion processes, that increase the cost of production. As this is clearly undesirable for a product that is to be mass-produced, several methods have been suggested to decrease the manufacturing costs, thereby focusing in particular on the creation of a selective emitter in one high temperature diffusion step.
United States patent 6,825,104, for example, describes a method of manufacturing a photovoltaic cell having two or more selectively diffused regions in a single diffusion step. The disclosed method includes the steps of (i) selectively applying a pattern of a solids-based dopant source to a first major surface of a semiconductor substrate, in particular by means of screen printing, and (ii) diffusing the dopant atoms from the solids-based dopant source into the substrate by a controlled heat treatment step in a gaseous environment surrounding the substrate. During the heat treatment step, the dopant from the solids-based dopant source diffuses directly into the substrate to form first diffusion regions while, at the same time, the dopant from the solids-based dopant source diffuses indirectly via the gaseous environment into said substrate to form second diffusion regions in at least some areas of said substrate not covered by the aforementioned pattern. A drawback associated with the process described in US'104 is that most currently available screen-printable solids-based pastes suitable for creating a selective emitter are relatively expensive, either due to proprietary formulas or to relatively expensive chemical constituents.
It is therefore an object of the present invention to provide for a more economical method of manufacturing a photovoltaic cell having a selective emitter in a single high-temperature diffusion step. Summary of the Invention
According to one aspect of the invention a method of manufacturing a semiconductor device, in particular a photovoltaic cell, is provided. The device comprises a substantially flat semiconductor substrate. The method includes the following three process steps. Step 1: applying a dopant source by selectively inkjetting a first pattern of a phosphoric acid or boric acid solution onto a main surface of said semiconductor substrate. Step 2: heating the substrate so as to diffuse phosphorus or boron atoms from said dopant source into said substrate, thereby forming first diffusion regions
immediately beneath the first pattern. And step 3: forming a metal contact pattern substantially in alignment with said first diffusion regions. The method according to the present invention employs common and inexpensive acid solutions for creating the selective emitter. This is a phosphoric acid solution for an n-type emitter (typically in combination with a p-type substrate), and a boric acid solution for a p-type emitter (typically in combination with an n-type substrate). The term 'phosphoric acid may be construed broadly, and includes both orthophosphoric acid (H3PO4) and polyphosphoric acids (such as diphosphoric acid (H4P2O7)) insofar as these acids may serve as phosphorus dopant source. Likewise, the term Tioric acid is intended to include both orthoboric acid (H3BO3), metaboric acid and
polyboric acids, insofar as these acids may serve as boron dopant source. Both types of acid, in particular the ortho-variants, are amply available and inexpensive, and therefore very suitable for the mass production of
photovoltaic cells. A respective acid is selectively applied in solution to a substrate by means of inkjetting, a technique that allows for the production of fine structures having accuracies on the order of tens of micrometers. The use of phosphoric or boric acid in combination with the method of inkjetting this source material to a substrate enables the highly economical and reasonably accurate formation of a selective emitter in a solar cell. It is noted that both aspects provide an improvement over US'104: the phosphoric acid solution is less expensive than the phosphorus containing paste proposed by US'104, while inkjetting in itself is a more economical method than screen printing as it reduces the amount of waste material. Furthermore inkjet printing is a contactless method that significantly diminishes the risk of substrate fracture. Although US'104 seems to hint that the solids-based paste might be applied to the substrate by means of inkjetting, it is unlikely that inkjetting a viscous paste, such as the exemplary PlOl from Soltech NV, Belgium, is practically viable.-The three process steps of the method according to the present invention will now be elucidated in turn.
Step 1 is concerned with the selective application of a phosphoric or boric acid solution to a major surface of the preferably silicon substrate in a first pattern. This first pattern corresponds to the metal contact pattern of electrodes that is applied to the substrate in step 3. At room temperature (about 25°C), both phosphoric acid and boric acid are solids. To obtain a solution that can be inkjetted onto the substrate the substances may be dissolved in a carrier liquid, such as, for example, water (H2O) or ethanol (C2H6O), both of which are inexpensive solvents. A mixture of water and ethanol may also be used. The solution may further comprise surfactants in the form of, inter alia, diethylene glycol, glycerin, isopropyl alcohol (IPA), and polyethylene glycol (PEG). Compared to plain water, ethanol has a lower surface tension and a lower vapor pressure, which gives it better wetting- behavior and inkjetability, and a generally improved uniformity of deposited dopant material. In addition, due to its lower boiling point (78 °C vs. 100 0C), ethanol dries up sooner during heating. Some of these effects may also be achieved using an aqueous solvent provided with the mentioned surfactants. An inkjetable solution may preferably have a viscosity in the range of 0.1-1 cPa-s (centi-Pascal second; said range corresponding to 1-10 cP (centi-Poise)).
Step 2 entails the single high-temperature diffusion step of the manufacturing process. The substrate is heated to a temperature of several hundreds of degrees Celcius, preferably to a temperature in the range of 800 - 1100 °C so as to enable swift diffusion. The substrate is preferably kept at its final temperature for about 4 - 120 minutes, depending, inter alia, on the diffusion rate of the employed dopant source (boron generally has a lower diffusion rate than phosphorus, so that achieving a desirable boron doping level may take longer). The chemistry involved in the diffusion process is known, and mentioned here only by way of example. In case orthophosphoric acid (H3PO4) was applied to a silicon substrate during step 1, the gradual heating of the substrate causes at least part of the phosphoric acid (H3PO4) deposited thereon to decompose into phosphorus penta oxide (P2O5) and water (H2O). The phosphorus penta oxide subsequently reacts with the silicon substrate to form silicon dioxide (SiCh) and metallic phosphorus (P), whereby the latter in turn diffuses into the substrate. A similar process occurs when orthoboric acid (H3BO3) is used: the acid decomposes into boron trioxide (B2O3) and water (H2O), whereby the former will subsequently react with the silicon substrate to form a mixture of silica (SiCh) and boron (B) atoms, which latter atoms may diffuse into the substrate. It is understood that the phosphorus or boron atoms diffuse into the substrate in agreement with the first pattern in which the phosphoric or boric acid solution was inkjetted onto the substrate, thereby forming first, highly doped regions immediately beneath said pattern. Regions of the substrate whose surface area was not treated with phosphoric or boric acid solution during step 1 do not undergo doping. There are several ways in which lightly doped regions may be created in between the highly doped regions, and these will be elaborated upon below.
Step 3 involves the formation of a metal contact pattern
substantially in alignment with said first, highly doped diffusion regions.
During this step a pattern of electrodes is applied to the main surface of the substrate, substantially overlaying the first, highly doped diffusion regions. Once the electrode pattern is applied, contacts may be fired to create ohmic connections between the metallization pattern and the first, highly doped regions in the underlying substrate. The metal contact pattern typically takes the form of elongate bus bars and fingers, though other shapes are possible. See for example the SunWeb solar cell- design by Solland Solar Cells B. V., The Netherlands, which features a both functional and decorative flowerish metal contact pattern.
According to an embodiment of the method according to the present invention, step 1 further comprises heating said main surface of the substrate at a temperature substantially equal to or greater than the boiling point of a solvent of the phosphoric acid or boric acid solution while inkjetting said solution onto the substrate.
Heating the main surface of the substrate that is being inkjetted on at a temperature substantially equal to or somewhat above the boiling point of a solvent of the acid solution effects the evaporation of the solvent. The solvent, used as a carrier to enable the inkjetting of the phosphoric or boric acid in the desired concentration, effectively becomes superfluous once the respective acid is deposited and may preferably be removed quickly thereafter to prevent it from spreading out the acid over the substrate's surface beyond the targeted first pattern. In case water is used as a solvent (and inkjetting takes place at atmospheric pressure), the substrate's main surface is preferably heated at about 90-110 °C, whereas a temperature of about 75-90 °C may be used when ethanol is the solvent.
According to one embodiment of the present invention, a print head used to jet the phosphoric or boric acid solution onto the substrate is, at least in part, made of a material that is chemically inert or resistant to the acid solution, such as PEEK. Suitable print heads include the PL128L print head marketed by PixDro B. V., Eindhoven (The Netherlands).
Thus far the formation of first, highly doped regions in the substrate was described. Attention is now invited to ways in which second, lightly doped regions may be formed in between the first, highly doped regions so as to complete the selective emitter.
According to a first manner, step 1 further comprises selectively inkjetting a second pattern of said phosphoric acid or boric solution onto said main surface of said semiconductor substrate, said second pattern having a lower concentration of acid solution per unit of substrate area than said first pattern. Step 2 further comprises forming second diffusion regions
immediately beneath the second pattern.
This first manner thus involves applying both a first and a second pattern of the phosphoric or boric acid solution by means of inkjetting, yet at different surface concentrations for the respective patterns. The first pattern, from which the highly doped regions associated with the electrodes are to be formed, is provided with a higher surface concentration of acid solution than the second pattern. Such a differentiation in surface concentration may be advantageously achieved in a single inkjetting pass using a single phosphoric or boric acid solution and a single print head capable of jetting the solution at different resolutions. The phosphoric or boric acid solution may, for example, be selectively inkjetted onto the main surface of the semiconductor substrate using a print head comprising a plurality of inkjet nozzles, whereby each of the nozzles can be activated independently of the other nozzles to produce a droplet of said acid solution. During application of the dopant source, the nozzles of the print head producing droplets for the first pattern may then be activated at a greater frequency than the nozzles producing droplets for the second pattern. This approach to selectively applying a dopant source in different concentrations provides an advantageous, simplified alternative to those based on the use of multiple print heads and/or multiple dopant source solutions of mutually different concentrations. The single phosphoric or boric acid solution, for example, may have a concentration of 0-20% of phosphoric or boric acid in ethanol, say 5%. This solution may be inkjetted onto the substrate in droplets of several picoliters, e.g. 10-20 pi, at resolutions of about 800-1200 and 400-800 droplets per inch (dpi) for the first and second pattern respectively, so as to obtain patterns having a different surface concentration of phosphoric or boric acid. As one skilled in the art will appreciate, these numbers are merely ball park figures that do not only exhibit a mutual dependency, but also depend on other factors such as the substrate
temperature during inkjetting (which affects running of the deposited solution), the temperature at which the diffusion step is performed, the desired volume concentrations of dopant atoms in the substrate, etc. In case both the first and the second pattern of acid solution are applied through inkjetting, the following high-temperature diffusion step may preferably be performed in a belt diffusion furnace, thereby enabling a continuous throughput that is desirable for the large-scale production of solar cells.
According to a second manner, during step 2, the main surface of the substrate is subjected to a gaseous dopant atmosphere comprising phosphorus or a phosphorus compound (e.g. POCI3), respectively boron or a boron compound (e.g. H2B), such that, while forming said first diffusion regions, second diffusion regions are formed through diffusion of phosphorus or boron from said gaseous atmosphere into said substrate via one or more areas of said main surface not covered by the first pattern.
It is understood that diffusion of dopant material from the gaseous atmosphere need not necessarily take place directly, but may, for example, include the formation of intermediate compounds on the surface of the substrate, from which the actual diffusion takes place. Gaseous dopant atmospheres are preferably used in combination with batch furnaces, such as, for example, a POCl3-closed tube diffusion furnace in the case of phosphorus doping. If desired, a phosphorus or boron comprising atmosphere may be effected by spraying/atomizing phosphoric acid or boric solution into the heated atmosphere to which the substrate is subjected. The concentration of dopant source material in the atmosphere may be determined independently of the concentration of the acid solution that is inkjetted onto the substrate, allowing the final volume concentrations of phosphorus or boron in the first and second diffusion regions to be set independently.
The invention further provides a method of manufacturing a semiconductor device including a substantially flat semiconductor substrate, said method comprising the following steps. Step 1: applying a dopant source by selectively inkjetting a first pattern and a second pattern of a same dopant source solution onto a main surface of said semiconductor substrate, said second pattern having a lower dopant source concentration per unit of substrate area than said first pattern. Step 2: heating the substrate so as to diffuse dopant atoms from said dopant source into said substrate, thereby forming first diffusion regions immediately beneath the first pattern and second diffusion regions immediately beneath the second pattern; and step 3: forming a metal contact pattern substantially in alignment with said first diffusion regions. The dopant source may be selectively inkjetted onto the main surface of the semiconductor substrate using a print head, said print head comprising a plurality of inkjet nozzles, each of which nozzles can be activated independently of the other nozzles to produce a droplet of dopant source solution. During the application of dopant source, the nozzles of the print head producing droplets for the first pattern are activated at a greater frequency than the nozzles producing droplets for the second pattern. This method of creating a selective emitter, which in itself is not limited to certain dopant source solutions/inks such as phosphoric acid and boric acid solutions, provides an advantageous alternative to methods that are based on the use of multiple print heads and/or multiple dopant source solutions of mutually different concentrations.
These and other features and advantages of the invention will be more fully understood from the following detailed description of certain embodiments of the invention, taken together with the accompanying drawings, which are meant to illustrate and not to limit the invention.
Brief Description of the Drawings
Fig. 1 schematically shows a top view of an exemplary photovoltaic cell having a selective emitter;
Fig. 2 schematically shows a cross- sectional side view of the photovoltaic cell shown in Fig. 1; and
Fig. 3 schematically shows a perspective view of a production line for carrying out the method according to the present invention. Detailed Description
The general construction of an exemplary photovoltaic cell 1 having a selective emitter will be discussed briefly with reference to Figs. 1 and 2, wherein Fig. 1 schematically shows a top view of the photovoltaic cell and Fig. 2 shows the same photovoltaic cell in a cross-sectional side view. The exemplary photovoltaic cell 1 is of a conventional design and based on a semiconductor substrate 2, more particularly a silicon wafer of p- type conductivity. At its front or top side 2a, i.e. the side that is exposed to (sun)light during use, the substrate 2 is successively provided with a selective emitter 3, an anti-reflection coating 8, and a metallization pattern 10. The selective emitter 3 includes both relatively deep, heavily doped regions 4 and relatively shallow, lightly doped regions 6 of n-type conductivity. In
accordance with the present invention, the regions 4 and 6 are doped with phosphorus. Volume concentrations of phosphorus atoms in the highly doped regions 4 may typically be on the order of 1020-1023 atoms/cm3, whereas those for the lightly doped regions may be on the order of 1018-1021 atoms/cm3. The ratio of the doping levels in the regions 4 and 6 is generally at least 2. On top of the selective emitter 3 lies a preferably passivating anti-reflection coating 8, which itself is partially topped with the metallization pattern 10. The metallization pattern 10 includes relatively wide bus bars 11 and relatively narrow fingers 12, and extends across the top side 2a of the substrate 2, overlaying the heavily doped regions 4. The bus bars 11 and fingers 12 preferably include ohmic contacts 13 to these heavily doped regions 4. At its back or bottom side 2b, the substrate 2 is provided with a back side
metallization pattern 14, which too may preferably makes good ohmic contact with the substrate. The latter pattern 14 is typically made of aluminium and substantially uniform across the back side 2b of the substrate 2.
As one skilled in the art will appreciate, the photovoltaic cell 1 of Figs. 1 and 2 is exemplary only. Some cell designs may include features not discussed above, such as, for example, separate passivating layers or back surface fields adjacent the back side metal contact 14 (e.g. a layer of p+-type conductivity added to reduce local electron-hole recombination in order to increase the cell's 1 efficiency), while others may not possess one or more of the described features, such as, for example, the anti-reflection coating 8. It is therefore explicitly noted that insofar as the present invention is concerned, all photovoltaic cell designs incorporating a selective emitter 3 that is based on phosphorus or boron doping are intended to fall within the scope of the appended claims. In fact, the method according to the present invention is not limited to the manufacture of photovoltaic cells; it is contemplated that it can be employed in other micro-electronic production processes as well.
Having discussed the general structure of a photovoltaic cell 1 with a selective emitter 3, the method according to the invention will now be elucidated further in relation to Fig. 3, which schematically shows an exemplary production line 30 for carrying out this method to produce the solar cell depicted in Figs. 1 and 2.
The production line 30 comprises several serially linked processing stations 34-46. A silicon substrate 2 is supplied to the production line 30 at a wafer entry point 32. From there it is conveyed to a station 34 for saw damage removal and texturization. The texturization of a substrate's surface, which serves to increase absorption of incident solar radiation, may be performed in any suitable manner, such as, for example, through laser- texturing or chemical etching. The substrate 2 is then transported on to inkjet station 36, where step 1 of the method according to the invention is performed. The inkjet station 36 applies a dopant source to the front main surface 2a of the substrate 2 by selectively inkjetting a first pattern, corresponding to the metal contact pattern 10 of the front surface electrodes 11, 12 to be applied at a later stage, of an phosphoric acid or boric acid solution onto this surface. During the inkjetting step, the substrate 2 may move relative to a print head, or vice versa. In an advantageous embodiment, the dimensions of the print head substantially correspond to those of one or more substrates, such that the dopant source can be applied to the substrate in one pass. Subsequently, the substrate 2 is advanced through a high- temperature diffusion belt furnace 38, wherein the substrate is heated to a temperature in the range of 800-1100 °C. Doing so executes step 2 of the method according to the present invention. The heating of the substrate 2 causes phosphorus atoms to diffuse from the applied dopant source into the substrate, thereby forming the first, heavily doped diffusion regions 4 immediately beneath the first pattern 10. The second, lightly doped diffusion regions 6 are also formed in the belt furnace 38. The phosphorus dopant source may either have been applied by the inkjet station 36, as discussed above, or be added through spraying a phosphoric acid solution onto the substrate inside the belt furnace 38. After the high-temperature diffusion stage, any remaining phosphosilicate glass (PSG, SiCh comprising P2O5) on the substrate's surface may be removed using a chemical etching solution, e.g. a hydrofluoric acid (HF) solution, at station 40. Subsequently, station 42 may apply a passivating anti-reflection coating, preferably a hydrogenated silicon nitride (SiNx:H) layer which may be applied by means of plasma enhanced chemical vapor deposition (PECVD) techniques. Alternatively, other anti- reflection coatings, e.g. made of TiCh, may be applied, or an anti-reflection coating may be omitted altogether. Following the deposition of the anti- reflection coating, the metallization pattern 10, typically of silver, may be applied to the front side 2a of substrate 2, while the metallization pattern 14, typically of aluminium, may be applied to the back side 2b of the substrate at station 44. The metallization patterns 10, 14 are preferably applied through screen printing and subsequent drying, though other thick film deposition techniques may be used as well. During screen printing of metallization pattern 10, metal paste is selectively applied to the front side of the substrate 2 in alignment with the heavily doped regions 4. Whether the back side metallization pattern 14 involves selective or non- selective application of a metal paste depends on the design of the particular photovoltaic cell 1. Once the paste of metallization pattern 10 has been dried, contacts may be fired through the anti-reflection coating 8 at station 46. The applied metal thereby penetrates the SiNx:H-coating 8 to form low-resistance ohmic contacts, while hydrogen from the coating diffuses into the bulk of the cell 1 to passivate impurities and defects. The contacts may be fired by means of a variety of techniques, such as laser-firing or conveying the substrate 2 through an infra-red conveyor belt furnace. Finally, the cell 1 may be tested before it exits the production line 1 at 48. It is understood that although the above description of the production line 30 is phrased in terms of a p-type silicon substrate being doped with phosphorus, the example is, mutatis mutandis, equally applicable to, inter alia, an n-type silicon substrate being doped with boron atoms.
Although illustrative embodiments of the present invention have been described above, in part with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted that particular features, structures, or characteristics of one or more embodiments may be combined in any suitable manner to form new, not explicitly described embodiments.
List of elements
1 photovoltaic cell
2 semiconductor substrate
2a,b front main surface (a) and back main surface (b) of semiconductor substrate
3 emitter
4 heavily doped substrate regions
6 lightly doped substrate regions
8 anti-reflection coating
10 metal contact pattern on front main surface
11 bus bars
12 fingers
13 contacts
14 back side metallization
30 production line
32 wafer entry
34 saw damage removal and texturization station 36 inkjet station
38 high-temperature diffusion belt furnace
40 PSG removal station
42 anti-reflection coating application station
44 metallization station
46 station for firing metal contacts
48 photovoltaic cell exit

Claims

Claims We claim:
1. A method of manufacturing a semiconductor device (1) including a substantially flat semiconductor substrate (2), said method comprising:
- step 1: applying a dopant source by selectively inkjetting a first
pattern of a phosphoric acid or boric acid solution onto a main surface (2a) of said semiconductor substrate;
- step 2: heating the substrate so as to diffuse phosphorus or boron atoms from said dopant source into said substrate, thereby forming first diffusion regions (4) immediately beneath the first pattern; and
- step 3: forming a metal contact pattern (10, 11, 12) substantially in alignment with said first diffusion regions.
2. The method according to claim 1, wherein
- step 1 further comprises: heating said main surface (2a) of the
substrate (2) at a temperature substantially equal to or greater than the boiling point of a solvent of the phosphoric acid or boric acid solution while inkjetting said solution onto the substrate.
3. The method according to any of the preceding claims, wherein the acid solution comprises ethanol as a solvent.
4. The method according to any of the preceding claims, wherein the acid solution comprises one or more of the following surfactants: diethylene glycol, glycerin, isopropyl alcohol, and polyethylene glycol.
5. The method according to any of the preceding claims, wherein the semiconductor device (1) is a photovoltaic cell.
6. The method according to any of the preceding claims, wherein
- step 1 further comprises: applying the dopant source by selectively inkjetting a second pattern of said phosphoric acid or boric acid solution onto said main surface (2a) of said semiconductor substrate (2), said second pattern having a lower concentration of acid solution per unit of substrate area than said first pattern; and
- step 2 further comprises: forming second diffusion regions (6)
immediately beneath the second pattern.
7. The method according to claim 6, wherein the phosphoric or boric acid solution is selectively inkjetted onto the main surface (2a) of the semiconductor substrate (2) using a print head, said print head comprising a plurality of inkjet nozzles, each of which nozzles can be activated
independently of the other nozzles to produce a droplet of said acid solution.
8. The method according to claim 7, wherein, during the application of the dopant source, the nozzles of the print head producing droplets for the first pattern are activated at a greater frequency than the nozzles producing droplets for the second pattern.
9. The method according to any of the claims 6-8, wherein
- step 2 further comprises: conveying said substrate (2) through a belt diffusion furnace (38).
10. The method according to any of the preceding claims, wherein
- during step 2, the main surface (2a) of the substrate (2) is subjected to a gaseous atmosphere comprising phosphorus or a phosphorus compound, respectively boron or a boron compound, such that, while forming said first diffusion regions (4), second diffusion regions (6) are formed through diffusion of phosphorus or boron from said gaseous atmosphere into said substrate via one or more areas of said main surface not covered by the first pattern.
11. The method according to claim 10, wherein said gaseous
atmosphere is a phosphoryl chloride or boron hydride atmosphere.
12. The method according to claim 10 wherein the gaseous atmosphere is effected by vaporizing a phosphoric acid or boric acid solution.
13. A photovoltaic cell (1) manufactured through a method according to any of the preceding claims.
14. A method of manufacturing a semiconductor device (1) including a substantially flat semiconductor substrate (2), said method comprising:
- step 1: applying a dopant source by selectively inkjetting a first pattern (10) and a second pattern of a same dopant source solution onto a main surface (2a) of said semiconductor substrate, said second pattern having a lower dopant source concentration per unit of substrate area than said first pattern;
- step 2: heating the substrate so as to diffuse dopant atoms from said dopant source into said substrate, thereby forming first diffusion regions (4) immediately beneath the first pattern and second diffusion regions (6) immediately beneath the second pattern; and
- step 3: forming a metal contact pattern (10, 11, 12) substantially in alignment with said first diffusion regions.
15. The method according to claim 14, wherein the dopant source is selectively inkjetted onto the main surface (2a) of the semiconductor substrate (2) using a print head, said print head comprising a plurality of inkjet nozzles, each of which nozzles can be activated independently of the other nozzles to produce a droplet of dopant source solution.
16. The method according to claim 15, wherein, during the application of dopant source, the nozzles of the print head producing droplets for the first pattern are activated at a greater frequency than the nozzles producing droplets for the second pattern.
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