DE102013108422A1 - Method for producing doped or metallized regions in a solar cell substrate and corresponding solar cell - Google Patents

Abstract

A method is described for producing doped and / or metallized regions on a surface (3) of a semiconductor substrate (1) for a solar cell. A pattern of various regions (5, 7), for example deeper subsections (5) and higher subregions (7), is formed on the surface (3). A flowable substance (11), for example in the form of a screen-printing paste or an inkjet ink, which contains dopants and / or metal particles and also contains a liquefying agent to increase its fluidity, is introduced into the deeper subareas (5). Because of their fluidity, the substance (11), which is initially introduced dropwise, for example, laterally dissipates and preferably fills the underlying regions (5) uniformly. Subsequently, the liquefying agent can be evaporated or converted from the flowable substance (11) and these are thus solidified. Subsequently, metal particles contained in the substance (11) may be fired, for example, to form electrical metal contacts, or dopants contained in the substance (11) may be diffused to form, for example, emitter regions for the solar cell. With the proposed method, a risk of short circuit current paths in solar cells can be reduced and manufacturing tolerances can be selected larger than in conventional production methods for solar cells.

Description

FIELD OF THE INVENTION

The present invention relates to a method for producing doped and / or metallized regions on a surface of a semiconductor substrate for a solar cell. In particular, the present invention relates to a method that can be used in the manufacture of a solar cell to form their doped and / or metallized areas. Furthermore, the present invention relates to a suitably producible solar cell.

TECHNICAL BACKGROUND

Solar cells serve as photovoltaic elements to convert light energy into usable electrical energy. By absorption of radiated light charge carrier pairs are generated within a semiconductor substrate. By providing oppositely doped regions in the semiconductor substrate, electrical potential differences occur at the transition between such regions, for example at a pn junction, with the aid of which the charge carriers of a charge carrier pair can be spatially separated. By means of, for example, formed by a metallization electrical contacts on the surface of the semiconductor substrate, the charge carriers can then be derived from the semiconductor substrate and fed to an external circuit.

In the manufacture of a solar cell, there is a general need to electrically isolate electrical contacts contacting oppositely doped portions of the solar cell semiconductor substrate from each other, for example, to avoid parasitic currents within the fabricated solar cell. Furthermore, an electrical contact which contacts a doped partial region should also be electrically insulated from an oppositely doped partial region of the semiconductor substrate.

In conventional solar cells, in which typically an electrical emitter contact, the emitter-like doped portions of the semiconductor substrate contacted, is provided exclusively on a front side of the semiconductor substrate and an electrical base contact, the opposite base-like doped portions of the semiconductor substrate contacted, exclusively on the back of the Semiconductor substrate is provided, there is usually a risk of formation of parasitic current paths possibly at the edge of the semiconductor substrate.

In so-called back contact solar cells such as IBC (Interdigitated Back Contact) solar cells, in which both types of electrical contacts are arranged on the back of the semiconductor substrate, for example in the form of nested grids with elongated contact fingers, there may be a risk of parasitic current paths however, be much higher. For example, finger-like base contacts and finger-like emitter contacts in such solar cells often extend over long distances parallel to each other. In this case, the geometries of the emitter contact grid and of the base contact grid are usually selected such that the emitter contact fingers are arranged only slightly laterally spaced from the base contact fingers in order in this way to move laterally a distance, the generated charge carrier within the semiconductor substrate, in order to reach one of the contact fingers. to minimize and thus to keep series resistance losses or a so-called electrical shading low. However, the closely spaced array of contact fingers of the emitter contact grid and contact fingers of the base contact grid may present a risk that, for example, due to manufacturing defects or manufacturing inaccuracies, base contact fingers may come into contact with emitter contact fingers, or at least insufficient isolation therebetween, and thus be detrimental parasitic short circuit current paths are generated within the solar cell. A similar effect can be detrimental if regions of an electrical contact of one polarity with no or poor electrical insulation are adjacent to partial regions of the semiconductor substrate of opposite polarity, that is to say with opposite doping.

In order to minimize the risk of the formation of parasitic short-circuit current paths, the geometry or the design of the electrical contacts in the case of back-contact solar cells has hitherto mostly been selected such that a certain safety distance remains between the fingers of the electrical contacts of different polarity. Among other things, the safety distance is often chosen such that, even with maximum permissible manufacturing tolerances, there is no risk of electrical insulation between electrical contacts of opposite polarity or between an electrical contact of one polarity and one doped partial region of the semiconductor substrate of opposite polarity. However, the safety margin should not be too large to avoid excessive series resistance losses for the solar cell due to excessive distances between adjacent contact fingers of a contact grid.

Therefore, it seems in the production of conventional back-contact solar cells previously necessary to be both the differently doped portions of the semiconductor substrate and the electrical contact grids of different polarity high positioning accuracy, ie with low positioning tolerances to be able to manufacture. This can require complicated and thus expensive manufacturing processes and / or tools.

SUMMARY OF THE INVENTION AND SOME EMBODIMENTS

There may therefore be a need for a method for producing doped and / or metallized regions on a surface of a semiconductor substrate for a solar cell, in which, inter alia, the abovementioned shortcomings of conventional methods are at least reduced. In particular, there may be a need for a method by which doped or metallized regions of high local accuracy can be formed on a surface of a semiconductor substrate for a solar cell and thus reduce the risk of short circuit current path formation, preferably manufacturing tolerances in forming structures for formation the doped or metallized areas may be selected to be comparatively high. There may also be a need for a solar cell, as can be made in particular by means of such a method.

Such a need can be met with the subject matters of the independent claims. Advantageous embodiments are given in the dependent claims and in the following description.

According to a first aspect of the present invention, a method for producing doped and / or metallized regions on a surface of a semiconductor substrate for a solar cell is described, wherein the method has the following steps. First, a pattern of different portions of a first and a second type is formed on the surface of the semiconductor substrate. The various partial regions are designed differently in such a way that a first type of partial region can be wetted with a flowable substance without the flowable substance flowing into an adjacent partial region of a second type of partial region. Subsequently, the flowable substance is applied exclusively to the first of the subregions. The flowable substance contains dopants and / or metal particles. The flowable substance also contains a fluidizing agent such as a solvent to increase its fluidity. Thereafter, the flowable substance is solidified by at least partial evaporation or conversion of the liquefying agent.

According to a second aspect of the present invention, there is proposed a solar cell as can be produced by the method according to the above first aspect of the invention. In particular, such a solar cell is proposed which has a semiconductor substrate, differently doped regions on partial areas of a surface of the semiconductor substrate and electrical contacts on partial areas of the surface of the semiconductor substrate. In this case, the semiconductor substrate has a pattern of lower and higher partial regions on the surface of the semiconductor substrate, wherein the lateral dimensions of the deeper partial regions are again substantially larger than a height difference between the deeper partial regions and the higher partial regions.

Ideas for the above-described aspects of the present invention may be considered, inter alia, as being based on the following observations and findings:
In conventionally manufactured solar cells, metallized areas on a surface of a solar cell semiconductor substrate are usually formed on an industrial scale using screen printing technologies. For this purpose, a viscous paste containing inter alia metal particles, printed on the areas to be metallized and then fired the paste in a subsequent high-temperature step. Similarly, doped regions can also be generated on the surface of the semiconductor substrate by applying a paste or liquid mixed with dopants to the substrate surface, for example by screen or inkjet printing, and then the dopants are diffused into the semiconductor substrate in a high-temperature step.

On the one hand, the screen printing pastes used should be sufficiently fluid for them to be printed; on the other hand, the printed structures should retain their shape as well as possible, as is the case during printing. B. are given by a geometry of openings in a sieve, to ensure that an actually formed on the substrate surface pattern of metallized or doped areas largely corresponds to a desired and vorzugebenden by the geometry of the screen pattern.

However, the two contradict each other for the screen printing pastes desired properties and in reality, the paste for printing must be sufficiently liquid, so that the printed structures at least partially dissipate after printing. In this way, for example, on a printed with metal-containing paste portion of the semiconductor substrate surface, which should have an electrical contact with a perfectly perfect quadrangular cross-section and the highest possible height-width ratio (aspect ratio) and for this purpose with a suitably formed screen was printed, by an after-removal of the screen beginning lateral deliquescence ultimately a wider, for example, approximately Gaussian metal contact arise.

This not only causes greater shadowing by the metal contact, but also increases the risk that the metal paste dissolves laterally so strong that it comes to unwanted short circuits with adjacent contacts or doped areas. The width of a screen-printed metallic contact depends not only on the viscosity of the screen printing paste but also on properties of the surface of the printed semiconductor substrate or on its wettability. If the parameters of the paste or those of the surface of the semiconductor substrate are poorly matched with respect to the screen printing paste used, the metal contacts ultimately produced can often have a considerably greater width than the associated openings provided in the screen used to print these contacts.

In order to minimize the effect of lateral deliquescence, a viscosity of screen printing pastes used is therefore usually chosen as high as possible. On the other hand, however, this may cause the cross-sectional area along a finger-like printed structure, such as a metal contact finger, to vary due to the screen printing technique used, for example, due to fine wires of the wire running in the screen used transverse to the longitudinal direction of the metal contact fingers to be formed. For example, over the length of a metal contact varying cross-sectional areas can result in an increased electrical series resistance of the metal contact.

It is now proposed to structure the surface of the locally to be metallized or doped semiconductor substrate specifically with a pattern of different subregions. The various subregions are designed in such a way that a flowable substance such as a thin paste or ink can distribute and spread within a subarea, but can not flow over a boundary between the subregions due to the different properties of the different subregions.

The different partial regions on the surface of the semiconductor substrate can be formed, for example, in the form of deeper and higher partial regions. Lateral dimensions of the deeper subregions can be substantially greater than a height difference between the deeper subregions and the higher subareas. Subsequently, the flowable substance can be introduced into the deeper parts.

Alternatively, the different partial regions on the surface of the semiconductor substrate can also be formed in the form of differently wettable partial surfaces. First partial regions may have a surface which, for example due to pretreatments or temporarily applied layers or masks, has a different wettability for flowable substances to be deposited thereon than second partial regions. A flowable substance may then preferably be applied to a portion of high wettability and spread thereon to the limit of a portion of lower wettability. Differently wettable subareas can be generated, for example, by local deposition of well or poorly wettable layers. In the case of water-based pastes or inks as a flowable substance, a poorly wettable portion z. B. by covering with suitable material or by suitable pretreatment hydrophobic properties, whereas an adjacent well wettable subregion may have hydrophilic properties.

It should also be noted that embodiments of the invention may also provide more than two different types of subregions. For example, two different patterns of deeper wettable portions may, for example, be nested within each other and separated from each other by one or more higher or lower wettable portions. In the different underlying or better wettable parts can then z. B. different flowable substances are introduced to z. B. to be able to form different and separate metal contacts or doped regions of a solar cell.

Subsequently, the various subregions and their effect in forming the metallized or doped regions for the solar cell will be explained using the example of a semiconductor substrate surface with deeper and higher subregions.

At the deeper subareas, the semiconductor substrate, including any additional layers applied thereto, generally has a smaller thickness than at the higher subareas. The pattern of subregions of different heights can, as explained in more detail below, be produced by means of different processing techniques, for example by local etching, local Laser ablation, local laser assisted etching, mechanical ablation or a combination of these measures. A height difference between the lower lying areas and the higher lying areas can be relatively small compared to their lateral dimensions.

A flowable substance, for example, similar to a thinned screen printing paste or an ink, is subsequently introduced into the lower part areas formed in this way, the flowable substance containing dopants and / or metal particles. The flowable substance also contains a liquefying agent such as a solvent, diluent or dispersant to increase its flowability, for example, a proportion of fluidizing agent may be substantially higher than conventional high viscosity screen printing pastes and the introduced substance may therefore flow away very laterally. Because of their good flowability, the introduced substance can therefore preferably be distributed over an entire deeper subarea. The difference in height between the lower and the higher partial areas, which is expressed for example in the form of a step between the lower and higher partial areas, can prevent flowable substance from flowing over the lower partial areas. From the thus introduced and laterally flowable flowable substance liquefying agent is then specifically evaporated, for example by the semiconductor substrate is dried in an oven at elevated temperatures. Alternatively, the liquefying agent may also be converted, e.g. B. if contained therein UV-curing, for example, polymerizing additives. In this case, the substance solidifies and the semiconductor substrate can be further processed, for example, below, without there being any risk of a change in the doping or metallization structure introduced in this way.

Hereinafter, possible features and advantages of embodiments of the invention will be further described in detail.

The semiconductor substrate processed in the context of the method can have any suitable geometry for a solar cell and can consist of any semiconductor material suitable for a solar cell. For example, a silicon wafer can serve as a semiconductor substrate.

The pattern of lower and higher partial areas can be formed on the surface of the semiconductor substrate in different ways. In the finished pattern, both the deeper subsections and the higher subregions can be flat in themselves and be separated from each other by an example steepest possible step.

If the method is used to produce metalized areas serving as electrical contacts, the deeper subareas may be formed, for example, as a finger-shaped grid having a multiplicity of elongate, narrow and preferably mutually parallel depressions. Between two adjacent finger-like deeper subareas is a higher-lying subregion, wherein the deeper subsections may be wider, narrower or the same width as the higher subsections.

Alternatively, the method can also be used to selectively dope large partial areas of the semiconductor substrate, for example to produce emitter areas for the solar cell, and to leave undoped or otherwise doped bars between doped partial areas, which in the example mentioned are exposed to the outside can serve contactable base areas. In such an embodiment, the deeper subregions may cover a major part of the surface of the semiconductor substrate and be separated from each other by narrow higher subregions lying therebetween.

The deeper subregions can be formed for example by local removal of material from the surface of the semiconductor substrate. From a semiconductor substrate having an originally planar or possibly microscopically textured surface, it is possible by means of suitable methods to remove material locally from the underlying subregions to be produced, for example by local etching and / or using laser irradiation.

In order to produce the deeper subareas by etching, the surface of the semiconductor substrate in the other subregions can be provided, for example, with an etch barrier mask. The etch barrier mask covers the later higher subareas. In the subregions not covered by the etching barrier mask, a corrosive medium, for example in the form of a corrosive solution or a corrosive plasma, can then act and etch material out of the semiconductor substrate in order to produce the underlying subregions. The Ätzbarrieremaske can subsequently be removed if necessary again. Alternatively, the deeper subregions can be formed by local coverage with a caustic mass, for example in the form of a corrosive screen printing paste. The Ätzbarrieremaske or the corrosive mass can be applied for example by screen printing or similar technologies.

As an alternative to removing material by etching, the pattern may also be generated on the semiconductor substrate surface using laser irradiation. By way of example, the deeper-lying partial regions can be produced by ablating material from the semiconductor substrate using high-energy laser radiation and possibly subsequently etching away laser damage. Alternatively, it is also possible, for example, to cover the entire surface of the semiconductor substrate with a corrosive substance and then to accelerate an etching process locally by laser irradiation in the deeper subregions to be generated, which is also referred to as laser-assisted etching.

Instead of locally removing material from the semiconductor substrate surface and thus producing the deeper subregions, a mask may alternatively be applied to the surface of the semiconductor substrate covering the surface only in areas intended to form the higher subregions. In other words, for example, the desired pattern can be generated on an originally planar surface of a semiconductor substrate by applying an additional layer as a mask to the surface in certain areas and thus producing the higher-lying partial areas there. In this case, the layer of the mask can be sufficiently thick that at its edge a step is created towards adjacent, deeper-lying regions, which prevents spreading of flowable substance introduced there.

Alternatively, a mask with a very small layer thickness, for example less than 10 μm or even less than 1 μm, can be applied. While such a layer generally does not produce a sufficient level, it may prevent spreading of flowable substance incorporated into the unmasked areas because of different wetting properties of the masking material used therefor.

The mask can be printed, for example. For example, a mask can be applied locally using screen printing technologies and using a suitable screen printing paste. The screen printing paste may contain, for example, a wax, a polymer, or the like.

Alternatively, the mask can also be applied in another way. For example, a layer of a masking material not only locally but initially over the entire surface are applied to the surface of the semiconductor substrate to be examined and then removed locally. For example, the layer may be printed similarly to the preceding example. Alternatively, a layer can also be formed using other deposition methods, for example by blanket deposition by means of chemical vapor deposition (CVD), for example PECVD, LPCVD, APCVD, etc.) or sputtering or by spray-on or spin-on methods (spin-on). on). The layer can be formed, for example, from a dielectric such as silicon nitride, silicon oxide, a wax or a polymer, etc.

The flowable substance used to form the doped or metallized regions may have a fluidity such that it propagates substantially uniformly solely due to surface adhesion forces and / or surface capillary forces within a deeper portion. In other words, in addition to the dopants that may serve to dope the desired regions, or in addition to the metal particles that may serve to form the metallization in the desired regions, the flowable substance may include sufficiently different flowable components, for example, in the form of a fluidizing agent in order to impart to the flowable substance-forming mixture of dopants or metal particles and liquid constituents a sufficiently high flowability, that is to say a low viscosity, so that the flowable substance, when applied locally to an approximately flat surface of a deeper subregion, becomes self-sufficient Distributed over this plane, that is, deliquesces and thereby ultimately covers the entire surface in the region of the lower part with a substantially constant layer thickness. As a liquefying agent z. B. low viscosity liquids such as solvents or water. For example, organic non-polar substances such as alkanes, alkenes, alkynes or benzenes or polar substances such as ketones (eg acetone), alcohols (eg ethanol, (iso) propanol, butanol, etc.) or various acids may be used become.

How easily and uniformly the flowable substance spreads over the substrate surface in a deeper area depends on the one hand on the rheology of the flowable substance and on the other hand on the properties of the surface to be covered. In particular, the wetting properties on this surface, which inter alia depend on the material of the surface to be coated, and the geometric structure, for example a measure of microscopic roughness or texturing of the surface to be coated, can influence the propagation behavior of the flowable substance. Adapted to these influences, the flowable substance is to be sufficiently free-flowing, that is to say fluid, in order to be able to disperse within the deeper subsections without any further forces, such as those otherwise caused, for example, by screen printing by a doctor blade. In particular, the flowable substance may have a substantially lower viscosity than conventional screen-printing pastes, which may be effected by a significantly higher proportion of liquefying agents.

In particular, owing to its good flowability, the flowable substance does not necessarily have to be uniformly applied to the entire surface of the deeper subareas, but it may be sufficient to introduce the flowable substance only in subareas of the deeper subareas. For example, it may be sufficient to apply the flowable substance only approximately in the middle of a deeper subarea and then to distribute it automatically over the surface of the entire deeper subarea. It may also be sufficient to introduce the flowable substance only as a series of dots, for example, by introducing it as an individual droplets in the deeper sub-areas with an inkjet printer, since the dots or droplets can then laterally dissolve and along with adjacent dots / droplets completely cover a deeper subarea.

The property that the flowable substance can distribute itself automatically within the deeper subareas can have an advantageous effect, in particular with regard to manufacturing tolerances to be maintained. For the formation of the deeper subareas other manufacturing technologies can be used as for an introduction of the flowable substance. While, for example, flowable substances are also applied in a conventional solar cell production, for example, in the screen printing of the metal contacts and doing as introductory running a lateral flow of screen printing negative and causes the screen printing paste with high spatial precision, ie low spatial tolerances are applied must, can be used in the method presented here, the fact that the deeper subregions can be relatively easily formed using laser irradiation with high spatial precision and then an introduction of flowable substance in these deeper subareas can be carried out spatially less precise. For the introduction of the flowable substance, screen printing technologies may also be used here, for example, but the flowable substance itself may be modified such that it flows even more laterally than conventional screen printing paste, since lateral flow is even advantageous in the process proposed herein. A spatial positioning accuracy when introducing the flowable substance, that is, for example, when screen printing the flowable substance in the deeper subsections, can remain much lower than in conventional Siebdruckmetallisierungsverfahren. In other words, positioning tolerances when applying the flowable substance may be relatively high, since it is generally sufficient if the flowable substance is introduced in sufficient quantity somewhere in a deeper subregion and then can be distributed in this self-contained.

The flowable substance can be introduced into the deeper subregions, for example by printing. In addition to the screen printing method already described, other printing methods such as, for example, inkjet printing technologies, injection molding technologies, extrusion printing technologies or the like can also be used here.

In order to further process the semiconductor substrate processed as described above and ultimately to produce a solar cell therefrom, after being solidified by at least partially evaporating or converting the fluidizing agent, the flowable substance may be brought to a higher temperature of, for example, more than 400 ° C is needed to effect the desired reaction described below.

If, for example, the substance applied to the deeper subregions contains dopants, for example in the form of dopants mixed in a carrier paste or carrier liquid, such as boron or phosphorus, these dopants can be present as atoms or molecules or even small particles at temperatures greater than 700 ° C., preferably greater 800 ° C, diffuse superficially into the semiconductor substrate and thus form the desired doped region there. This is also known as a drive-in step. In this way, for example, local or selective emitter or base regions can be generated for a solar cell.

Alternatively, with the aid of a substance in which metal particles and optionally other particles, for example glass frit, are mixed into a carrier mass, desired metallized regions for electrical contacts of a solar cell are produced, wherein at temperatures greater than 400 ° C., preferably greater than 600 ° C., a good electrical contact between the metal particles with each other and / or the semiconductor substrate can be effected. This may involve locally removing or converting dielectric or other coatings by constituents of the applied substance, and / or a mono- or multiphase Alloying occurs between semiconductor substrate material and metals contained in the paste, such as a eutectic phase of aluminum and silicon or metal silicides, and / or metal particles in the applied substance combine with one another in such a way that a sufficiently good transverse conductivity arises. This is sometimes referred to as a sintering or fire step.

A solar cell proposed according to the second aspect of the present invention can be produced in particular with the aid of embodiments of the method described above. The described method steps can lead to corresponding structural and functional features for the solar cell.

Like any solar cell, a solar cell produced in this way has at least one semiconductor substrate, differently doped regions, for example in the form of emitter regions and base regions at partial regions of a surface of the semiconductor substrate, and electrical contacts with which these regions are electrically contacted at the partial regions of the surface of the semiconductor substrate.

In contrast to conventional solar cells, however, the semiconductor substrate has a pattern of lower and higher partial areas on its surface, wherein this pattern preferably extends over the entire surface of the semiconductor substrate at least on one side thereof, but a height difference between the lower partial areas and the higher partial areas is relatively small in relation to the lateral dimensions of these subregions. For example, the minimum lateral dimensions of the partial regions, that is, for example, a width of these partial regions, may be in a range of 10 μm to 10 mm, preferably in the range of 100 μm to 3 mm, and thus more than ten times as large, preferably more than be fifty times as large as the difference in height between the various sub-areas, which may for example be in a range of 50 nm to 50 microns, preferably in a range of 100 nm to 10 microns (each of the lower and higher subsections can each be by approximately flat surfaces be formed and adjoin one step to a correspondingly different portion.

When the deeper portions have been formed to form a metallization therein to form electrical contacts, the correspondingly fabricated solar cell may differ from conventional solar cells in that the deeper portions are covered with a metal-containing layer having a substantially constant thickness. In other words, the deeper areas may include a metal-containing layer having an outwardly directed, approximately planar surface, which differs in particular from conventional screen-printed metal contacts in which a screen-printed metal contact by the lateral flow of the screen printing paste after printing usually an approximately gaussian Cross-section and the outwardly directed surface is thus convexly curved. A solar cell produced in this way can also be characterized in that the metal layer serving as an electrical contact extends exclusively within the deeper subregions, but does not protrude beyond an edge which delimits these deeper subareas from the higher subareas. A geometry of the electrical contacts formed by the metal-containing layer can thus be specified very precisely by forming the deeper subregions with the desired geometry.

In an alternative embodiment, in which the deeper subareas are used to deposit there a dopant-added flowable substance, the solar cell thus produced may differ from conventional solar cells in that the semiconductor substrate adjacent to a surface at the higher subregions another doping has as adjacent to a surface at the lower partial areas. The pattern of higher-lying and lower-lying subareas thus substantially corresponds to a pattern of differently doped areas on the surface of the semiconductor substrate, that is, a pattern of, for example, emitter areas and base areas.

It should be noted that possible features and advantages of embodiments of the invention are described herein in part with reference to a method according to the invention and in part with reference to a solar cell according to the invention. A person skilled in the art will recognize that the features described can be suitably combined or exchanged with one another and in particular can also be transferred from the method to the solar cell or vice versa in order to arrive at further embodiments and possibly synergy effects.

A person skilled in the art will also recognize that a method for fabricating a solar cell may contain further method steps in addition to the method steps described here, for example method steps for forming other differently doped regions in the semiconductor substrate, method steps for forming dielectric layers, for example as passivation, antireflection or protective layers, process steps for texturing a surface of the Semiconductor substrate and for cleaning this surface, process steps for metallization and contact formation, etc ..

In addition, a person skilled in the art will recognize that a solar cell according to the invention can have, in addition to the described features, further features and properties, for example further doped regions, metallization structures, dielectric layers, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described and other possible aspects, features and advantages of the present invention will become apparent from the following description of specific embodiments, in which neither the description nor the drawings are to be construed as limiting the invention.

1 FIG. 14 illustrates sequential steps of a method of forming metallized regions on a surface of a semiconductor substrate for a solar cell according to an embodiment of the present invention.

2 illustrates steps of a method of manufacturing a solar cell according to another embodiment of the present invention.

The figures are only schematic and not to scale. Like reference numerals in the figures indicate the same or equivalent features.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to the 1 (a) to 1 (c) describes steps of a method according to the invention for producing metallized regions on a surface of a semiconductor substrate for a solar cell.

First, a semiconductor substrate 1 provided in the form of a silicon wafer ( 1 (a) ). On a surface 3 of the semiconductor substrate 1 , which later serves as the back surface of a solar cell to be produced, then becomes a pattern of deeper subregions 5 and higher parts 7 formed as in cross section in 1 (b) is shown. The deeper parts 5 and higher parts 7 can move along the surface 3 extending in the form of elongated fingers, for example, two nested comb-like grids.

As in the enlarged view 1 (c1) are the deeper and higher subareas 5 . 7 formed such that their lateral dimensions, for example, a width b, are significantly greater than a height difference h between the sub-areas 5 . 7 ,

The deeper parts 5 can in the semiconductor substrate 1 for example using a laser, in particular by laser ablation, and / or by selective etching of the silicon of the semiconductor substrate 1 opposite the surrounding higher-lying sections 7 be lowered.

For example, the shallow trench structure created in this way can be created using an inkjet printer 9 a flowable substance in the trench-like deeper parts 5 be introduced. As in 1 (c) schematically illustrates the inkjet printer 9 this a variety of nozzles 15 through which he targeted a beam 13 of droplets of a flowable substance 11 locally in the deeper areas 5 can secrete. A printhead of the inkjet printer and the substrate can be moved relative to one another in order to be able to introduce flowable substance, for example, along an elongated finger-like deeper area. The flowable substance 11 may be, for example, an inkjet ink, are accommodated in the smallest metal particles in a liquefying agent.

Alternatively, a flowable substance 11 also by, for example, other printing techniques such as, for example, screen printing on the surface of the semiconductor substrate in the region of the deeper subareas 5 be applied. In alternative embodiments, instead of a flowable substance containing metal particles, a flowable substance mixed with dopants can be applied.

As shown in the image sequence 1 (c1) to (c3), the introduced flowable substance 11 immediately after applying a drop-like geometry on first. Due to the deliberately chosen low viscosity of the flowable substance 11 and due to acting surface adhesion forces and surface capillary forces, however, this drop-like shape will flow sideways within a short time ( 1 (c2)) and finally the deeper part 5 as a layer of substantially uniform thickness ( 1 (C3)).

After the metal-containing substance 11 in this way in the deeper parts 5 has been introduced and distributed therein, a liquefying agent contained therein may be at least partially evaporated, for example, by keeping the entire semiconductor substrate in an elevated temperature range for a certain period of time. The substance 11 solidifies such that the thus prepared semiconductor substrate 1 then be processed further. For example, the metal particle-containing solidified substance 11 are then fired in a further high temperature step at about 400 ° C or above 600 ° C and in this way long-term stable metal contacts for one from the semiconductor substrate 1 to be formed solar cell.

With reference to the 2 (a) to (h), a possible manufacturing process for manufacturing wafer-based solar cells using the presented method will be outlined below by way of example.

First, a silicon wafer as a semiconductor substrate 1 provided ( 2 (a) ). The wafer can be textured on its surfaces (not shown for clarity). In the example shown, the semiconductor substrate 1 be an n-type wafer.

Subsequently, on both surfaces of the semiconductor substrate 1 Phosphorus, for example, diffused with a POCl ₃ diffusion and thus a heavily doped n ⁺⁺ zone 21 generated, which can serve as FSF (front surface field) or as area for a base contact. ( 2 B) ).

Subsequently, the surface becomes 3 the future back of the solar cell with a dielectric 23 For example, in the form of a silicon nitride or silicon oxide, wherein the dielectric layer can serve as a temporary or permanent protection of the back surface. The heavily n ⁺⁺ doped area 21 on the front side 4 of the semiconductor substrate 1 can then be unilaterally etched back to attenuate the n ⁺⁺ layer to form, for example, an n ⁺ FSF. Subsequently, the front can 4 with a dielectric layer 25 be coated, for example, can cause a good surface passivation and / or an anti-reflection property ( 2 (c) ).

Below, for example, by means of a laser on the back 3 provided dielectric 23 be opened locally in areas where later the deeper subareas 5 are to be formed and that should be covered in the example described in the manufactured solar cell with a p ⁺ -Emitter ( 2 (d) , for simplicity, only a back part of the semiconductor substrate 1 shown).

One in the silicon of the semiconductor substrate 1 resulting laser damage can then together with the remaining n ⁺⁺ layer 21 etched, for example in a liquid etching solution or a corrosive plasma. The locally opened dielectric layer 23 can act as an etching barrier for the areas of the back covered by it 3 serve. By etching into the non-dielectric layer 23 Protected areas can there a sufficiently deep trench structure to form the deeper subareas 5 be generated ( 2 (e) ).

Hereinafter, in the underlying subregions formed in this way 5 For example, by inkjet printing or screen printing mixed with dopants flowable substance 11 be introduced ( 2 (f) ; as an enlargement from the in 2 (e) shown area shown). In the example presented, such a substance can be a boron-containing liquid which can serve as p ⁺ doping source in a subsequent high-temperature step.

The boron-containing liquid can be found within the entire deeper subregion 5 evenly distributed and can then be dried and solidified by evaporation of the liquefying agent contained therein.

Subsequently, the semiconductor substrate 1 together with the substance thus applied as a pattern serving as a doping source 11 be heated in a high-temperature step, for example, above 900 ° C, so that boron from the boron-containing layer of the substance 11 in adjacent areas of the semiconductor substrate 1 can diffuse and there is a p ^± emitter region 27 can form ( 2 (g) ). The dielectric layer 23 can serve as a diffusion barrier in order to protect the n ^{+ +} areas covered by it from an inward diffusion of boron, especially from the gas phase.

After removal of any residues, the back of the semiconductor substrate 1 optionally coated with a dielectric to the generated exposed p ⁺ emitter 27 to passivate (in 2 not shown for clarity). Subsequently, electrical contacts 29 . 31 for example, by local metallization of the emitter regions 27 as well as the base areas 21 at the back 3 of the semiconductor substrate 1 be formed for example by local screen printing. If necessary, a metal-containing screen printing paste can be passed through a dielectric layer 23 be fired through 2 (h) ). Alternatively, for example by means of a short-pulse, short-wave laser, the dielectric layer 23 be removed at desired locations and metal contacts, for example, galvanically or de-energized plated.

A solar cell produced in this way thus has higher lying partial areas 7 and deeper parts 5 on, with a surface adjacent to the higher-lying sections 7 a base-like doped layer 21 and thus contains a different doping than a surface adjacent to the underlying areas 5 in which an emitter-like layer 27 is trained.

Finally, it should be pointed out that the embodiments of the method according to the invention described with reference to the figures are merely exemplary and that individual process steps can also be supplemented or replaced, for example, by similarly acting process steps.

For example, as an alternative to using a boron-containing liquid as a flowable substance 11 which is to serve as a doping source, also a flowable substance mixed with aluminum particles, for example in the form of an aluminum-containing paste, are processed, which can then be alloyed in a subsequent high-temperature step and locally form p ⁺ zones. Aluminum particles can be alloyed in at relatively low temperatures of little more than 570 ° C. and thereby form both a p-doped region in the semiconductor substrate and a metal layer serving as an electrical contact.

In addition, the lower and higher subareas 5 . 7 on the semiconductor substrate 1 are formed not only by local removal of material from the semiconductor substrate, but alternatively also by local deposition of additional material, for example in the form of a on the back surface 3 applied mask. For example, subregions of the back 3 be coated with a dielectric layer or deposited such a dielectric layer over the entire surface and subsequently removed in some areas again. Alternatively, for example, a mask may be printed using wax-containing pastes, polymer-containing pastes or the like.

It should also be noted that in the figures the recessed areas 5 and the flowable substance introduced therein 11 Although shown so that the uniformly thick layer formed by the flowable substance, the recessed portions 5 does not completely fill, that is, not up to an edge to the higher subareas 7 enough. However, this is not absolutely necessary. It could also be more flowable substance 11 be introduced into the deeper areas, so that these over the edge to adjacent higher-lying areas 7 surmounted, assuming that the flowable substance 11 for example due to surface tension effects and / or a different wettability in the deeper subareas 5 compared to the higher subareas 7 not in the higher parts 7 will flow into it.

Finally, it should be noted that the terms "comprise", "exhibit" etc. are not intended to exclude the presence of additional elements. The term "a" does not exclude the presence of a plurality of elements or objects. Furthermore, in addition to the process steps mentioned in the claims further process steps may be necessary or advantageous to z. B. to finally finish a solar cell. The reference signs in the claims are only for better readability and are not intended to limit the scope of the claims in any way.

LIST OF REFERENCE NUMBERS

1

Semiconductor substrate

3

back surface

4

front surface

5

deeper parts

7

higher subareas

9

Inkjet Printers

11

flowable substance

13

Printer jet or drops

15

jet

21

n ⁺⁺ layer

23

protective dielectric layer

25

front dielectric layer

27

p ⁺ emitter layer

29

emitter contact

31

base contact

Claims (16)

Method for producing doped and / or metallized regions on a surface ( 3 ) of a semiconductor substrate ( 1 ) for a solar cell, the method comprising: forming a pattern from different subregions of a first ( 5 ) and a second type ( 7 ) on the surface of the semiconductor substrate, wherein the different subregions ( 5 . 7 ) are formed so differently that a first type of subregion ( 5 ) with a flowable substance ( 11 ) can be wetted without the flowable substance in an adjacent subregion ( 7 ) of a second type of sub-region flows; Application of the flowable substance ( 11 ) exclusively to one of the subsections ( 5 . 7 ), wherein the flowable substance contains dopants and / or metal particles, and wherein the flowable substance contains a liquefying agent for increasing a flowability; At least partial evaporation or conversion of the liquefying agent of the flowable substance ( 11 ) for solidifying the substance. Method according to claim 1, wherein the subareas ( 5 ) of the first type as lower lying ( 5 ) and the subareas ( 7 ) of the second type as higher lying subregions ( 7 ) are formed on the surface of the semiconductor substrate, wherein lateral dimensions (b) of the deeper subregions ( 5 ) are significantly larger than a height difference (h) between the deeper subregions ( 5 ) and the higher subareas ( 7 ). The method of claim 2, wherein for forming the deeper portions of locally material is removed from the surface of the semiconductor substrate. The method of claim 3, wherein the material is ablated locally by etching. The method of claim 3, wherein the material is ablated locally using laser irradiation. Method according to one of claims 2 to 5, wherein for forming the higher-lying portions of a mask is applied to the surface of the semiconductor substrate, which covers the surface only in the higher part areas forming areas. The method of claim 6, wherein the mask is printed. Method according to one of claims 1 to 7, wherein the flowable substance has a flowability such that it propagates substantially uniformly due to surface adhesion forces and / or surface capillary forces within a deeper subarea alone. Method according to one of claims 1 to 8, wherein the subregions ( 5 ) of the first type having a different wettability than the subregions ( 7 ) of the second type. Method according to one of claims 1 to 9, wherein the flowable substance is introduced only in partial areas of the subregions of the first type. Method according to one of claims 1 to 10, wherein the flowable substance is introduced into the subregions of the first type by printing. The method of any one of claims 1 to 9, further comprising: Heating the substance solidified by evaporation or conversion of liquefier to a temperature higher than 400 ° C. Solar cell, produced by a method according to one of claims 1 to 12. A solar cell, in particular according to claim 13, comprising: a semiconductor substrate ( 1 ), differently endowed regions ( 21 . 27 ) to subareas ( 5 . 7 ) of a surface ( 3 ) of the semiconductor substrate; and electrical contacts ( 29 . 31 ) at portions of the surface of the semiconductor substrate; wherein the semiconductor substrate has a pattern of underlying ( 5 ) and higher parts ( 7 ) on the surface of the semiconductor substrate, wherein lateral dimensions (b) of the deeper subregions are substantially larger than a height difference (h) between the deeper subregions and the higher subregions. Solar cell according to claim 14, wherein the deeper subsections with a metal-containing layer ( 11 ) are covered with spatially substantially constant thickness. A solar cell according to claim 14 or 15, wherein the semiconductor substrate is in a region ( 21 ) has a different doping adjacent to a surface at the higher subregions than in a region ( 27 ) adjacent to a surface at the lower parts.

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