Wafer-based solar cell comprising passivated contacts, 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 semiconductor material and 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 are each electrically contacted, via opening regions and current contact regions located therein, with a metal electrode structure attached to the semiconductor wafer surface, and each metal electrode structure covers an electrode coverage 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 this passivation layer has a double layer consisting of a polycristalline silicon layer and an SiO2 layer, and wherein the current contact regions are at least in part guided into the polycristalline silicon layer. 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: providing, at least in some sections, through-contact regions (9) in at least some of the current contact regions (5), in which through-contact regions (9) the SiO2 layer is penetrated; and forming, at the through-contact region (9), a metal-silicon mixture in a mixing region at the transition between the metal electrode structure (4), the polycrystalline silicon layer (6b), the SiO2 layer (6a), and the semiconductor material.

Description

Both p-type wafer-based solar cells and n-type wafer-based solar cells are known. In the case of p-type wafer-based solar cells the semiconductor wafer has a positively charged silicon basis whose top surface is negatively charged via doping with a foreign material. The negatively charged top surface forms the emitter of the p-type wafer-based solar cell. Furthermore in the case of p-type wafer-based solar cells the poly-Si layer of the passivation layer is highly doped and positively charged. In the case of n-type wafer-based solar cells the semiconductor wafer has a negatively charged silicon basis whose top surface is positively charged via doping with a foreign material. The positively charged top surface forms the emitter of the n-type wafer-based solar cell. Furthermore, in the case of n-type wafer-based solar cells the poly-Si layer of the passivation layer is highly doped and negatively charged. The passivation of the contacts may be either one-sided or two-sided. Wafer-based solar cells may have passivated contacts only on the front side, only on the reverse side, or else on the front side and reverse side. The passivated contacts may optionally also be selectively formed. In the case of selectively formed passivated contacts the double layer of SiO2 layer and poly-Si layer is arranged only below the metal electrode structure. Regions without a metal electrode structure are implemented without the double layer of SiO2 layer and poly-Si layer by corresponding process steps during construction of the wafer-based solar cell. To form the metal electrode structure of a wafer-based solar cell having a poly-Si / SiO2 passivation layer metal pastes are printed onto the semiconductor wafer surface in a screen printing process for example and in a subsequent heat treatment step the metal pastes are etched or burnt at least into regions of the poly-Si layer. This forms the opening regions comprising the electrical contact regions arranged therein. The SiO2 layer remains completely intact even in these electrical contact regions. Especially in the case of metal pastes containing metal particles composed of silver, burning of the metal paste into the poly-Si layer in the electrical contact regions causes local formation of silver crystallites in the poly-Si layer. These local silver crystallites primarily form the current paths for the passage of the charge carriers produced in the semiconductor wafer to the metal electrode structure. The charge carrier transport of these charge carriers through the SiO2 layer is based essentially on the quantum mechanical tunneling effect. The metal pastes may in principle also be applied to the semiconductor wafer surface in other processes. Metal pastes which comprise further metallic particles in addition to silver or other metallic particles instead of silver are also known. The electrical contact regions have a defining influence on the electrical contact resistance between the metal electrode structure and the doped semiconductor wafer surfaces of the wafer-based solar cell. This contact resistance should be as low as possible. It is an object of the present invention to provide a wafer-based solar cell having passivated contacts, wherein the wafer-based solar cell has a reduced electrical contact resistance to increase the efficiency of the wafer-based solar cell. This object is achieved when through-contact regions are at least partially provided in at least some of the electrical contact regions and the SiO2 layer is penetrated in these through-contact regions. Furthermore according to the invention a metal-silicon mixture is formed in a mixing region in the transition between the metal electrode structure, the poly-Si layer, the SiO2 layer (6a) and the semiconductor material at the through-contact regions. The penetration the SiO2 layer at the through-contact regions forms a point-like direct current path between the semiconductor material of the semiconductor wafer and the metal electrode structure. This leads to a marked reduction in the contact resistance between the metal electrode structure and the doped semiconductor wafer surfaces of the wafer-based solar cell. The formation of the mixing regions then leads to a further reduction in the contact resistance between the metal electrode structure and the doped semiconductor wafer surfaces of the wafer-based solar cell. According to the invention the through-contact regions comprising the mixing regions are produced by local heating of the opening regions and / or electrical contact regions to temperatures in the range from 1000°C to 1500°C for a period in the range from 10 ns to 1 s. The local heating may be effected for example by means of a LECO treatment (Laser Enhanced Contact Optimization) or by means of local radiation-assisted direct heating. In a LECO treatment the wafer-based solar cell is electrically contacted at both polarities, a reverse voltage is applied and a light source such as for example a laser beam is passed over the surface of the wafer-based solar cell so that the resulting local current flows bring about a local heating of the wafer-based solar cell. In a local radiation-assisted direct heating the relevant region of the wafer-based solar cell is locally irradiated for example by means of laser radiation, wherein the laser energy of the laser radiation is directly coupled into the wafer-based solar cell as thermal energy and thus results in a direct local heating. In an advantageous embodiment the through-contact regions in the plane of the SiO2 layer each have an area in the range from 0.05 pm2 to 1.50 pm2, preferably in the range from 0.1 pm2 to 1.0 pm2, particularly preferably in the range from 0.15 pm2 to 0.6 pm2. A further advantageous embodiment provides that the through-contact regions in the plane of the SiO2 layer each have a lateral extent in the range from 50 nm to 500 nm, preferably in the range from 100 nm to 250 nm, particularly preferably in the range from 150 nm to 200 nm. It is further proposed to provide 200 to 2000 through-contact regions per mm2, preferably 400 to 1000 through-contact regions per mm2, particularly preferably 500 to 800 through-contact regions per mm2, over the electrode covered area. A process for producing the wafer-based solar cell according to the invention is further proposed. This process comprises initially providing the wafer-based solar cell comprising the semiconductor material, the passivation layer and the formed metal electrode structure and according to the invention subsequently locally heating the electrical contact regions to a temperature in the range from 1000°C to 1500°C for a period in the range from 10 ns to 1 s in a heat-treatment step. In a preferred embodiment the temperature is preferably in the range from 1100°C to 1500°C, more preferably in the range from 1200°C to 1500°C. In an advantageous embodiment the local heating is effected by means of a LECO treatment or by means of a local radiation-assisted direct heating. In a LECO treatment the wafer-based solar cell is electrically contacted at both polarities, a reverse voltage is applied and a light source such as for example a laser beam is passed over the surface of the wafer-based solar cell so that the resulting local current flows bring about a local heating of the wafer-based solar cell. In a local radiation-assisted direct heating the relevant region of the wafer-based solar cell is locally irradiated for example by means of laser radiation, wherein the laser energy of the laser radiation is directly coupled into the waferbased solar cell as thermal energy and thus results in a direct local heating. An exemplary embodiment of the invention shall now be elucidated with reference to the figures. Fig. 1 A cross-sectional view of a p-type wafer-based solar cell having passivated contacts according to the prior art Fig. 2 A cross-sectional view of an n-type wafer-based solar cell having passivated contacts according to the prior art Fig. 3 A detail view of section A of the cross-sectional view according to fig. 1 and fig. 2 Fig. 4      A plan view of section A according to fig. 3 Fig. 5     A detail view of section A of the cross-sectional view analogous to fig. 3 in a wafer-based solar cell according to the invention Fig. 6 A plan view of section A according to fig. 5 Fig. 7 A detail view of section B of the cross-sectional view according to fig. 5 Fig. 1 shows a schematic cross-sectional view of a p-type wafer-based solar cell having passivated contacts according to the prior art. The wafer-based solar cell comprises a semiconductor wafer 1 having a semiconductor wafer surface comprising a front side and a reverse side. In the region of the front side an n-doped region 3 is formed, while in the region of the reverse side a p-doped region 2 is formed. An antireflective layer 7 is applied on the n-doped region 3 and a passivation layer 6 is applied on the p-doped region 2. The passivation layer 6 comprises an SiO2 layer 6a (silicon dioxide layer) and a poly-Si layer 6b (polycrystalline silicon layer). Starting from the p-doped region 2 first the SiO2 layer 6a and then the poly-Si layer 6b is arranged. In this embodiment the poly-Si layer 6b of the passivation layer 6 is highly doped and positively charged. A metal electrode structure 4 in the form of finger electrodes is arranged on each of the passivation layer 6 and the antireflective layer 7. In the embodiment shown the wafer-based solar cell has the anti-reflective layer 7 only on the top surface. However, the invention is not limited to such wafer-based solar cells. In other embodiments an antireflective layer 7 may also be provided on the passivation layer 6 at the bottom surface or only on the passivation layer 6 at the bottom surface. When viewed in a plan view of the front side or reverse side the metal electrode structure 4 covers an electrode covered area. The metal electrode structures 4 on the passivation layer 6 are electrically connected to the p-doped region 2 via electrical contact regions 5 (fig. 3) which are located within opening regions 8 (fig. 3) of the passivation layer 6. Fig. 2 shows a schematic cross-sectional view of an n-type wafer-based solar cell having passivated contacts according to the prior art. The wafer-based solar cell comprises a semiconductor wafer 1 having a semiconductor wafer surface comprising a front side and a reverse side. In the region of the front side a p-doped region 2 is formed, while in the region of the reverse side an n-doped region 3 is formed. An antireflective layer 7 is applied on the p-doped region 2 and a passivation layer 6 is applied on the n-doped region 3. In the embodiment shown the wafer-based solar cell has the anti-reflective layer 7 only on the top surface. However, the invention is not limited to such wafer-based solar cells. In other embodiments an antireflective layer 7 may also be provided on the passivation layer 6 at the bottom surface or only on the passivation layer 6 at the bottom surface. The passivation layer 6 comprises an SiO2 layer 6a (silicon dioxide layer) and a poly-Si layer 6b (polycrystalline silicon layer). Starting from the n-doped region 3 first the SiO2 layer 6a and then the poly-Si layer 6b is arranged. In this embodiment the poly-Si layer 6b of the passivation layer 6 is highly doped and negatively charged. A metal electrode structure 4 in the form of finger electrodes is arranged on each of the passivation layer 6 and the antireflective layer 7. When viewed in a plan view of the front side or reverse side the metal electrode structure 4 covers an electrode covered area. The metal electrode structures 4 on the passivation layer 6 are electrically connected to the n-doped region 3 via electrical contact regions 5 (fig. 3) which are located within opening regions 8 (fig. 3) of the passivation layer 6. The invention is moreover not limited to wafer-based solar cells having whole-area double layers of SiO2 layer and poly-Si layer. In further embodiments (not shown) the invention also provides wafer-based solar cells having selectively passivated contacts. In the case of selectively formed passivated contacts the double layer of SiO2 layer and poly-Si layer is arranged only in the regions of the wafer-based solar cell covered by the metal electrode structure. The regions not covered by the metal electrode structure do not have a poly-Si layer. Fig. 3 shows a detail view of section A of the cross-sectional view according to fig. 1 and fig. 2 of the wafer-based solar cell having passivated contacts known from the prior art. In this detail view an opening region 8 which has the feature that the material of the metal electrode structure 4 in this region has partially penetrated into the poly-Si layer 6b is apparent on the side of the metal electrode structure 4 facing the semiconductor material 2, 3. Five electrical contact regions 5 are also apparent within the opening region 8. In these regions the material of the metal electrode structure 4 has penetrated deeper into the poly-Si layer 6b. The electrical contact regions 5 differ in their lateral extents. Fig. 4 shows a plan view of a schematic representation of the distribution of the electrical contact regions 5 of section A according to fig. 3 which is visible after removal of the electrode structure 4. Fig. 5 shows a detail view of the section A of the cross-sectional view analogous to fig. 3 in a wafer-based solar cell according to the invention. Through-contact regions 9 are at least partially provided in at least some of the electrical contact regions 5. The SiO2 layer is penetrated in these through-contact regions 9. The material of the metal electrode structure 4 has partially penetrated through the SiO2 layer into the semiconductor material 2, 3 in these through-contact regions 9. Fig. 6 shows a plan view of a schematic diagram of the distribution of the through-contact regions 9 in the electrical contact regions 5 of section A according to fig. 5. The through-contact regions 9 in the plane of the SiO2 layer each have an area in the range from 0.05 pm2 to 1.50 pm2, preferably in the range from 0.1 pm2 to 1.0 pm2, particularly preferably in the range from 0.15 pm2 to 0.6 pm2. The through-contact regions 9 in the plane of the SiO2 layer each have a lateral extent in the range from 50 nm to 500 nm, preferably in the range from 100 nm to 250 nm, particularly preferably in the range from 150 nm to 200 nm. 200 to 2000 through-contact regions 9 per mm2, preferably 400 to 1000 through-contact regions 9 per mm2, particularly preferably 500 to 800 through-contact regions 9 per mm2, are provided over the electrode covered area. Fig. 7 shows a detail view of section B of the cross-sectional view according to fig. 5. A metal-silicon mixture 10 is formed in a mixing region in the transition between the metal electrode structure 4, the poly-Si layer 6b, the SiO2 layer 6a and the semiconductor material 2, 3 at the through-contact region 9. The mixing region 10 in the plane of the SiO2 layer has a lateral extent in the range from 50 nm to 300 nm, preferably in the range from 100 nm to 250 nm, particularly preferably in the range from 150 nm to 200 nm. The mixing region 10 in a plane perpendicular to the plane of the SiO2 layer has a lateral extent in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, particularly preferably in the range from 400 nm to 700 nm. Producing the wafer-based solar cell according to the invention comprises initially providing the wafer-based solar cell comprising the semiconductor material, the passivation layer and the formed metal electrode structure and subsequently locally heating the electrical contact regions 5 to a temperature in the range from 1000°C to 1500°C for a period in the range from 10 ns to 1 s in a heat-treatment step. The local heating of the current contact regions 5 is carried out by means of a LECO treatment or by means of a local radiation-assisted direct heating. The temperature is preferably in the range from 1100°C to 1500°C, more preferably in the range from 1200°C to 1500°C. The metal electrode structures 4 recited in the exemplary embodiments are formed for example using metal paste which has been applied via a screen printing process. However, the invention is not limited thereto. The metal paste may also be applied in another process. The metal paste preferably comprises silver particles. However, the invention is not limited thereto. By way of example the metal paste contains not only silver particles but also further metallic particles (e.g. aluminum, copper, etc.). In further embodiments the metal paste contains other metallic particles (e.g. aluminum, copper, etc.) instead of the silver particles. List of reference numerals Semiconductor wafer p-doped region n-doped region Metal electrode structure Electrical contact region Passivation layer SiO2 layer (silicon dioxide layer) poly-Si layer (polycrystalline silicon layer) Anti-reflective layer Opening regions Through-contact regions Mixing region

Claims

1. A wafer-based solar cell comprising a semiconductor wafer (1) composed of semiconductor material comprising a semiconductor wafer surface and at least one p-doped region (2) and at least one n-doped region (3), wherein the p-doped region (2) and / or the n-doped region (3) are in each case electrically contacted with a metal electrode structure (4) mounted on the semiconductor wafer surface via opening regions (8) and electrical contact regions (5) arranged therein and each metal electrode structure (4) covers an electrode covered 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 (4) and this passivation layer is a double layer composed of a poly-Si layer (6b) and an SiO2 layer (6a) and wherein the electrical contact regions (5) are at least partially passed into the poly-Si layer (6b), characterized in that through-contact regions (9) are at least partially provided in at least some of the electrical contact regions (5) and the SiO2 layer (6a) is penetrated in these through-contact regions (9) and in that a metal-silicon mixture is formed in a mixing region in the transition between the metal electrode structure (4), the poly-Si layer (6b), the SiO2 layer (6a) and the semiconductor material at the through-contact region (9).

2. The wafer-based solar cell as claimed in claim 1, characterized in that the through-contact regions (9) in the plane of the SiO2 layer each have an area in the range from 0.05 pm2 to 1.50 pm2, preferably in the range from 0.1 pm2 to 1.0 pm2, particularly preferably in the range from 0.15 pm2 to 0.6 pm2.

3. The wafer-based solar cell as claimed in claim 1, characterized in that the through-contact regions (9) in the plane of the SiO2 layer each have a lateral extent in the range from 50 nm to 500 nm, preferably in the range from 100 nm to 250 nm, particularly preferably in the range from 150 nm to 200 nm.

4. The wafer-based solar cell as claimed in any of the preceding claims, characterized in that 200 to 2000 through-contact regions (9) per mm2, preferably 400 to 1000 through-contact regions (9) per mm2, particularly preferably 500 to 800 through-contact regions (9) per mm2, are provided over the electrode covered area.

5. The wafer-based solar cell as claimed in any of the preceding claims, characterized in that the mixing region (10) in the plane of the SiO2 layer has a lateral extent in the range from 50 nm to 300 nm, preferably in the range from 100 nm to 250 nm, particularly preferably in the range from 150 nm to 200 nm.

6. The wafer-based solar cell as claimed in any of the preceding claims, characterized in that the mixing region (10) in a plane perpendicular to the plane of the SiO2 layer has a lateral extent in the range from 100 nm to 1000 nm, preferably in the range from 200 nm to 800 nm, particularly preferably in the range from 400 nm to 700 nm.

7. A process for producing a wafer-based solar cell as claimed in any of the preceding claims, characterized in that it comprises initially providing the wafer-based solar cell comprising the semiconductor material, the passivation layer (6) and the formed metal electrode structure (4) and subsequently locally heating the electrical contact regions (5) to a temperature in the range from 1000°C to 1500°C for a period in the range from 10 ns to 1 s in a heat-treatment step.

8. The process as claimed in claim 7, characterized in that the temperature is preferably in the range from 1100°C to 1500°C, more preferably in the range from 1200°C to 1500°C.

9. The process as claimed in claim 7 or 8, characterized in that the local heating is effected by means of a LECO treatment or by means of a local radiation-assisted direct heating.

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