Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F71/00—Manufacture or treatment of devices covered by this subclass
  • H10F71/121—The active layers comprising only Group IV materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table 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
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F10/00—Individual photovoltaic cells, e.g. solar cells
  • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
  • H10F10/14—Photovoltaic cells having only PN homojunction potential barriers
• • 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.

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