JPH0661516A - Manufacture of solar battery - Google Patents

Abstract

PURPOSE:To improve the photoelectric conversion efficiency of a solar battery so as to reduce the cost of the battery by diffusing impurities in a semiconductor substrate after reducing the impurity concentration of the substrate by heat treatment. CONSTITUTION:The method contains a process for lowering the impurity concentration of the surface section of a semiconductor substrate containing impurities of first conductivity by heat treatment, process for forming a p-n junction by diffusing impurities of second conductivity in the surface section of the substrate, and process for forming a first electrode on the front surface of the substrate and, at the same time, a second electrode on the rear surface of the substrate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a solar cell using a single crystal substrate, and more particularly to a method of manufacturing a solar cell which has high photoelectric conversion efficiency and is suitable for mass production.

[0002]

2. Description of the Related Art Generally, this type of solar cell is classified into two types, a thin film type solar cell and a bulk type solar cell, as shown in FIGS. Here, FIGS. 3A and 4A are cross-sectional views, and FIGS. 3B and 4B are band diagrams.

A thin-film solar cell is one in which a pn junction to be an operating layer is formed by an epitaxial growth method,
For example, p ⁺ -type InP layer 20, p-type InP layer 30, n ⁺ -type InP layer 40 are successively epitaxially grown on the surface of the p ⁺ -type InP substrate 10.

On the other hand, a bulk type solar cell is one in which a pn junction is formed in a bulk crystal, and impurities are introduced from the outside into the p ⁺ type InP substrate 50 by a diffusion method,
An n ⁺ layer 51 is formed on the surface portion. According to this, since the diffusion device is inexpensive and the number of wafers that can be processed at one time can be increased, mass productivity is high and manufacturing cost is low.

In the thin film solar cell, the carrier concentration of each layer is constant, and a potential barrier φp is formed at the pp ⁺ interface. The sunlight on the short wavelength side absorbed by the n ⁺ type InP layer 40 generates electrons and holes, and the n ⁺ type InP layer 4
Holes that become minority carriers at 0 diffuse toward the pn junction. Similarly, sunlight on the long wavelength side absorbed by the p-type InP layer 30 generates electrons and holes, and the p-type InP layer 3
Electrons that become minority carriers at 0 diffuse toward the pn junction. Therefore, the short-circuit current Isc, which represents the characteristics of the solar cell, can efficiently convert the minority carriers generated in the above layers to p
It depends on whether they can be separated by an n-junction. That is, the major factor that determines the magnitude of the short-circuit current is the minority carrier diffusion length (lifetime), which is related to whether or not the pn junction can be reached.

Regarding the spectral sensitivity of the solar cell, which is related to the short circuit current Isc, the following is expected. For n ⁺ -p bulk type solar cell and n ⁺ -pp ⁺ thin film type solar cell, n ⁺ layer, p
Since the carrier concentration and thickness of the layer are almost the same,
The minority carriers derived from impurities in the film have similar lifetimes. In the n ⁺ -pp ⁺ thin-film solar cell, the p ⁺ -type InP layer 20 is below the p-type InP layer 30.
The diffusion of electrons generated in the InP layer 30 in the opposite direction is suppressed by the potential barrier φp, so that the apparent diffusion length increases and the number of electrons reaching the pn junction increases. Therefore, in the n ⁺ -PP ⁺ thin film solar cell, the spectral sensitivity on the long wavelength side due to the light absorbed by the p-type InP layer 30 becomes large.

On the other hand, in the case of an n ⁺ -p bulk type solar cell, p
^Since there is no potential barrier in the ⁺ type Inp substrate 50,
The spectral sensitivity on the long wavelength side is not as high as that of the thin film type, but the n ⁺ layer 5
Since 1 is a graded layer, an apparent diffusion length is increased by the built-in electric field, and holes generated in the n ⁺ layer 51 can efficiently reach the pn junction. Therefore, the spectral sensitivity on the short wavelength side of the n ⁺ -p bulk type solar cell is higher than that of the n ⁺ -pp ⁺ thin film type solar cell in which the carrier concentration gradient is not formed in the n ⁺ -type InP layer 40.

Next, the open circuit voltage Voc representing another characteristic of the solar cell will be compared. The open circuit voltage of the solar cell is closely related to the built-in potential Vbi of the pn junction. Vbi
Is expressed by the following equation (1).

Vbi = KT / qIn (Na.Nb / ni ² ) ... (1) where K is Boltzmann's constant, T is temperature, q is elementary charge, Na is carrier concentration of p layer, and Nb is n layer. , Ni is the intrinsic carrier concentration (1.8 × 10 ⁶ cm ^{−3 in} the case of InP).

Therefore, it is understood that Vbi increases as the carrier concentration of the p layer and the n layer sandwiching the pn junction increases.

On the other hand, Voc is not only Vbi but also the reverse saturation current Jo and short-circuit current Jsc (per unit area) of the pn junction.
And the following equation (2).

Voc = nKT / qIn (Jsc / Jo) (2) where n is an ideal coefficient of the pn junction, and 1 when the diffusion current is dominant in the current flowing through the pn junction. To an intermediate value between 2 when the recombination current is dominant. Further, n and Jo are not independent variables, and if the characteristics of the pn junction are improved, Jo will be smaller and n will be closer to the ideal value 1.
In the above formula (2), the increase of Voc due to the decrease of Jo is n
Since it is larger than the effect of increasing Voc, it is necessary to increase Jsc and decrease Jo in order to increase Voc.

Next, Vbi and Voc will be compared using an InP solar cell as an example. The carrier concentration of the p layer is Na = 5 × 1
0 ¹⁶ cm ⁻³ , the carrier concentration of the n ⁺ layer is Nb = 5 × 10 ¹⁸
Assuming that cm ⁻³ is calculated using the above equation (1), Vbi is 1.25V. On the other hand, the measured Voc of the n ⁺ -p bulk Inp solar cell is 0.82 V, and the n ⁺ -pp ⁺ thin film type I
The measured Voc of the np solar cell is 0.87V. That is,
Although Vbi determined by the carrier concentration of the p layer and the n ⁺ layer sandwiching the pn junction is the same, Voc is n ⁺ -p bulk type and n ⁺ -pp ⁺ due to the factor expressed by the above equation (2). Thin film type.
There will be a difference of 05V. This is because the reverse saturation current Jo is significantly different between the n ⁺ -p bulk type and the n ⁺ -pp ⁺ thin film type, as described above. That is, in the n ⁺ -pp ⁺ thin film type, the potential barrier φp is formed under the p-type Inp layer 30.
Since the p ⁺ -type InP layer 20 having
Therefore, Voc becomes n ⁺ -P according to the above equation (2).
It is larger than the bulk type.

Finally, the fill factor FF of the solar cell is compared. FF is the series resistance of the solar cell and the ideal coefficient n of the pn junction.
As the series resistance becomes smaller and the n value becomes smaller, the FF becomes larger and the photoelectric conversion efficiency of the solar cell is improved. The series resistance of a solar cell depends on various factors such as the shape of the front electrode, the contact resistance of the front / back electrodes n ⁺ layer thickness, the resistance of the substrate, etc. The n ⁺ -p bulk solar cell currently being discussed. And n ⁺ -pp ⁺ thin-film solar cells are the same except for the substrate resistance, so n ⁺ -p
-p ⁺ The series resistance of thin-film solar cells is reduced. Also, p
As for the ideal coefficient n of the n-junction, as described above, n ⁺ -p
Since the reverse saturation current Jo of the -p ⁺ thin film solar cell is smaller than that of the n ⁺ -p bulk solar cell, a smaller n value is taken.
Therefore, n ⁺ -pp ⁺ with a small series resistance and n value
The thin film solar cell had a larger FF than the n ⁺ -p bulk solar cell.

[0015]

However, in the above-described conventional thin-film solar cell, the epitaxial growth apparatus is expensive, and the number of wafers that can be processed at one time is limited, so that the mass productivity is low and the manufacturing is difficult. There is a problem that the cost becomes high.

The bulk type solar cell is a simple n ⁺ -P
Since only the structure can be taken, there is a problem that the photoelectric conversion efficiency becomes lower than that of the thin film solar cell.

The object of the present invention is to solve the above problems.
It is intended to provide a method of manufacturing a solar cell having high photoelectric conversion efficiency and capable of reducing cost.

[0018]

In order to achieve the above-mentioned object, the present invention comprises a step of heat-treating a semiconductor substrate containing an impurity of the first conductivity type to reduce the impurity concentration of the surface portion of the semiconductor substrate, Forming a pn junction by diffusing a second conductivity type impurity on the front surface of the semiconductor substrate; forming a first electrode on the front surface of the semiconductor substrate; and forming a second electrode on the back surface of the semiconductor substrate. And the step of performing.

[0019]

In the present invention, after heat treatment is performed to reduce the impurity concentration of the semiconductor substrate, impurities are diffused into the semiconductor substrate to form a pn junction, so that the photoelectric conversion efficiency is improved and the mass productivity is increased. , The manufacturing cost is reduced.

[0020]

EXAMPLE An example of the method for manufacturing a solar cell according to the present invention will be described below with reference to FIGS.

First, the InP substrate has a thickness of 0.4 mm and has a carrier concentration of about 5 × 10 ¹⁸ cm ⁻³ by doping with zinc (Zn) (FIG. 1a). This p
By heat-treating the ⁺ -type InP substrate in advance, the carrier concentration gradually decreases from the inside of the substrate toward the surface over 5 μm from the surface, and 5 × 10 5
A doping profile of about ¹⁶ cm ^-3 is formed (Fig. 1b).

PSG (phosphosilicagat) on InP substrate
An insulating film such as e) is formed and heat-treated at an appropriate temperature. As a result, impurities in the substrate are taken into the insulating film, and a layer in which the carrier concentration gradually decreases toward the surface is formed. At this time, it is important to prevent decomposition of the InP surface due to heat treatment in order to manufacture a highly efficient solar cell. In the above example, PSG containing phosphorus in the insulating film is used to suppress thermal decomposition. The doping profile can be controlled by appropriately selecting the heat treatment temperature and time. In the above example, in order to obtain the desired profile,
Heat treatment is performed at 00 ° C. for 1 hour.

Then, the PSG used for the heat treatment is buffered with HF.
After the removal by the method described above, the sulfur (S) is diffused in the gas phase at 670 ° C. for 3 hours to obtain a surface carrier concentration of 5 × 10 ¹⁸ cm ⁻³ or more on the substrate surface of 0.3 μm.
An n ⁺ layer of about m is formed. Since this n ⁺ layer is formed by the diffusion method, it is a graded layer in which the carrier concentration gradually decreases from the substrate surface toward the inside.
Also in this sulfur diffusion, it is important to prevent thermal decomposition of the InP surface, and therefore diffusion is performed in a sealed tube to which phosphorus vapor pressure is applied. Also, the diffusion temperature is considerably lower than the heat treatment temperature of 900 ° C. 67 so that the profile of the substrate surface formed previously does not change during sulfur diffusion.
The temperature is 0 ° C. (FIG. 1c).

By the above method, a structure of n ⁺ layer, p layer and p ⁺ layer is formed in the InP bulk crystal in order from the surface, and then the 20th IEEE Solar Cell Expert International Conference Proceedings (Conference Record of). 20th IEEE Photovoltaic Spec
ialists Conference, 1988, Las Vegas) by the method described on page 886, the front and back electrodes and the antireflection film are formed to complete the bulk type double graded solar cell.

A band diagram of the solar cell according to FIG. 2 is shown. Both the n ⁺ layer and the p layer are graded layers, and a gradual potential barrier φp is formed in the graded p layer.

The Isc of this bulk type double graded solar cell is n for light of a short wavelength related to the n ⁺ layer.
^It has the same spectral sensitivity as ⁺ -P type solar cells and is graded p for long-wavelength light with p-layer.
Because of the layers, it has a greater spectral sensitivity than n ⁺ -pp ⁺ thin film solar cells. In addition, since Voc has the potential barrier φp formed, the reverse saturation current Jo is small, and Voc has a large Voc which is comparable to that of the n ⁺ -pp ⁺ thin film solar cell. The FF has the same characteristics as the n ⁺ -pp ⁺ thin film solar cell due to the use of the p ⁺ substrate and the small n value. Therefore, in the bulk solar cell, n ⁺
-pp ⁺ It becomes possible to manufacture a solar cell having a photoelectric conversion efficiency (17 to 19%) that is equal to or higher than that of a thin film solar cell.

Further, regarding InP solar cells, In
Regarding the radiation resistance, which is a feature of P solar cells, it is better that the substrate resistance is as small as possible, that is, it is better to use a substrate having a high carrier concentration. Japanese Journal of Upright Physics Vol.
984 (Japanese Journal of Applied Physics Vol.
23 no. 3 1984) pp. It is clarified by the articles such as 302-307.

When InP, GaAs or the like, which is a direct transition type material, is used as the substrate of the solar cell, the light absorption coefficient of these materials is large. It is desirable to form a doping profile in which the carrier concentration gradually decreases toward the surface within 10 μm from the surface on which sunlight is incident.

[0029]

As described above, according to the present invention, a photoelectric conversion efficiency higher than that of a conventional n ⁺ -p bulk type solar cell can be achieved, and at the same time, an n ⁺ -pp ⁺ thin film type solar cell can be obtained. Equivalent or higher photoelectric conversion efficiency can be realized.
Further, by using the diffusion method and the heat treatment method suitable for mass production, the cost can be significantly reduced as compared with the conventional n ⁺ -pp ⁺ thin film solar cell.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a method of the present invention.

FIG. 2 is a band diagram of the solar cell of the present invention.

FIG. 3 is a cross-sectional view and band diagram of a conventional thin film solar cell.

FIG. 4 is a cross-sectional view and band diagram of a conventional bulk solar cell.

[Explanation of symbols]

10 p ⁺ type InP substrate 20 p ⁺ type InP layer 30 p type InP layer 40 n ⁺ type InP layer 50 p ⁺ type InP substrate 51 n ⁺ layer

Claims (1)

[Claims] 1. A step of heat-treating a semiconductor substrate containing a first conductivity type impurity to reduce an impurity concentration on a surface portion of the semiconductor substrate, and diffusing a second conductivity type impurity on the surface portion of the semiconductor substrate. A method of manufacturing a solar cell, comprising: forming a pn junction; and forming a first electrode on the front surface of the semiconductor substrate and forming a second electrode on the back surface of the semiconductor substrate. .