BACK-SIDE CONTACT SOLAR CELL

Abstract

The invention relates to a back-side contact solar cell including a semiconductor substrate, in particular a silicon wafer, including a front side and a back side, the solar cell having electrodes of a first polarity and electrodes of a second polarity on the back side, wherein a tunnel layer and a highly doped silicon layer are positioned under the electrodes of a first polarity, and the electrodes of the second polarity make direct electrical and mechanical contact with the semiconductor substrate.

Claims

1. A back-side contact solar cell, comprising a semiconductor substrate comprising a front side and a back side, wherein the solar cell comprises, on the back side, electrodes of a first polarity and electrodes of a second polarity, characterized in that a tunnel layer and a highly doped silicon layer are positioned under the electrodes of a first polarity, and the electrodes of the second polarity make direct electrical and mechanical contact with the semiconductor substrate in highly doped base regions (26), wherein the highly doped base regions (26) comprise selectively overcompensated regions of the highly doped silicon layer (20).

2. (canceled)

3. The solar cell according to claim 1, characterized in that an uncontacted, lightly doped region separates the highly doped silicon layer from the base region.

4. A method for producing a back-side contact solar cell, wherein a semiconductor substrate of the solar cell comprises a, in particular polished or textured, back side and a textured, front side, wherein the method comprises the steps of: applying a tunnel layer comprising silicon dioxide to a surface of the front side and/or to a surface of the back side, and depositing a full-area, highly doped silicon layer of a first polarity on the tunnel layer on the back side, characterized in that the method further comprises a step of applying a precursor layer comprising a dopant on the highly doped silicon layer of the first polarity on the back side, wherein a concentration of the dopants in the highly doped silicon layer of the first polarity and of the precursor layer is selected such that an amount of the dopant of a second polarity in the precursor layer is higher than the amount of the dopant of the first polarity in the highly doped silicon layer of the first polarity, and in that the method comprises a step of laser irradiation of the back side for producing locally highly doped base regions, wherein the dopant of the second polarity overcompensates for the dopant of the first polarity during the laser irradiation, and the locally highly doped base regions are accordingly produced by overcompensation.

5. (canceled)

6. The method according to claim 4, characterized in that the depositing of the highly doped silicon layer of the first polarity comprises the depositing of undoped silicon and subsequent introduction of a dopant.

7. The method according to claim 4, characterized in that the method comprises a step of removing the tunnel layer and/or the highly doped silicon layer of the first polarity from the front side.

8. The method according to claim 4, characterized in that the method comprises a step of applying a precursor layer comprising a dopant, in particular phosphorus, on the highly doped silicon layer of the first polarity on the front side.

9. (canceled)

10. (canceled)

11. The method according to claim 4, characterized in that the method comprises a step of selectively removing the highly doped silicon layer of the first polarity and/or the precursor layer.

12. The method according to claim 4, characterized in that the method comprises a step of applying a passivation layer to the surface of the front side and/or back side.

13. The method according to claim 4, characterized in that the method comprises a step of applying electrodes to the back side of the solar cell.

14. The method according to claim 4, characterized in that the method comprises a step of selectively removing the passivation layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the drawings:

[0025] FIG. 1 is a schematic view of a section of a solar cell according to the invention, and

[0026] FIGS. 2a to 2h show a solar cell according to FIG. 1 in various steps of a method for producing the solar cell.

DETAILED DESCRIPTION

[0027] FIG. 1 shows a section of a solar cell 10 having a semiconductor substrate 12, in particular a silicon wafer, a back side 14, and a front side 16 which faces the sun. The silicon wafer 12 can be doped either n-type or p-type. The solar cell 10 is explained by way of example using an n-type doping of the silicon wafer 12, the “base”.

[0028] A polycrystalline highly doped p-type silicon layer 20 is provided on the back side 14. This forms a first polarity on the back side 14. In the region of the highly doped p-type silicon layer 20, a tunnel layer 18, in particular comprising silicon dioxide, 18 passivates the surface of the silicon wafer 12.

[0029] The solar cell 10 comprises, on the back side 14, electrodes 34 of a first polarity and electrodes 36 of a second polarity. Below the electrodes 34 of the first polarity is the tunnel layer 18 and the highly doped p-type silicon layer 20. The electrodes 36 of the second polarity make direct electrical and mechanical contact with the semiconductor substrate 12. The electrodes 36 of the second polarity make contact with the semiconductor substrate 12 according to the illustrated embodiment in highly doped base regions 26. An uncontacted edge region 28 separates the highly doped p-type silicon layer 20 and the highly doped n-type base region 26.

[0030] The production process for the solar cell 10 is explained below with reference to FIGS. 2a to 2h. FIGS. 2a to 2h illustrate the process flow for the mask-free production of a solar cell 10 with a single type of selective contacts. As a starting material, the silicon wafer 12 having a front side 14 and a back side 16 can be doped either n-type or p-type. The process sequence is explained using an n-type doping of the wafer, the “base”.

[0031] FIG. 2a shows the silicon wafer 12 with a polished back side 14 and a textured front side 16. A tunnel layer 18, for example a silicon dioxide, with a maximum thickness of preferably about four nanometers, is produced on both surfaces, or preferably only on the back side 14, for example in a thermal or wet-chemical process, or by deposition.

[0032] In a next step of the method, a highly doped silicon layer 20 of a first polarity is deposited on the tunnel layer 18 on the back side 14, in particular over the whole area thereof. In the following, a p-type doping is assumed to be the first polarity. FIG. 2b shows the full-area deposition of the highly doped p-type silicon layer 20 on the tunnel layer 18 on the back side 14. The highly doped p-type silicon layer 20 can be deposited, for example, by means of plasma-enhanced chemical vapor deposition, PECVD, atmospheric chemical vapor deposition, APCVD, low-pressure chemical vapor deposition (LPCVD), or sputtering. The highly doped p-type silicon layer 20 has a thickness of approximately 100 nanometers to 300 nanometers.

[0033] The highly doped p-type silicon layer 20 can also be deposited in two steps instead of in one step. In this case, depositing the p-type silicon layer 20 comprises depositing undoped silicon and then introducing a dopant. The dopant is introduced, for example, by means of ion implantation, furnace diffusion or laser diffusion. The dopant is, for example, boron, aluminum, or gallium.

[0034] If there is a tunnel layer 18 and/or a highly doped p-type silicon layer 20, in particular an unintentionally deposited one, on the front side 16, the tunnel layer 18 and/or the highly doped p-type silicon layer 20 is removed from the front side 16 in a next step of the method. The tunnel layer 18 and/or the highly doped p-type silicon layer 20 can be etched away over the entire area or locally. This is shown in FIG. 2c.

[0035] In a next step of the method, which is shown in FIG. 2d, a precursor layer 22 comprising a dopant, in particular phosphorus, is applied to the highly doped silicon layer 20 of the first polarity on the back side 14 and/or on the front side 16. According to the illustrated embodiment, the second polarity dopant is phosphorus. For example, a furnace diffusion process or a PECVD deposition is carried out to apply the precursor layer 22, in which process phosphorus silicate glass 22 grows on the highly doped p-type silicon layer 20 on the back side 14 and on the front side 16. During furnace diffusion, phosphorus from the phosphosilicate glass 22 diffuses into the front side 16 of the solar cell 10 and dopes the front side 16 surface to n-type. On the back side 14, the phosphorus also diffuses into the highly doped p-type silicon layer 20. The furnace diffusion process is advantageously carried out in such a way that a high proportion of phosphorus is contained in the phosphorus silicate glass 22 after the furnace diffusion process.

[0036] A concentration of the dopants in the highly doped p-type silicon layer 20 and the precursor layer 22 is advantageously selected such that, after the furnace diffusion, in the highly doped p-type silicon layer 20 the dopant concentration of the first dopant is higher than the dopant concentration of the second dopant, but in the precursor layer the amount of dopant of the second dopant is higher than the amount of dopant of the first dopant in the highly doped p-type silicon layer 20. This prevents overcompensation of the first dopant during furnace diffusion. The highly doped p-type silicon layer therefore remains p-type doped. In this way, however, sufficient dopant is present in the phosphorus silicate glass 22 for the following step of laser irradiation in order to overcompensate for the dopant of the first polarity during the laser irradiation.

[0037] According to one embodiment, the method comprises a step of laser irradiation of the back side 14, in particular for producing locally highly doped n-type base regions 26. FIG. 2e shows the solar cell 10 after the back side 14 has been locally irradiated with a laser. The phosphorus silicate glass 22 serves as a doping source during the laser irradiation for the production of a locally highly doped n-type base surface in the locally highly doped n-type base regions 26. The energy of the laser radiation locally melts the phosphorus silicate glass 22, the highly doped p-type silicon layer 20, the tunnel layer, namely the silicon dioxide 18 and the surface of the back side 14, so that the locally highly doped n-type base regions 26 are created. The dopants from the phosphorus silicate glass 22 and the highly doped p-type silicon layer 20 diffuse into the silicon melt on the surface of the back side 14. After the laser radiation has subsided, the melt cools and solidifies. Both dopants remain in the silicon, with one or both dopants accumulating at the surface. Due to the greater amount of dopant of the second polarity dopant in the phosphorus silicate glass 22, the concentration of the second dopant in the solidified silicon surface is significantly higher than the concentration of the first dopant from the highly doped p-type silicon layer 20, and thus leads to a local high n-type doping due to overcompensation in locally highly doped n-type base regions 26. By a suitable choice of the laser parameters, portions of n-type silicon with locally-selectively different levels of doping can also be produced in the locally highly doped n-type base regions 26.

[0038] In the case of back-side contact solar cells with selective contacts of both polarities, high levels of recombination can occur if the two differently doped silicon layers come into contact. The method therefore includes a step of selectively removing the highly doped silicon layer 20 of the first polarity and/or the precursor layer 22.

[0039] FIG. 2f shows the solar cell 10 after the highly doped p-type silicon layer 20 in the edge regions 28 of the locally highly doped n-type base regions 26 has been removed. The p-type silicon layer 20 in the edge regions 28 can be removed, for example, by laser irradiation, wet-chemical etching, or a combination of both processes. Furthermore, a wet chemical process can be used to remove the phosphosilicate glass 22 from the highly doped p-type silicon layer and the front side 16 of the solar cell. Etching back a few nanometers of the highly doped p-type silicon layer 20 can remove a silicon layer with a high phosphorus content, and limit an increase in layer resistance of the highly doped p-type silicon layer 20 by phosphorus diffusion.

[0040] FIG. 2g shows the solar cell 10 after a passivation layer 30, 32 has been applied to the surface of the front side 16 and/or back side 14. The passivation layer 30, 32 comprises, for example, thermally grown silicon dioxide, silicon nitride, aluminum oxide, or a layer stack made up of two or more dielectric layers. The thickness, refractive index, and composition of the passivation layer 30 on the back side 14 can differ from the thickness, refractive index, and composition of the passivation layer 32 on the front side 16. The thicknesses, composition, and refractive indices of the passivation layers 30, 32 are advantageously optimized to reduce front side 16 reflection and increase back side 14 reflection.

[0041] FIG. 2h shows the solar cell 10 after the electrodes 34, 36 have been applied to the back side 14 of the solar cell 10. The electrodes 34, 36 can be applied, for example, by means of screen printing, vapor deposition, sputtering, or galvanic deposition of one or more metals or other conductive layers. The electrodes 34, 36 can, for example, be silver paste, silver/aluminum paste, aluminum paste or pure aluminum, copper, tin, palladium, silver, titanium, nickel, or layer stacks or alloys of the metals mentioned, or other conductive layers, in particular conductive polymers or oxides, or a combination of such layers with metals. The composition and deposition process of the electrodes 34, 36 may differ for both polarities. The electrodes 34, 36 can penetrate the passivation layer locally, particularly in a high-temperature step after screen printing, and contact either the highly doped p-type silicon layer 20 or the locally highly doped n-type base regions 26, depending on the polarity of the electrodes.

[0042] Optionally, before the electrodes 34, 36 are applied, the passivation layer can be removed selectively, for example by laser irradiation, so that the electrodes make direct contact with the p-type silicon layer 20 or the locally highly doped n-type base regions 26.