Wafer-based solar cell, and method for manufacturing such a wafer-based solar cell

Abstract

The invention relates to a wafer-based solar cell comprising a semiconductor wafer that is made of a semiconductor material consisting of silicon and that has a semiconductor wafer surface, at least one p-doped region, and at least one n-doped region, wherein the p-doped region and / or the n-doped region is electrically contacted, via opening regions and current contact regions located therein, with a metal electrode structure attached to the semiconductor wafer surface, and the metal electrode structure covers an electrode cover area on the semiconductor wafer surface, and wherein the semiconductor wafer surface of the wafer-based solar cell has a passivation layer arranged between the semiconductor material and the metal electrode structure, and wherein this passivation layer consists at least in part of an aluminium oxide and / or the metal electrode structure is formed from a metallic material containing, at least in part, aluminium, and wherein the current contact regions pass through the passivation layer and wherein, at the current contact regions, a metal-silicon mixture is formed in a first mixing region in the transition region between the metal electrode structure, the passivation layer, and the semiconductor material. The aim of the invention is to provide a wafer-based solar cell, wherein the wafer-based solar cell has a reduced electrical contact resistance in order to increase the efficiency of the wafer-based solar cell. This aim is achieved by forming, in the current contact regions (5), a second mixing region in the semiconductor material at an end of the first mixing region (9) facing the semiconductor material, and embedding aluminium atoms in the semiconductor material within this second mixing region (10).

Description

The present invention relates to a wafer solar cell. A wafer solar cell in the context of the present invention means a solar cell which is produced from a silicon semiconductor wafer which forms the structural framework of the solar cell. The semiconductor wafer formed from semiconductor material (silicon) has a semiconductor wafer surface and at least one p-doped region and at least one n-doped region, wherein electrical contact is made with the p-doped region and / or the n-doped region in each case with a metal electrode structure on the semiconductor wafer surface via current contact regions, and each metal electrode structure covers an electrode coverage area on the semiconductor wafer surface. The metal electrode structure is important for removing charge carriers in the form of a flow of electrical current from the wafer solar cell. The silicon can be p-doped with boron, gallium or aluminum. It is known that wafer solar cells may have at least one dielectric passivation layer arranged between the metal electrode structure and the semiconductor wafer surface. The passivation layer is typically formed from the materials selected from the group consisting of: AlOx (aluminum oxide), SiNx (silicon nitride), SiOxNy (silicon oxynitride), SiOx (silicon oxide) and / or poly-Si (polycrystalline silicon). This passivation layer typically has a layer thickness in the range from 50 nm to 200 nm. Further layers may optionally be provided on this passivation layer - for example an antireflection layer. p-type wafer solar cells and n-type wafer solar cells are known. In p-type wafer solar cells, the semiconductor wafer has a positively charged silicon base, the top side of which is negatively charged by doping with an extrinsic material. The negatively charged top side forms the emitter of the p-type wafer solar cell. In n-type wafer solar cells, the semiconductor wafer has a negatively charged silicon base, the top side of which is positively charged by doping with an extrinsic material. The positively charged top side forms the emitter of the n-type wafer solar cell. In order to form the metal electrode structure of a wafer solar cell with a passivation layer, metal pastes are printed onto the semiconductor wafer surface, for example in a screen printing process, and in a subsequent heat-treatment step these metal pastes are etched or baked into the passivation layer at least into some regions. The baking in at least in some regions forms opening regions in the passivation layer. In at least some of these opening regions, current contact regions are also formed, which fully penetrate the passivation layer. It is the current contact regions here that primarily form the current paths for the route of the charge carriers produced in the semiconductor wafer to the metal electrode structure. The current contact regions are thus crucial for electrical contact resistance between the metal electrode structure and the doped semiconductor wafer surfaces of the wafer solar cell. This contact resistance should be as low as possible. It is known from the prior art that low contact resistances to the metal electrode structure on emitters p-doped with boron or gallium are achieved only when the metal pastes used to produce the metal electrode structure contain at least a small portion of aluminum. By contrast, when aluminum-free metal pastes (for example aluminum-free silver paste) are used, only poor electrical contact is usually created between the semiconductor material and the metal electrode structure. On the other hand, the aluminum particles used in metal pastes are usually relatively large, which in turn limits the minimum finger width in screen printing and also restricts the process window in the baking of these metal pastes. It is an object of the invention to provide a wafer solar cell, wherein the wafer solar cell has reduced electrical contact resistance coupled with reduced impairment of the semiconductor material in order to increase the efficiency of the wafer solar cell. This object is achieved in the case of wafer solar cells which have an aluminum p-doped region of the semiconductor material in that a second mixing region is formed in the semiconductor material in the current contact regions at an end of the first mixing region facing the semiconductor material and aluminum atoms are embedded in the semiconductor material in this second mixing region and a concentration of the aluminum atoms in the second mixing region is greater than a concentration of the aluminum atoms in the semiconductor material outside the first and second mixing regions. At the junction between the first mixing region and the semiconductor material, an intermediate layer (second mixing region) is thus provided, in which the semiconductor material is doped with aluminum atoms in a higher concentration. This leads to a significant reduction in the contact resistance between the metal electrode structure and the doped semiconductor wafer surfaces of the wafer solar cell. The reduction in contact resistance can be seen in that the semiconductor material formed from silicon is more highly locally doped with the embedded aluminum atoms in the second mixing region. Furthermore, in the case of wafer solar cells which have a region of the semiconductor material that is p-doped with boron or gallium, the object is achieved in that a second mixing region is formed in the semiconductor material in the current contact regions at an end of the first mixing region facing the semiconductor material, and aluminum atoms are embedded in the semiconductor material in this second mixing region. At the junction between the first mixing region and the semiconductor material, an intermediate layer (second mixing region) is thus provided, in which the semiconductor material is doped with aluminum atoms. This leads to a significant reduction in the contact resistance between the metal electrode structure and the doped semiconductor wafer surfaces of the wafer solar cell. The reduction in contact resistance can be seen firstly in that the semiconductor material formed from silicon is locally doped with the embedded aluminum atoms in the second mixing region. If the region of the semiconductor material that has been p-doped with boron or gallium should already incorporate aluminum atoms from the outset in the sense of an impurity in the starting material, the invention should be considered to mean that a concentration of the aluminum atoms in the second mixing region is greater here too than a concentration of the aluminum atoms in the semiconductor material outside the first and second mixing regions. With regard to the second mixing regions, it is made clear once again that these are provided only locally at the current contact regions. The individual, local second mixing regions below the metal electrode structure are separated from one another. The second mixing regions together do not form a continuous layer beneath the metal electrode structure. In one embodiment, the passivation layer is formed at least partly from an aluminum oxide and / or the metal electrode structure is formed from an at least partly aluminum-containing metallic material. According to the invention, the second mixing regions are produced by local heating of the opening regions and / or current contact regions. In the case of local heating of the opening regions and / or current contact regions, temperatures in the range from 600°C to 1500°C are achieved in particular in the aluminum oxide layer and in a zone of the semiconductor material adjoining the aluminum oxide layer. The heating in this temperature range should last for a period of time in the range from 10 ns to 1 s. The local heating can be effected, for example, by a LECO (laser enhanced contact optimization) treatment or by local radiation-assisted direct heating. In the case of LECO treatment, the wafer solar cell is electrically contacted at both polarities, a reverse voltage is applied and a light source or point light source, such as a laser beam for example, is guided over the surface of the wafer solar cell such that the local current flows that arise achieve local heating of the wafer solar cell. In the case of local radiation-assisted direct heating, the relevant region of the wafer solar cell is irradiated locally, for example by means of laser radiation, wherein the laser energy of the laser radiation is coupled directly into the wafer solar cell as thermal energy and hence leads to direct local heating. In an advantageous embodiment, the second mixing regions have a lateral extent in a plane parallel to the passivation layer in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, more preferably in the range from 400 nm to 700 nm. In an advantageous configuration, the second mixing regions have a lateral extent perpendicular to a plane parallel to the passivation layer in the range from 25 nm to 250 nm, preferably in the range from 50 nm to 200 nm, more preferably in the range from 75 nm to 150 nm. It is proposed that the aluminum atoms are embedded in the second mixing region with a concentration in the range from 0.1 wt% to 2.0 wt%, preferably in the range from 0.3 wt% to 1.8 wt%, more preferably in the range from 0.5 wt% to 1.5 wt% (the unit wt% is percent by weight for specification of a proportion by mass of a substance mixture). It is also proposed that the first mixing regions in the plane of the passivation layer each have an area in the range from 0.05 pm2 to 1.00 pm2, preferably in the range from 0.1 pm2 to 0.8 pm2, more preferably in the range from 0.2 pm2 to 0.5 pm2. In an advantageous embodiment, the first mixing regions in the plane of the passivation layer each have a lateral extent in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, more preferably in the range from 400 nm to 700 nm. In a further advantageous configuration, 500 to 45 000 first mixing regions per mm2 are provided over the electrode coverage area, preferably 2,000 to 15 000 first mixing regions per mm2, more preferably 3000 to 6000 first mixing regions per mm2. Also proposed is a method of producing the wafer solar cell of the invention. In this method, the wafer solar cell with the semiconductor material, the passivation layer and the metal electrode structure formed is firstly provided and then, according to the invention, in a heat treatment step, the current contact regions are heated locally to a temperature in the range from 600°C to 1500°C for a period in the range from 10 ns to 1 s. It is proposed that the temperature is in the range from 900°C to 1500°C, preferably in the range from 1100°C to 1500°C, more preferably in the range from 1250°C to 1500°C. Particularly when aluminum-free metal pastes are used, the local heating can release a portion of the aluminum from a passivation layer formed from Al2O3 and said portion can also react with an aluminum-free metal paste. In this case, the aluminum is at least temporarily also available for a possible reduction in the melting temperature of an alloy formed from this aluminum and the aluminum-free metal paste. In an advantageous embodiment, the local heating is effected by a LECO treatment or by local radiation-assisted direct heating. In the case of LECO treatment, the wafer solar cell is electrically contacted at both polarities, a reverse voltage is applied and a light source or point light source, such as a laser beam for example, is guided over the surface of the wafer solar cell such that the local current flows that arise achieve local heating of the wafer solar cell. In the case of local radiation-assisted direct heating, the relevant region of the wafer solar cell is irradiated locally, for example by means of laser radiation, wherein the laser energy of the laser radiation is coupled directly into the wafer solar cell as thermal energy and hence leads to direct local heating. A working example of the invention is elucidated hereinafter with reference to the drawings. Fig. 1 a cross-sectional view of a wafer solar cell of the invention Fig. 2      a detail view of detail A of the cross-sectional view according to figure 1 Fig. 3      a detail view of detail B of the cross-sectional view according to figure 1 Fig. 1 shows a schematic cross-sectional view of an n-type wafer solar cell of the invention. The wafer solar cell has a silicon semiconductor wafer 1 having a semiconductor wafer surface comprising a front side and a back side. A region 3 p-doped with boron, gallium or aluminum is formed in the region of the front side, while an n-doped region 2 is formed in the region of the back side. A passivation layer 6 is respectively applied to the n-doped region 2 and to the p-doped region 3. The passivation layer 6 applied to the p-doped region 3 is additionally covered with an antireflection layer 7. However, the antireflection layer 7 is optional and not mandatory. The passivation layer 6 includes at least one layer formed from an aluminum oxide (for example Al2O3). In other words, the passivation layer 6 may consist entirely of a layer formed from aluminum oxide or else may have further layers in addition to a layer formed from aluminum oxide. These further layers may be formed, for example, from SiNx (silicon nitride), SiOxNy (silicon oxynitride), SiOx (silicon oxide) and / or poly-Si (polycrystalline silicon). There is a metal electrode structure 4 in the form of finger electrodes atop the passivation layer 6 applied to the n-doped region 2 and atop the antireflection layer 7. The metal electrode structure 4 covers an electrode coverage area when viewed in the plan view of the front side or back side. The metal electrode structures 4 on the antireflection layer are electrically connected to the p-doped region 3 via current contact regions 5 (fig. 2) located within opening regions 8 (fig. 2) of the passivation layer 6 and antireflection layer 7. The metal electrode structures 4 on the passivation layer 6 are likewise electrically connected to the n-doped region 2 via current contact regions 5 located within opening regions 8 of the passivation layer 6 applied to the n-doped region 2. In the embodiment shown, the metal electrode structure 4 is formed from an at least partly aluminum-containing metallic material or from an aluminum-free metallic material. For example, the metallic material may consist of silver or the metallic material consists of silver admixed with aluminum. Fig. 2 shows a detail view of detail A of the cross-sectional view according to fig. 1. In this detail view, a conductor track of the metal electrode structure 4 and details of the passivation layer 6, of the antireflection layer 7 and of the p-doped region 3 of the semiconductor material consisting of silicon are visible. Also apparent is an opening region 8 with a current contact region 5 therein. In the opening region 8, the material of the metal electrode structure 4 has partially penetrated into the antireflection layer 7. In the current contact region 5, the material of the metal electrode structure 4 has penetrated through the passivation layer 6 and the antireflection layer 7 into the p-doped region 3 of the semiconductor material. A metal-silicon mixture is formed in a first mixing region 9 at the current contact region 5 at the junction between the metal electrode structure 4, the passivation layer 6 and the semiconductor material. Furthermore, a second mixing region 10 is formed in the semiconductor material in the current contact region 5 at an end of the first mixing region 9 facing the semiconductor material. If the p-doped region of the semiconductor material is doped with boron or gallium, aluminum atoms are now embedded in the p-doped region of the semiconductor material in the second mixing region 10. If the p-doped region of the semiconductor material was already doped with aluminum in the construction of the wafer solar cell, a concentration of the aluminum atoms in the second mixing region 10 will then be greater than a concentration of the aluminum atoms in the semiconductor material outside the first and second mixing regions 9, 10. Fig. 3 shows a detail view of detail B of the cross-sectional view according to fig. 1. In this detail view, a conductor track of the metal electrode structure 4 and sections of the passivation layer 6 and of the n-doped region 2 of the semiconductor material consisting of silicon are visible. Also apparent is an opening region 8 with a current contact region 5 therein. In the opening region 8, the material of the metal electrode structure 4 has partially penetrated into the passivation layer 6. In the current contact region 5, the material of the metal electrode structure 4 has penetrated through the passivation layer 6 into the n-doped region 2 of the semiconductor material. A metal-silicon mixture is formed in a first mixing region 9 at the current contact region 5 at the junction between the metal electrode structure 4, the passivation layer 6 and the semiconductor material. Furthermore, a second mixing region 10 is formed in the semiconductor material in the current contact region 5 at an end of the first mixing region 9 facing the semiconductor material. In this second mixing region 10, aluminum atoms are embedded in the n-doped region of the semiconductor material. Both in the case of the metal electrode structure 4 on the p-doped side of the semiconductor material and in the case of the metal electrode structure 4 on the n-doped side of the semiconductor material, a multitude of opening regions 8 and current contact regions 5 therein are provided. The second mixing regions 10 have a lateral extent in a plane parallel to the passivation layer in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, more preferably in the range from 400 nm to 700 nm. Perpendicular to a plane parallel to the passivation layer, the second mixing regions 10 have a lateral extent in the range from 25 nm to 250 nm, preferably in the range from 50 nm to 200 nm, more preferably in the range from 75 nm to 150 nm. The aluminum atoms are embedded in the second mixing region 10 with a concentration in the range from 0.1 wt% to 2.0 wt%, preferably in the range from 0.3 wt% to 1.8 wt%, more preferably in the range from 0.5 wt% to 1.5 wt%. The first mixing regions 9 in the plane of the passivation layer 6 each have an area in the range from 0.05 pm2 to 1.00 pm2, preferably in the range from 0.1 pm2 to 0.8 pm2, more preferably in the range from 0.2 pm2 to 0.5 pm2. Perpendicular to the plane of the passivation layer 6, the first mixing regions 9 each have a lateral extent in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, more preferably in the range from 400 nm to 700 nm. 500 to 45 000 first mixing regions 9 per mm2 are provided over the electrode coverage area, preferably 2000 to 15 000 first mixing regions 9 per mm2, more preferably 3000 to 6000 first mixing regions 9 per mm2. The invention is not limited to the embodiment shown. By way of example, the wafer solar cell may also be a p-type wafer solar cell, where the p-doped and the n-doped region are interchanged with respect to the n-type wafer solar cell. Furthermore, both in the case of a p-type wafer solar cell and in the case of an n-type wafer solar cell, the passivation layer 6 formed from aluminum oxide can be provided only on the n-doped side or only on the p-doped side. In contrast to the embodiments described above, the passivation layer 6 may also not be formed from aluminum oxide. For example, the passivation layer 6 may be formed form SiNx (silicon nitride), SiOxNy (silicon oxynitride), SiOx (silicon oxide) and / or poly-Si (polycrystalline silicon). For production of the wafer solar cell of the invention, the wafer solar cell with the semiconductor material, the passivation layer 6 and the metal electrode structure 4 formed is first provided. Subsequently, in a heat treatment step, the current contact regions 5 are heated locally to a temperature in the range from 600°C to 1500°C for a period in the range from 10 ns to 1 s. In further embodiments, the temperature is in the range from 900°C to 1500°C, preferably in the range from 1100°C to 1500°C, more preferably in the range from 1250°C to 1500°C. The local heating is effected here by means of a LECO treatment or by means of local radiation-assisted direct heating. The aluminum atoms embedded in the semiconductor material in the second mixing region 10 may originate, for example, from the passivation layer 6 formed at least partly from an aluminum oxide and / or from the metal electrode structure 4 formed from an at least partly aluminum-containing metallic material, but without the invention being limited thereto. List of reference numerals 1 semiconductor wafer 2     p-doped region 3     n-doped region 4      metal electrode structure 5      current contact region 6      passivation layer 7      antireflection layer 8     opening regions 9      first mixing region 10    second mixing region

Claims

1. A wafer solar cell having a semiconductor wafer (1) composed of silicon-consisting semiconductor material, having a semiconductor wafer surface and at least one region (2) p-doped with boron or gallium and at least one n-doped region (3), wherein the p-doped region (2) and / or the n-doped region (3) are / is electrically contacted to a metal electrode structure (4) on the semiconductor wafer surface via opening regions (8) and current contact regions (5) therein, and the metal electrode structure (4) covers an electrode coverage area on the semiconductor wafer surface, and wherein the semiconductor wafer surface of the wafer solar cell has a passivation layer (6) between the semiconductor material and the metal electrode structure (4), and wherein the current contact regions (5) are routed through the passivation layer (6), and wherein a metal-silicon mixture is formed in a first mixing region (9) at the current contact regions (5) at the junction between the metal electrode structure, the passivation layer (6) and the semiconductor material, characterized in that a second mixing region is formed in the semiconductor material in the current contact regions (5) at an end of the first mixing region (9) facing the semiconductor material, and aluminum atoms are embedded in the semiconductor material in said second mixing region (10).

2. A wafer solar cell having a semiconductor wafer (1) composed of silicon-consisting semiconductor material, having a semiconductor wafer surface and at least one region (2) p-doped with aluminum and at least one n-doped region (3), wherein the p-doped region (2) and / or the n-doped region (3) are / is electrically contacted to a metal electrode structure (4) on the semiconductor wafer surface via opening regions (8) and current contact regions (5) therein, and the metal electrode structure (4) covers an electrode coverage area on the semiconductor wafer surface, and wherein the semiconductor wafer surface of the wafer solar cell has a passivation layer (6) between the semiconductor material and the metal electrode structure (4), and wherein the current contact regions (5) are routed through the passivation layer (6), and wherein a metal-silicon mixture is formed in a first mixing region (9) at the current contact regions (5) at the junction between the metal electrode structure, the passivation layer (6) and the semiconductor material, characterized in that a second mixing region is formed in the semiconductor material in the current contact regions (5) at an end of the first mixing region (9) facing the semiconductor material, and aluminum atoms are embedded in the semiconductor material in said second mixing region (10), and a concentration of the aluminum atoms in the second mixing region is greater than a concentration of thealuminum atoms in the semiconductor material outside the first and second mixing regions (9, 10).

3. The wafer solar cell as claimed in any of the preceding claims, characterized in that the passivation layer (6) is formed at least partly from an aluminum oxide and / or the metal electrode structure (4) is formed from an at least partly aluminum-containing metallic material.

4. The wafer solar cell as claimed in any of the preceding claims, characterized in that aluminum atoms are embedded in the second mixing region with a concentration in the range from 0.1 wt% to 2.0 wt%, preferably in the range from 0.3 wt% to 1.8 wt%, more preferably in the range from 0.5 wt% to 1.5 wt%.

5. The wafer solar cell as claimed in claim 1, characterized in that the second mixing regions (10) have a lateral extent in a plane parallel to the passivation layer in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, more preferably in the range from 400 nm to 700 nm.

6. The wafer solar cell as claimed in any of the preceding claims, characterized in that the second mixing regions (10) have a lateral extent perpendicular to a plane parallel to the passivation layer in the range from 25 nm to 250 nm, preferably in the range from 50 nm to 200 nm, more preferably in the range from 75 nm to 150 nm.

7. The wafer solar cell as claimed in any of the preceding claims, characterized in that the first mixing regions (9) in the plane of the passivation layer each have an area in the range from 0.05 pm2 to 1.00 pm2, preferably in the range from 0.1 pm2 to 0.8 pm2, more preferably in the range from 0.2 pm2 to 0.5 pm2.

8. The wafer solar cell as claimed in any of the preceding claims, characterized in that the first mixing regions (9) perpendicular to the plane of the passivation layer each have a lateral extent in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, more preferably in the range from 400 nm to 700 nm.

9. The wafer solar cell as claimed in any of the preceding claims, characterized in that 500 to 45 000 first mixing regions (9) per mm2 are provided over the electrode coverage area, preferably 2000 to 15 000 first mixing regions (9) per mm2, more preferably 3000 to 6000 first mixing regions (9) per mm2.

10. A method of producing a wafer solar cell as claimed in any of the preceding claims, characterized in that the wafer solar cell with the semiconductor material, the passivation layer (6) and the metal electrode structure (4) formed is first provided and then, in a heat treatment step, the current contact regions (5) are heated locally to a temperature in the range from 600°C to 1500°C for a period in the range from 10 ns to 1 s.

11. The method as claimed in claim 9, characterized in that the temperature is in the range from 900°C to 1500°C, preferably in the range from 1100°C to 1500°C, more preferably in the range from 1250°C to 1500°C.

12. The method as claimed in either of claims 9 and 10, characterized in that the local heating is effected by a LECO treatment or by local radiation-assisted direct heating.

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