EP3241243A1 - Procédé pour doper des semiconducteurs - Google Patents

Procédé pour doper des semiconducteurs

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Publication number
EP3241243A1
EP3241243A1 EP15805391.8A EP15805391A EP3241243A1 EP 3241243 A1 EP3241243 A1 EP 3241243A1 EP 15805391 A EP15805391 A EP 15805391A EP 3241243 A1 EP3241243 A1 EP 3241243A1
Authority
EP
European Patent Office
Prior art keywords
doping
silicon
diffusion
printing
substrate
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP15805391.8A
Other languages
German (de)
English (en)
Inventor
Oliver Doll
Ingo Koehler
Sebastian Barth
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Merck Patent GmbH
Original Assignee
Merck Patent GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Merck Patent GmbH filed Critical Merck Patent GmbH
Publication of EP3241243A1 publication Critical patent/EP3241243A1/fr
Withdrawn legal-status Critical Current

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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
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/028—Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic Table
    • H01L31/0288—Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic Table characterised by the doping material
    • 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
    • H01L21/2225—Diffusion sources
    • 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
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02167—Coatings 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
    • H01L31/18—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 Table
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/186—Particular post-treatment for the devices, e.g. annealing, impurity gettering, short-circuit elimination, recrystallisation
    • 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 method and means for producing structured, highly efficient solar cells and of
  • Photovoltaic elements having regions of different doping.
  • the invention also relates to the solar cells thus produced with increased efficiency.
  • a silicon wafer (monocrystalline, multicrystalline or quasi-monocrystalline, p- or n-type base doping) is freed of adherent saw damage by means of an etching process and "simultaneously", usually in the same etching bath, texturized in this case, the creation of a preferred
  • the surface of the wafer now acts as a diffuse reflector, thus reducing the directional, wavelength-dependent, and angle-of-impact reflection, as a result of the etching step or simply the targeted roughening of the wafer surface.
  • etching solutions for treating the silicon wafers typically consist of thinner ones in the case of monocrystalline wafers Potassium hydroxide, which is added as solvent isopropyl alcohol. Instead, other alcohols having a higher vapor pressure or higher boiling point than isopropyl alcohol may be added, provided that the desired etching result can be achieved thereby.
  • the desired etch result is a morphology characterized by randomly-spaced, or rather, squared-out pyramids etched from the original surface.
  • the density, the height and thus the base area of the pyramids can be influenced by a suitable choice of the above-mentioned constituents of the etching solution, the etching temperature and the residence time of the wafers in the etching basin.
  • the texturing of the monocrystalline wafers is carried out in the temperature range from 70 to less than 90 ° C., whereby etch removal of up to 10 ⁇ m per wafer side can be achieved.
  • the etching solution may consist of potassium hydroxide solution with an average concentration (10-15%).
  • this etching technique is hardly used in industrial practice. More often, an etching solution consisting of nitric acid, hydrofluoric acid and water is used.
  • This etching solution can be modified by various additives, such as, for example, sulfuric acid, phosphoric acid, acetic acid, N-methylpyrrolidone and also surfactants, with which, inter alia, wetting properties of the etching solution and its etching rate can be influenced in a targeted manner.
  • These acid etch mixtures produce a morphology of interstitially arranged etch pits on the surface.
  • the etching is typically carried out at temperatures in the range between 4 ° C to less than 10 ° C and the ⁇ tzabtrag here is usually 4 pm to 6 pm.
  • the wafers typically between 750 ° C and less than 1000 ° C, treated with steam consisting of phosphorus oxide.
  • the wafers are exposed in a tube furnace in a controlled atmosphere quartz glass tube consisting of dried nitrogen, dried oxygen and phosphoryl chloride.
  • the wafers are introduced at temperatures between 600 and 700 ° C in the quartz glass tube.
  • the gas mixture is transported through the quartz glass tube. During the transport of the gas mixture through the highly heated tube, the phosphoryl chloride decomposes into a
  • This mixed oxide is called phosphosilicate glass (PSG).
  • PSG phosphosilicate glass
  • the mixed oxide serves the silicon wafer as a diffusion source, wherein in the course of the diffusion, the phosphorus oxide diffuses in the direction of the interface between PSG glass and silicon wafer and is reduced there by reaction with the silicon on the wafer surface (silicothermally) to phosphorus.
  • the resulting phosphor has a solubility that is orders of magnitude higher in silicon than in the glass matrix from which it is formed and thus dissolves due to the very high segregation coefficient preferably in silicon.
  • the phosphorus in silicon diffuses along the concentration gradient into the volume of silicon. In this diffusion process, concentration gradients of the order of 10 5 arise between typical surface concentrations of 10 21 atoms / cm 2 and the base doping in the range of 10 16
  • the typical depth of diffusion is from 250 to 500 nm and depends on the chosen diffusion temperature, for example at about 880 ° C., and the total exposure time (heating, coating phase, driving phase and cooling) of the wafers in the highly heated atmosphere.
  • a PSG layer is formed, which typically has a layer thickness of 40 to 60 nm.
  • the drive-in phase follows. This can be decoupled from the occupancy phase, but is conveniently coupled in terms of time usually directly to the occupancy and therefore usually takes place at the same temperature.
  • the composition of the gas mixture is adjusted so that the further supply of phosphoryl chloride is suppressed.
  • the surface of the silicon is further oxidized by the oxygen contained in the gas mixture, whereby a phosphorous oxide is depleted between the actual doping source, the phosphoric oxide highly enriched PSG glass and the silicon wafer
  • Silicon dioxide layer is generated, which also contains phosphorus oxide.
  • the growth of this layer is proportional to the mass flow of the
  • Oxide growth is accelerated by the high surface doping of the wafer itself (acceleration by one to two orders of magnitude). As a result, in some way, a depletion or separation of the doping source is achieved, whose penetration with diffusing Phosphorus oxide is influenced by the flow of material, which is dependent on the temperature and thus the diffusion coefficient. In this way, the doping of the silicon can be controlled within certain limits.
  • a typical duration of diffusion consisting of occupancy and driving phase is for example 25 minutes. Following this treatment, the tube furnace is automatically cooled and the wafers can join
  • boron doping of the wafers in the form of an n-type base doping another method is used, which will not be explained separately here.
  • the doping is carried out in these cases, for example with boron trichloride or boron tribromide.
  • the formation of a so-called boron skin on the wafers can be determined. This boron skin is dependent on various influencing factors, specifically the doping atmosphere, the temperature, the doping time, the source concentration and the coupled (or linearly combined) aforementioned parameters.
  • in-line doping in which the corresponding pastes and inks are applied to the side of the wafer to be doped by suitable methods.
  • the solvents contained in the compositions used for doping are removed by temperature and / or vacuum treatment.
  • the actual dopant remains on the wafer surface.
  • dilute solutions of phosphoric or boric acid, as well as sol-gel-based systems or else solutions of polymeric borazil compounds can be used as liquid doping sources.
  • Corresponding doping pastes are almost exclusively by the use of additional thickening polymers characterized, and contain dopants in a suitable form.
  • the evaporation of the solvents from the abovementioned doping media is usually followed by treatment at high temperature, during which the undesired and interfering additives which cause the formulation are either "burned" and / or pyrolyzed
  • the coated substrates usually pass through a continuous furnace at temperatures between 800 ° C. and 1000 ° C.
  • the temperatures can be slightly increased in comparison with the gas phase diffusion in the tube furnace
  • Gas atmosphere may be different according to the requirements of the doping and from dry nitrogen, dry air, a mixture of dry oxygen and dry nitrogen and / or, depending on the design of the furnace to be passed, from zones of one and other of the above gas atmosphere Other gas mixtures are conceivable, but currently have no major industrial significance.
  • a characteristic of the in-line diffusion is that the assignment and the driving of the dopant can in principle be decoupled from each other.
  • the wafers are on the one hand transhipped in batches in wet process boats and with their help in a solution of dilute hydrofluoric acid, typically 2% to 5%, immersed and left in this until either the surface is completely removed from the glasses, or the process cycle has expired, representing a cumulative parameter of the required etch time and machine process automation.
  • the complete removal of the glasses for example, based on the complete dewetting of the
  • Silicon wafer surface can be detected by the dilute aqueous hydrofluoric acid solution.
  • Hydrofluoric acid solution reached within 210 seconds at room temperature.
  • the etching of corresponding BSG glasses is slower and requires longer process times and possibly also higher concentrations of the hydrofluoric acid used.
  • the wafers are rinsed with water.
  • the etching of the glasses on the wafer surfaces can also be carried out in a horizontally operating method in which the wafers are introduced in a constant flow into an etching system in which the wafers pass through the corresponding process tanks horizontally (inline system).
  • the wafers are conveyed on rollers and rollers either through the process tanks and the etching solutions contained therein or the etching media are transported onto the wafer surfaces by means of roller application.
  • the typical residence time of the wafers in the case of etching the PSG glass is about 90 seconds, and the hydrofluoric acid used is somewhat more concentrated than in the batch process to compensate for the shorter residence time due to an increased etch rate.
  • the concentration of hydrofluoric acid is typically 5%.
  • the pool temperature may be slightly higher than the room temperature (greater than 25 ° C less than 50 ° C).
  • Edge insulation -> glass etching The edge insulation is a process engineering necessity, which results from the system-inherent characteristics of the double-sided diffusion, even with intentional unilateral back-to-back diffusion.
  • On the (later) back side of the solar cell there is a large-area parasitic pn junction which, although due to process technology, is partly, but not completely, removed in the course of the later processing.
  • the front and back of the solar cell will be short-circuited via a parasitic and residual pn junction (tunnel junction) that reduces the conversion efficiency of the future solar cell.
  • the wafers are guided on one side via an etching solution consisting of nitric acid and hydrofluoric acid.
  • the etching solution may contain as minor constituents, for example, sulfuric acid or phosphoric acid.
  • the etching solution is transported via rollers mediated on the back of the wafer.
  • the etching removal typically achieved with these methods amounts to approximately 1 ⁇ m silicon (including the glass layer present on the surface to be treated).
  • the glass layer still present on the opposite side of the wafer serves as a mask, which exerts some protection against etching attacks on this side. This glass layer is subsequently removed using the glass etching already described.
  • the edge isolation can also be done with the help of
  • Plasma etching processes are performed. This plasma etching is then usually carried out before the glass etching. For this purpose, several wafers are stacked on each other and the outer edges become the plasma
  • the plasma is filled with fluorinated gases, for example
  • Tetrafluoromethane fed.
  • an antireflection coating which usually consists of amorphous and hydrogen-rich silicon nitride.
  • Alternative antireflection coatings are conceivable. Possible coatings may be titanium dioxide, magnesium fluoride, tin dioxide and / or corresponding stacked layers of silicon dioxide and silicon nitride. But it is technically possible also differently composed antireflection coatings.
  • the coating of the wafer surface with the above-mentioned silicon nitride fulfills essentially two functions: on the one hand, the layer generates an electric field due to the numerous incorporated positive charges that charge carriers in silicon can keep away from the surface and can significantly reduce the recombination speed of these charge carriers on the silicon surface (Field effect passivation), on the other hand, this layer generates depending on their optical parameters, such as refractive index and layer thickness, a reflection-reducing property, which contributes to that in the later solar cell more light can be coupled. Both effects can increase the conversion efficiency of the solar cell.
  • the antireflection reduction is most pronounced in the wavelength range of the light of 600 nm.
  • the directional and non-directional reflection shows a value of about 1% to 3% of the originally incident light (perpendicular incidence to the surface normal of the silicon wafer).
  • the above-mentioned silicon nitride films are currently deposited on the surface generally by direct PECVD method.
  • a plasma is ignited in a gas atmosphere of argon, in which silane and ammonia are introduced.
  • the silane and the ammonia are converted in the plasma by ionic and radical reactions to silicon nitride and thereby deposited on the wafer surface.
  • the properties of the layers can z. B. adjusted and controlled by the individual gas flows of the reactants.
  • the deposition of the above-mentioned silicon nitride layers can also be carried out using hydrogen as the carrier gas and / or the reactants alone. Typical deposition temperatures are in the range between 300 ° C to 400 ° C.
  • Alternative deposition methods may be, for example, LPCVD and / or sputtering.
  • Silicon nitride coated wafer surface defines the front electrode.
  • the electrode has been established using the screen printing method using metallic
  • a paste heavily enriched with silver particles is generally used.
  • the sum of the residual constituents results from the necessary for the formulation of the paste theological aids, such as solvents, binders and thickeners.
  • the silver paste contains a special Glasfrit mixture, mostly oxides and mixed oxides based on silica, borosilicate glass and lead oxide and / or bismuth oxide.
  • the glass frit fulfills essentially two functions: on the one hand it serves as a bonding agent between the wafer surface and the mass of the silver particles to be sintered, on the other hand it is responsible for the penetration of the silicon nitride covering layer in order to enable the direct ohmic contact to the underlying silicon.
  • the penetration of the silicon nitride takes place via an etching process with subsequent diffusion of silver present dissolved in the glass frit matrix into the silicon surface, whereby the ohmic contact formation is achieved.
  • the silver paste is deposited by screen printing on the wafer surface and then dried at temperatures of about 200 ° C to 300 ° C for a few minutes.
  • double-printing processes also find industrial application, which make it possible to print on an electrode grid generated during the first printing step, a congruent second.
  • the strength of the silver metallization is increased, which can positively influence the conductivity in the electrode grid.
  • the solvents contained in the paste are expelled from the paste.
  • the printed wafer passes through a continuous furnace.
  • Such an oven generally has several heating zones, which can be independently controlled and tempered.
  • passivating the continuous furnace the wafers are heated to temperatures up to about 950 ° C.
  • the single wafer is typically exposed to this peak temperature for only a few seconds.
  • the wafer has temperatures of 600 ° C to 800 ° C.
  • Customized metallization processes are based on two consecutive screen printing processes with optional composition of two distinctive metal pastes
  • the rear bus buses are usually also applied and defined by screen printing. This is one of the front metallization used similar silver paste application. This paste is similarly composed, but contains an alloy of silver and aluminum, wherein the proportion of aluminum is typically 2%. In addition, this paste contains a lower glass frit content.
  • the bus busses usually two, will be screenprinted with a typical width of 4mm on the back side of the wafer and densified and sintered as described in paragraph 5 above. 7.) Generation of the back electrode
  • the back electrode is defined following the pressure of the bus buses.
  • the electrode material is made of aluminum, which is why an aluminum-containing paste is printed by screen printing on the remaining free area of the wafer backside with an edge distance smaller than 1mm to define the electrode.
  • the paste is composed of greater than or equal to 80% aluminum.
  • the remaining components are those already mentioned in paragraph 5 (such as solvents, binders, etc.).
  • the aluminum paste is bonded to the wafer during co-firing by causing the aluminum particles to begin to melt during heating and dissolve silicon from the wafer into the molten aluminum.
  • the melt mixture acts as a dopant source and gives aluminum to the silicon (solubility limit: 0.016 atomic percent), whereby the silicon is p + doped as a result of this drive-in.
  • a eutectic mixture of aluminum and silicon which solidifies at 577 ° C. and has a composition with a mole fraction of 0.12 Si, is deposited on the wafer surface, inter alia.
  • a highly doped p-type layer is formed on the back side of the wafer, which acts as a kind of mirror on parts of the free charge carriers in the silicon ("electric These charge carriers can not overcome this potential barrier and are thus very efficiently kept away from the rear wafer surface, which thus manifests itself in an overall reduced recombination rate of charge carriers on this surface. designated.
  • edge isolation of the wafer has not already been carried out as described under point 3, this is typically carried out after co-firing with the aid of laser beam methods.
  • a laser beam is directed to the front of the solar cell and the front p-n junction is severed by means of the energy coupled in by this beam.
  • This trench with a depth of up to 15 ⁇ generated as a result of the action of the laser.
  • silicon is removed from the treated area via an ablation mechanism or thrown out of the laser ditch.
  • this laser trench is 30 pm to 60 pm wide and about 200 pm away from the edge of the solar cell.
  • the solar cells After production, the solar cells are characterized and classified according to their individual performance in individual performance categories.
  • solar cell architectures with both n-type and p-type base material. These solar cell types include • PERC solar cells
  • structured diffusion barriers may be deposited on the silicon wafers prior to depositing the glasses to define the regions to be doped.
  • a disadvantage of this method is that in each case only one polarity (n or p) of
  • Doping can be achieved. Somewhat simpler than the structuring of the doping sources or that of any diffusion barriers is the direct laser-beam driven driving-in of dopants from previously onto the
  • Wafer surfaces deposited dopant sources This method makes it possible to save costly structuring steps. Nevertheless, the disadvantage of a possible simultaneous simultaneous doping of two polarities on the same surface at the same time (Co- Diffusion), since this method is also based on a predeposition of a dopant source, which is only activated later for the release of the dopant. Disadvantage of this (post-) doping from such sources is the inevitable laser damage to the substrate: the laser beam must by absorbing the radiation into heat
  • the silicon located under the optically transparent glasses is used as the absorption source.
  • the silicon is partially heated to the melt, and as a result heats the glass above it. This allows diffusion of the dopants - much faster than what would be expected at normal diffusion temperatures, resulting in a very short silicon diffusion time (less than 1)
  • the silicon should after the absorption of the laser radiation due to the strong dissipation of heat in the remaining, not irradiated
  • Defects are accompanied, which may be due to incomplete epitaxial solidification and thus the formation of crystal defects. This can be attributed, for example, to dislocations and formation of voids and voids as a result of the shocking process.
  • Another disadvantage of laser-assisted diffusion is the relative inefficiency when larger areas are to be rapidly doped because the laser system scans the surface in a dot-matrix process. For narrow, doped areas of this disadvantage has a natural lower weight.
  • laser doping requires sequential deposition of the aftertreatable glasses.
  • a cost-effective structuring is desirable, whereby an improved competitiveness compared to the currently technologically predominant doping process can be achieved.
  • the present invention is a new method for direct doping of a silicon substrate, by
  • a doping paste which is suitable as a sol-gel for the formation of oxide layers and at least one doping element selected from the group
  • a doping of the substrate is carried out by laser irradiation
  • the steps b) to e) depending on the desired doping result in different order of magnitude and optionally can be carried out repeatedly.
  • the temperature treatment is carried out in the diffusion step after laser irradiation at temperatures in the range of 750 to 1100 ° C for doping, at the same time there is a healing of the laser irradiation induced damage in the substrate.
  • the present invention also relates to a method as characterized by claims 2 to 9, and thus forms part of the present description.
  • the solar cells and photovoltaic elements produced by these process steps are also the subject matter of the present invention which, owing to the process described here, have substantially improved properties, such as better light yield and thus improved efficiency, ie. H. have a higher current efficiency.
  • FIG. 1 shows a schematic and simplified, Unscaled representation of the front side of a conventional solar cell (back side ignored). Shown is the two-stage emitter, which results from two doped regions, in the form of differing sheet resistances. The different sheet resistances are due to different profile depths of the two doping profiles, and are therefore usually also associated with different doses of dopants. The metal contacts of the solar cells to be produced from such structural elements always contact the more heavily doped regions.
  • the front of the solar cell at least as a rule, is with the
  • emitter doping provided. Depending on the base material used, this can be either n-type or p-type (the basis is then doped in reverse).
  • the emitter forms the pn junction, which can collect and separate the charge carriers formed in the solar cell via an electric field present above it.
  • the minority carriers are injected from the base into the emitter, where they then belong to the majorities. These majorities are transported further in the emitter zone and can be removed as current from the cell via the electrical contacts located at the emitter zone.
  • the emitter forms the pn junction, which can collect and separate the charge carriers formed in the solar cell via an electric field
  • Recombination rate of the minorities simplified inversely proportional to the doping concentration of the respective region in the silicon is dependent; d. h., That in the emitter region of a solar cell, which itself represents a highly doped zone in silicon, the carrier life of the respective minorities very low, d. H. can be much smaller than in the relatively low-doped base. For this reason, as far as possible, the emitter regions become relatively thin, i. H. produced little deep in relation to the thickness of the entire substrate, the silicon wafer, so that the minorities produced in this region, which then have a very short lifetime inherent in the system, have ample opportunity, or just time, to transition the pn reach, be collected at this, separated and now injected as majorities in the base. As a rule, the majorities have a carrier lifetime which is to be regarded as infinite.
  • Recombination activity may be due to the creation and disposal of electronic passivation layers are greatly reduced (up to seven orders of magnitude, as measured by the
  • the carrier lifetime of the minorities in these areas is so low that their average lifetime allows only a very low quasi-static concentration. Since the recombination of charge carriers is based on the combination of minorities and majorities, in this case there are simply too few minorities that can recombine with majorities directly on the surface. A much better electronic passivation than that of an emitter is achieved with dielectric passivation layers. On the other hand, however, the emitter is also responsible for creating the electrical contacts to the solar cell, which must be ohmic contacts. They are obtained by driving the contact material, usually silver, into the silicon crystal, the so-called
  • Silicon is dependent on the surface to be contacted. The higher the doping of the silicon, the lower the contact resistance can be. The metal contacts on the silicon are also very strong
  • the silicon zone under the metal contacts should have a very strong and very deep emitter doping. This doping shields the minorities from the metal contacts, at the same time a low contact resistance and thus a very good ohmic conductivity is achieved.
  • the emitter doping should be very low and relatively flat (ie low depth) so that sufficient incidental solar radiation can be generated with sufficient lifetime that can be injected into the base via the separation at the pn junction as majorities.
  • the present process is a simplified method of preparation compared to the previously described two-stage and selective emitter structures, respectively.
  • the method describes a simplification of the production of differently strongly and deeply doped zones (n and p) starting from the surface of a
  • the simplified method of preparation is made possible by using easily and inexpensively printable doping media.
  • the doping media correspond at least to those disclosed in the patent applications WO2012 / 119686 A1 and WO2014 / 101989 A1, but may have different compositions and formulations.
  • the doping media have a viscosity of preferably greater than 500 mPa * s, measured at a shear rate of 25 1 / s and a temperature of 23 ° C, and are therefore due to their viscosity and other formulation properties perfectly adapted to the individual requirements of screen printing. They are structurally viscous and may also exhibit thixotropic behavior.
  • the printable Dotiermedien be applied over the entire surface to be doped using a conventional screen printing machine. Typical, however non-limiting pressure settings will be mentioned throughout the present description.
  • the printed doping media in a temperature range between 50 ° C and 750 ° C, preferably between 50 ° C and 500 ° C, more preferably between 50 ° C and 400 ° C, using one or more, sequentially carried out Temper suitsen ( Tempering by means of a step function) and / or an annealing ramp, dried and compacted for glazing, whereby a grip and abrasion resistant layer with a thickness of up to 500 nm is formed.
  • the further processing for achieving two-stage doping of the substrates treated in this way can comprise two possible process sequences, which are to be outlined briefly below.
  • Doping selectively influenced beneficial. It is particularly advantageous in this case that due to the heat treatment of the printed substrate, the dopants are transported in dependence on the duration of treatment in depths of up to 1 ⁇ and electrical sheet resistances of less than 10 ⁇ / sqr be achieved.
  • the surface concentration of the Dotierstoffs can assume values greater than or equal to 1 * 10 19 to more than 1 * 10 21 atoms / cm 3 and is of the type in the printable
  • a thin so-called boron skin is formed on the silicon surface, which is generally understood as a phase consisting of silicon boride, which is formed as soon as the solubility limit of boron in silicon is exceeded (this is typically 3-4 * 10 20 atoms / cm 3 ).
  • the formation of this boron skin is dependent on the diffusion conditions used, but can not be prevented in the context of classical gas phase diffusion and doping. However, it was found that by choosing the
  • Formulation of the printable doping media can exert a significant influence on the formation and the formed thickness of the boron skin.
  • the existing on the silicon substrate Borhaut can by means of
  • suitable laser irradiation as a dopant source for locally selective further and the doping profile deepening input of the dopant boron can be used.
  • the wafers treated in this way must be removed from the diffusion and doping furnace and treated by means of laser irradiation. At least the remaining surface areas of the silicon wafer not exposed to the laser irradiation subsequently still have an intact boron skin. Since the boron skin has proven in many studies to be counterproductive for the electronic surface passivatability of the silicon surfaces, it seems to be unavoidable to eliminate them in order to prevent disadvantageous diffusion and doping processes.
  • the successful elimination of this phase can be achieved by various oxidative processes, such as low-temperature oxidation (typically at temperatures between 600 ° C to 850 ° C), a short oxidation step under diffusion and doping temperature by the gas atmosphere targeted and controlled by enrichment of Oxygen is "tilted", or by the Constant supply of a small amount of oxygen during the diffusion and doping process.
  • Dopant source probably no, or if, only little dopant in the silicon after.
  • the oxidation of the silicon surface and the boron skin present on it can also be carried out and significantly accelerated by the additional introduction of water vapor and / or chlorine-containing vapors and gases.
  • Elimination of the boron skin consists of a wet-chemical oxidation by means of concentrated nitric acid and subsequent etching of the silicon dioxide layer obtained on the surface. This treatment must be carried out in several cascades to completely remove the borne skin, this cascade is not worth mentioning
  • the drying and compression of the dopant applied over the entire surface are followed by local irradiation of the substrate by means of laser radiation.
  • the layer present on the surface does not necessarily have to be completely compacted and glazed.
  • the parameters characterizing the laser radiation treatment such as pulse length, illuminated area in the radiation focus, repetition rate using pulsed laser radiation
  • the doped dopants contained in the printed and dried layer of the dopant source can be applied to the surrounding, preferably underneath the printed layer , Silicon are discharged.
  • the sheet resistance of the substrate can be specifically influenced and controlled.
  • the printed layer of the dopant source with the help of both hydrofluoric acid and hydrofluoric acid and phosphoric acid-containing aqueous solutions or by appropriate solutions based on organic solvents, as well as by Use of mixtures of the aforementioned etching solutions are removed without residue from the surface of the wafer.
  • the removal of the dopant source can be accelerated and promoted by the action of ultrasound during the use of the etching mixture.
  • the printed dot source may be left on the surface of the silicon wafer.
  • the thus-coated wafer may be doped in a conventional doping furnace on the entire covered silicon wafer surface by thermally induced diffusion.
  • This doping can take place in customarily used doping furnaces. These can be either tubular furnaces (horizontal and / or vertical) or horizontally operating continuous furnaces in which the gas atmosphere used can be set in a targeted manner.
  • a doping of the entire wafer is achieved in conjunction with a change in the sheet resistance.
  • the strength of the doping depends on the particular process parameters used, such as process temperature, plateau time, gas flow, the type of heat source used and the temperature ramps for setting the respective process temperature.
  • a gas flow of five standard liters N2 per minute achieves sheet resistances of about 75 ohms / sq. R at a diffusion time of 30 minutes at 950 ° C.
  • the wafers may optionally be pre-dried at temperatures of up to 500 ° C.
  • the oxidative removal of the so-called boron skin, as well as the redistribution of the boron dissolved in the silicon for adaptation and manipulation of the adjustable doping profile are linked directly to the diffusion.
  • the aforementioned sheet resistance can be reproducibly obtained based on the above-described procedure. Further details on implementation and corresponding further process parameters are described in more detail in the following examples.
  • dopants are also stimulated due to the thermally induced diffusion of the dopants also for further diffusion. Due to this additional diffusion, the dopants can penetrate deeper into the silicon at these points and accordingly form a deeper doping profile. Dopant can be replenished to the silicon from the dopant source located on the wafer surface at the same time.
  • doped zones are formed which have a significantly lower doping profile and also a significantly higher dose of the dopant boron than those areas which were exclusively exposed to a thermally induced diffusion in a doping furnace. In other words, there are two-stage, or also referred to as selective doping.
  • the latter can be used, for example, in the manufacture of selective emitter solar cells, in bifacial solar cells (selective emitter / uniform (single-stage) BSF, uniform emitter / selective BSF, and selective emitter / selective BSF) in the case of PERT cells, or in the application of IBC solar cells application.
  • selective emitter solar cells in bifacial solar cells (selective emitter / uniform (single-stage) BSF, uniform emitter / selective BSF, and selective emitter / selective BSF) in the case of PERT cells, or in the application of IBC solar cells application.
  • the comparable principle is also applicable to the thermally induced postdiffusion of the silicon wafers pretreated by means of laser radiation, which were previously freed from the presence of the printed dopant source by means of etching.
  • the dopant boron is driven deeper into the silicon. Due to the removal of the printed-on dopant source which occurred before this process, however, no dopant can be added to the silicon any more be redelivered.
  • the dose dissolved in the silicon will remain constant, while the average concentration of the dopant in the doped zone will decrease due to increasing tread depth and concomitant decrease in the direct surface concentration of the dopant. This procedure can be used to produce IBC
  • Strips of one polarity are produced from the dried doping paste by means of laser beam doping in addition to those of the opposite polarity, which in turn can be obtained by means of laser beam doping from a printed and dried phosphorus-containing doping paste.
  • the thus described sequence for the production of locally selectively or two-stage doped regions is characterized by the following at least eight steps: printing the dopant source -
  • the two process cascades described above represent possibilities for producing two-stage, or so-called selective, doping.
  • the second embodiment described makes the process more attractive and due to the smaller number of process steps preferred alternative.
  • the doping effect of the printed dopant source by the choice of the respective process parameters, in particular those of laser beam treatment or laser beam doping, can be influenced.
  • the doping effect can also be significantly influenced and controlled by the composition of the printable dopant source (see Fig. 2).
  • two-step doping can not be accomplished exclusively by using only one printable dopant source followed by another, but can also be generated by using two printable dopant sources.
  • the dose of dopants which is to be introduced into the silicon to be doped can be influenced and controlled in a targeted manner via the dopant concentrations present in the dopant sources used.
  • FIG. 2 shows a schematic and simplified, non-scaled representation of the doping process according to the invention, induced by laser radiation treatment (see FIG. 3) of printable doping pastes on silicon wafers, where printable doping pastes of different compositions (for example doping pastes with different concentrations of dopant) can be used.
  • the method according to the invention can easily and inexpensively easily and inexpensively produce two-stage, as well as structured and opposite polarity dopants on silicon wafers using the novel printable doping pastes to be characterized in the following, the sum of which is a single classic high-temperature step (thermally induced diffusion) required (see Figure 4).
  • the opposite polarities may both be on one side of a wafer, or on opposite, or ultimately a mixture of both of the aforementioned structural motifs.
  • both polarities may have two-level doping regions, but not necessarily both Must have polarities.
  • structures can be made with the polarity 1 having a two-stage nature, while the polarity 2 does not include it. This means that the method described here is very variable feasible.
  • the structures of the areas provided with opposite doping are no further limits, except the limits of the respective structure resolution during the printing process as well as those which are inherent in the laser beam treatment.
  • FIGS. 3, 4 and 5 show various embodiments of the method according to the invention:
  • FIG. 3 shows a schematic and simplified, non-scaled representation of the inventive doping process induced by laser radiation treatment of printable doping pastes on silicon wafers.
  • FIG. 5 shows a schematic and simplified, non-scaled representation of the inventive doping process induced by laser radiation treatment of printable doping pastes on silicon wafers
  • the printed and dried dopant sources may be in one of the possible
  • cover layers may, inter alia, both after the laser beam treatment, as well as before this on the printed and dried-on dopant sources be applied.
  • cover view is the cover view after the laser beam treatment with the printed and dried
  • Dopant source supplemented by thermal diffusion thus comprises an alternative inexpensive and easy to carry out process for the production of solar cells with a more efficient charge generation, but also the production of alternative and inexpensive producible, printable dopant sources, their deposition on the silicon substrate and its selective single-stage as well as the selective two-stage doping ,
  • the selective doping of the silicon substrate may hereby, but not necessarily, by means of a combination of initial
  • Laser beam treatment of silicon wafers may be associated with damage to the substrate itself and thus immanent
  • the laser beam treatment can undergo thermal diffusion
  • the metal contacts (cf., FIG. 1) are deposited directly on the areas exposed to the laser radiation.
  • the silicon-metal interface is characterized by a very high recombination rate
  • This doping can be achieved locally and without further activation of the dopants, as is usually achieved by classical thermal diffusion.
  • a conventional thermal diffusion the dopant introduced into the silicon can either be driven deeper or the already dissolved dopant can be driven deeper and further dopant from the dopant source can be tracked into the silicon, whereby in the latter case the dose of the in increases the silicon dissolved dopant.
  • the dopant source printed and dried on the wafer may have a homogeneous dopant concentration.
  • this dopant source can be applied over the entire surface of the wafer or selectively printed.
  • dopant sources may be used
  • the sources can be printed in two consecutive printed and
  • Patent Applications WO 2012/119686 A1 or WO 2014/101989 A1 as the compositions described in these applications are particularly suitable for use in the present invention.
  • Example 1 A textured 6 "CZ wafer with a phosphorus base doping, with a resistivity of 2 ohm * cm, is treated with a boron doping paste as described in the patent applications WO 2012/119686 A1 and WO 2014/101989 A1, with a steel screen ( Tension angle 22.5 °) with 25 ⁇ m thread thickness and an emulsion thickness of 10 ⁇ m using a doctor blade speed of 110 mm / s, a squeegee pressure of 1 bar and a printing screen of 1 mm printed, depending on the other printing parameters, a layer thickness between 100 After being completely dried, the printed paste is dried for 3 minutes at 300 ° C.
  • the wafer is placed in predefined fields with the aid of an Nd: YAG nanosecond laser Wavelength of 532 nm and using different, applied to the dried dopant source Laserfluenzen beha
  • the dopants of the different fields on the wafer are subsequently determined by means of four-point measurements and electrochemical capacitance voltage measurements (ECV).
  • ECV electrochemical capacitance voltage measurements
  • the wafer is subjected to thermal diffusion in a conventional tube furnace using an inert gas atmosphere, N 2, at 930 ° C for 30 minutes.
  • N 2 inert gas atmosphere
  • the Borhauthaut formed during the boron diffusion is, following the diffusion, but still during the furnace process, by means of dry oxidation at a constant process temperature and by controlled tilting due to the introduction of 20% vol.
  • FIG. 6 shows ECV doping profiles as a function of different ones
  • Diffusion conditions after laser diffusion as well as after laser diffusion and subsequent thermal diffusion.

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Abstract

La présente invention concerne un procédé de fabrication de cellules solaires structurées, hautement efficaces, et d'éléments photovoltaïques comportant des régions présentant des dopages différents. La présente invention concerne également des cellules solaires à efficacité accrue fabriquées selon ledit procédé.
EP15805391.8A 2014-12-30 2015-12-01 Procédé pour doper des semiconducteurs Withdrawn EP3241243A1 (fr)

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CN107393850A (zh) * 2017-08-16 2017-11-24 君泰创新（北京）科技有限公司 太阳能电池浆料的干燥方法及系统
CN109411341B (zh) * 2018-09-29 2021-07-27 平煤隆基新能源科技有限公司 一种改善se电池扩散方阻均匀性的方法
CN111739956B (zh) * 2020-06-30 2022-04-26 常州时创能源股份有限公司 激光se电池的制备方法
CN113035976B (zh) * 2021-03-17 2023-01-17 常州时创能源股份有限公司 硼掺杂选择性发射极及制法、硼掺杂选择性发射极电池
CN113471314A (zh) * 2021-05-07 2021-10-01 盐城工学院 一种利用镓掺杂硅纳米浆料制备选择性发射极的方法
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DE102010024308A1 (de) * 2010-06-18 2011-12-22 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Verfahren zur Erzeugung einer selektiven Dotierstruktur in einem Halbleitersubstrat zur Herstellung einer photovoltaischen Solarzelle
DE102012101359A1 (de) * 2011-02-18 2012-08-23 Centrotherm Photovoltaics Ag Verfahren zur Herstellung einer Solarzelle mit einem selektiven Emitter sowie Solarzelle
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CN102842646A (zh) * 2012-05-30 2012-12-26 浙江晶科能源有限公司 一种基于n型衬底的ibc电池的制备方法
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SG11201504937VA (en) * 2012-12-28 2015-07-30 Merck Patent Gmbh Doping media for the local doping of silicon wafers

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TW201635348A (zh) 2016-10-01
WO2016107662A1 (fr) 2016-07-07

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