NL2003324C2

NL2003324C2 - Photovoltaic cell with a selective emitter and method for making the same.

Info

Publication number: NL2003324C2
Application number: NL2003324A
Authority: NL
Inventors: Woutherus Johannes Maria Brok; Emerentius Maria Josephus Antonius Dijk; Franciscus Cornelius Dings; Wouterus Johannes Paulus Carolus Vugt
Original assignee: Otb Solar Bv
Priority date: 2009-07-31
Filing date: 2009-07-31
Publication date: 2011-02-02
Also published as: EP2460188A1; TW201110404A; WO2011014068A1; CN102549776A

Description

Title: Photovoltaic cell with a selective emitter and method for making the same

Field of the Invention 5 The present invention relates to a photovoltaic cell, also referred to as a solar cell, having a selective emitter, and to a method for manufacturing such a photovoltaic cell employing only one high-temperature diffusion step.

Background 10 To improve the efficiency of a photovoltaic cell, it may be fitted with a selective instead of a homogeneous emitter. A selective emitter comprises two kinds of doped regions of the same conductivity (i.e. n-type or p-type) that differ mutually in doping level. One kind is heavily doped, whereas the other is doped only lightly. The heavily doped regions are disposed below a 15 metallization pattern of electrodes provided on a major, light receiving surface of the cell, and enable the electrodes to form ohmic contacts to the underlying substrate. The lightly doped regions, on the other hand, extend between the electrodes and promote light collection and conversion. The selective emitter thus provides a solution to conflicting electrical and optical 20 requirements on the emitter of a photovoltaic cell. In general, the use of a selective emitter may improve the efficiency of a photovoltaic cell by approximately 0.5%-l%. Unfortunately however, the formation of differently doped regions implies additional process steps, e.g. more relatively lengthy high-temperature diffusion processes, that increase the cost of production. As 25 this is clearly undesirable for a product that is to be mass-produced, several methods have been suggested to decrease the manufacturing costs, thereby focusing in particular on the creation of a selective emitter in one high temperature diffusion step.

United States patent 6,825,104, for example, describes a method of 30 manufacturing a photovoltaic cell having two or more selectively diffused 2 regions in a single diffusion step. The disclosed method includes the steps of (i) selectively applying a pattern of a solids-based dopant source to a first major surface of a semiconductor substrate, in particular by means of screen printing, and (ii) diffusing the dopant atoms from the solids-based dopant 5 source into the substrate by a controlled heat treatment step in a gaseous environment surrounding the substrate. During the heat treatment step, the dopant from the solids-based dopant source diffuses directly into the substrate to form first diffusion regions while, at the same time, the dopant from the solids-based dopant source diffuses indirectly via the gaseous 10 environment into said substrate to form second diffusion regions in at least some areas of said substrate not covered by the aforementioned pattern. A drawback associated with the process described in US’104 is that most currently available screen-printable solids-based pastes suitable for creating a selective emitter are relatively expensive, either due to proprietary 15 formulas or to relatively expensive chemical constituents.

It is therefore an object of the present invention to provide for a more economical method of manufacturing a photovoltaic cell having a selective emitter in a single high-temperature diffusion step.

20 Summary of the Invention

According to one aspect of the invention a method of manufacturing a semiconductor device, in particular a photovoltaic cell, is provided. The device comprises a substantially flat semiconductor substrate. The method includes the following three process steps. Step 1: applying a dopant source 25 by selectively inkjetting a first pattern of a phosphoric acid or boric acid solution onto a main surface of said semiconductor substrate. Step 2: heating the substrate so as to diffuse phosphorus or boron atoms from said dopant source into said substrate, thereby forming first diffusion regions immediately beneath the first pattern. And step 3: forming a metal contact 30 pattern substantially in alignment with said first diffusion regions.

3

The method according to the present invention employs common and inexpensive acid solutions for creating the selective emitter. This is a phosphoric acid solution for an n-type emitter (typically in combination with a p-type substrate), and a boric acid solution for a p-type emitter (typically in 5 combination with an n-type substrate). The term ‘phosphoric acid’ may be construed broadly, and includes both orthophosphoric acid (H3PO4) and polyphosphoric acids (such as diphosphoric acid (H4P2O7)) insofar as these acids may serve as phosphorus dopant source. Likewise, the term ‘boric acid’ is intended to include both orthoboric acid (H3BO3), metaboric acid and 10 polyboric acids, insofar as these acids may serve as boron dopant source. Both types of acid, in particular the ortho-variants, are amply available and inexpensive, and therefore very suitable for the mass production of photovoltaic cells. A respective acid is selectively applied in solution to a substrate by means of inkjetting, a technique that allows for the production of 15 fine structures having accuracies on the order of tens of micrometers. The use of phosphoric or boric acid in combination with the method of inkjetting this source material to a substrate enables the highly economical and reasonably accurate formation of a selective emitter in a solar cell. It is noted that both aspects provide an improvement over US’104: the phosphoric acid solution is 20 less expensive than the phosphorus containing paste proposed by US’104, while inkjetting in itself is a more economical method than screen printing as it reduces the amount of waste material. Furthermore inkjet printing is a contactless method that significantly diminishes the risk of substrate fracture. Although US’104 seems to hint that the solids-based paste might be applied 25 to the substrate by means of inkjetting, it is unlikely that inkjetting a viscous paste, such as the exemplary P101 from Soltech NV, Belgium, is practically viable. - The three process steps of the method according to the present invention will now be elucidated in turn.

Step 1 is concerned with the selective application of a phosphoric or 30 boric acid solution to a major surface of the preferably silicon substrate in a 4 first pattern. This first pattern corresponds to the metal contact pattern of electrodes that is applied to the substrate in step 3. At room temperature (about 25°C), both phosphoric acid and boric acid are solids. To obtain a solution that can be inkjetted onto the substrate the substances may be 5 dissolved in a carrier liquid, such as, for example, water (H2O) or ethanol (C2H6O), both of which are inexpensive solvents. A mixture of water and ethanol may also be used. The solution may further comprise surfactants in the form of, inter alia, diethylene glycol, glycerin, isopropyl alcohol (IPA), and polyethylene glycol (PEG). Compared to plain water, ethanol has a lower 10 surface tension and a lower vapor pressure, which gives it better wetting- behavior and inkjetability, and a generally improved uniformity of deposited dopant material. In addition, due to its lower boiling point (78 °C vs. 100 °C), ethanol dries up sooner during heating. Some of these effects may also be achieved using an aqueous solvent provided with the mentioned surfactants.

15 An inkjetable solution may preferably have a viscosity in the range of 0.1 — 1 cPa-s (centi-Pascal second; said range corresponding to 1-10 cP (centi-Poise)).

Step 2 entails the single high-temperature diffusion step of the manufacturing process. The substrate is heated to a temperature of several hundreds of degrees Celcius, preferably to a temperature in the range of 800 -20 1100 °C so as to enable swift diffusion. The substrate is preferably kept at its final temperature for about 4 - 120 minutes, depending, inter alia, on the diffusion rate of the employed dopant source (boron generally has a lower diffusion rate than phosphorus, so that achieving a desirable boron doping level may take longer). The chemistry involved in the diffusion process is 25 known, and mentioned here only by way of example. In case orthophosphoric acid (H3PO4) was applied to a silicon substrate during step 1, the gradual heating of the substrate causes at least part of the phosphoric acid (H3PO4) deposited thereon to decompose into phosphorus penta oxide (P2O5) and water (H2O). The phosphorus penta oxide subsequently reacts with the silicon 30 substrate to form silicon dioxide (SiCb) and metallic phosphorus (P), whereby 5 the latter in turn diffuses into the substrate. A similar process occurs when orthoboric acid (H3BO3) is used: the acid decomposes into boron trioxide (B2O3) and water (H2O), whereby the former will subsequently react with the silicon substrate to form a mixture of silica (SiCh) and boron (B) atoms, which latter 5 atoms may diffuse into the substrate. It is understood that the phosphorus or boron atoms diffuse into the substrate in agreement with the first pattern in which the phosphoric or boric acid solution was inkjetted onto the substrate, thereby forming first, highly doped regions immediately beneath said pattern. Regions of the substrate whose surface area was not treated with phosphoric 10 or boric acid solution during step 1 do not undergo doping. There are several ways in which lightly doped regions may be created in between the highly doped regions, and these will be elaborated upon below.

Step 3 involves the formation of a metal contact pattern substantially in alignment with said first, highly doped diffusion regions.

15 During this step a pattern of electrodes is applied to the main surface of the substrate, substantially overlaying the first, highly doped diffusion regions. Once the electrode pattern is applied, contacts may be fired to create ohmic connections between the metallization pattern and the first, highly doped regions in the underlying substrate. The metal contact pattern typically takes 20 the form of elongate bus bars and fingers, though other shapes are possible. See for example the SunWeb solar cell-design by Solland Solar Cells B.V.,

The Netherlands, which features a both functional and decorative flowerish metal contact pattern.

According to an embodiment of the method according to the present 25 invention, step 1 further comprises heating said main surface of the substrate at a temperature substantially equal to or greater than the boiling point of a solvent of the phosphoric acid or boric acid solution while inkjetting said solution onto the substrate.

Heating the main surface of the substrate that is being inkjetted on 30 at a temperature substantially equal to or somewhat above the boiling point 6 of a solvent of the acid solution effects the evaporation of the solvent. The solvent, used as a carrier to enable the inkjetting of the phosphoric or boric acid in the desired concentration, effectively becomes superfluous once the respective acid is deposited and may preferably be removed quickly thereafter 5 to prevent it from spreading out the acid over the substrate’s surface beyond the targeted first pattern. In case water is used as a solvent (and inkjetting takes place at atmospheric pressure), the substrate’s main surface is preferably heated at about 90-110 °C, whereas a temperature of about 75-90 °C may be used when ethanol is the solvent.

10 According to one embodiment of the present invention, a print head used to jet the phosphoric or boric acid solution onto the substrate is, at least in part, made of a material that is chemically inert or resistant to the acid solution, such as PEEK. Suitable print heads include the PL128L print head marketed by PixDro B.V., Eindhoven (The Netherlands).

15 Thus far the formation of first, highly doped regions in the substrate was described. Attention is now invited to ways in which second, lightly doped regions may be formed in between the first, highly doped regions so as to complete the selective emitter.

According to a first manner, step 1 further comprises selectively 20 inkjetting a second pattern of said phosphoric acid or boric solution onto said main surface of said semiconductor substrate, said second pattern having a lower concentration of acid solution per unit of substrate area than said first pattern. Step 2 further comprises forming second diffusion regions immediately beneath the second pattern.

25 This first manner thus involves applying both a first and a second pattern of the phosphoric or boric acid solution by means of inkjetting, yet at different surface concentrations for the respective patterns. The first pattern, from which the highly doped regions associated with the electrodes are to be formed, is provided with a higher surface concentration of acid solution than 30 the second pattern. Such a differentiation in surface concentration may be 7 advantageously achieved in a single inkjetting pass using a single phosphoric or boric acid solution and a single print head capable of jetting the solution at different resolutions. The phosphoric or boric acid solution may, for example, be selectively inkjetted onto the main surface of the semiconductor substrate 5 using a print head comprising a plurality of inkjet nozzles, whereby each of the nozzles can be activated independently of the other nozzles to produce a droplet of said acid solution. During application of the dopant source, the nozzles of the print head producing droplets for the first pattern may then be activated at a greater frequency than the nozzles producing droplets for the 10 second pattern. This approach to selectively applying a dopant source in different concentrations provides an advantageous, simplified alternative to those based on the use of multiple print heads and/or multiple dopant source solutions of mutually different concentrations. The single phosphoric or boric acid solution, for example, may have a concentration of 0-20% of phosphoric 15 or boric acid in ethanol, say 5%. This solution may be inkjetted onto the substrate in droplets of several picoliters, e.g. 10-20 pi, at resolutions of about 800-1200 and 400-800 droplets per inch (dpi) for the first and second pattern respectively, so as to obtain patterns having a different surface concentration of phosphoric or boric acid. As one skilled in the art will appreciate, these 20 numbers are merely ball park figures that do not only exhibit a mutual dependency, but also depend on other factors such as the substrate temperature during inkjetting (which affects running of the deposited solution), the temperature at which the diffusion step is performed, the desired volume concentrations of dopant atoms in the substrate, etc. In case 25 both the first and the second pattern of acid solution are applied through inkjetting, the following high-temperature diffusion step may preferably be performed in a belt diffusion furnace, thereby enabling a continuous throughput that is desirable for the large-scale production of solar cells.

According to a second manner, during step 2, the main surface of 30 the substrate is subjected to a gaseous dopant atmosphere comprising 8 phosphorus or a phosphorus compound (e.g. POCI3), respectively boron or a boron compound (e.g. H2B), such that, while forming said first diffusion regions, second diffusion regions are formed through diffusion of phosphorus or boron from said gaseous atmosphere into said substrate via one or more 5 areas of said main surface not covered by the first pattern.

It is understood that diffusion of dopant material from the gaseous atmosphere need not necessarily take place directly, but may, for example, include the formation of intermediate compounds on the surface of the substrate, from which the actual diffusion takes place. Gaseous dopant 10 atmospheres are preferably used in combination with batch furnaces, such as, for example, a POCh-closed tube diffusion furnace in the case of phosphorus doping. If desired, a phosphorus or boron comprising atmosphere may be effected by spraying/atomizing phosphoric acid or boric solution into the heated atmosphere to which the substrate is subjected. The concentration of 15 dopant source material in the atmosphere may be determined independently of the concentration of the acid solution that is inkjetted onto the substrate, allowing the final volume concentrations of phosphorus or boron in the first and second diffusion regions to be set independently.

The invention further provides a method of manufacturing a 20 semiconductor device including a substantially flat semiconductor substrate, said method comprising the following steps. Step 1: applying a dopant source by selectively inkjetting a first pattern and a second pattern of a same dopant source solution onto a main surface of said semiconductor substrate, said second pattern having a lower dopant source concentration per unit of 25 substrate area than said first pattern. Step 2: heating the substrate so as to diffuse dopant atoms from said dopant source into said substrate, thereby forming first diffusion regions immediately beneath the first pattern and second diffusion regions immediately beneath the second pattern; and step 3: forming a metal contact pattern substantially in alignment with said first 30 diffusion regions. The dopant source may be selectively inkjetted onto the 9 main surface of the semiconductor substrate using a print head, said print head comprising a plurality of inkjet nozzles, each of which nozzles can be activated independently of the other nozzles to produce a droplet of dopant source solution. During the application of dopant source, the nozzles of the 5 print head producing droplets for the first pattern are activated at a greater frequency than the nozzles producing droplets for the second pattern. This method of creating a selective emitter, which in itself is not limited to certain dopant source solutions/inks such as phosphoric acid and boric acid solutions, provides an advantageous alternative to methods that are based on the use of 10 multiple print heads and/or multiple dopant source solutions of mutually different concentrations.

These and other features and advantages of the invention will be more fully understood from the following detailed description of certain embodiments of the invention, taken together with the accompanying 15 drawings, which are meant to illustrate and not to limit the invention.

Brief Description of the Drawings

Fig. 1 schematically shows a top view of an exemplary photovoltaic cell having a selective emitter; 20 Fig. 2 schematically shows a cross-sectional side view of the photovoltaic cell shown in Fig. 1; and

Fig. 3 schematically shows a perspective view of a production line for carrying out the method according to the present invention.

25 Detailed Description

The general construction of an exemplary photovoltaic cell 1 having a selective emitter will be discussed briefly with reference to Figs. 1 and 2, wherein Fig. 1 schematically shows a top view of the photovoltaic cell and Fig. 2 shows the same photovoltaic cell in a cross-sectional side view.

10

The exemplary photovoltaic cell 1 is of a conventional design and based on a semiconductor substrate 2, more particularly a silicon wafer of p-type conductivity. At its front or top side 2a, i.e. the side that is exposed to (sun)light during use, the substrate 2 is successively provided with a selective 5 emitter 3, an anti-reflection coating 8, and a metallization pattern 10. The selective emitter 3 includes both relatively deep, heavily doped regions 4 and relatively shallow, lightly doped regions 6 of n-type conductivity. In accordance with the present invention, the regions 4 and 6 are doped with phosphorus. Volume concentrations of phosphorus atoms in the highly doped 10 regions 4 may typically be on the order of 1020-1023 atoms/cm3, whereas those for the lightly doped regions may be on the order of 1018-1021 atoms/cm3. The ratio of the doping levels in the regions 4 and 6 is generally at least 2. On top of the selective emitter 3 lies a preferably passivating anti-reflection coating 8, which itself is partially topped with the metallization pattern 10. The 15 metallization pattern 10 includes relatively wide bus bars 11 and relatively narrow fingers 12, and extends across the top side 2a of the substrate 2, overlaying the heavily doped regions 4. The bus bars 11 and fingers 12 preferably include ohmic contacts 13 to these heavily doped regions 4. At its back or bottom side 2b, the substrate 2 is provided with a back side 20 metallization pattern 14, which too may preferably makes good ohmic contact with the substrate. The latter pattern 14 is typically made of aluminium and substantially uniform across the back side 2b of the substrate 2.

As one skilled in the art will appreciate, the photovoltaic cell 1 of Figs. 1 and 2 is exemplary only. Some cell designs may include features not 25 discussed above, such as, for example, separate passivating layers or back surface fields adjacent the back side metal contact 14 (e.g. a layer of p+-type conductivity added to reduce local electron-hole recombination in order to increase the cell’s 1 efficiency), while others may not possess one or more of the described features, such as, for example, the anti-reflection coating 8. It is 30 therefore explicitly noted that insofar as the present invention is concerned, 11 all photovoltaic cell designs incorporating a selective emitter 3 that is based on phosphorus or boron doping are intended to fall within the scope of the appended claims. In fact, the method according to the present invention is not limited to the manufacture of photovoltaic cells; it is contemplated that it can 5 be employed in other micro-electronic production processes as well.

Having discussed the general structure of a photovoltaic cell 1 with a selective emitter 3, the method according to the invention will now be elucidated further in relation to Fig. 3, which schematically shows an exemplary production line 30 for carrying out this method to produce the 10 solar cell depicted in Figs. 1 and 2.

The production line 30 comprises several serially linked processing stations 34-46. A silicon substrate 2 is supplied to the production line 30 at a wafer entry point 32. From there it is conveyed to a station 34 for saw damage removal and texturization. The texturization of a substrate’s surface, 15 which serves to increase absorption of incident solar radiation, may be performed in any suitable manner, such as, for example, through lasertexturing or chemical etching. The substrate 2 is then transported on to inkjet station 36, where step 1 of the method according to the invention is performed. The inkjet station 36 applies a dopant source to the front main 20 surface 2a of the substrate 2 by selectively inkjetting a first pattern, corresponding to the metal contact pattern 10 of the front surface electrodes 11, 12 to be applied at a later stage, of an phosphoric acid or boric acid solution onto this surface. During the inkjetting step, the substrate 2 may move relative to a print head, or vice versa. In an advantageous embodiment, 25 the dimensions of the print head substantially correspond to those of one or more substrates, such that the dopant source can be applied to the substrate in one pass. Subsequently, the substrate 2 is advanced through a high-temperature diffusion belt furnace 38, wherein the substrate is heated to a temperature in the range of 800-1100 °C. Doing so executes step 2 of the 30 method according to the present invention. The heating of the substrate 2 12 causes phosphorus atoms to diffuse from the applied dopant source into the substrate, thereby forming the first, heavily doped diffusion regions 4 immediately beneath the first pattern 10. The second, lightly doped diffusion regions 6 are also formed in the belt furnace 38. The phosphorus dopant 5 source may either have been applied by the inkjet station 36, as discussed above, or be added through spraying a phosphoric acid solution onto the substrate inside the belt furnace 38. After the high-temperature diffusion stage, any remaining phosphosilicate glass (PSG, SiC>2 comprising P2O5) on the substrate’s surface may be removed using a chemical etching solution, e.g. 10 a hydrofluoric acid (HF) solution, at station 40. Subsequently, station 42 may apply a passivating anti-reflection coating, preferably a hydrogenated silicon nitride (SiNx:H) layer which may be applied by means of plasma enhanced chemical vapor deposition (PECVD) techniques. Alternatively, other antireflection coatings, e.g. made ofTiC>2, maybe applied, or an anti-reflection 15 coating may be omitted altogether. Following the deposition of the antireflection coating, the metallization pattern 10, typically of silver, may be applied to the front side 2a of substrate 2, while the metallization pattern 14, typically of aluminium, may be applied to the back side 2b of the substrate at station 44. The metallization patterns 10, 14 are preferably applied through 20 screen printing and subsequent drying, though other thick film deposition techniques may be used as well. During screen printing of metallization pattern 10, metal paste is selectively applied to the front side of the substrate 2 in alignment with the heavily doped regions 4. Whether the back side metallization pattern 14 involves selective or non-selective application of a 25 metal paste depends on the design of the particular photovoltaic cell 1. Once the paste of metallization pattern 10 has been dried, contacts may be fired through the anti-reflection coating 8 at station 46. The applied metal thereby penetrates the SiNx:H-coating 8 to form low-resistance ohmic contacts, while hydrogen from the coating diffuses into the bulk of the cell 1 to passivate 30 impurities and defects. The contacts may be fired by means of a variety of 13 techniques, such as laser-firing or conveying the substrate 2 through an infra-red conveyor belt furnace. Finally, the cell 1 may be tested before it exits the production line 1 at 48. It is understood that although the above description of the production line 30 is phrased in terms of a p-type silicon 5 substrate being doped with phosphorus, the example is, mutatis mutandis, equally applicable to, inter alia, an n-type silicon substrate being doped with boron atoms.

Although illustrative embodiments of the present invention have been described above, in part with reference to the accompanying drawings, it 10 is to be understood that the invention is not limited to these embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Reference throughout this specification to "one embodiment" or "an embodiment" means that a 15 particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted 20 that particular features, structures, or characteristics of one or more embodiments may be combined in any suitable manner to form new, not explicitly described embodiments.

14

List of elements 1 photovoltaic cell 2 semiconductor substrate 5 2a,b front main surface (a) and back main surface (b) of semiconductor substrate 3 emitter 4 heavily doped substrate regions 6 lightly doped substrate regions 10 8 anti-reflection coating 10 metal contact pattern on front main surface 11 busbars 12 fingers 13 contacts 15 14 back side metallization 30 production line 32 wafer entry 34 saw damage removal and texturization station 20 36 inkjet station 38 high-temperature diffusion belt furnace 40 PSG removal station 42 anti-reflection coating application station 44 metallization station 25 46 station for firing metal contacts 48 photovoltaic cell exit

Claims

A method for manufacturing a semiconductor device (1) comprising a substantially flat semiconductor substrate (2), the method comprising: - step 1: applying a dopant source (dopant source) by the selective inkjetting a first pattern of a phosphoric or boric acid solution onto a major surface (2a) of said semiconductor substrate; Step 2: heating the substrate to diffuse phosphorus or boron atoms from said doping source into the substrate, first diffusion zones (4) being formed immediately below the first pattern; and - step 3: forming a metal contact pattern (10, 11, 12) substantially in line with said first diffusion zones 15.

The method of claim 1, wherein - step 1 further comprises: heating the main surface (2a) of the substrate (2) to a temperature that is substantially equal to or greater than the boiling point of a solvent of the phosphoric or boric acid solution while said solution is inked on the substrate.

3. The method according to any of the preceding claims, wherein the acid solution comprises ethanol as a solvent.

The method of any one of the preceding claims, wherein the acid solution comprises one or more of the following surfactants: diethylene glycol, glycerin, isopropyl alcohol, and polyethylene glycol. 30

The method according to any of the preceding claims, wherein the semiconductor device (1) is a photovoltaic cell.

The method according to any of the preceding claims, wherein step 1 further comprises: applying the doping source by selectively inking a second pattern of said phosphoric or boric acid solution onto the main surface (2a) of the semiconductor substrate (2) wherein the second pattern has a lower concentration of the acid solution per unit of substrate area than the first pattern; and - step 2 further comprises: forming second diffusion zones (6) immediately below the second pattern.

7. The method according to claim 6, wherein the phosphoric acid or boric acid solution is selectively inked on the main surface (2a) of the semiconductor substrate (2) with the aid of a printhead, said printhead having a plurality of inkjet nozzles ), and wherein each of the nozzles can be activated independently of the other nozzles to produce a drop of said acid solution.

The method according to claim 7, wherein, during the application of the doping source, the nozzles of the print head producing droplets for the first pattern are activated at a greater frequency than the nozzles producing droplets for the second pattern.

9. The method according to any of claims 6-8, wherein: - step 2 further comprises: transporting the substrate (2) through a conveyor diffusion oven (38)

10. The method according to any of the preceding claims, wherein - during step 2 the main surface (2a) of the substrate (2) is exposed to a gaseous atmosphere comprising phosphorus or a phosphorus compound, respectively boron or a boron compound, while the first diffusion zones (4) are formed, second diffusion zones (6) are formed by diffusion of phosphorus or boron from the gaseous atmosphere into the substrate through one or more regions of the main surface that are not covered by the first pattern 10.

The method of claim 10, wherein said gaseous atmosphere is a phosphoryl chloride or borohydride atmosphere.

The method of claim 10, wherein the gaseous atmosphere is effected by evaporating or spraying a phosphoric or boric acid solution.

13. A photovoltaic cell (1) manufactured using a method according to any of the preceding claims.

A method of manufacturing a semiconductor device (1) comprising a substantially flat semiconductor substrate (2), the method comprising: 25. step 1: applying a doping source by selectively inking a first cartridge (10) and a second pattern of the same doping source solution on a main surface (2a) of said semiconductor substrate, wherein the second pattern per unit of substrate surface has a lower doping source concentration than the first pattern; - step 2: heating the substrate to diffuse dopant atoms from the dopant source into the substrate, first diffusion zones (4) being formed immediately below the first pattern and second diffusion zones (6) being formed immediately below the second pattern; and - step 3: forming a metal contact pattern (10, 11, 12) substantially in line with said first diffusion zones.

The method of claim 14, wherein the doping source 10 is selectively inked on the main surface (2a) of the semiconductor substrate (2) using a printhead, wherein said printhead comprises a plurality of inkjet nozzles, and wherein each of the nozzles is independent of the other nozzles can be activated to produce a drop of dopant source solution. 15

16. The method of claim 15, wherein, during the application of the doping source, the nozzles of the print head producing droplets for the first pattern are activated at a greater frequency than the nozzles producing droplets for the second pattern.