JP2005154795A - Method for producing thin film, and solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently producing a thin film used for a solar cell with high conversion efficiency, and to provide a solar cell including the thin film produced with the method. <P>SOLUTION: The method for producing the thin film comprises the steps of: introducing hydrogen into a silicon substrate; heating the silicon substrate so that the surface temperature of the silicon substrate becomes 350°C to 500°C; spraying a solution containing the raw material of the thin film onto the surface of the silicon substrate of 350°C to 500°C. The method for introducing hydrogen into the silicon substrate can employ one method selected from the group consisting of a plasma method, a foaming gas annealing method and a Cat-CVD method, and uses a hydrogen-containing gas or a hydrogen gas. The solar cell includes the thin film produced with the above method. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a method for producing a thin film and a solar cell, and more particularly, a method for producing a thin film capable of efficiently producing a solar cell having high conversion efficiency using a spray pyrolysis method, and a solar including the thin film produced using the method. It relates to batteries.

Conventionally, as a method for forming a thin film used for surface protection of a glass substrate and a semiconductor device, for example, a sputtering method or a plasma CVD method is generally used.

In these methods, a high-performance thin film having a very small impurity and a dense structure can be formed. However, on the other hand, an expensive vacuum apparatus is required, and it takes a very long time to grow the thin film, so that the thin film cannot be formed efficiently in terms of cost and time.

As a method for forming a thin film without using a vacuum apparatus, there is a thermal CVD method. However, when this method is used, it is necessary to increase the temperature of the substrate on which the thin film is formed. Further, when a titanium oxide film is formed using a thermal CVD method, the formation rate of the titanium oxide film is very slow, about 30 to 100 liters / min, and titanium alkoxide and water are used as a raw material solution for the titanium oxide film. In the case of using a mixed solution with the above, there is a problem in terms of productivity and reliability such that titanium alkoxide and water react in the nozzle of the thermal CVD apparatus to generate titanium oxide and the nozzle is clogged. .

In Patent Document 1, a tantalum alkoxide solution or a niobium alkoxide solution or the like is applied to a solar cell substrate using a spin method, a spray method, a dip method, or the like, and the solar cell substrate after application is fired. A method for forming an antireflection film at low cost is disclosed.

However, in this method, it is difficult to form an antireflection film having a uniform thickness on fine irregularities on the surface of a single crystal or polycrystalline silicon substrate subjected to texture etching or the like. Therefore, in the solar cell produced using this method, the unevenness of the surface of the antireflection film becomes gentle, and the antireflection effect of sunlight may not be sufficiently obtained. This method also has a problem in that the use efficiency of the solution used for forming the antireflection film is poor.
JP 58-23486 A JP 2001-26885 A JP 7-330336 A JP-A-8-41646 Japanese Patent Laid-Open No. 10-130097 Japanese Patent Laid-Open No. 10-265960 JP 7-182939 A

In view of the above circumstances, an object of the present invention is to provide a method for producing a thin film capable of efficiently producing a solar cell having high conversion efficiency and a solar cell including a thin film produced using the method.

The present invention includes a step of introducing hydrogen into a silicon substrate, a step of heating the silicon substrate so that the surface temperature of the silicon substrate is 350 ° C. or more and 500 ° C. or less, and 350
And a step of spraying a solution containing the raw material of the thin film on the surface of the silicon substrate at a temperature of from ℃ to 500 ℃.

Here, in the thin film manufacturing method according to the present invention, the method of introducing hydrogen into the silicon substrate includes any one hydrogen selected from the group consisting of a plasma method, a forming gas annealing method, and a Cat-CVD method. It may be a method using gas or hydrogen gas.

Moreover, this invention is a solar cell containing the thin film produced using said manufacturing method of a thin film.

As described above, according to the present invention, it is possible to provide a method for producing a thin film capable of efficiently producing a solar cell having high conversion efficiency, and a solar cell including a thin film produced using the method.

(Process to introduce hydrogen into silicon substrate)
Hydrogen is introduced into the silicon substrate used in the present invention. This is because the dangling bonds existing on the surface of the single crystal silicon substrate or the dangling bonds existing on the surface and / or grain boundaries of the polycrystalline silicon substrate are terminated with hydrogen, so that the solar cell receives sunlight. This is because the carriers generated by the incident are prevented from being captured by dangling bonds, and a solar cell having high conversion efficiency is manufactured.

Here, for introducing hydrogen into the silicon substrate, a gas containing hydrogen or a method using hydrogen gas selected from the group consisting of a plasma method, a forming gas annealing method, and a Cat-CVD method may be used. When any one of these methods is used, hydrogen can be efficiently introduced into the silicon substrate.

The plasma method is a method of introducing hydrogen into a silicon substrate using hydrogen radicals and / or hydrogen ions decomposed by applying a high-frequency voltage to a gas containing hydrogen or hydrogen gas.

Hydrogen is introduced into the silicon substrate by the plasma method, for example, as follows. First, a silicon substrate is installed on one of two opposing electrodes installed in a reaction vessel in a vacuum atmosphere, and a gas containing hydrogen or hydrogen gas is allowed to flow into the reaction vessel. Next, hydrogen in the reaction vessel is decomposed by applying a high-frequency voltage between these electrodes using a high-frequency power source to generate hydrogen radicals and / or hydrogen ions. The generated hydrogen radicals and / or hydrogen ions terminate dangling bonds existing on the surface and / or grain boundaries of the silicon substrate, whereby hydrogen is introduced into the silicon substrate.

Here, in the plasma method, after heating the silicon substrate to about 400 ° C. in a reaction vessel in a vacuum atmosphere of several millitorr to several torr, hydrogen gas or the like is introduced.
It is desirable to decompose hydrogen by applying a voltage between the electrodes with a high-frequency power source having a high frequency (about MHz to GHz).

The forming gas annealing method is a method of introducing hydrogen into a silicon substrate using hydrogen radicals and / or hydrogen ions decomposed by heating a gas containing hydrogen or hydrogen gas existing in a reaction vessel.

Hydrogen is introduced into the silicon substrate by the forming gas annealing method, for example, as follows. First, a silicon substrate is placed in a reaction vessel made of quartz or the like in an atmospheric pressure atmosphere, and a gas containing hydrogen or hydrogen gas is allowed to flow into the reaction vessel. And the silicon substrate and the reaction vessel are heated to about 400 ° C.,
Hydrogen radicals and / or hydrogen ions generated by decomposing hydrogen in the reaction vessel terminate dangling bonds existing on the surface and / or grain boundaries of the silicon substrate, whereby hydrogen is introduced into the silicon substrate.

In the forming gas annealing method, since the inside of the reaction vessel is in an atmospheric pressure atmosphere when forming a thin film, hydrogen can be introduced into the silicon substrate at a lower cost than the plasma method or the Cat-CVD method. There is no plasma damage.

The Cat-CVD method is a method in which hydrogen is introduced into a silicon substrate using hydrogen radicals and / or hydrogen ions decomposed by bringing a gas containing hydrogen or hydrogen gas into contact with a heated catalyst (Appl. Phys. Lett. 71 (15), 13, pp. 2169-2
171 (1997)).

Hydrogen is introduced into the silicon substrate by the Cat-CVD method, for example, as follows. First, a tungsten wire serving as a catalyst installed in the reaction vessel is heated to about 1700 to 2000 ° C. Next, a gas containing hydrogen or hydrogen gas is sprayed onto the heated tungsten wire to decompose the hydrogen in the reaction vessel into hydrogen radicals and / or hydrogen ions. Then, hydrogen radicals and / or hydrogen ions are brought into contact with the silicon substrate heated to about 200 to 400 ° C. to terminate dangling bonds where the hydrogen radicals and / or hydrogen ions exist on the surface and / or grain boundaries of the silicon substrate. By doing so, hydrogen is introduced into the silicon substrate.

(Process of heating the silicon substrate)
In FIG. 1, the typical schematic of an example of the manufacturing apparatus of the thin film by the spray pyrolysis method used for this invention is shown. In FIG. 1, a silicon substrate 11 into which hydrogen has been introduced is heated by a heater 12 so that the surface temperature of the silicon substrate 11 is 350 ° C. or more and 500 ° C. or less.

This is because, when the surface temperature of the silicon substrate 11 is less than 350 ° C., a solution (hereinafter referred to as “thin film raw material solution”) 13 containing a thin film raw material to be described later is sprayed on the surface of the silicon substrate 11. At this time, the solvent in the thin film raw material solution 13 is difficult to evaporate, and the thin film raw material solution 13 can flow on the surface of the silicon substrate 11, so that it is difficult to form the thin film 16 having a uniform thickness. Because it is in. When the surface temperature of the silicon substrate 11 is less than 350 ° C., the silicon substrate 1
This is because the adhesion strength between the thin film 16 and the formed thin film 16 becomes very weak and the formed thin film 16 tends to be easily peeled off. Further, when the surface temperature of the silicon substrate 11 is heated to be higher than 500 ° C., hydrogen introduced into the silicon substrate 11 is detached from the silicon substrate 11 and dangling bonds are likely to be regenerated. It is because it tends to become.

(Process of spraying raw material solution of thin film)
After heating the silicon substrate 11, the silicon substrate 11 is sprayed from the spray nozzle 14.
A thin film raw material solution 13 is sprayed on the surface of the silicon substrate 11 to form a thin film 16 on the surface of the silicon substrate 11.

The raw material solution 13 for the thin film used in the present invention is preferably an alkoxide solution of a metal such as titanium, copper, indium or tin that can be formed by a spray pyrolysis method, or a non-metallic alkoxide solution such as silicon. Used for. For example, as the thin film raw material solution 13 used in the present invention, a solution containing titanium alkoxide, an organic solvent such as isopropyl alcohol or ethanol, and a stabilizer such as acetylacetone can be used.

In particular, it is preferable to use a solution containing titanium as the thin film raw material solution 13. In general, when a thin film 16 having a refractive index of about 2.2 is formed on the surface of the silicon substrate 11, a high antireflection effect against sunlight is obtained. In this case, the refractive index is increased on the surface of the silicon substrate 11. This is because the thin film 16 containing about 2.0 to 2.4 of titanium oxide is formed.

As shown in FIG. 1, a thin film raw material solution 13 is stored in a solution tank 17. The thin film raw material solution 13 is jetted from the spray nozzle 14 onto the surface of the silicon substrate 11 together with the compressed air supplied from the air compressor 18. Here, the raw material solution 13 of the thin film becomes a mist by colliding with the compressed air, and is ejected onto the surface of the silicon substrate 11.

When the fine particles of the thin film raw material solution 13 sprayed on the surface of the silicon substrate 11 adhere to the heated surface of the silicon substrate 11, a solid phase is immediately deposited. Accordingly, since the thin film raw material solution 13 cannot flow on the surface of the silicon substrate 11, the thin film 16 having a uniform thickness can be formed on the unevenness of the surface of the silicon substrate 11.

The surface temperature of the silicon substrate 11 when the thin film raw material solution 13 is sprayed is set to 350 ° C. or more and 500 ° C. or less by being heated by the heater 12. However, when the thin film raw material solution 13 is sprayed, the surface temperature of the silicon substrate 11 decreases due to the heat of vaporization of the solvent constituting the thin film raw material solution 13.

Therefore, the control unit 15 controls the spraying of the thin film raw material solution 13 at a predetermined interval, and raises the heating temperature of the heater 12 when the thin film raw material solution 13 is not sprayed. The surface temperature of the silicon substrate 11 is preferably controlled so as to recover to 350 ° C. or more and 500 ° C. or less.

The spraying of the thin film raw material solution 13 and the heating of the silicon substrate 11 are alternately repeated a plurality of times. By this repetition, a plurality of extremely thin layers are stacked on the surface of the silicon substrate 11 to have a desired thickness. A thin film 16 may be formed. Therefore, it is preferable to spray the raw material solution 13 repeatedly for a plurality of times in order to obtain the thin film 16 having a uniform desired thickness.

(Action / Effect)
When a thin film is formed by sputtering or plasma CVD as in the prior art, a vacuum atmosphere is required when forming the thin film. When a thin film is formed by thermal CVD, the silicon substrate is heated to a high temperature when forming the thin film. It needs to be heated. Therefore, when these methods are used, hydrogen once introduced into the silicon substrate tends to be easily detached and dangling bonds tend to be regenerated.

However, in the present invention, since the thin film is formed by using the spray pyrolysis method, it is not necessary to form the thin film in a vacuum atmosphere like the sputtering method or the plasma CVD method. It is not necessary to form a thin film by heating to a high temperature. Accordingly, since hydrogen is less likely to be detached from the silicon substrate than when these methods are used, it is possible to effectively prevent dangling bonds from being regenerated in the silicon substrate during the formation of the thin film.

(Solar cell)
FIG. 2 shows a flowchart of a preferred example of a method for manufacturing a solar cell according to the present invention. First, in step 1, a texture structure is formed on the surface of a p-type polycrystalline silicon substrate for the purpose of removing a damaged layer by cutting the substrate and increasing the antireflection effect. Here, as a method of forming the texture structure, there are a method of forming by dry etching such as reactive ion etching, a method of forming by wet etching using an alkaline solution, or a method of forming mechanically.

Next, in step 2, an n-type impurity such as phosphorus is diffused in the p-type polycrystalline silicon substrate to form a pn junction. The pn junction can be formed, for example, by applying a solution containing n-type impurities such as phosphorus on the surface of a p-type polycrystalline silicon substrate and then heat-treating the silicon substrate.

In step 3, hydrogen is introduced into the silicon substrate on which the pn junction is formed. Here, as a method for introducing hydrogen into the silicon substrate, for example, the above-described plasma method, forming gas annealing method, Cat-CVD method, or the like can be used.

Thereafter, in step 4, the surface temperature of the silicon substrate is 350 ° C. or more and 500 ° C.
The silicon substrate is heated by a heater or the like so that the temperature is not higher than ° C.

In step 5, a thin film raw material solution is obtained from a spray nozzle or the like.
A thin film is formed by spraying on the surface of a silicon substrate at 0 ° C. or more and 500 ° C. or less. The heating of the silicon substrate in step 4 and the spraying of the raw material solution for the thin film in step 5 can be alternately repeated a plurality of times.

Finally, in step 6, a front electrode and a back electrode are formed on the silicon substrate on which the thin film has been formed. These electrodes can be formed as follows, for example. First, a silver paste containing silver powder is printed on the surface of the thin film. Also, an aluminum paste containing aluminum powder is printed on the back surface of the silicon substrate.
Then, by performing a heat treatment on the entire silicon substrate, a front electrode made of silver and a back electrode made of aluminum can be formed. After that, on the surface electrode,
Solder coating can be applied.

The solar cell according to the present invention thus obtained has a large open circuit voltage and short circuit current, and has high conversion efficiency.

The reason for this is that when the carriers generated by the incidence of sunlight on the solar cell according to the present invention move inside the silicon substrate, the hydrogen introduced into the silicon substrate sufficiently causes dangling bonds inside the silicon substrate. Since it can be terminated, it is considered that carriers can freely move inside the silicon substrate without being trapped by dangling bonds.

In addition, since a thin film having a uniform thickness can be formed on the surface of a p-type polycrystalline silicon substrate having fine irregularities due to a texture structure, the surface of the thin film on the sunlight receiving surface side also corresponds to the irregularities of the silicon substrate. It is considered that the minute irregularities are formed and reflection of incident sunlight can be effectively prevented.

In the above description, a solar cell is manufactured by applying an n-type impurity to a p-type polycrystalline silicon substrate. However, a solar cell is applied by applying a p-type impurity to an n-type single crystal or polycrystalline silicon substrate. It goes without saying that the same effects as described above can be obtained even if a solar cell is manufactured by applying an n-type impurity to a p-type single crystal silicon substrate.

(Example 1)
First, a texture structure was formed on the surface of the prepared p-type polycrystalline silicon substrate.
Next, after forming a pn junction on the silicon substrate, the silicon substrate is placed in a reaction vessel, and hydrogen gas (volume ratio 3%) and nitrogen gas (volume ratio 9%) are placed in the reaction vessel.
7%) and a silicon substrate and a reaction vessel were heated by a heater. Then, the atmosphere in the silicon substrate and the reaction vessel is changed to 2 at 400 ° C.
Hydrogen was introduced into the silicon substrate by holding for a period of time.

Next, the silicon substrate is taken out from the reaction vessel, and the silicon substrate after introduction of hydrogen is placed on the heater of the thin film production apparatus schematically shown in FIG. 1, so that the surface temperature of the silicon substrate becomes 400 ° C. by the heater. The silicon substrate was heated. And about 4
A solution containing titanium (MOF-Ti-30000s manufactured by Tokyo Ohka Kogyo Co., Ltd.) was sprayed from the spray nozzle onto the surface of the silicon substrate at 00 ° C. Then, spraying of the solution and heating of the silicon substrate were alternately repeated a plurality of times to form an antireflection film containing titanium oxide having a predetermined thickness.

Finally, a back electrode and a front electrode were formed on a silicon substrate on which an antireflection film was formed to produce a solar cell. Here, each of these electrodes was formed by printing a silver paste on the surface of the antireflection film and printing an aluminum paste on the back surface of the silicon substrate, and then heat-treating the entire silicon substrate. Thereafter, a solder coating was applied on the surface electrode. In addition, the area of the sunlight receiving surface of these solar cells was set to 19 cm ² , respectively.

(Comparative Example 1)
In Comparative Example 1, a solar cell was produced in the same manner as in Example 1 except that hydrogen was not introduced into the silicon substrate.

(Comparative Example 2)
In Comparative Example 2, a solar cell was produced in the same manner as in Example 1 except that hydrogen was not introduced into the silicon substrate and an antireflection film was formed using a spin coating method.

(Evaluation of solar cells)
In FIG. 3, the current-voltage characteristic of the solar cell produced in Example 1 and Comparative Examples 1-2 is shown. Table 1 shows the shunt resistance values and reverse saturation current amounts of the solar cells produced in Example 1 and Comparative Examples 1 and 2. Here, the shunt resistance value is an actual measurement value, and the reverse saturation current amount is a numerical value predicted by performing simulation from the current-voltage curve of FIG.

As shown in FIG. 3, the solar cell of Example 1 had a higher open circuit voltage than the solar cells of Comparative Examples 1 and 2. This is presumably because, in the solar cell of Example 1, hydrogen introduced into the silicon substrate by the forming gas annealing method is terminated and dangling bonds in the silicon substrate are terminated. This means
As shown in Table 1, the shunt resistance value in the solar cell of Example 1 was 90Ω, which was larger than 50Ω in the solar cells of Comparative Examples 1 and 2, and the reverse saturation current in the solar cell of Example 1 The amount is 2.0 × 10 ⁻⁷ A and Comparative Examples 1 to
This is demonstrated by less than 4.0 × 10 ⁻⁷ A in 2 solar cells.

In addition, as shown in FIG. 3, the solar cell of Comparative Example 2 in which the antireflection film was formed using the spin coating method had a remarkably smaller short circuit current than the solar cells of Example 1 and Comparative Example 1. This is because, in the solar cell of Comparative Example 2, unlike the solar cells of Example 1 and Comparative Example 1, since the spin coating method was used for forming the antireflection film, the film thickness of the antireflection film varied greatly. This is considered to be because the surface of the antireflection film became smooth and the antireflection effect of sunlight could not be sufficiently obtained.

In this embodiment, the case where a polycrystalline silicon substrate is used has been described. However, even when a single crystal silicon substrate is used, the same effects as in this embodiment can be exhibited.

Needless to say, the current-voltage characteristics and the shunt resistance values of the solar cells of Example 1 and Comparative Examples 1 and 2 were all measured under substantially the same conditions.

It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a typical schematic diagram of an example of a manufacturing device of a thin film used for the present invention. It is a flowchart of a preferable example of the manufacturing method of the solar cell which concerns on this invention. It is a figure which shows the current-voltage characteristic of the solar cell produced in Example 1, the comparative example 1, and the comparative example 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Silicon substrate, 12 Heater, 13 Thin film raw material solution, 14 Spray nozzle, 15 Control means, 16 Thin film, 17 Solution tank, 18 Air compressor.

Claims (3)

A step of introducing hydrogen into the silicon substrate, a step of heating the silicon substrate so that the surface temperature of the silicon substrate is 350 ° C. or more and 500 ° C. or less,
Spraying a solution containing the raw material of the thin film onto the surface of the silicon substrate at 350 ° C. or higher and 500 ° C. or lower. A method for introducing hydrogen into a silicon substrate is a method using any one of hydrogen-containing gas or hydrogen gas selected from the group consisting of a plasma method, a forming gas annealing method, and a Cat-CVD method. The manufacturing method of the thin film of Claim 1. The solar cell containing the thin film produced using the manufacturing method of the thin film of Claim 1 or 2.

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