EP3241242A1 - Laser doping of semiconductors - Google Patents

Abstract

The invention relates to a method for producing structured high-efficiency solar cells and photovoltaic elements that have differently doped regions. The invention also relates to the solar cells that are produced by said method and have increased efficiency.

Description

 Laser doping of semiconductors

The present invention relates to a method for the production of structured, highly efficient solar cells as well as photovoltaic elements having regions of different doping. Subject of the

Invention are also the solar cells thus produced with increased efficiency.

State of the art

The production of simple solar cells or those currently represented in the market with the largest market share comprises the essential production steps outlined below: 1) saw damage etch and texture

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 As a result of texturing, 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. which ultimately leads to an increase in the absorbed fraction of the light incident on the surface and thus increases the conversion efficiency of the solar cell The above-mentioned 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. Typically, 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. Typically, the texturing of the monocrystalline wafers is carried out in the temperature range from 70 ° to 90 ° C., whereby etching removals of up to 10 μm per wafer side can be achieved. in the case of multicrystalline silicon wafers, the etching solution may consist of potassium hydroxide at medium concentration (10-15%). However, 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 <10 ° C and the Ätzabtrag here is usually 4 pm to 6 pm.

Immediately following the texturing, the silicon wafers are included

Water was thoroughly cleaned and treated with dilute hydrofluoric acid to absorb and adsorb the chemical oxide layer resulting from the previous treatment steps, as well as from it Impurities to prepare the subsequent

To remove high temperature treatment.

2) diffusion and doping

The wafers etched and cleaned in the previous step (in this case p-type base doping) are heated at higher temperatures,

typically between 750 ° C and <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

Steam, consisting of phosphorus oxide (eg P205) and chlorine gas. The vapor of phosphorus oxide u. a. on the wafer surfaces down (occupancy). At the same time, the silicon surface is oxidized at these temperatures to form a thin oxide layer. In this layer, the deposited phosphorus oxide is embedded, whereby a mixed oxide of silicon dioxide and phosphorus oxide formed on the wafer surface.

This mixed oxide is called phosphosilicate glass (PSG). This PSG glass has different softening points and, depending on the concentration of the contained phosphorus oxide

different diffusion constants with respect to the phosphorus oxide. 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. After dissolution, 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 atoms / cm ² . The typical depth of diffusion is 250 to 500 nm and is of the selected diffusion temperature, for example at about 880 ° C, and the total exposure time (heating &

Occupancy phase & drive-in phase & cooling down) of the wafers in the strongly heated atmosphere. During the coating phase, a PSG layer is formed, which typically has a layer thickness of 40 to 60 nm. Following the wetting of the wafers with the PSG glass, during which diffusion into the volume of the silicon already takes place, 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

Phosphorylchlorids is prevented. During driving in, the surface of the silicon is contained by that contained in the gas mixture

 Oxygen further oxidized, whereby between the actual doping source, the strongly enriched in phosphorus oxide PSG glass and the

Silicon wafer is a phosphorus depleted silicon dioxide layer is generated, which also contains phosphorus oxide. The growth of this layer is much faster relative to the mass flow of the dopant from the source (PSG glass), because 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, a depletion or separation of the doping source is achieved in a certain way, whose penetration with diffusing phosphorus oxide is influenced by the material flow, which depends on the temperature and thus the diffusion coefficient. In this way, the doping of the silicon in certain Limits are controlled. A typical duration of diffusion consisting of occupancy and driving phase is for example 25 minutes. in the

Following this treatment, the tube furnace is automatically cooled and the wafers can be removed from the process tube at temperatures between 600 ° C to 700 ° C.

 In the case of a 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. Depending on the choice of the composition of the gas atmosphere used for the doping, the formation of a so-called boron skin on the wafers can be determined. This boron skin is dependent on various factors influencing the doping atmosphere, the temperature, the doping time, the source concentration and the coupled (or linearly combined) aforementioned parameters.

In such diffusion processes, it goes without saying that there can be no regions of preferred diffusion and doping in the wafers used (with the exception of those which have arisen due to inhomogeneous gas flows and inhomogeneous gas packets resulting therefrom), provided the substrates are not in advance of a corresponding pretreatment (for example, their structuring with diffusion-inhibiting and / or -unterbindenden layers and materials).

For the sake of completeness, it should be pointed out here that there are still other diffusion and doping technologies which have established themselves to varying degrees in the production of crystalline solar cells based on silicon. So be mentioned

The ion implantation, the doping mediated via the gas phase deposition of mixed oxides, such as PSG and BSG (borosilicate) glass, by APCVD, PECVD, MOCVD and LPCVD methods,

 (Co) sputtering of mixed oxides and / or ceramic materials and hard materials (eg boron nitride),

 the purely thermal gas phase deposition starting from solid dopant sources (eg boron oxide and boron nitride)

 the sputtering of boron on the silicon surface and its thermal input into the silicon crystal

 Laser doping of dielectric passivation layers of different compositions, such as AI2O3, SiOxNy, the latter containing the dopants in the form of mixed P2O5 and B2O3

 and the liquid phase deposition of doping liquids (inks) and pastes.

The latter are often used in so-called in-line doping in which the corresponding pastes and inks are applied to the side of the wafer to be doped by suitable methods. After or even during application, the solvents contained in the compositions used for doping are removed by temperature and / or vacuum treatment. As a result, the actual dopant remains on the wafer surface. For example, 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 characterized almost exclusively by the use of additional thickening polymers, and contain dopants in a suitable form. The evaporation of the solvents from the aforementioned doping media is usually followed by a Treatment at high temperature, during which undesirable and interfering, but the formulation-related, additives are either "burned" and / or pyrolyzed.The removal of solvents and the burnout may, but need not, be simultaneous, and then the coated substrates usually pass a continuous furnace at temperatures between 800 ° C. and 1000 ° C., in order to shorten the throughput time, the temperatures may be slightly increased compared to the gas phase diffusion in the tube furnace The gas atmosphere prevailing in the continuous furnace may vary according to the requirements of the doping and of dry nitrogen, dry air, a mixture of dry oxygen and dry nitrogen and / or, depending on the design of the furnace to be passed, zones of one and other of the above-mentioned gas atmospheres Other gas mixtures are conceivable, but currently have no industrial g A more important feature of iniine diffusion is that the assignment and the driving in of the dopant can in principle be decoupled from each other. 3) Dopant source removal and optional edge isolation

The wafers present after the doping are coated on both sides with more or less glass on both sides of the surface. More or less in this case refers to modifications that can be applied in the context of the doping process: double-sided diffusion vs. quasi one-sided diffusion mediated by back-to-back arrangement of two wafers in a parking space of the process boats used. The latter

Variant allows a predominantly one-sided doping, but does not completely prevent the diffusion on the back. In both cases, it is currently the state of the art after the Doping existing glasses by means of etching in dilute hydrofluoric acid to remove from the surfaces. For this purpose, 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 Process cycle has expired, which represents a sum parameter of the necessary Ätzdauer and the automatic process automation. The complete removal of the glasses can be determined, for example, by the complete dewetting of the silicon wafer surface by the dilute aqueous hydrofluoric acid solution. The complete removal of a PSG glass is achieved under these process conditions, for example with 2% hydrofluoric acid solution 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. After etching, the wafers are rinsed with water.

On the other hand, 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). In this case, 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%. Optionally, in addition, the pool temperature may be slightly elevated compared to the room temperature (> 25 ° C <50 ° C). In the method outlined last, it has been established to carry out the so-called edge isolation sequentially simultaneously, which results in a slightly modified process flow:

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-scale parasitic p-n transition, which, although due to process technology, is partly, but not completely, removed in the course of the later processing. As a result, the front and back of the solar cell will be short-circuited via a parasitic and residual p-n junction (tunnel junction) that reduces the conversion efficiency of the future solar cell. To remove this transition, 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. Alternatively, the etching solution is transported via rollers mediated on the back of the wafer. At temperatures between 4 ° C. and 8 ° C., the etching removal typically achieved with these methods amounts to approximately 1 μm silicon (including the glass layer present on the surface to be treated). In this method, 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.

In addition, 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

exposed. The plasma is filled with fluorinated gases, for example Tetrafluoromethane, fed. The reactive species occurring in the plasma decay of these gases etch the edges of the wafer. Following the plasma etch, then generally the glass etch

carried out.

4.) Coating the front with an antireflection coating

Subsequent to the etching of the glass and the optional edge insulation, the coating of the front side of the later solar cells takes place with 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. Typical properties of the layers currently used are: a layer thickness of about 80 nm using only the above-mentioned silicon nitride, which has a refractive index of about 2.05. The antireflection reduction is most evident in the wavelength range of the light of 600 nm. The directed and non-directional reflection shows in this case a value of about 1% to 3% of the originally incident light (vertical 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. For this purpose, 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.

5.) Generation of front panel grid

After depositing the antireflection coating is on with

Silicon nitride coated wafer surface defines the front electrode. In industrial practice, the electrode has been established using the screen printing method using metallic

To produce sinter pastes. This is just one of many

different ways to produce the desired metal contacts. In screen printing metallization, a paste heavily enriched with silver particles (silver content> = 80%) 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. Furthermore, 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 Glasfrit basically fulfills two functions: on the one hand, it serves as a On the other hand, it is responsible for the penetration of the Siliziumnitriddeckschicht to allow the direct ohmic contact with 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. In practice, 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. For the sake of completeness, it should be mentioned that 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. Thus, the strength of the silver metallization is increased, which can positively influence the conductivity in the electrode grid. During this drying, the solvents contained in the paste are expelled from the paste. Subsequently, the printed wafer passes through a continuous furnace. Such an oven generally has several heating zones, which can be independently controlled and tempered. When passivating the continuous furnace, the wafers are heated to temperatures up to about 950 ° C. However, the single wafer is typically exposed to this peak temperature for only a few seconds. During the remaining run-up phase, the wafer has temperatures of 600 ° C to 800 ° C. At these temperatures, organic impurities contained in the silver paste, such as binder, are burned out and the etching of the silicon nitride layer is initiated. During the short time interval of the prevailing peak temperatures, contact with silicon occurs. Subsequently, the wafers are allowed to cool. The process of establishing contact thus briefly described is usually simultaneous with the two remaining contact formations (compare 6 and 7). which is why one speaks of a co-firing process in this case.

The front electrode grid consists of thin fingers

(typical number greater than or equal to 68 with an emitter layer resistance> 50 Ω / sqr), which have a width of typically 60 pm to 140 pm, as well as collecting buses with widths in the range of 1, 2 mm to 2.2 mm (depending on their Number, typically two to three). The typical height of the printed silver elements is usually between 10 pm and 25 pm. The aspect ratio is rarely greater than 0.3, but can be significantly increased by the choice of alternative and / or adapted metallization. Among alternative metallization is the

Dispensing of metal paste called. Customized metallization processes are based on two consecutive screen printing processes with optional composition of two distinctive metal pastes

(Dual-print or print-on-print). In particular, in the latter method can be used with so-called floating buses, which ensure the removal of the current of the charge carrier collecting fingers, however, do not contact the silicon crystal itself directly ohm'sch.

6.) Generation of the back buses

The rear bus buses are also usually applied and defined by screen printing. For this purpose, one of the similar silver paste used for front metallization is used. This paste is similar in composition, but contains an alloy of silver and aluminum, in which the proportion of aluminum is typically 2%. In addition, this paste has a lower glass frit content. The bus busses, usually two pieces, are printed with a typical width of 4 mm on the back of the wafer by screen printing be compacted and sintered as already described under point 5.

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 surface of the wafer backside with an edge distance <1mm to define the electrode. The paste is composed of> = 80% aluminum. The remaining components are those already mentioned under point 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. As the wafer cools, it will precipitate on the wafer surface u. a. a eutectic mixture of aluminum and silicon, which solidifies at 577 ° C and has a composition with a mole fraction of 0.12 Si.

As a result of the driving of the aluminum into the silicon, 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 mirror"). 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. This potential wall is generally referred to as the back surface field or "back surface field". The sequence of process steps described in items 5, 6 and 7 may or may not be the same as outlined herein. The person skilled in the art will appreciate that the sequence of the described process steps can, in principle, be carried out in any imaginable combinatorics.

8.) Optional edge isolation If the 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 techniques. For this purpose, a laser beam is directed to the front of the solar cell and the front pn junction is severed by means of the energy injected by this beam. Here, trenches are generated with a depth of up to 15 pm as a result of the action of the laser. In this case, silicon is removed from the treated area via an ablation mechanism or thrown out of the laser ditch. Typically, this laser trench 30 is ^{η μη} to 60 pm wide and about 200 pm from the edge of the solar cell.

After production, the solar cells are characterized and classified according to their individual performance in individual performance categories. The person skilled in the art knows solar cell architectures with both n-type and p-type base material. These solar cell types include

• PERC solar cells

 • PERT solar cells

· PERL solar cells

• MWT solar cells • From this, MWT-PERC, MWT-PERT and MWT-PERL solar cells

• Bifacial solar cells with homogeneous and selective back field

• backside contact cells

 • Rear contact cells with interdigitating contacts

The choice of alternative doping technologies, as an alternative to the gas phase doping already described above, can usually be the

Do not resolve the problem of creating locally differently doped regions on the silicon substrate. As alternative technologies here are the landfill doped glasses, or mentioned by amorphous mixed oxides, by means of PECVD and APCVD method. From these glasses, a thermally induced doping of the silicon located under these glasses can be easily achieved. To create locally differently doped regions, however, these glasses must be etched by means of mask processes in order to extract the corresponding structures from them

prepare. Alternatively, 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, however, 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, it can not compensate for 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 likewise based on a predeposition of a dopant source, which is activated only subsequently for the emission 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 being transformed. Since the conventional dopant sources off

Mixed oxides of the silicon and the dopants to be introduced, ie of boron oxide in the case of boron, are consequently the optical properties of these mixed oxides quite similar to those of the silicon oxide.

Therefore, these glasses (mixed oxides) have a very low

Absorption coefficients for radiation in the relevant

Wavelength range. For this reason, 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)

Second). The silicon should after the absorption of the laser radiation due to the strong dissipation of heat in the remaining, not irradiated

Cool the volume of silicon relatively quickly again and thereby epitaxially solidify on the unmelted material. The overall process, however, is in reality the formation of laser induced radiation

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 areas to be doped, this disadvantage naturally has a lower weight. However, laser doping requires sequential deposition of the aftertreatable glasses. Object of the present invention

It is an object of the present invention to provide a method for producing more efficient solar cells, which improve the current efficiency from the light incident on the solar cells and the charge carriers generated by this in the solar cell. In this context, a cost-effective structuring is desirable, whereby a competitiveness can be achieved with currently technologically predominant doping process.

Brief description of the invention

The present invention is a new method for direct doping of a silicon substrate, by

a) a low-viscosity dopant ink, which is suitable as sol-gel for the formation of oxide layers and at least one doping element selected from the group boron, gallium, silicon, germanium, zinc, tin, phosphorus, titanium, zirconium, yttrium, nickel, cobalt, iron , Cerium, niobium, arsenic and lead, is printed on the substrate surface over the entire surface or selectively and dried,

b) optionally repeating this step with a low viscosity ink of the same or different composition, and

c) if appropriate by temperature treatment at temperatures in the range of 750 to 1100 ° C doping by diffusion is carried out, and

d) a doping of the substrate is carried out by laser irradiation, and

e) if appropriate, an annealing of the laser irradiation induced damage in the substrate by a tube furnace or inlined diffusion step at elevated temperature is performed

and

f) after doping, the ink formed from the applied ink Glass layer is removed again,

wherein the steps b) to e) depending on the desired doping result in different order of magnitude and optionally can be carried out repeatedly. Preferably, 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. In particular, however, the present invention also relates to a method as characterized by claims 2 to 9, which thus form part of the present description.

However, the solar cells and photovoltaic elements produced by these process steps are also the subject matter of the present invention which, due to the process described here, have substantially improved properties, such as better light yield and thus improved efficiency. Detailed description of the invention

As such, increasing the charge carrier generation improves the short-circuit current of the solar cell. Now, the ability of the person skilled in the art to improve the performance over conventional solar cells due to many technological advances while still present, but not extraordinary, since the silicon substrate, even as an indirect semiconductor, is able to absorb the majority of the incident solar radiation. A significant increase in the current efficiency is only possible with, for example solar radiation concentrating solar cell concepts. Another parameter which characterizes the performance of the solar cell is the so-called open terminal voltage or, for simplifying the voltage, which can deliver the cell maximum. The magnitude of this voltage is dependent on several factors, including the maximum achievable short-circuit current density, but also the so-called effective charge carrier life, which in turn is a function of the material quality of the silicon but also one of the electronic passivation of the surfaces of the semiconductor. In particular, the two last-mentioned properties and parameters play an essential role in the design of highly efficient solar cell architectures and are primarily responsible for the possibility of increasing the performance of new solar cell types. Some new solar cell types have already been mentioned at the beginning. If one refers back to the concept of the so-called selective or two-stage emitter (cf., FIG. 1), the principle can be sketched schematically on the basis of the mechanism hiding behind the increase in efficiency as follows with reference to FIG.

Fig. 1: Schematic and simplified, non-scaled representation of the front 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

so-called emitter doping provided. Depending on the base material used, this can be either n-type or p-type (the base is then reversely doped). In contact with the base, 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 same applies to the minorities who are in the

 Emitters are generated and can be removed via the base. In contrast to the minorities in the base, these have a very low effective carrier lifetime in the emitter in the range of up to only a few nanoseconds. That stems from the fact that the

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.

If you want to make this process more efficient, then inevitably the emitter doping and depth must be reduced to generate more minorities with a higher carrier lifetime than the current

transporting majorities can be injected into the base. Conversely, the emitter shields the minorities from the surface. The surfaces of a semiconductor are always very recombination-active. These

Recombination activity can be greatly reduced by the creation and deposition of electronic passivation layers (up to seven orders of magnitude, as measured by the effective Surface recombination rate compared to, for example, an unpassivated surface).

The creation of an emitter with a sufficiently steep doping profile supports the passivation of the surface in one point:

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

Contact resistance silicon - silver from the height of the doping of the

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

Therefore, 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. Wherever the incident sunlight falls directly onto the solar cell, the emitter doping should be very low and relatively shallow (that is, not very deep), so that the incident light can be used

Solar radiation sufficient minorities with a sufficient

Life can be generated, which can be injected via the separation at the pn junction as majorities in the base. Surprisingly, it has now been found through experiments that a solar cell which has two different emitter dopings, namely a region with shallow doping and a region with very deep and very high doping, which has substantially higher efficiencies directly under the metal contacts. This concept is referred to as a selective or two-stage emitter. The corresponding concept is based on the so-called selective backside fields. Consequently, two differently doped regions in the surface of the solar cell, structured dopants, must be achieved.

The experiments have shown that the present problem can be achieved, in particular by achieving these structured dopants. The input method described doping methods are generally based on a two-dimensional deposition and a likewise flat driving the deposited dopant. A selective trigger to reach

different doping levels is usually not provided and even in the absence of further structuring and masking methods not readily available. Accordingly, the present method is a simplified one

Production method in comparison to the previously described two-stage or selective emitter structures. In a more general way, 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 term "strong" may describe the height of the achievable surface concentration, but need not necessarily describe it, which in both cases may be the same for two-stage doped regions The present method thus at the same time claims the cost-effective and simplified production of solar cell structures at least one structural motif having a two-stage doping. These are briefly repeated below:

• PERC solar cells

 • PERT solar cells

 • PERL solar cells

 • MWT solar cells

 • From this, MWT-PERC, MWT-PERT and MWT-PERL solar cells

• Bifacial solar cells with homogeneous and selective back field

• backside contact cells

 • Rear contact cells with interdigitating contacts

The simplified method of preparation in the present case is based on simple and inexpensive printable doping media. The doping media correspond at least to those disclosed in patent applications WO 2012/119685 A1 and WO 2014/101990 A1, but may have different compositions and formulations. The doping media have a viscosity of preferably less than 500 mPa * s, but typically in a range of greater than 1 to 50 mPa * s, measured at a shear rate of 25 1 / s and a temperature of 23 ° C, and are therefore due their viscosity and other formulation properties are excellently 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, but not limiting, pressure settings will be mentioned throughout this description. Subsequently, 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 to be performed Temperschritten (tempering by means of a step function) and / or an annealing ramp, dried and compacted for glazing, thereby forming a grip and abrasion resistant layer with a thickness of up to 500 nm. Following this, 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.

The process sequence will be described exclusively for the possible doping of the silicon substrate with boron as dopant. Analogous but slightly different descriptions are applicable to phosphorus as a dopant.

1. A heat treatment of the printed on the surfaces, compacted and vitrified layers at a temperature in the range between 750 ° C and 1100 ° C, preferably between 850 ° C and 00 ° C, more preferably between 850 ° C and 1000 ° C. , As a result, atoms doped with silicon, such as boron, pass through

 silicothermal reduction of their oxides (insofar as the dopants are present in the form of free and / or bound oxides in the matrix of the dopant source) at the substrate surface to give this, whereby the conductivity of the silicon substrate due to the onset

Doping selectively influenced beneficial. In this case, it is particularly advantageous that due to the heat treatment of the printed substrate, the dopants are transported in depths of up to 1 μm as a function of the treatment duration and electrical sheet resistances of less than 10 Ω / D are achieved. In this case, the surface concentration of the dopant 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 that can be printed

Oxide medium used dopant dependent. In the case of doping with boron forms on the silicon surface, a thin so-called boron skin, which generally as a phase consisting of silicon boride is understood as soon as the solubility limit of boron in silicon is exceeded (this is typically 3-4 ^* 10 ²⁰ atoms / cm ³ ). The emergence of this boron skin depends on the diffusion conditions used, but it can occur in the

Classical gas phase diffusion and doping can not be prevented. However, it has been found that by choosing the formulation of the printable dopant media a significant impact on the formation and the formed thickness of the boron skin can be exercised. By means of suitable laser irradiation, the boron skin present on the silicon substrate can be used as a dopant source for the locally selective further penetration of the dopant boron which deepens the doping profile. For this purpose, however, 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.

 The choice of oxidation conditions affects the obtained

doping:

in the case of low-temperature oxidation, only the boron skin is oxidized at a sufficiently low temperature, and only one of them is found Low surface depletion of the dopant boron, which in principle dissolves better in the resulting during the oxidation of silica, while in the remaining two

 Oxidation steps not only exclusively the boron skin, but also parts of the actually desired, doped silicon, which has a significantly increased oxidation rate due to the high doping

 (Increase in the rate by a factor of 200), and with oxidized and consumed. There may be a significant depletion of the dopant on the surface, which requires a thermal aftertreatment, a distribution or Eintreibeschritt already diffused into the silicon dopant atoms. However, in this case, the

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. An alternative method to

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

Surface depletion of the dopant brings with it.

The sequence described here for the production of locally selectively or two-stage doped regions is characterized by the following at least ten steps:

Printing the Dopant Source -

Compaction ->

 Insertion into doping furnace ->

thermal diffusion and doping of the substrate

Discharge of samples -> Laser irradiation for selective doping from the boron skin

Introduce the samples into the oven ->

oxidative removal of boron skin ->

further drive-in treatment ->

Out of the oven. 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. For this, the layer present on the surface does not necessarily have to be completely compacted and glazed. By a suitable choice of the parameters characterizing the laser radiation treatment, such as pulse length, illuminated area in the radiation focus, repetition rate using pulsed laser radiation, the printed and dried layer of the dopant source may contain the doping dopants contained therein at the surrounding, preferably below the printed layer, Silicon are discharged. By choosing the laser energy coupled onto the surface of the printed substrate, the sheet resistance of the substrate can be specifically influenced and controlled. In this case, higher laser energies produce a lower sheet resistance, which, in simple terms, corresponds to a higher dose of the introduced dopant and a greater depth of the doping profile. If required, the printed layer of the dopant source can then be removed without residue from the surface of the wafer by means of aqueous solutions containing hydrofluoric acid as well as hydrofluoric acid or solutions based on organic solvents, or by using mixtures of both aforementioned etching solutions , The removal of the dopant source can be accelerated and promoted by the action of ultrasound during the use of the etching mixture. Alternatively, the printed dot source may be on the surface of the silicon wafer be left. 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. As a result of the thermally induced diffusion of the dopants from the printed dopant source into the underlying silicon of the wafer, 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. Typically, in such a process, subject to laser beam doped areas and using a dopant ink formulation of the invention, 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. In the aforementioned treatment, the wafers may optionally be pre-dried at temperatures of up to 500 ° C. Directly connected to the diffusion, as described in more detail in paragraph 1 above, is the oxidative removal of the so-called boron skin but also, if appropriate, the redistribution of the boron dissolved in the silicon for adaptation and manipulation of the adjustable doping profile. 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. The previously defined by laser beam treatment areas and the dissolved in these areas 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. Thus, in the areas which were previously exposed to the laser radiation treatment, 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.

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. In this case, 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, it is no longer possible to replenish any dopant to the silicon. 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 Find solar cells application. Strips of one polarity are produced from the dried-on doping ink 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 ink. 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

 Drying ->

 Laser irradiation from the dopant source ~>

 Insertion into doping furnace

 thermal diffusion and (further) doping of the substrate ->

 oxidative removal of the boron skin - further rubbing treatment -

Removing the samples from the oven (see Figure 3).

The two process cascades described above represent possibilities for producing two-stage, or so-called selective, doping. On the basis of the aforementioned embodiments and the associated number of process steps to be carried out, the second embodiment described makes the more attractive and preferable because of the smaller number of process steps Alternative dar.

In both embodiments, the doping effect of the printed-on dopant source can be influenced by the choice of the respective process parameters, in particular the laser beam treatment or laser beam doping. However, the doping effect can also be significantly influenced and controlled by the composition of the printable dopant source (see Fig. 2). If desired, two-step dopants can not be exclusively used only by using a printable one Dopant source followed by another, but they can also be generated by using two printable dopant sources. In particular, by the aforementioned embodiment, 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.

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) on silicon wafers, wherein printable doping inks of different compositions (such as different concentrations of the dopant, for example) can be used. As described, by means of the method according to the invention using the novel, printable doping inks to be further characterized, both simple and cost-effective dopants can easily be produced on silicon wafers, which in sum only comprise a classic high-temperature step (US Pat. thermally induced diffusion) (see Figure 4).

Advantageously, 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. Furthermore, it is possible that both polarities may have two-level doping regions but need not necessarily have both polarities. Likewise, 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 regions provided with opposite doping are none set further limits, except the limits of the respective structure resolution during the printing process as well as those who are immanent the laser beam treatment. The illustrations of 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 inks on silicon wafers.

FIG. 4 shows a schematic and simplified, non-scaled illustration of the inventive doping process induced by laser radiation treatment of printable doping inks on silicon wafers, taking into account the generation of adjacent dopants of different polarities, each of which has two stages (light = weak doping, dark = stronger doping).

FIG. 5 shows a schematic and simplified, non-scaled illustration of the inventive doping process induced by laser radiation treatment of printable doping inks on silicon wafers

Consideration of the generation of adjacent dopants

different polarities, each in two stages (light = weak

Doping, dark = stronger doping) are executed. The printed and dried dopant sources may be in one of the possible

Process variants are sealed with possible cover layers. The cover layers can u. a. Both after the laser beam treatment, and before being applied to the printed and dried-on dopant sources. In the present Figure 5, the cover layer was after the LaserstrabJbändernJung with the printed and dried

Dopant source supplemented by thermal diffusion. The present invention thus comprises an alternative inexpensive and easy to carry out process for the production of solar cells with a more effective charge generation, but also the production of alternative and inexpensive producible, printable Dotierstoff- sources, their deposition on the silicon substrate and its selective one-stage and 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 the printed and dried Dotierstoffquelle and subsequent thermal diffusion can be achieved. The

Laser beam treatment of silicon wafers may be associated with damage to the substrate itself and thus immanent

Disadvantage of this method, insofar as these, sometimes deep penetrating into the silicon damage can not be gegehen by subsequent treatment at least partieff, in the present process may be to the laser beam treatment, a thermal diffusion

which helps to heal the radiation-induced damage. Furthermore, in this type of production of two-stage doped structures, the metal contacts (see Figure 1) are deposited directly on the areas exposed to the laser radiation. As a rule, the silicon-metal interface is characterized by a very high recombination rate

characterized in the order of 2 * 10 ⁷ cm / s), whereby a possible damage in the heavily doped zone of the two-stage doped region due to the overall limitation of the

 Charge carrier life at the metal contact not for the

Performance of the component is significant.

Surprisingly, it has thus been found that the use of printable doping media, as described in the patent applications WO 2012/1 19685 A1 and WO 2014/101990 A1, affords the possibility of silicon substrates being printed by laser beam treatment on a printed and to directly dope dried medium. This doping can be achieved locally and without further activation of the dopants, as is usually achieved by classical thermal diffusion. In a subsequent step, 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. For this purpose, this dopant source can be applied over the entire surface of the wafer or selectively printed. Alternatively, dopant sources may be used

different compositions and different polarities are printed on the wafer in any order. For example, the sources can be printed in two consecutive printed and

 Drying shreds are processed. In the following examples, the preferred embodiments of the present invention are

played. As stated above, the present description enables a person skilled in the art to comprehensively apply the invention. Without further explanation, it is therefore assumed that a person skilled in the art can make the most of the above description. Any ambiguity goes without saying, the quoted

 Publications and patent literature. Accordingly, these documents are considered part of the disclosure of the present application

Description. This applies in particular to the disclosures of

Patent Applications WO 2012/119685 A1 or WO 2014/101990 A1, as the compositions described in these applications are particularly suitable for use in the present invention. For a better understanding and clarification of the invention, examples are given below which are within the scope of the present invention. These examples are also used for

Illustration of possible variants. Due to the general validity of the described principle of the invention, however, the examples are not suitable for reducing the scope of protection of the present application only to these.

Furthermore, it is self-evident to the person skilled in the art that both in the given examples and in the remainder of the description, the amounts of components contained in the compositions amount in the sum to only 100% by weight, molar or vol

Aggregate composition and can not exceed it, even if higher values could result from the indicated percentage ranges. Unless otherwise stated,% data are therefore in the form of% by weight, moi or vo (% by weight).

The temperatures given in the examples and the description as well as in the claims are always in ° C.

Examples:

Example 1 :

A brightly etched 6 "CZ wafer with a resistivity of 2 ohm * cm is coated with a boron doping ink according to one of the patent applications WO 2012/119685 A1 or WO 2014/101990 A1 via spin coating, wherein a layer thickness between 50 nm and 200 nm after whose complete drying is set at 600 ° C. The sample is dried for five minutes at 300 ° C. on a conventional laboratory hotplate and then subjected to a further drying step at 600 ° C. for 20 minutes after introduction into the doping oven, after which the sample is subjected to boron diffusion and heated for 30 minutes in an inert gas atmosphere (nitrogen gas) to a temperature of 930 ° C. To drift deeper boron doping, individual sites of the sample are treated by means of an Nd: YAG nanosecond laser with a pause of 532 nm and different laser fluence (pulse power) After the laser treatment, the glass layer is removed by means of ver The resulting doping profiles are characterized by electrochemical capacitance voltage (ECV) measurement and secondary ion mass spectrometry (SIMS). The doped reference sample has a sheet resistance of 52 ohms / sqr, whereas the laser radiation treated samples after four-tip measurement have a sheet resistance of 28 ohms / sqr, 10 ohms / sqr, and 5 ohms / sqr (in the order of their appearance in Figure 6) ) ·

Figure 6 shows ECV doping profiles before and after the treatment of already thermally diffused and not subsequently oxidized samples. The

Doping has been carried out by means of a boron ink according to the invention. The abbreviation "OV" noted in the legend stands for the punctiform The laser beam scanning the doped wafer, and designates the degree of overlap of the nominally juxtaposed diameter of the laser radiation. The values given behind the degree of overlap correspond to those introduced on the silicon surface

Energy densities. The reference curve corresponds to the doping, which due to the thermal diffusion already before the onset of

Laser radiation treatment was achieved.

FIG. 7 shows SIMS doping profiles before (black) and after treatment already thermally diffused and subsequently unoxidized samples (blue). The doping has been carried out by means of a boron ink according to the invention. The abbreviation "Ox" in the legend stands for the pointwise laser beam scanning the doped wafer, and designates the degree of overlap of the nominally adjacent laser radiation diameters The values given behind the degree of overlap correspond to the respective energy densities applied to the silicon surface the doping, which has been achieved as a result of the thermal diffusion before the onset of laser radiation treatment.

On the basis of the ECV profiles it can be seen that from a laser fluence of 1, 1 J / cm ^2, an increased doping of the substrate of the so-called boron skin occurs. The complementary SIMS profiles show a reduction in the boron surface concentration for the sample aftertreated by the laser radiation. The boron contained in the boron is further driven into the silicon wafer. The depth of the doping profile of the boron increases to ~ 1.5 pm as a result of treatment with the laser radiation of 1 pm. Example 2:

A brightly etched 6 "CZ wafer with a resistivity of 2 ohm * cm is coated with a boron doping ink according to one of the patent applications WO 2012/119685 A1 or WO 2014/101990 A1 via spin coating, wherein a layer thickness between 50 nm and 200 nm after whose complete drying is set at 600 ° C. The sample is dried for five minutes at 300 ° C. on a conventional laboratory hotplate and then subjected to a further drying step at 600 ° C. for 20 minutes after introduction into the doping oven, after which the sample is subjected to boron diffusion and heated for 30 minutes in an inert gas atmosphere, nitrogen, to a temperature of 930 ° C. To remove the boron-rich layer which occurs during boron diffusion, the so-called boron skin, after the diffusion, a moist oxidation in situ at 850 ° C. for 25 For deeper drifting of the boron doping, individual samples of the sample are analyzed by means of e Ines Nd.YAG nanosecond laser with a wavelength of 532 nm and different laser fluence (pulse power) treated. After laser treatment, the glass layer is removed by means of dilute hydrofluoric acid, and the doping profiles resulting in the silicon are characterized by means of electrochemical capacitance voltage measurement (ECV) and secondary ion mass spectrometry (SIMS). The four-tip measurement sheet resistance of the reference sample is 85 ohms / sqr, whereas those of the laser radiation treated samples are sheet resistances of 85 ohms / sqr and 100 ohms / sqr (in order of appearance in Figure 8).

FIG. 8 shows ECV doping profiles before and after the treatment of already thermally diffused and subsequently oxidized samples. The doping has been carried out by means of a boron ink according to the invention. The abbreviation "OV" noted in the legend stands for the punctiform laser beam scanning the doped wafer, and denotes the degree of overlap of the nominally juxtaposed

Diameter of the laser radiation. The values given behind the degree of overlap correspond to the respective energy densities introduced onto the silicon surface. The reference curve corresponds to the doping which has already been achieved as a result of the thermal diffusion before the onset of the laser radiation treatment.

FIG. 9 shows SIMS doping profiles before (black) and after treatment already thermally diffused and subsequently oxidized samples (red & blue) as a function of applied laser irradiation parameters. The doping has been carried out by means of a boron ink according to the invention. The abbreviation "Ox" in the legend stands for the pointwise laser beam scanning the doped wafer, and denotes the degree of overlap of the nominally juxtaposed laser radiation diameters.The values given behind the overlap degree correspond to the respective energy densities introduced onto the silicon surface the doping, which has been achieved as a result of the thermal diffusion before the onset of laser radiation treatment.

On the basis of the measured film resistances no information is given on a significant change of the dopings compared to the reference sample. It can be seen in comparison to the samples with the present on the wafer Borhaut no Nachdotierung. The doping profiles determined by means of ECV and SIMS show a slight reduction in the surface concentration of the dopant as the energy density irradiated by the laser increases, as well as a slight increase in profile depth. The average doses of dopants determined by integration from the SIMS profiles give the following values: 1.2 * 10 ¹⁵ atoms / cm ² for the reference and 0.8 * 10 ¹⁴ atoms / cm ² and 0.9 * 10 ^14, respectively Atoms / cm ² for the aftertreated samples using laser beam treatment.

Example 3:

A brightly etched 6 "CZ wafer with a resistivity of 2 ohm * cm is coated with a boron doping ink according to one of the patent applications WO 2012/119685 A1 or WO 2014/101990 A1 via spin coating, wherein a layer thickness between 50 nm and 200 nm after the complete drying is set at 600 ° C. The sample is dried on a conventional laboratory hotplate for five minutes at 300 ° C. Subsequently, the sample is treated by laser radiation to induce doping, with individual sites of the sample being detected using an Nd: YAG nanosecond laser In order to investigate the pure diffusion by laser treatment, the sheet resistances are determined by four-point measurement, and the doping profiles are checked by means of ECV After laser beam treatment, the sample is subjected to thermal boron diffusion the sample for 30 minutes in an inert gas atmosphere, nitrogen, to a temperature of 930 ° C is heated. To remove the boron-rich layer which occurs during boron diffusion, the so-called boron skin, a dry oxidation is carried out in situ at 930 ° C. for 5 minutes after the diffusion. After thermal diffusion, the glass layer is removed by means of dilute hydrofluoric acid and the resulting doping profiles are characterized by means of electrochemical capacitance voltage measurement (ECV) and four-terminal measurement. The sheet resistances of the doped samples are (in order of appearance in Figure 10 - the sheet resistance of the base doped wafer was 60 ohms / sqr): Processing sheet resistance [ohms / sqr]

 Laser diffusion, field 33 (LD 82

 ink, 33), 66% overlap

 adjacent laser spots,

Energy density: 2.8 J / cm ²

 No laser diffusion & 60

 thermal diffusion, field 11

 (LD & diff ink, 11)

 Laser diffusion & thermal 35

 Diffusion, field 33 (LD & diff ink,

 33), 66% overlap

 adjacent laser spots,

Energy density: 2.8 J / cm ²

 Laser diffusion & thermal 65

 Diffusion, field 38 (LD & diff ink,

 38), 20% overlap

 adjacent laser spots,

Energy density: 1, 53 J / cm ²

Table 1: Compilation of measured film resistances in

Dependence of different process controls: after laser diffusion as well as after laser diffusion and subsequent thermal diffusion.

FIG. 10 shows ECV doping profiles as a function of various

Diffusion conditions: after laser diffusion and after laser diffusion followed by thermal diffusion. Due to the laser irradiation of the printed and dried ink is a doping of the

Silizumwafers been induced, as can be clearly shown by the doping profile in the irradiated field 33 (LD, 33). By subsequent thermal treatment (diffusion) of the samples, the Doping increased and the doping profile deeper into the volume of

Silicon wafers are driven.

On the basis of further sheet resistance measurements, it can be stated that starting from a laser fluence of 1.4 J / cm ^2, doping of the silicon wafer from the printed and dried-on doping ink layer is achieved, which requires no further activation by means of thermal diffusion. A subsequent to the laser irradiation of the samples thermal diffusion causes an increase in the tread depth and a reduction of the

Sheet resistance. Treatment with high laser energy density (> 2 J / cm ² ) results in very deep and heavily doped areas.

Claims

P A T E N T A N S P R E C H E

Process for the direct doping of a silicon substrate, characterized in that

a) a low-viscosity dopant ink, which is suitable as sol-gel for the formation of oxide layers and at least one doping element selected from the group boron, gallium, silicon, germanium, zinc, tin, phosphorus, titanium, zirconium, yttrium, nickel, cobalt, iron , Cerium, niobium, arsenic and lead, is printed on the substrate surface over the entire surface or selectively and dried,

b) optionally repeating this step with a low-viscosity ink of the same or different composition, c) optionally carrying out doping by diffusion at temperatures in the range from 750 to 1100 ° C., by diffusion,

d) a doping of the substrate is carried out by laser irradiation,

and

e) if appropriate, an annealing of the laser irradiation induced damage in the substrate by a tube furnace or inlined diffusion step is carried out at elevated temperature, and

f) after doping, the glass layer formed from the applied ink is removed again,

wherein the steps b) to e) depending on the desired doping result in different order of magnitude and optionally can be carried out repeatedly.

A method according to claim 1, characterized in that a temperature treatment at temperatures in the range of 750 to 1100 ° C for doping by diffusion after a laser irradiation for Doping of the substrate is carried out, wherein at the same time there is a healing of the laser irradiation induced damage in the substrate.

3. The method according to claim 1 or 2, characterized in that a low-viscosity dopant ink, which is suitable as a sol-gel for the formation of oxide layers, is printed, which contains at least one doping element selected from the group boron, phosphorus, antimony, arsenic, gallium ,

4. The method according to one or more of claims 1, 2 or 3, characterized in that the low-viscosity ink by a printing process selected from the group spin coating, dip coating, drop casting, curtain coating, slot-dye coating, screen Printing, Flexo Printing, Gravure Printing, Ink Jet Printing, Aerosol Jet Printing, Offset Printing, Micro Contact Printing, Electrohydrodynamic Dispensing, Roller Coating, Spray Coating, Ultrasonic Spray Coating, Pipe Jetting, Laser Transfer Printing, Päd Printing, Flatbed Screen Printing and Rotary Screenprinting becomes.

5. The method according to one or more of claims 1, 2 or 3, characterized in that the low-viscosity ink is printed by ink jet printing.

6. The method according to one or more of claims 1 to 5, characterized in that a doping takes place directly from the printed and dried glass following a boron diffusion under exclusion of an oxidation process of the "skin".

7. The method according to one or more of claims 1 to 6, characterized in that by at least a two-stage doping with only a thermal diffusion or high-temperature treatment of Substrate structured, highly efficient solar cells are produced, which have regions of different doping.

8. The method according to one or more of claims 1 to 7, characterized in that in step a) by vapor deposition by means of PECVD (plasma-enhanced chemical vapor deposition), APCVD (atmospheric pressure chemical vapor deposition), ALD (atom / c layer deposition ) or cathode sputtering (sputtering) a glass layer on the substrate surface is generated over the entire surface or selectively, the at least one dopant selected from the group boron, gallium, silicon, germanium, zinc, tin, phosphorus, titanium, zirconium, yttrium, nickel, cobalt, iron , Cerium, niobium, arsenic and lead.

9. The method according to one or more of claims 1 to 8, characterized in that after doping the glass layer is removed by means of hydrofluoric acid.

10. Solar cells, produced by a process according to one or more of claims 1-9.

11. Photovoltaic elements, produced by a method according to one or more of claims 1-9.