KR100777717B1 - Method for manufacturing silicon solar cell - Google Patents

Abstract

The present invention discloses a method of manufacturing a silicon solar cell. The manufacturing method includes forming a pn junction on a front surface of a silicon substrate and then removing an oxide film formed on a back surface of the silicon substrate; Sequentially coating a metal thin film and an amorphous silicon thin film having the same polarity as the silicon substrate on a back surface of the silicon substrate from which the oxide film is removed; And heat treating the resultant at a temperature lower than the eutectic temnperature of the metal and silicon for the metal thin film, and then cooling to solid phase epitaxy of the crystallized p ⁺ or n ⁺ silicon layer on the back surface of the silicon substrate. And forming a metal thin film thereon. By using the method of forming the backside field of the present invention, the heat treatment temperature at the time of forming the backside field can be significantly lowered compared to the case of the prior art, thereby reducing the manufacturing cost and introducing impurities and heat load from the outside during the backside field formation process. By reducing the budget, the conversion efficiency of the solar cell can be improved. In addition, according to the present invention it is possible to obtain a rear field layer having a uniform thickness compared to the case of the prior art to maximize the rear field effect. In addition, a smooth interface is formed between the backside field layer and the metal thin film to increase light reflection at the back surface of the silicon substrate. As a result, the light conversion efficiency of the silicon solar cell is improved by improving the light trapping effect.

Description

Method for manufacturing silicon solar cell

1 is a view for explaining a method for forming a back field of a silicon solar cell according to the prior art,

2 is a view for explaining a method for forming a back field of a silicon solar cell according to an embodiment of the present invention,

Figure 3a-b is a view showing an electron scanning micrograph before and after the heat treatment in the silicon solar cell manufactured according to an embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

10, 20 ... p-type silicon substrate 11, 11a, 21 ... n-type junction

12, 22a .. Front side of silicon substrate 12a, 22 ... Rear side of silicon substrate

13, 13a, 23 ... silicon oxide 14a, 24 ... aluminum thin film

15, 28 ... back junction 16, 26 ... conductive layer

17, 27 ... backplane 25 ... amorphous silicon thin film

The present invention relates to a method for manufacturing a silicon solar cell, and more particularly, to form a smooth interface between a backside field layer and a metal layer to increase light reflection at the backside, thereby improving light trapping effects. It relates to an electric field formation method.

The back field process of silicon solar cells is widely used in the manufacture of silicon solar cells, and the thinner the thickness of the silicon substrate and the higher the quality of the substrate, the more effectively the back field is formed.

Currently, in order to lower the price of the silicon substrate, which occupies the largest portion of the silicon solar cell, it is a trend to reduce the thickness of the silicon substrate and use the polycrystalline silicon substrate instead of the single crystal silicon substrate. In the case of a polycrystalline silicon substrate, the quality of the substrate is also improved with increasing silicon grain size. Therefore, the backside electric field process of silicon solar cell is an essential process for high efficiency and low price of silicon solar cell.

According to US Pat. No. 4,056,879, a method of forming a backside field in a silicon solar cell is disclosed. Referring to FIG. 1, a method of forming a backside field according to the related art is as follows.

The diffusion of phosphorus (P) into the p-type silicon substrate 10 forms n-type junctions 11 and 11a into the silicon substrate 10, and the front and rear surfaces 12a and 12a of the silicon substrate 10 are formed. Silicon oxide films 13 and 13a containing phosphorus are formed. Subsequently, after the aluminum thin film 14a is formed on the rear surface 12a of the silicon substrate 10 and heat-treated at a temperature higher than the eutectic temperature of silicon-aluminum, the inside of the rear surface 12a of the silicon substrate 10 is formed. As a result, a backside field layer 17 is formed to form a backside junction 15 and a conductive layer 16 made of aluminum, oxygen, and phosphorus.

As described above, when the back side electric field is formed, reduction of recombination rate of carriers on the back side of the silicon substrate, hydrogen passivation, impurity gettering, and light trapping due to texturing on the back side Effects can improve the photoelectric conversion efficiency of solar cells (Rohatgi et. Al, 23rd IEEE Photovoltaic Specialists Conference, pp. 52-57, 1993).

On the other hand, in order to increase the rear field effect, it is necessary to form the rear field layer by increasing the doping concentration of the material for forming the rear field layer while forming the rear field layer thickly (J. Gee et. Al, 26th Photovoltaic Specialists Conference, pp 275-278). , 1997). At this time, the thickness of the backside field layer is proportional to the thickness of the aluminum thin film, and the doping concentration is proportional to the heat treatment temperature (S. Narasimha. Al, 26th IEEE Photovoltaic Specialists Conference, pp. 63-66, 1997).

In order to effectively remove impurities by the conventional aluminum backside electric field process, high purity aluminum should be used when manufacturing an aluminum thin film, and heat treatment should be performed at a high temperature for a long time. If metal elements such as copper and nickel that are well diffused in silicon are contained in aluminum, they may diffuse into the silicon substrate during the heat treatment process, and heat treatment time much longer than the diffusion time of impurities is required to sufficiently remove impurities from the silicon substrate. (H. Hieslmair et al, 25th IEEE Photovoltaic Specilists Conference, pp. 441-444, 1996).

As described above, the conventional aluminum backside electric field process requires a long heat treatment at a high temperature with a small amount of impurities and a thick aluminum thin film, and thus may not only be disadvantageous in terms of manufacturing cost, but may also degrade the performance of the solar cell. In addition, inflow of impurities from a heat treatment furnace or an aluminum thin film may be promoted during a long time heat treatment at a high temperature. In particular, in the case of a polycrystalline silicon solar cell, the performance of the solar cell may be reduced due to excessive heat load. Therefore, in order to enhance the light trapping effect, the reflectance at the rear side needs to be high, and it needs to be heat-treated as fast as 100 seconds or less at a temperature lower than 580 ° C (M. Cudzinovic et. Al, 25th IEEE Photovoltaic Specialists Conference, pp 501-). 503, 1996).

Another problem with conventional aluminum backside field processes is the nonuniformity of backside layer thickness. Although the thickness of the backside field layer is generally higher as the temperature rises faster during heat treatment, the thickness variation is often more than 50% no matter how fast the heat treatment temperature rises (S. Narasimha et al, 26th IEEE Photovoltaic Specialists Conference, pp 63-66, 1997). As such, if the thickness of the backside field layer is non-uniform, even if the backside recombination rate calculated from the doping concentration distribution is low, the aluminum backside field process may not contribute to improving the efficiency of the solar cell at all.

The technical problem to be solved by the present invention is to solve the above problems by lowering the heat treatment temperature when forming the rear field electric field to reduce impurities inflow and heat load from the outside during the heat treatment process while reducing the thickness variation of the rear electric field layer to improve the rear field effect silicon It is to provide a method of manufacturing a solar cell.

In order to achieve the above technical problem, in the present invention, (a) forming a pn junction on the front surface of the silicon substrate, and then removing the oxide film formed on the back surface of the silicon substrate;

(b) sequentially coating a metal thin film and an amorphous silicon thin film having the same polarity as the silicon substrate on the back surface of the silicon substrate from which the oxide film has been removed; And

(c) heat treating the resultant at a temperature lower than the eutectic temnperature of the metal and silicon for the metal thin film and then cooling the solid phase epitaxy of the p ⁺ or n ⁺ silicon layer crystallized on the back surface of the silicon substrate. And a metal thin film formed thereon, the method for manufacturing a silicon solar cell comprising: a.

When the silicon substrate is a p-type silicon substrate, the metal thin film is preferably made of aluminum (Al). In this case, the thickness of the aluminum thin film is preferably 100 to 1000 nm, and the thickness of the amorphous silicon thin film is 100 to 1000 nm. When the silicon substrate is an n-type silicon substrate, the metal thin film includes one metal thin film selected from palladium and nickel, and an antimony thin film. In this case, the thickness of one metal thin film selected from the palladium and nickel is 1 to 1500 kPa, the thickness of the antimony thin film is 1 to 10 kPa, and the thickness of the amorphous silicon thin film is 0.1 to 50 m.                     

In the aluminum backside field process according to the prior art, by coating an aluminum thin film on the back surface of the silicon substrate, heat treatment at a temperature higher than the process temperature of silicon and aluminum, the liquid phase growth of the silicon layer doped with aluminum from the molten silicon-aluminum ( LPE: Liquid Phase Epitaxy. On the other hand, in the method of forming a backside field according to the present invention, aluminum and an amorphous silicon thin film are sequentially formed on the back side of a silicon substrate, and then heat-treated at a temperature lower than the process temperature of silicon and aluminum to solid-grow the silicon layer doped with aluminum (SPE). : Solid Phase Epitaxy

Solid phase growth is a method of depositing crystals from a solid solution. The principle is that silicon constituting an amorphous silicon thin film is diffused into an aluminum thin film and epitaxially grown on a silicon substrate.

As a method of solid-phase growth of silicon, it can be classified into a method of using a transport medium such as metal or silicide and a method of not using such a transport medium. In the present invention, the former method is used for the solid state growth of silicon, because the method has a lower heat treatment temperature of about 500 ° C. than the latter method, and it is possible to apply photolithography technology, Ohmic contact is well formed between the metal (or silicide) layers formed thereon, and it is easy to pattern with a shadow mask or the like.

Hereinafter, referring to FIG. 2, a method of forming a back field according to the present invention will be described.

First, phosphor is diffused to the front surface 22a of the p-type silicon substrate 20 to form an emitter junction. Next, the oxide film 23 formed on the front surface 22a of the silicon substrate 20 is left as it is, and the oxide film formed on the rear surface 22 of the substrate 20 is removed.

Then, the aluminum thin film 24 and the amorphous silicon thin film 25 made of aluminum having the same polarity as the silicon substrate are sequentially coated on the rear surface 22 of the silicon substrate 20. At this time, the aluminum thin film 24 is used as a transport medium during the silicon solid phase growth, and the thickness of this layer is preferably the same as that of the amorphous silicon thin film 25. If the thickness of the aluminum thin film 24 is thicker than the thickness of the amorphous silicon thin film 25, the grown thin film is discontinuous, and if thin, the surface of the grown thin film is rough.

The aluminum thin film 24 and the amorphous silicon thin film 25 may be coated by a vacuum deposition method using an electron beam, a sputtering method, or the like. Any of these methods may be used. Preferably, it is preferable to coat the two thin films continuously, in order to increase the productivity of the manufacturing process and to suppress the formation of aluminum oxide film at the interface between the aluminum thin film 24 and the amorphous silicon thin film 25. .

As described above, after sequentially coating the aluminum thin film 24 and the silicon thin film 25 on the back surface 22 of the silicon substrate 20, and then heated to 150 to 580 ℃ in an inert gas or hydrogen gas atmosphere, It is cooled slowly. Through this process, the aluminum-doped p ⁺ type silicon layer 27 is grown in solid phase, and an aluminum layer 26 is formed thereon to form a back junction 28 region. At this time, the thickness of the p ⁺ type silicon layer 27 is determined by the thickness of the amorphous silicon thin film 25.

On the other hand, Figure 3a-b is an electron scanning micrograph showing the cross-section of the solar cell specimen formed with the back-field in accordance with a preferred embodiment of the present invention, Figure 3a shows the state before the heat treatment, Figure 3b shows the state after the heat treatment The figure shown.

Referring to FIG. 3A, it can be seen that the aluminum layer and the amorphous silicon layer are sequentially formed on the silicon substrate before the heat treatment. 3B, it can be easily seen that the silicon layer is epitaxially grown on the silicon substrate because the boundary between the silicon substrate and the p + silicon epitaxial layer is not fine after the heat treatment. It was also confirmed that the interface between the p + silicon layer and the aluminum layer was very smooth.

On the other hand, unlike the method described above, when using an n-type silicon substrate instead of a p-type silicon substrate as a semiconductor substrate, an n ⁺ -type silicon layer must be formed as a backside field layer. Therefore, a metal or silicide other than Al must be used. do. Referring to a preferred embodiment for this case is as follows.

On the back of the n-type silicon substrate, one metal thin film selected from a palladium (Pd) thin film or a nickel (Ni) thin film and an antimony (Sb) thin film are sequentially coated, followed by coating an amorphous silicon thin film. Among the metal thin films, palladium and nickel metals contained in the palladium thin film and the nickel thin film are metals having the same polarity as the n-type silicon substrate, and serve to transfer silicon (Si) to the silicon substrate. It is used as an n-type impurity.

The thickness of the metal thin film such as the palladium thin film and the nickel thin film is preferably about 1 to 1500 kPa, particularly about 500 kPa, and the thickness of the antimony thin film is preferably about 1 to 10 kPa, particularly about 5 kPa. The thickness of the amorphous silicon thin film is preferably 0.1 to 50 µm, particularly about 1 µm. Here, when the thickness of the metal thin film such as palladium thin film, nickel thin film, etc. is out of the above range, there is a problem that the heat treatment time is long or the material is wasted, and when the thickness of the antimony thin film is out of the above range, the doping concentration becomes too low or grows. There is a problem that the electrical properties of the silicon thin film is deteriorated. In addition, when the thickness of the amorphous silicon thin film is out of the above range, there is a problem that the rear field effect is degraded or the substrate is deformed.

Thereafter, the resultant obtained by the above process is heat-treated at 200 to 550 ° C., preferably about 280 ° C. At this time, if the heat treatment temperature is higher than 550 ° C, the surface of the grown film may be rough or silicide may remain in the grown silicon thin film. If the heat treatment temperature is lower than 200 ° C, the silicide film as a transport medium may not be formed. Can not do it.

If a palladium thin film is formed as a metal thin film on the back of the silicon substrate, palladium reacts with the silicon substrate to form palladium silicide (Pd ₂ Si), and when the nickel thin film is formed as the metal thin film, nickel reacts with the silicon substrate. Nickel silicide (Ni ₂ Si) is formed.

Subsequently, the heat treatment temperature of the resultant product is raised to a range of 200 to 600 ° C., preferably about 550 ° C., and then gradually cooled to allow the n ⁺ type silicon layer doped with antimony to grow on a silicon substrate. According to the silicon solar cell is completed.

According to the method of forming the backside field of the present invention described above, the thickness variation of the backside field layer formed by the backside field formation method of FIG. 1 is about 50% or more, whereas the thickness variation can be reduced to within 5%. In addition, the growth temperature may be considerably lowered to about 500 ° C. as compared with the conventional technology of growing the doped silicon layer at a temperature higher than the process temperature of aluminum and silicon, that is, 800 ° C. or more.

By using the method of forming the backside field of the present invention, the heat treatment temperature at the time of forming the backside field can be drastically lowered compared to the case of the related art, so that the cost of the heat treatment equipment can be reduced and energy costs can be reduced. In addition, the conversion efficiency of the solar cell can be improved by reducing the inflow of impurities from the outside and the thermal budget during the backside field forming process. In addition, according to the present invention, since the thickness of the backside field layer is determined by the thickness of the amorphous silicon thin film, the backside electric field layer having a uniform thickness corresponding to the thickness distribution of the amorphous silicon thin film can be obtained, thereby maximizing the backside electric field effect. . In addition, a smooth interface is formed between the backside field layer and the metal thin film to increase light reflection at the back surface of the silicon substrate. As a result, the light conversion efficiency of the silicon solar cell is improved by improving the light trapping effect.

Although the present invention has been described with reference to the above embodiments, it is merely illustrative, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. . Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

Claims (8)

(a) forming a pn junction on the front surface of the silicon substrate and then removing the oxide film formed on the back surface of the silicon substrate; (b) sequentially coating a metal thin film and an amorphous silicon thin film having the same polarity as the silicon substrate on the back surface of the silicon substrate from which the oxide film has been removed; And (c) heat treating the resultant at a temperature lower than the eutectic temnperature of the metal and silicon for the metal thin film and then cooling the solid phase epitaxy of the p ⁺ or n ⁺ silicon layer crystallized on the back surface of the silicon substrate. And a metal thin film formed thereon. 2. The method of claim 1, wherein when the silicon substrate is a p-type silicon substrate, The method of manufacturing a silicon solar cell, wherein the metal thin film is made of aluminum (Al). The method of claim 2, wherein the thickness ratio of the aluminum thin film to the thickness of the amorphous silicon thin film is 0.8 to 1.2. The method of claim 1, wherein when the silicon substrate is an n-type silicon substrate, The metal thin film is a method of manufacturing a silicon solar cell, characterized in that it comprises one metal thin film selected from palladium and nickel, and an antimony thin film. The method of claim 4, wherein the thickness of one metal thin film selected from palladium and nickel is 1 to 1500 kPa, The antimony thin film has a thickness of 1 to 10Å, The amorphous silicon thin film has a thickness of 0.1 to 50㎛ manufacturing method of a silicon solar cell. The method of claim 1, wherein the metal thin film and the amorphous silicon thin film are continuously coated in a vacuum state. The method of claim 1, wherein the metal thin film and the amorphous silicon thin film are coated by a vacuum deposition method or a sputtering method using an electron beam. The method of claim 1, wherein the heat treatment temperature of the step (c) is 150 to 580 ℃.

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