JP2000183379A - Method for manufacturing solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively manufacture a p-n junction-type solar cell with high optoelectric conversion efficiency, using a crystal system semiconductor by reducing the number of the manufacturing processes. SOLUTION: A diffusion film 11 containing impurity is formed on the surface of the non-electrode part of a crystal system semiconductor (semiconductor substrate 1), then the semiconductor is subjected to thermal diffusion in an atmosphere of diffusion gas containing the impurity after forming the film 11, a lightly-doped layer (n+ layer 2a) based on the diffusion film of the impurity from the film 11 is formed at a non-electrode part, and at the same time, a heavily-doped layer (n++ layer 2b) based on the diffusion of the impurity from the diffusion gas is formed at an electrode part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for thermally diffusing impurities into a crystalline semiconductor, and diffusing impurities into the electrode portion on the surface side thereof at a higher concentration than the non-electrode portion, thereby increasing the p-to-p conversion efficiency.
The present invention relates to a method for manufacturing a solar cell for forming an n-junction type solar cell.

[0002]

2. Description of the Related Art Conventionally, a pn junction type standard solar cell has a resistivity of 0.3 to 0.3, for example, as a p-type crystalline semiconductor.
1 Ωcm p-type crystalline silicon (hereinafter referred to as crystalline silicon c-
It is manufactured by thermally diffusing an n-type impurity to the surface side (light-receiving surface side) of the p-type c-Si substrate to form a pn junction.

In general, this solar cell has a diffusion layer depth (junction depth) of 0.5 μm and an n-layer impurity density of 2 μm.
4 × 10 ⁻²⁰ cm ⁻³ . In this case, the impurity density (surface layer concentration) becomes extremely high at 10 ²¹ cm ^{−3 in} the outermost surface region of the n-layer. There is a disadvantage that the length is shortened by the coupling and the photoelectric conversion efficiency is low.

On the other hand, from the aspect of extracting electricity, it is desirable to increase the impurity density and reduce the resistance as much as possible.

Therefore, the diffusion surface is oxidized and etched to reduce the depth of the diffusion layer to 0.2 to 0.3 μm and to reduce the impurity concentration of the n-layer by one digit to improve the optical response in a short wavelength region. In order to improve or reduce the series resistance of the electrode portion where the electrode is formed, a large amount of impurities are added to the n ⁺ layer instead of the n ⁺ layer.
Attempts have been made to improve the photoelectric conversion efficiency by forming a ⁺⁺ layer, and as a result, a pn junction type solar cell having a high photoelectric conversion efficiency with the cell structure shown in FIG. 2 has been devised.

In the solar cell shown in FIG. 2, an n-layer 2 is formed on the surface side of a p-type semiconductor substrate 1, the surface of a non-electrode portion of the n-layer 2 is covered with an antireflection film 3, and the surface of the electrode portion is covered. Is the surface Ag of the grid electrode pattern as a so-called comb-shaped electrode
An electrode 4 is provided.

On the back surface of the semiconductor substrate 1, a back Al electrode 6 and a back Ag electrode 7 are provided as back electrodes via ap ⁺ layer (highly doped layer) 5 in order to prevent recombination of carriers. I have.

The non-electrode portion (light receiving portion) of the n-layer 2 where the electrode is not formed is a lightly doped layer, that is, the n ⁺ layer 2a, in order to prevent recombination of carriers, and is almost directly under the surface Ag electrode 4. In order to reduce the resistance of the electrode part of
^A highly doped layer containing impurities in a larger amount than the ⁺ layer 2a, that is, an n ⁺⁺ layer 2b.

Next, a conventional method for manufacturing the solar cell of FIG. 2 will be described with reference to a manufacturing process diagram of FIG. First,
As shown in FIG. 3A, 3 × 10 ¹⁵ to 4 × 10 ¹⁶
A p-type semiconductor substrate 1 having an impurity concentration of cm ⁻³ is prepared.

The semiconductor substrate 1 generally has (100)
The light-receiving surface is formed on a textured surface having many fine pyramid-shaped irregularities in order to reduce light reflection.

This textured surface has a few percent of N
The semiconductor substrate 1 is formed by heating the semiconductor substrate 1 at 80 ° C. to 90 ° C. for 20 to 30 minutes while adding isopropyl alcohol in a solution containing aOH.

Then, for example, POCl is used as a diffusion gas.
_In an atmosphere containing _three gases, 85
A heat diffusion process is performed at 0 ° C. for 30 minutes, and an n ⁺ layer 2 a having a thickness of, for example, 0.4 μm is formed on the entire surface of the semiconductor substrate 1 as shown in FIG.

Further, the semiconductor substrate 1 is oxidized at 1000 ° C. for 60 minutes in an oxygen gas atmosphere to form a diffusion mask S covering the n ⁺ layer 2a as shown in FIG.
An oxide film 8 of iO ₂ is formed.

Then, a resist film 9 shown in FIG. 3D is formed on the surface of the oxide film 8 by a photolithography process so as to cover only the non-electrode portion on the surface side of the semiconductor substrate 1, and is oxidized by a dry etching process. The film 8 is subjected to mask pattern processing, the oxide film 8 on the electrode portion on the surface of the substrate 1 is removed, and after this processing, the resist film 9 is removed.

Next, the semiconductor substrate 1 is subjected to a thermal diffusion treatment at, for example, 900 ° C. for 40 minutes in an atmosphere containing, for example, a POCl ₃ gas, and as shown in FIG. the ^{n ++} layer 2b is formed in a portion immediately below the electrode, after the formation of the semiconductor substrate 1 was immersed for 1 minute in a solution containing 10% HF, the resist film 9 and the ^{n +} layer 2a becomes ^{unnecessary, n ++} The phosphorus glass formed on the surface of the layer 2b is removed.

Further, as shown in FIG. 3 (f), an anti-reflection film 3 is formed on the surface of the n ⁺ layer 2a, n ^{+ +} An acid-resistant resist film 1 is formed on the surface of the antireflection film 3 in order to remove the unnecessary n ⁺⁺ layer 2b outside the broken line in the figure.
0 is applied by a screen printing method and dried.

Then, the semiconductor substrate 1 is etched with a mixed acid (HF: HNO ₃ = 1: 3) containing HF and HNO _{3 at} a ratio of 1: 3, and the unnecessary n ⁺⁺ layer 2b of the substrate 1 is etched.
After the removal, the resist film 10 is also removed with a solvent such as toluene or xylene, and an n-layer 2 and an antireflection film 3 are formed on the surface of the semiconductor substrate 1 as shown in FIG. .

Next, after using a paste containing Ag and a paste containing Al on the back surface of the semiconductor substrate 1 in a predetermined pattern by a screen printing method or the like, and drying, the semiconductor substrate 1 is heated to 700 ° C. A heat treatment is performed at 800 ° C. to form a back Al electrode 6 having a thickness of about 20 μm on the back of the semiconductor substrate 1.
Is fired.

At this time, the back Ag electrode 7 is also formed.
At this time, aluminum and silicon are alloyed, and ap ⁺ layer 5 is formed in a portion of the semiconductor substrate 1 in contact with the Al electrode 6.
⁺ Layer 5 has a thickness of about 5 μm and a BSF (Back Sur
face Field) effect.

Further, a paste containing Ag is applied to the surface of the antireflection film 3 by a screen printing method or the like and dried,
The semiconductor substrate 1 is heat-treated at 600 to 700 ° C. for about 20
The surface Ag electrode 4 having a thickness of μm is fired.

At this time, the Ag-containing paste (silver paste) contains glass frit, and the surface Ag electrode 4 is ohmically connected to the n ⁺⁺ layer 2b through the antireflection film 3.

[0022]

In the case of the conventional method for manufacturing a pn junction type solar cell using a crystalline semiconductor shown in FIG. 3, an n ⁺⁺ layer 2b is formed to remove impurities in an electrode portion of the n layer 2. In order to make the diffusion concentration higher than the n ⁺ layer 2a in the non-electrode portion, separately from the step of forming the n ⁺ layer 2a, FIG.
An oxide film 8 for a diffusion mask is formed, and the oxide film 8 is subjected to a photoresist treatment shown in FIG.
Is required to form the n ⁺⁺ layer 2b by performing the thermal diffusion process described above.

The thermal diffusion is performed by forming the n ⁺ layer 2a and n
^Since the formation of the ⁺⁺ layer 2b is performed twice in total, the number of manufacturing steps is large, the processing is complicated, and the manufacturing cost is increased.

An object of the present invention is to simplify the processing by reducing the number of manufacturing steps and to manufacture a pn junction type solar cell having high photoelectric conversion efficiency at low cost.

[0025]

In order to solve the above-mentioned problems, in a method of manufacturing a solar cell according to the present invention, a diffusion film containing impurities is formed on the surface of a non-electrode portion of a crystalline semiconductor, and After the diffusion film is formed, the semiconductor is thermally diffused in an atmosphere of a diffusion gas containing impurities to form a lightly doped layer based on the diffusion of the impurities in the diffusion film in the non-electrode portion, and the impurity from the diffusion gas is formed in the electrode portion. To form a highly doped layer based on the diffusion of

Therefore, if the crystalline semiconductor is a p-type semiconductor, the lightly doped layer on the non-electrode portion on the surface side of the semiconductor is an n ⁺ layer, and the heavily doped layer on the electrode portion is n ⁺ layer.
⁺⁺ layer, and both of these layers are formed by a single thermal diffusion process, so that the number of manufacturing steps is smaller than before, and this type of pn junction type solar cell can be manufactured at low cost.

Preferably, the diffusion film is formed to a thickness such that impurities contained in the diffusion gas hardly reach the crystalline semiconductor.

Further, the condition of the impurity concentration in the diffusion film, the impurity concentration in the diffusion gas or the condition of the thermal diffusion is set so that the amount of the impurity diffused from the diffusion gas to the electrode portion is more than the amount of the impurity diffused from the diffusion film to the non-electrode portion. It is desirable to adjust the amount to be larger.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to FIGS. (One Embodiment) First, one embodiment of the present invention will be described. 1A to 1F, the same reference numerals as those in FIGS. 3A to 3H denote the same or corresponding components.

In this embodiment, a p-type semiconductor substrate 1 shown in FIG. 1A is prepared as a crystalline semiconductor.

The semiconductor substrate 1 is made of, for example, a c-Si substrate having a (100) main surface, like the conventional substrate.
Its surface is formed on the textured surface.

Then, as shown in FIG. 3B, a film of an appropriate concentration (low concentration) of an n-type dopant agent, that is, an n-type dopant is formed on the surface of the non-electrode portion of the semiconductor substrate 1 as the n ⁺ layer 2a. The diffusion film 11 containing impurities is formed by applying a screen printing method or the like.

Specifically, the dopant of the diffusion film 11 is, for example, 200 ml of isopropyl alcohol and 4 ml of isopropyl alcohol.
It is a paste-like dopant agent containing 5 g of phosphorus pentoxide (P ₂ O ₅ ), and contains other components such as ethyl acetate, carboxylic acid, and silicon dioxide.

In addition, the amount of phosphorus pentoxide as the dopant agent is actually set to a desired low diffusion concentration (10
^The thickness is adjusted in the range of ²⁰ g to 70 g according to the diffusion conditions so that the n ⁺ layer 2 a is formed at ¹⁹ cm ⁻³ to 10 ²⁰ cm ⁻³ ).

Next, after the temperature of the semiconductor substrate 1 is increased and the diffusion film 11 is temporarily dried, the semiconductor substrate 1 is set in a diffusion jig, and thermal diffusion is performed in an atmosphere containing a diffusion gas such as POCl ₃ gas. Apply.

By performing the thermal diffusion in this manner, the impurity (phosphorus) contained in the diffusion film 11 diffuses into the non-electrode portion covered with the diffusion film 11 on the surface side of the semiconductor substrate 1,
An n ⁺ layer 2a shown in FIG. 1C is formed.

On the other hand, the impurity (phosphorus) in the diffusion gas is diffused into the electrode portion on the surface side of the semiconductor substrate 1 which is not covered with the diffusion film 11, and the n ⁺⁺ layer 2b shown in FIG. Is done.

At this time, in the non-electrode portion where the diffusion film 11 is formed, diffusion of impurities in the diffusion gas is performed by the diffusion film 1.
Suppressed by 1. Therefore, by adjusting the thickness of the diffusion film 11, diffusion of impurities contained in the diffusion gas into the semiconductor substrate 1 can be suppressed. In the present invention, the thickness of the diffusion film 11 is set so that impurities contained in the diffusion gas hardly reach the semiconductor substrate 1. By adjusting the thickness of the diffusion film 11 in this manner, the amount of impurities in the n ⁺ layer 2a formed in the non-electrode portion can be controlled by the amount of impurities diffused from the diffusion film 11.

According to the method described above, the impurity concentration of the n ⁺ layer 2a can be controlled by the impurity concentration of the dopant in the diffusion film 11 and the condition of thermal diffusion.
Further, the impurity concentration of the n ⁺⁺ layer 2b can be controlled by the impurity concentration in the diffusion gas and the condition of thermal diffusion. According to the present invention, the diffusion film 1 is formed such that the impurity concentration of the n ⁺⁺ layer 2b is higher than the impurity concentration of the n ⁺ layer 2a.
1, the impurity concentration in the diffusion gas, and the conditions of thermal diffusion are determined.

Therefore, by the one-time thermal diffusion,
And n ⁺⁺ layer 2b of n ⁺ layer 2a and the electrode portion of the non-electrode portion of the surface side of the semiconductor substrate 1 are formed at the same time.

The n ⁺ formed by this one-time thermal diffusion
Layer ^2a, the concentration of the ^{n ++} layer 2b (surface density) and the diffusion layer depth, 900 ° C. in an atmosphere containing _{POCl 3} gas, was subjected to a thermal diffusion of 40 minutes, ^{n +} layer 2a concentration: 1-5 ×
10 ¹⁹ cm ⁻³ , the depth of the diffusion layer is 0.2 to 0.3 μm, and the concentration of the n ⁺⁺ layer 2b is 2 × 10 ²⁰ cm ⁻³ to 1 × 1.
0 ²¹ cm ⁻³ , diffusion layer depth 0.5-2.0 μm,
Good results were obtained.

The high-concentration doped layer (n ⁺⁺ layer 2b) is also formed on the side and back surfaces of the semiconductor substrate 1 not covered with the diffusion film 11 by the thermal diffusion.

After completion of the diffusion process, for example, 10%
The semiconductor substrate 1 is immersed in a solution containing HF for 1 minute, and n
Unnecessary phosphorus glass formed on the surface of the ⁺ layer 2a, n ⁺⁺ layer 2b is removed.

Next, as shown in FIG. 1D, the anti-reflection film 3 is formed on the surface of the n layer 2 formed by the n ⁺ layers 2a and ^{2 +} b on the surface side of the semiconductor substrate 1 as in the conventional case. About 70-8
It is formed to a thickness of 0 nm, and then unnecessary n outside the broken line in FIG.
^In order to remove the ⁺⁺ layer 2b, the anti-reflection
An acid-resistant resist film 10 is applied to the surface of the substrate by a screen printing method, and dried to form a resist film.

Further, the semiconductor substrate 1 is etched with a mixed acid (HF: HNO ₃ = 1: 3) containing HF and HNO _{3 at} a ratio of 1: 3, and the unnecessary n ⁺⁺ of the semiconductor substrate 1 is etched.
The layer 2b is removed, the resist film 10 is also removed with a solvent such as toluene or xylene, and an n-layer 2 and an anti-reflection film 3 are formed on the surface of the semiconductor substrate 1 as shown in FIG.

Next, a paste containing Ag and a paste containing Al are applied on the back surface of the semiconductor substrate 1 in a predetermined pattern by a screen printing method or the like and dried, and then the semiconductor substrate 1 is heated at 700 to 800 ° C. Heat treatment is performed to form a back Al electrode 6 and a back Ag electrode 7 on the back surface of the semiconductor substrate 1 as before, and a thickness of about 5 μm is formed on a portion of the semiconductor substrate 1 in contact with the Al electrode 6 by alloying aluminum and silicon. forming a ^{p +} layer 5.

Then, a paste containing Ag is applied to the surface of the antireflection film 3 by a screen printing method or the like, and dried.
The semiconductor substrate 1 is heat-treated at 600 ° C. to 700 ° C., and the surface Ag electrode 4 similar to the conventional one is fired to form a solar cell.

Therefore, in the case of this embodiment, the diffusion film 11 is applied to the surface of the non-electrode portion of the substrate 1 and subjected to the thermal diffusion process, whereby the n ⁺ layer 2 can be formed in one thermal diffusion process.
a and the n ⁺⁺ layer 2b are formed at the same time, and FIG.
As is clear from the comparison between (f) and (a) to (g) of FIG. 3, the step of forming the conventional n ⁺⁺ layer 2b can be omitted, and the photoelectric conversion can be performed using a p-type crystalline semiconductor. This type of pn junction type solar cell having high conversion efficiency can be easily and inexpensively manufactured.

(Other Embodiments) In the first embodiment, a p-type semiconductor substrate is used as a crystalline semiconductor, and
Although the present invention is applied to the case where a pn junction is formed by thermally diffusing a type impurity, the present invention employs a crystalline semiconductor as an n-type semiconductor substrate,
The same can be applied to the case where a pn junction is formed by thermally diffusing an n-type impurity into this substrate.

In this embodiment, the n-type semiconductor substrate has p
When a pn junction is formed by thermally diffusing a type impurity,
This will be described with reference to FIG.

First, an n-type semiconductor substrate is prepared in place of the semiconductor substrate 1 of FIG. The semiconductor substrate has a resistivity of 0.3.
An n-type c-Si of 5 to 3.0 Ωcm can be used.

Then, a p-type diffusion film corresponding to the diffusion film 11 of FIG. 1 is applied and formed on the surface of the non-electrode portion of the n-type semiconductor substrate.

This p-type diffusion film is made of a dopant containing p-type impurities.
It is a paste containing 00 ml of isopropyl alcohol and 30 g of boron trioxide (B ₂ O ₃ ).

The amount of boron trioxide is adjusted to a desired low concentration (10 ¹⁹ cm ^{−3) in} the non-electrode portion on the surface side of the semiconductor substrate.
In order to form a p ⁺ layer of 〜１０10 ⁻²⁰ cm ⁻³ ), the amount is adjusted up or down in the range of 10 to 60 g according to diffusion conditions and the like.

Then, after a diffusion film is applied and temporarily dried,
The semiconductor substrate is set in a diffusion jig, and thermal diffusion is performed in an atmosphere of, for example, BBr ₃ gas as a diffusion gas.

By performing the thermal diffusion as described above, the impurity (boron) contained in the diffusion film is diffused into the non-electrode portion covered by the diffusion film on the surface side of the semiconductor substrate, and the impurity (boron) in FIG. The p ⁺ layer is formed at the position of the n ⁺ layer 2a.

On the other hand, the diffusion film on the front surface side of the semiconductor substrate
The electrode parts that are not covered with
(Boron) diffuses and n in FIG.⁺⁺Layer 2b
P ⁺⁺A layer is formed.

At this time, in the present embodiment, as in the first embodiment, the thickness of the diffusion film is set such that impurities contained in the diffusion gas hardly reach the semiconductor substrate.

[0059] Thus, the impurity concentration of the dopant agent impurity concentration of p ⁺ layer in the diffusion layer, and can be controlled by the conditions of the thermal diffusion, also the impurity concentration and thermal diffusion in the gas impurity concentration of the p ⁺⁺ layer It can be controlled by diffusion conditions. And also in this embodiment, p
The impurity concentration in the diffusion film, the impurity concentration in the diffusion gas, and the conditions of thermal diffusion are determined so that the impurity concentration of the ⁺⁺ layer is higher than the impurity concentration of the p ⁺ layer.

[0060] Then, the surface concentration of the ^{p +} layer, ^{p ++} layer of the electrode portion of the non-electrode portion, the diffusion layer depth, 1000 ° C. in an atmosphere containing BBr ₃ gas, where subjected to thermal diffusion of 60 minutes ^{p +} The concentration of the layer is 1-5 × 10 ¹⁹ cm ⁻³ , and the depth of the diffusion layer is:
0.2-0.3 μm, concentration of p ⁺⁺ layer: 2 × 10 ²⁰ cm
⁻³ to 1 × 10 ²¹ cm ⁻³ , diffusion layer depth: 0.5 to 2.
0 μm, and good results were obtained.

After the completion of the thermal diffusion, the semiconductor substrate is
Immersed in a solution containing 0% HF for 1 minute, p ⁺ layer, p ⁺
Unnecessary boron glass formed on the surface of the ⁺⁺ layer is removed.

Thereafter, an antireflection film and a resist film corresponding to the antireflection film 3 and the resist film 10 of FIG. 1D are formed, and the semiconductor substrate is etched with a mixed acid (HF: HNO ₃ = 1: 3). , Unnecessary parts are removed, and FIG.
The electrodes corresponding to the respective electrodes 3, 6, 7 are formed.

[0063] Thus, a low concentration of p ⁺ layer formed on the non-electrode portion of the surface side of the n-type semiconductor substrate is subjected to even one thermal diffusion in this embodiment, the high-concentration p ⁺⁺ layer of the electrode portion And the same effects as in the case of the first embodiment can be obtained.

Incidentally, the diffusion film or the like may be formed by a printing method other than the screen printing method, for example, using a jet printer, or may be formed by another method.

The crystalline semiconductor, impurities, etc. are not limited to those of the above-described embodiments, and the processing time, concentration, etc., in each step may vary depending on the type, size, amount, etc. of the semiconductors, impurities, etc. Needless to say, it may be set appropriately according to the situation.

[0066]

The present invention has the following effects. Since a diffusion film is applied to the surface of the non-electrode portion of the crystalline semiconductor and then subjected to thermal diffusion treatment, the non-electrode portion on the front surface side of the semiconductor is lightly doped by diffusion of impurities from the diffusion film. The (n ⁺ layer 2a) can be formed, and a highly doped layer (n ^{+ +} layer 2b) can be formed at the electrode portion on the front surface side of the semiconductor by diffusing impurities from the diffusion gas.

Therefore, both doped layers can be formed by one heat diffusion treatment, and the number of manufacturing steps can be reduced as compared with the conventional one, and a pn junction type solar cell can be manufactured at low cost.

By forming the diffusion film to a thickness such that the impurities contained in the diffusion gas hardly reach the crystalline semiconductor, the amount of impurities in both doped layers can be appropriately controlled.

Further, the condition of the impurity concentration in the diffusion film, the impurity concentration in the diffusion gas, or the condition of thermal diffusion is set so that the amount of the impurity diffused from the diffusion gas to the electrode portion is more than the amount of the impurity diffused from the diffusion film to the non-electrode portion. By adjusting the amount to be larger, the amount of impurities in both doped layers can be more accurately controlled.

[Brief description of the drawings]

FIGS. 1A to 1F are explanatory views of a manufacturing process according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a pn junction type solar cell.

FIGS. 3 (a) to 3 (h) are explanatory views of manufacturing steps of a conventional method.

[Explanation of symbols]

Reference Signs List 1 p-type semiconductor substrate 2 n-layer 2 an ⁺ layer 2 bn ⁺⁺ layer 11 diffusion film

Claims (3)

[Claims] 1. A method of manufacturing a solar cell in which a pn junction is formed by diffusing an impurity into an electrode portion at a higher concentration than a non-electrode portion on a surface side of a crystalline semiconductor, wherein the impurity is added to a surface of the non-electrode portion. Forming a diffusion film containing the diffusion film, performing thermal diffusion on the semiconductor in an atmosphere of a diffusion gas containing the impurity after the formation of the diffusion film, and lowering the non-electrode portion based on diffusion of the impurity from the diffusion film. A method for manufacturing a solar cell, comprising: forming a doped layer; and forming a highly doped layer based on diffusion of the impurity from the diffusion gas in the electrode portion. 2. The method for manufacturing a solar cell according to claim 1, wherein the diffusion film is formed to a thickness such that impurities contained in the diffusion gas hardly reach the crystalline semiconductor. 3. The condition of the impurity concentration in the diffusion film, the impurity concentration in the diffusion gas, or the condition of the thermal diffusion is determined by comparing the amount of the impurity diffusing from the diffusion gas to the non-electrode portion with respect to the amount of the impurity diffusing to the non-electrode portion. 3. The method for manufacturing a solar cell according to claim 1, wherein the amount is adjusted so as to be larger.