EP2556545A2

EP2556545A2 - Method for producing a solar cell

Info

Publication number: EP2556545A2
Application number: EP11703234A
Authority: EP
Inventors: Tim Boescke
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2010-04-09
Filing date: 2011-02-16
Publication date: 2013-02-13
Also published as: KR20130050301A; JP2013524524A; WO2011124409A3; CN102822988A; US20130089942A1; DE102010003784A1; CN102822988B; JP5656095B2; WO2011124409A2

Abstract

The invention relates to a method for producing a solar cell from a silicon substrate, which has a first main surface serving as a light incidence side during use and a second main surface serving as a rear side, having a passivation layer on the second main surface, comprising the steps of: applying an oxide-containing layer to the second main surface of the silicon substrate; and heating the silicon substrate to a temperature of at least 800°C to compact the oxide-containing layer and to oxidize the interface between the oxide-containing layer and the second main surface of the silicon substrate to form a thermal oxide, wherein an oxygen source delivers oxygen for the oxidation.

Description

description

title

The present invention relates to a method for producing a solar cell from a silicon substrate according to claim 1.

PRIOR ART Solar cells usually consist of a silicon substrate. In order to ensure the long-term stability of solar cells and to prevent the penetration of foreign atoms into the substrate, solar cells are provided with a passivation layer. To passivate silicon surfaces of solar cells, dielectric thin films have hitherto been used. In industrial practice, silicon nitride films deposited primarily with a plasma process have become established. However, it is known that thermally grown silicon oxide layers offer significantly better passivation properties. This is the case in particular for the passivation of p-doped surfaces, since high positive charges in silicon nitride have a performance-reducing effect (generation of an inversion layer and "parasitic shunting"), in particular for the passivation of back sides of PERC (passivated emitter and rear cell ) - / PERT (passivated emitter, rear totally-diffused) - / PERL (passivated emitter, rear locally-diffused) cells is therefore the use of thermal oxide desirable. There are many drawbacks in the processes known hitherto for the production of thermal oxides in solar cell production: the process takes a very long time, since the process time increases quadratically with the layer thickness, which leads to high process costs. In addition, the process requires a high thermal budget, which can adversely affect the diffusion profiles. Another disadvantage is that the process is inherently two-sided. However, since the passivation layer is typically required only on one side of the solar cell, the other side of the solar cell must be masked.

For PERC cells, e.g. a process known as "All-Screen Printed 120-pm Thin Large-Area Silicon Solar Cells Applying Dielectric Rear Passivation and Laser-Fired Contacts Reaching 18% Efficiency", L. Gautero et al., 24th EU PVSEC 2009, Hamburg , Session 2D0.2.5), which - summarized in brief - comprises the following steps:

1) texture

2) Cleaning (HN03)

3) Diffusion of POCl ₃ with driving step

4) Path etching of PSG (phosphosilicate glass)

5) SiN deposition front

6) Emitter removal back side

7) Cleaning Standard Cleaning 1 / Standard Cleaning 2

8) oxidation

9) SiO 2 deposition rear side

10) SiN deposition backside

Hereby, the one-sidedness of the oxidation is achieved by front side masking with deposited SiN. To reduce the process time, only a thin layer (~ 20 nm) of oxide is grown and then thickened by deposited oxide or nitride. Since the boundary layer between Si0 ₂ and Si is primarily relevant for the passivation, a passivation quality comparable to the purely thermal oxide is achieved by the layer stack. The disadvantage of this, however, is that the method is technically complex and expensive. The present invention relates to a method for producing a solar cell from a silicon substrate, which has a first main surface serving as a light incident side in use and a second main surface serving as a back side, with a passivation layer on the second main surface, comprising the following steps: Applying an oxide-containing layer to the second main surface of the silicon substrate; and heating the silicon substrate to a temperature of at least 800 ° C to densify the oxide-containing layer and to oxidize the interface between the oxide-containing layer and the second major surface of the substrate

Silicon substrate for the formation of thermal oxide, wherein an oxygen source releases oxygen for the oxidation. An advantage of this method is that it is technically simple and inexpensive.

A process atmosphere of the silicon substrate, in particular 0 ₂ and / or H ₂ 0 comprising, can act as an oxygen source. The oxide-containing layer can be applied such that it, in particular Si0 ₂ , Zr0 ₂ , SiO _a N _b and / or SiO _a C _b , wherein each b << a, comprising, is oxygen-permeable.

An advantage of this is that the method is technically further simplified and cheaper. The oxide-containing layer, in particular comprising SiO ₂ , can be applied to the second main surface of the silicon substrate by a CVD or a PECVD method, in particular using SiH ₄ . This further reduces the cost of the process since the CVD and PECVD processes are very cost effective. In addition, the oxide-containing layer is applied uniformly to the second main surface.

The oxide-containing layer may be a superstoichiometric oxide, in particular SiO _{2 + x} : H and / or a low-density oxide and / or a hygroscopic oxide Oxide, preferably BSG, PSG and / or TEOS oxide, and the oxide-containing layer may function as the oxygen source. As a result, the process is further simplified technically, since no additional source of oxygen is needed. In the method, further, a silicon oxide layer formed during heating of the silicon substrate therefrom may be etched away from the first main surface and a portion of the oxide-containing layer may be etched away from the second main surface. An advantage of this is that, in a simple manner, the silicon substrate is exposed on the first major surface, while the passivation layer on the second major surface is only partially removed.

In the method, a dopant, in particular boron, preferably by means of boron tribromide, and / or phosphorus, preferably by means of phosphorus oxychloride, can be further diffused into the two main surfaces after the application of the oxide-containing layer, wherein the dopant is diffused during the step of heating the silicon substrate the first major surface diffuses, and wherein the oxide-containing layer acts as a masking layer of the second major surface during heating. As a result, a doped layer can easily be formed on the first main surface of the silicon substrate, which can function as an emitter, while the dopant does not diffuse into the second main surface of the silicon substrate. Dopant-silicon compound layers formed during heating of the silicon substrate may be etched away from the first main surface and / or the second main surface. An advantage of this is that the silicon of the silicon substrate is exposed on the first major surface and the oxide-containing layer is exposed on the second major surface.

In the method, furthermore, a surface structure may be applied to the first main surface and / or the second main surface before the application of the oxide-containing layer. An advantage of this is that targeted on Parts of the first and / or second major surface no oxide-containing layer can be applied.

In the method, furthermore, the second main surface can be planarized before the application of the oxide-containing layer. As a result, the application of the oxide-containing layer on the second main surface is significantly improved. Furthermore, in the proposed method, the first main surface and / or the second main surface can be cleaned before the application of the oxide-containing layer, in particular with HNO ₃ . An advantage of this is that the application of the oxide-containing layer is further improved.

In the method, boron or phosphorus may also be diffused into the second major surface or implanted by ion implantation, which is activated upon heating of the silicon substrate, to create a back surface field (BSF) layer. The back-surface field improves the efficiency of the solar cell because the back-surface field is a barrier to the electrons, which therefore does not gain access to the surface of the silicon substrate.

In the method, a SiN anti-reflection layer may further be applied to the first main surface and / or the oxide-containing layer of the second main surface. The anti-reflection layer reflects less light from the silicon substrate, thereby providing more light into the substrate

Silicon substrate penetrates. This increases the efficiency of the solar cell.

Furthermore, one or more holes can be produced by means of a laser through the silicon substrate for connecting the first main surface to the second main surface, in particular by means of a laser, before the application of the oxide-containing layer. The advantage of this is that an electrical connection is formed from the first main surface to the second main surface or vice versa through the holes in a simple manner. In the method, the following method steps can be carried out before the application of the oxide-containing layer: a dopant, in particular boron, preferably by means of boron tribromide, and / or phosphorus, preferably by means of phosphorus oxychloride, is diffused into both main surfaces; the dopant is diffused by heating the silicon substrate into the silicon substrate to form an emitter layer on the first major surface and an emitter layer on the second major surface; dopant-silicon compound layers formed by heating the silicon substrate are etched away from the first main surface and / or the second main surface; a masking layer, preferably SiN, is applied to the first major surface; and the emitter layer of the second main surface is removed, in particular by etching, wherein the SiN layer functions as a masking layer of the first main surface during the removal. An advantage of this is that over the prior art, the step of cleaning the silicon substrate, ie standard cleaning

1 / Standard Cleaning 2, is omitted or can be. This saves time and money and simplifies the process technically.

Drawings Further advantages and advantages of the invention are illustrated by the drawings and explained in the following description. It should be noted that the drawings have only descriptive character and are not intended to limit the invention in any way. Show it:

Fig. La to ld a silicon substrate after successive steps of a method according to the invention for producing a

Solar cell made of a silicon substrate with a passivation layer;

2a to 2d, a silicon substrate after successive steps of a further method according to the invention for the production development of a solar cell made of a silicon substrate with a passivation layer; and

3a to 3d a silicon substrate after successive steps of a further inventive method for producing a solar cell from a silicon substrate with a passivation layer.

In the following description, the same reference numerals will be used for the same and like parts.

FIGS. 1 a to 1 d show a silicon substrate 1 in each case after steps of a method according to the invention for producing a solar cell from a silicon substrate with a passivation layer on the rear side of the silicon substrate

Substrate. In Fig. La, a silicon wafer or silicon substrate 1 is shown. The silicon substrate 1 is made of crystalline silicon 2 and has a first main surface 3, also called the front side, and a second main surface 4, also referred to as a back side, which is opposite the first main surface 3. FIG. 1b shows the silicon substrate 1 after the first method step. In the first method step, silicon dioxide is applied to the second main surface 4 of the silicon substrate 1 by a PECVD method. Instead of silicon dioxide, other oxide-containing layers are conceivable. Other methods for applying the layer are conceivable.

The silicon substrate 1 is in a second process step to a

Temperature of at least 800 ° C heated. As a result, the oxide-containing

Layer 5 compacted and the boundary layer between the oxide-containing layer 5 and the silicon 2 of the silicon substrate 1 (re) oxidized. As a result, a thin layer of high-quality thermal oxide which has good passivation properties is formed at the boundary layer. The source of oxygen can be the

Process atmosphere of the silicon substrate 1 (e., 0 ₂ or H ₂ 0). In this case, the deposited oxide-containing layer 5 is permeable to oxygen, which is the case, for example, for SiO ₂ and SiO _a N _b or SiO _a C _b , when b is much smaller than a. Also conceivable as an oxide-containing layer are other oxygen-conducting metal oxides, such as. B. Zr0 ₂ .

The oxygen source may also be the oxide-containing layer 5 itself. In this case, a superstoichiometric oxide is applied as the oxide-containing layer to the second main surface 4 of the silicon substrate 1. The superstoichiometric oxide releases water and / or oxygen during heating of the silicon substrate. The superstoichiometric oxide may be, for example

Si0 _{2 + x} : H or a hygroscopic oxide such as BSG, PSG, or TEOS oxide. In addition, a low density oxide is useful to facilitate oxygen diffusion. This is typically the case in SiH ₄ processes at low temperatures.

An amorphous Si0 ₂ layer on the silicon substrate is produced by means of Si H ₄ and an oxygen source by a PECVD method. For example, nitrous oxide or pure oxygen can act as an oxygen source for this purpose.

The SiH ₄ processes take place at temperatures between room temperature and about 500 ° C, preferably at a temperature around 200 ° C. Fig. Lc shows the silicon substrate 1 after heating. On the first main surface 3, a silicon dioxide layer 6 has formed. On the opposite second main surface 4, a thermal oxide 6 has formed at the interface between the silicon 2 and the oxide-containing layer 5. A one-sided oxide, ie a solar cell with a passivation layer on only one side of the silicon 2, is now formed by etching the two main surfaces 3, 4. The etching removes the silicon dioxide layer on the first main surface 3 of the silicon substrate 1. On the second main surface 4, only part of the oxide-containing layer 5 is removed by the etching. Thus, a solar cell is produced which comprises only on one side, namely on the back, a passivation layer with a high-quality thermal oxide 6. FIGS. 2 a to 2 d show a silicon substrate 1 after successive steps of a further method according to the invention for producing a solar cell with a passivation layer on the rear side. In FIG. 2 a, a silicon-containing substrate 1, which comprises a wafer made of silicon 2, already has an oxide-containing layer 5 applied to the second main surface 4, which is opposite to the first main surface 3. In a second process step phosphorus is diffused. This forms PSG 7,

Phosphorus silicate glass, on the first main surface 3 of the silicon substrate 1 and on the silicon dioxide 5 on the second main surface 4. The diffused phosphorus is driven by heating the silicon substrate 1 in the silicon 2 of the silicon substrate 1, to form an emitter. 8 on the first main surface 3 of the silicon substrate 1. During this driving step, a thermal oxide layer 6 is formed at the interface between the silicon 2 and the silicon dioxide 5 deposited on the second main surface 4 of the silicon substrate 1. The state of the layer sequence after this process step is shown in Fig. 2b.

By etching the first main surface 3 and second main surface 4, the PSG 7 is removed from the two main surfaces 3, 4. In Fig. 2c, the result after the etching of the two main surfaces 3, 4 is shown. On the first main surface 3, the silicon 2 is now exposed, which comprises a thin layer 8 doped with phosphorus. On the second main surface 4 of the silicon substrate 1 is the thermal oxide 6 and thereon a layer of silicon dioxide 5. In Fig. 2c, the state of the silicon substrate 1 is shown after this process step.

In a further method step, a SiN antireflection layer 9 is then applied to the first main surface 3 of the silicon substrate 1. In Fig. 2d, the silicon substrate 1 is shown after completion of the process. The silicon substrate 1 has only on the back of a passivation layer comprising a thermal oxide 6. FIGS. 3 a to 3d show a silicon substrate 1 after successive steps of a further method according to the invention for producing a solar cell with a passivation layer on one side of the silicon substrate 1. In a first step, a boron layer 10 is introduced as a back-surface field into the second main surface 4 of the silicon substrate 1, for example by diffusion. The silicon substrate 1 after this first step is shown in FIG. 3a.

Thereafter, a silicon dioxide layer 5 is applied to the second main surface 4 of the silicon substrate 1. The sequence of layers after this step is shown in FIG. 3b.

Now, phosphorus is diffused to form an emitter 8. As a result, PSG 7 is formed on the first main surface 3 and on the silicon dioxide 5 on the second main surface 4. During the driving step of the phosphorus into the silicon 2, a thermal heat is generated at the interface between the silicon 2 and the silicon dioxide 5 deposited on the second main surface 4 Oxide layer 6. The thermal step of heating the silicon substrate 1 also activates the boron of the boron layer 10, and damages from the implantation steps are healed. The silicon substrate 1 after this process step is shown in FIG. 3c.

Now, by etching the first main surface 3 and the second main surface 4, PSG 7 is removed from the two main surfaces 3, 4. As the last method step, a SiN antireflection layer 9 is then applied to the first main surface 3 of the silicon substrate 1.

The process described here can be combined with the aforementioned process, which considerably simplifies the process flow, since no additional oxidation step is required and the number of

Cleaning steps is reduced. In addition, the required oxidation time / temperature can be reduced by the method according to the invention in combination with the previously known process flow. Modified process according to the present invention:

1) texture

2) Cleaning (HN0 ₃ )

3) Diffusion of POCl ₃ with driving step

4) Path etching of PSG

5) SiN deposition front

6) Emitter removal RS

7) Si0 ₂ deposition backside

8) oxidation

9) Si deposition rear side

Compared to the previously known process, steps 8) and 9) (now steps 8) and 7)) are reversed. Step 7) of the previously known Sinto process, i. Standard Cleaning 1 / Standard Cleaning 2-step, which is expensive and time-consuming to remove metal contamination, is omitted or can be omitted.

The PERC cell produced by this process can be expanded to a PERT cell using a boron implant. In this case, the POCI ₃ / BBr ₃ diffusion additionally fulfills the function of activation of the implanted dose, so that a total of two high-temperature steps can be saved.

New PERC process according to the present invention: 1) texture (+ backside planarization)

2) Cleaning (HN0 ₃ , maybe more)

3) SiO: H deposition rear

4) Diffusion of POCl ₃ with a driving step

5) Path etching of PSG

6) SiN deposition front

7) SiN deposition rear side This new PERC process saves an additional oxidation step.

In addition, this process can be combined with a MWT (metal wrap through) process flow.

New PERC-MWT process according to the present invention:

1) texture (+ backside planarization)

2) Cleaning

3) SiO: H deposition rear

4) laser holes (+ possibly ablation of the back in the busbar area)

5) Diffusion of POCl ₃ with driving step

6) Removal of PSG

7) SiN deposition front

8) SiN deposition rear side

New PERT process with ion implantation according to the present invention

1) texture (+ backside planarization)

2) Cleaning

3) implantation BSF (phosphorus or boron)

4) SiO: H deposition rear side

5) diffusion of BBr ₃ or POCl ₃ with driving step

6) Path etching of PSG

7) SiN deposition front

8) SiN deposition rear side

Since the POCl ₃ or BBr ₃ diffusion or the Einrreibschritt simultaneously causes the activation of Borimplantation, this will be two

Saved high temperature steps.

The proposed process flows are also applicable without restriction to a cell process flow with selective front diffusion. Here can the quality of the backside passivation can be further improved by a long drive-in step of the front diffusion.

It should be noted at this point that all of the above-described steps of the method taken alone and in any combination, in particular the details shown in the drawings are claimed as essential to the invention. Variations thereof are familiar to the person skilled in the art.

Incidentally, the embodiment of the invention is not limited to the above-described examples and emphasized aspects, but only by the scope of the appended claims.

Claims

Method for producing a solar cell from a silicon substrate (1), which has a first main surface that serves as the light incidence side when in use

(3) and a second main surface (4) serving as a back, with a passivation layer on the second main surface (4), comprising the following steps:

- Applying an oxide-containing layer (5) to the second main surface (4) of the silicon substrate (1); and

- Heating the silicon substrate (1) to a temperature of at least 800 ° C to densify the oxide-containing layer (5) and to oxidize the interface between the oxide-containing layer (5) and the second main surface (4) of the silicon substrate (1 ) to form thermal oxide (6), with an oxygen source releasing oxygen for oxidation.

Method according to claim 1, wherein a process atmosphere,

in particular comprising 0 ₂ and/or H ₂ 0, acts as an oxygen source.

Method according to claim 1 or 2, wherein the oxide-containing layer is applied in such a way that it is permeable to oxygen, in particular SiO ₂ , Zr0 ₂ , SiO _a N _b and/or SiOaCb, where b<<a in each case.

Method according to one of the preceding claims, wherein the oxide-containing layer (5), in particular comprising SiO ₂ , is applied to the second main surface by a CVD or a PECVD process, in particular using SiH ₄

(4) of the silicon substrate (1) is applied.

5. The method according to any one of the preceding claims, wherein the oxide-containing layer (5) is a superstoichiometric oxide, in particular

Si0 ₂ +x:H and/or a low density oxide and/or a hygroscopic oxide, preferably BSG, PSG and/or TEOS oxide, and the oxide-containing layer (5) acts as the oxygen source.

6. The method according to any one of the preceding claims, further comprising a silicon oxide layer formed there during the heating of the silicon substrate (1) from the first main surface (3) and a part of the oxide-containing layer (5) from the second main surface (4). is etched away.

7. The method according to any one of the preceding claims, wherein a dopant, in particular boron, preferably using boron tribromide, and / or phosphorus, preferably using phosphorus oxychloride, is further diffused into the two main surfaces (3) after the oxide-containing layer (5) has been applied , wherein the dopant diffuses into the first main surface (3) during the step of heating the silicon substrate (1), and wherein the oxide-containing layer (5) acts as a masking layer of the second main surface (4) during the heating.

8. The method according to claim 7, wherein further dopant-silicon connection layers formed during the heating of the silicon substrate (1) are etched away from the first main surface (3) and / or the second main surface (4).

9. The method according to any one of the preceding claims, wherein furthermore a surface structure is applied to the first main surface (3) and/or the second main surface (4) before the oxide-containing layer (5) is applied.

10. The method according to any one of the preceding claims, wherein the second main surface (4) is further planarized before the oxide-containing layer (5) is applied.

11. The method according to any one of the preceding claims, wherein further the first main surface (3) and/or the second main surface (4) are cleaned before applying the oxide-containing layer (5), in particular with HN0 ₃ .

12. The method according to any one of the preceding claims, wherein boron or phosphorus is further diffused or implanted into the second main surface (4) to produce a back surface field (BSF) layer (10) by heating the silicon substrate (1) is activated.

13. The method according to any one of the preceding claims, in particular according to any one of claims 6-12, wherein after etching the two main surfaces (3, 4), a SiN anti-reflection layer (9) is applied to the first main surface (3) and/or the oxide-containing layer (5) of the second main surface (4) is applied.

14. The method according to any one of the preceding claims, wherein before applying the oxide-containing layer (5), one or more holes through the silicon substrate (1) for connecting the first main surface (3) to the second main surface (4), in particular by means of a Laser.

15. The method according to any one of the preceding claims, wherein the following process steps are carried out before the oxide-containing layer (5) is applied:

- a dopant, in particular boron, preferably using boron tribromide, and/or phosphorus, preferably using phosphorus oxychloride, is diffused into both main surfaces (3, 4), - the dopant is diffused into the silicon substrate (1) by heating the silicon substrate (1) to form an emitter layer on the first main surface (3) and an emitter layer on the second main surface (4),

- dopant glass layers created by heating the silicon substrate (1) are etched away from the first main surface (3) and/or the second main surface (4),

- a masking layer, preferably SiN, is applied to the first main surface (3), and

- The emitter layer of the second main surface (4) is removed, in particular by etching, the SiN layer acting as a masking layer of the first main surface (3) during the removal.