Classifications

• • 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
• • 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
  • H01L31/02—Details
  • H01L31/0224—Electrodes
  • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
  • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for 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
  • H01L31/02—Details
  • H01L31/0224—Electrodes
• • 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
• • 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

Definitions

• the invention relates to a silicon solar cell with an emitter silicon layer and to a method for producing such a solar cell.
• a silicon solar cell has an n-doped and p-doped silicon layer. If photons impinge on a side of the solar cell acting as an emitter, a charge balance or current flow occurs between the two layers, which can be dissipated via contacts. On the upper side of such a solar cell usually a contact band made of metal with many contact fingers is applied, whereas on the underside a continuous metal layer is present as a contact. The contact band and the metal surface form the electrical poles of the solar cell.
• a contact band and metal fingers are formed by a silver paste, which are applied in a printing process on the surface.
• a disadvantage here is that the electrical leads formed from the silver paste have a relatively high line resistance, since the silver paste is porous.
• the contact resistance of the silver paste to the underlying silicon layer is relatively high and the adhesion to the ground is relatively poor.
• Another approach to improving solar cells is to increase the efficiency.
• a higher efficiency can be achieved for example by a smaller thickness of the emitter layer.
• the thickness of the emitter layer is only a few hundred nanometers. If an even thinner emitter layer is used, the efficiency can theoretically be increased, however, the metallic electrical leads can penetrate such a thin emitter layer, so that such a solar cell is electrically short-circuited.
• the silicon solar cell according to the invention has a silicon layer with an emitter layer and at least one region which is porosified by chemical or electrochemical etching within the emitter layer, wherein at least a part of the porosified region is formed as a metal silicide and at least one metal layer is applied thereon.
• a directly applied to silicon metal has a relatively low electrical conduction resistance, but the mechanical adhesion is still relatively poor, since the metal applied can easily dissolve in subsequent process steps from the silicon.
• a metal-silicon compound in the form of a metal silicide by providing a metal-silicon compound in the form of a metal silicide, a low electrical contact resistance between the metal and silicon can be achieved.
• the metal silicide is formed in the solar cell according to the invention in an area porosified by chemical or electrochemical etching within the emitter layer. This is advantageous because the etching creates a fissured structure which can be metallized well and ensures good adhesion between the metal and the silicon.
• the metal silicide forms faster than in the adjacent, non-porosified region, so that much of the metal silicide formed can be confined to the porosified region.
• the porosified region thus causes a limitation in the height of the metal silicide.
• the height of the porous area can be adjusted very precisely in a simple manner and only up to a small height, since an etching process can be precisely limited in time.
• the metal silicide does not extend along the entire emitter height, but only in a region extending from an upper side of the emitter layer whose height is less than the height of the entire emitter layer. A through metallization or a short circuit with the silicon layer adjacent to the emitter layer can thus be prevented.
• the etching thus allows a very controlled porosification, so that an area can be produced with a metal silicide in a very thin emitter layer whose thickness is only a few hundred nanometers to about 50 nanometers.
• a metal layer is applied to the metal silicide.
• a metal layer adheres very well to a metal silicide layer, achieving a low electrical contact resistance between these two layers.
• the metal layer then forms an electrical supply line of the solar cell together with the metal silicide. It is advantageous if the metal layer has nickel, copper or silver, and it is particularly advantageous if this metal layer is applied chemically or electrochemically and thus is non-porous in comparison to a printed paste and thus has a lower line resistance.
• the silicon layer preferably has a layer with highly n-doped silicon. It has been observed that such a silicon layer is attacked differently when exposed to an etching medium in comparison to a lightly doped silicon layer. A layer with high n-doping can be etched in a shorter time than a layer with low doping. This observation can be used advantageously for the production of the solar cell according to the invention. With a highly n-doped silicon layer it is achieved that the etch depth in the emitter and thus the height of the porosified region or the height of the subsequently formed metal silicide can be limited in a simple manner.
• an emitter layer which has a thickness of only a few hundred nanometers up to 50 nanotransmitters without an electrical short circuit of the solar cell occurring.
• an antireflection layer is applied to the emitter layer. This causes a smaller proportion of the incident light is reflected by the solar cell, so that a higher efficiency of the solar cell can be achieved.
• the antireflection layer preferably comprises silicon nitride, silicon oxide or tin oxide, the latter, in contrast to the others, being electrically conductive.
• the anti-reflection layer is formed of porous silicon.
• This layer is formed over the entire surface of the surface of the solar cell.
• this layer is a part of the emitter and is limited to a thickness of a few hundred nanometers up to 50 nanometers.
• the advantage is that such an antireflection layer does not need to be patterned to perform the metallization.
• the porosification can be unstructured and without masking layer. Thus, no masking layer is required, which must be resistant to hydrofluoric acid. After the porosification, a masking layer can then be applied, which is patterned so that regions with and without a masking layer are formed. In the areas without masking layer, a coating with a metal layer can take place. This is followed by heating of the emitter layer and metal layer, so that a metal silicide layer is formed
• the metal of the metal silicide and the metal of the metal layer deposited on the metal silicide are identical.
• the metal of the metal silicide and the metal of the metal layer deposited on the metal silicide are identical.
• For a relatively simple production is possible.
• the invention also relates to a method for producing a silicon solar cell as described above from a wafer having a silicon layer, the method comprising the steps:
• the masking layer can either be applied in a structured manner, for example by means of a screen printing method, or applied over the entire surface and subsequently structured, for example, by using a laser.
• the structuring takes place in such a way that a region with masking layer and a region without masking layer are formed, wherein in the region without masking layer the structuring is carried out until the emitter layer has been exposed. If another layer, for example an antireflection layer, is present under the masking layer, the structuring is carried out until the antireflection layer in the region without masking layer is removed and the emitter layer underneath is exposed.
• the structuring makes it possible to produce a narrow region or channel whose bottom or base, in this embodiment the exposed emitter layer, can be attacked with an etching medium.
• the etching in the region without the masking layer or the exposed emitter layer results in that a porous structure can be produced on the surface of the layer.
• Such etching and generation of a porous structure on the surface can take place with very limited time, so that the etching attack of the emitter layer takes place only to a small depth.
• the porosification is particularly advantageous, since a surface treated in this way has a fissured structure which can easily absorb and mechanically anchor a metal layer to be applied.
• a small electrical contact resistance between the metal layer and the silicon emitter layer is achieved.
• a material which is porosified by etching is very reactive, so that when heating the emitter layer and the metal layer in a simple manner at low temperatures, a metal silicide layer is formed which adheres well and has a low electrical conductivity Has resistance. Only by the porous structure can a satisfactory adhesion and a low electrical resistance be achieved.
• the order described can also be changed.
• the emitter layer is etched with an etching medium, so that the emitter layer is porosified.
• the etching attack may be limited to one zone or take place over the entire surface of the emitter layer.
• the step in which a structured masking layer is produced, so that a region with masking layer and a region without masking layer are present can follow.
• the coating of the porosified emitter layer in the area without masking layer is performed with a first metal layer, then heating the emitter layer and the first metal layer, and then removing the masking layer. This is advantageous because thus the etching without masking takes place and no masking layer is required, which must be resistant to an etching medium.
• the etching may take place of a silicon layer which is not yet an emitter layer and is formed by doping to form an emitter layer only in a further method step.
• the doping thus does not take place in this embodiment before the etching, as described above, but only after the etching. If the emitter layer is formed, then, as mentioned above, the steps may follow that a structured masking layer is produced, a coating is carried out with a first metal layer, then a metal silicide is formed and then the masking layer is removed.
• the coating with a metal layer preferably takes place without external current or chemically or electrochemically. This can be done for example in such a way that with an n-doped emitter silicon layer, the coating is carried out by contacting the porous silicon with a metal-containing electrolyte and depositing metal in and / or on the porous and cathodically poled silicon.
• the masking layer causes the lateral extent of the metal layer to be built up to take place within the etched-away channel of the masking layer.
• the deposited metal Partially penetrates into the depths of the porous structure and, depending on the selected process parameters (voltage, current, electrolyte concentration or time), can be built up to a predetermined height. However, it is also possible that the deposition of the metal takes place in such a way that the metal only reaches the depths of the porous structure and fills the free spaces there, but then no further layer is built up.
• the patterning of the masking layer makes it possible to produce a narrow channel, so that the metal silicide and any metal layer formed thereon have a narrow line width. This is advantageous because it is thus possible to form a solar cell with supply lines which, due to their narrow width, only cause a slight coverage of the irradiated area of the solar cell.
• a feed line produced in this way has a smaller coverage than in the conventional method, in which for a feed line, a silver paste is applied to the surface in a printing process. With such a silver paste only metal fingers or leads can be produced, the width of which is greater than their height, so that should not be less than a minimum height for a sufficiently low electrical resistance of a metal finger.
• the structured masking layer makes it possible to produce line widths of less than 100 ⁇ m, so that a smaller area is covered with the same dimensions of a solar cell and greater efficiency of the solar cell can be achieved.
• the metal deposition can also take place under the action of light on the emitter layer.
• the back side of the silicon wafer receives a cathodic contact with the anode in the electrolyte.
• the cell is then poled in the reverse direction, while by the action of light on the emitter layer, an increased photocurrent flows, which ensures the electroplating on the emitter surface.
• This is advantageous, since thus no contacting of the emitter layer within the medium must take place and the usual problems such as galvanization of the contact or layer thickness variation on the emitter layer due to the external field influence of the contact point are avoided.
• the photocurrent avoids the disadvantage that on the surface of the emitter layer no highly conductive starting layer is present, as is otherwise usual in a galvanic coating.
• the silicon layer can be porosified by electrochemical etching with anodic polarization of the silicon layer, the silicon layer being immersed in the etching medium.
• This is advantageous because the same arrangement as in the coating can be selected, but with a reversed polarity.
• the silicon layer is anodically poled during the etching so that the layer is porosified.
• For the subsequent coating there is now no need to rebuild or expand the silicon wafer and no rinsing steps u.a., Which saves time. There are also no wetting problems, since the electrolyte is already contained in the pores during the etching and does not have to penetrate for the coating first.
• the electrical contacting for the cathodic or anodic polarity of the silicon can take place outside of the electrolyte, so that the contacts and the lead are not exposed to an electric field in the electrolyte and are not galvanized or etched.
• the etching medium is the metal-containing electrolyte, so that the same electrolyte can be used for the etching and the coating.
• the etching medium may include hydrofluoric acid, a relaxant such as a surfactant, alcohol or acetic acid, and a metal salt, preferably in the composition 25 vol.% Hydrofluoric acid, 25 vol.% Ethanol, 25 vol.% Nickel sulfate solution or nickel chloride solution with 80 to 185 g / l nickel, and 25 vol.% water.
• Another advantageous composition dispenses with ethanol, makes do with less than 25% by volume of hydrofluoric acid and provides nickel acetate up to the limit of solubility.
• the concentration of hydrofluoric acid may be 25% by volume, with nickel acetate included up to the solubility limit, so that higher current densities of up to 60mA / cm 2 are achievable in the porosification.
• concentrations between 5% by volume and 10% by volume of hydrofluoric acid it is possible to porosify with a current density of up to 30 mA / cm 2 .
• a high nickel content allows high current densities in the Nickel deposition, with a lower nickel concentration, the current carrying capacity of the electrolyte drops correspondingly lower.
• a further advantageous composition comprises:. 150g / l hydrofluoric acid with 5 vol .-% to 10 vol%, up to 180g / l nickel acetate up to the solubility dissolved, an expansion means, such as anionic sodium dodecyl sulfate (C 2 H 25 NaO 4 S), or cationic with cetyltrimethylammonium bromide (Ci 9 H 42 BrN) or nonionic with a / 7-terf-octylphenol derivative, for example in the product Triton® X-100.
• an expansion means such as anionic sodium dodecyl sulfate (C 2 H 25 NaO 4 S), or cationic with cetyltrimethylammonium bromide (Ci 9 H 42 BrN) or nonionic with a / 7-terf-octylphenol derivative, for example in the product Triton® X-100.
• the step of heating for forming the metal silicide layer may preferably be in a range from 250 0 C to 400 ° C in a range from 250 0 C to 700 ° C.
• silicide formation is already possible in the low temperature range of 250 ° C to 400 ° C, so that less thermal stress is applied to the silicon.
• a selectivity can be achieved in the depth between porous silicon and nonporosified silicon.
• the low temperature is sufficient to form a metal silicide with the porous silicon, but the formation of a metal silicide with the nonporous region requires higher temperatures or runs at a significantly slower rate at the temperature.
• the remaining first metal layer can be removed and a first metal layer can be deposited thereon again. This ensures that reaction layers such as oxide layers are removed after the metal silicide formation, so that the subsequently applied first metal layer can adhere securely.
• a second metal layer is subsequently formed in the region in which the metal silicide layer is formed.
• the metal silicide layer is a nickel silicide, for example, three further metal layers may be used in the order of nickel, copper and tin or in the order nickel, copper, silver are applied.
• the second metal layer may comprise silver.
• no heating is carried out after the application of the first metal layer to the porous silicon, but a second metal layer is formed on this first metal layer.
• the masking layer can remain on the wafer, in particular on an antireflection layer, so that there is no incorrect deposition at porous sites in the antireflection layer.
• the deposition still takes place in the etched area of the masking layer, whereby the lateral spread of the metal layers can be reliably limited to the width of the etched area.
• the heating then takes place after the complete deposition of the metal layers and removal of the masking layer. Because the metallization is on porous silicon with good adhesion, the masking layer can be removed without peeling off the metal layers.
• the formation of metal silicide would not be possible after removal of a masking layer. Because the silicon is in a porous form, upon heating, the first metal layer can reliably form a metal silicide with the silicon without loss of adhesion.
• the masking layer preferably has a lacquer which can be applied by means of screen printing. It is not necessary to use a photoimageable varnish. Rather, the paint may have vacancies in the areas where the metallization is to take place later.
• the paint is resistant to hydrofluoric acid, acetone, ethanol, nitric acid, alkalis such as potassium hydroxide or sodium hydroxide.
• Fig. 1 is a cross-sectional view of a wafer as a starting material of a
• FIG. 2 shows a cross-sectional view of the wafer according to the first embodiment after a second method step
• FIG. 3 shows a cross-sectional view of the wafer according to the first embodiment after a third method step
• FIG. 4 shows a cross-sectional view of the wafer according to the first embodiment after a fourth method step
• FIG. 5 shows a cross-sectional view of the wafer according to the first embodiment after a fifth method step
• FIG. 6 shows a cross-sectional view of the wafer according to the first embodiment after a sixth method step
• FIG. 7 is a cross-sectional view of the wafer according to the first embodiment after a seventh process step
• Fig. 8 is a cross-sectional view of the wafer as a starting material of a
• FIG. 9 is a cross-sectional view of the wafer according to the second embodiment after a sixth process step
• FIG. 10 shows a cross-sectional view of the wafer according to a third embodiment after a first method step
• FIG 11 is a cross-sectional view of the wafer according to the third embodiment after a fifth process step.
• FIG. 1 shows a cross-sectional view of a wafer 20 which has an n-doped silicon layer 1 and a p-doped silicon layer 2.
• the n-doped layer has a height of, for example, 200 nanometers and acts as an emitter layer, while the p- doped layer as a carrier material (bulk) has a height of 100 to 200 micrometers.
• an antireflection layer 3 of, for example, silicon nitride is formed, which has a height of, for example, 100 nanometers.
• the antireflection layer 3 is not absolutely necessary, but advantageous because it serves to reduce a reflection of the incident light on the n-doped emitter layer 1.
• FIG. 1 shows a cross-sectional view of a wafer 20 which has an n-doped silicon layer 1 and a p-doped silicon layer 2.
• the n-doped layer has a height of, for example, 200 nanometers and acts as an emitter layer, while
• a masking layer 4 is applied on the surface of one side of the wafer 20, which has the emitter layer 1. Since an antireflection layer 3 is provided in this embodiment, the masking layer 4 is not located directly on the n-doped silicon layer 1 but on the antireflection layer 3.
• the masking layer 4 has at least one free space 5 which extends to the underlying layer, in this case Embodiment, the anti-reflection layer 3, enough.
• the free space can be created by structuring the masking layer. However, it is also possible that the masking layer is applied to the anti-reflection layer with an already existing clearance.
• the etching medium is allowed to act in a second process step until the antirefiction layer is completely etched away at the bottom of the free space 5, see FIG. 2. With prolonged exposure of the etching medium this also the underlying layer, here the n-doped silicon layer 1, to, see Fig. 3. This n-doped silicon layer 1 is then porous in structure, see reference numeral 7. With a short exposure time of the etching medium on the n-doped Silicon layer 1, the etching attack can be limited in depth.
• the first metal layer 9 can also be further applied, so that the first metal layer 9 is provided not only inside but also on the porous structure 7, see FIG. 4.
• the metal layer 9 is located inside of the masking layer 4 existing free space 5 and has a width which corresponds to the width of the free space 5.
• the masking layer 4 is removed in the first embodiment. Subsequently, a heating of at least the porous silicon layer takes place together with the first metal layer, so that a metal silicide layer 10, for. As a result of the porous structure 7, a good adhesion arises between the metal 9 and the silicon 1.
• a second metal layer 11 can be applied to the first metal layer 9 be constructed, whereby a solar cell 30 is formed.
• a second metal layer 11 can be applied to the first metal layer 9 in a second embodiment with a masking layer 4 still present, see FIG further method step, the heating of at least the porous silicon 7 with the first metal layer 9, see Figure 9, so that a metal silicide 10 is formed. Thereafter, the masking layer 4 is removed so that a solar cell 30 is formed also in this second embodiment, see Fig. 7. Should the masking layer 4 not survive the temperature generated upon heating to form the metal silicide without being damaged, it will become before the step of heating away.
• the wafer has a silicon layer which can simultaneously act as an antireflection layer.
• a first method step the entire surface of the silicon layer 1 is etched, so that the layer is porosified.
• a masking layer 4 is applied, which is already structured or still structured got to. This results in a region without a masking layer, which can be coated with a first metal layer 9 and a second metal layer 11.
• the silicon layer 1 and the first metal layer 9 are heated so that a metal silicide 7 forms, see FIG. 11.
• the two metal layers 9 and 11 remain on the surface, whereby the solar cell according to the invention is formed.

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• 2009
  • 2009-02-09 DE DE102009008152A patent/DE102009008152A1/de not_active Withdrawn
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