JPH0548133A - Laminated solar battery - Google Patents

Abstract

PURPOSE:To improve the initial energy conversion efficiency of a laminated solar battery and, at the same time, to reduce the deterioration by light of the battery. CONSTITUTION:The first and second pin photoelectric conversion element units 3 and 4 of this laminated a-Si solar battery 100 are directly joined to each other by an ohmic contact. In order to obtain the ohmic contact, the p-type a-SiC:H8 of the second unit 4 viewed from the light incident direction side is doped with boron by 0.6-1.5%. In addition, in order to control the quantities of the light rays made incident to the units 3 and 4 and to make the electric currents generated by the units 3 and 4 equal to each other, the film thicknesses of the i-type (intrinsic-type) semiconductors of the units 3 and 4 are respectively set at 30-100nm and 200-600nm. Therefore, the initial energy conversion efficiency of the battery 100 can be improved and the deterioration by light of the battery can be reduced even when the battery is exposed to the sunlight for a long time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photovoltaic system for converting sunlight into electric energy and a laminated solar cell which is a main power source for stably operating electric equipment for a long period of time.

[0002]

2. Description of the Related Art Conventionally, an amorphous silicon solar cell (hereinafter referred to as an a-Si solar cell) can easily manufacture a solar cell having a large area as compared with a single crystal silicon solar cell. Further, it is positioned at the center of solar cell development because it has advantages of simple manufacturing process and low cost.

[0003]

However, the present a-
The Si solar cell has a problem that the initial energy conversion efficiency is low, and the energy conversion efficiency is lowered (hereinafter referred to as photodegradation) during long-term outdoor use. Among them, with respect to the improvement of the initial energy conversion efficiency, an attempt has been made to improve the conversion efficiency by using a laminated structure,
For example, the one disclosed in Japanese Patent Laid-Open No. 64-59966, “Multilayer Amorphous Solar Cell” is known. In this structure, both the p-type layer and the n-type layer of the pin photoelectric conversion element unit are bonded via a thin film layer capable of ohmic coupling. Then, it has been proposed as a method for promoting the recombination of carriers, that is, a high initial energy conversion efficiency when the laminated structures are combined. However, in addition to the a-Si film forming technique, a complicated semiconductor manufacturing process of providing a very thin film in the middle of two pin photoelectric conversion element units is required. Furthermore, it has not reached the sufficient effect with respect to another problem, that is, photodegradation.

The present invention has been made to solve the above problems, and an object of the present invention is to improve the initial energy conversion efficiency and reduce the photodegradation due to the irradiation of sunlight for a long time. It is to provide a laminated solar cell capable of achieving the above.

[0005]

The structure of the invention for solving the above-mentioned problems is a p-type amorphous semiconductor material,
In a laminated solar cell in which two pin photoelectric conversion element units in which i (intrinsic) -type and n-type amorphous semiconductor films are sequentially formed are laminated, a p-type layer facing each other between the two pin photoelectric conversion element units or The n-type layer is heavily doped with impurities and directly joined by ohmic contact, and the film thickness of the i (intrinsic) type semiconductor of the two pin photoelectric conversion element units viewed from the light incident direction side is in the order of 30 to 100 nm, 200 to It is characterized in that they are formed so that the carriers excited in the unit of the two pin photoelectric conversion elements are equal in the range of 600 nm.

[0006]

[Function] Two pin photoelectric conversion element units are opposite to each other.
The type layer or the n-type layer is heavily doped with impurities and directly bonded by ohmic contact. That is, since a complicated semiconductor manufacturing process is not required for making ohmic contact between two pin photoelectric conversion element units, the cost is low.
Further, the initial energy conversion efficiency is improved by joining the two pin photoelectric conversion element units with low resistance. For this purpose, for example, the second pi seen from the light incident direction side
As a p-type layer material in an n photoelectric conversion element unit, a-Si
C: H with 0.6 to 1.5% of boron added as a dopant is used. Also, the above two p's viewed from the light incident direction side
The film thickness of the i (intrinsic) type semiconductor in the in photoelectric conversion element unit is in the order of 30 to 100 nm and 200 to 600 nm.
Carriers excited in units of n photoelectric conversion elements are formed so as to be equal to each other. As a result, the two pin photoelectric conversion element units can control the amount of incident light and can generate the same current. Then two p
Photodegradation of each in photoelectric conversion element unit can be suppressed to the same extent, and a change with time of the laminated solar cell can be suppressed. That is, the photodegradation of the stacked solar cell is affected by the larger of the two pin photoelectric conversion element units.

[0007]

EXAMPLES The present invention will be described below based on specific examples. FIG. 1 is a schematic diagram showing the structure of a laminated a-Si solar cell according to the present invention. Multilayer a-Si solar cell 100
Is a transparent conductive film (hereinafter referred to as TCO) on the glass substrate 1.
After forming 2, the first pin photoelectric conversion element unit 3 and the second pin photoelectric conversion element unit 4 are formed, and the metal electrode 11 is formed thereon. The first pin photoelectric conversion element unit 3 is a p-type a-SiC: H5, which is formed by a plasma CVD method.
An i (intrinsic) type a-SiC: H6 and an n-type μc-Si: H7 are sequentially formed. In addition, the second pin photoelectric conversion element unit 4 is also made of p-type a by the plasma CVD method.
-SiC: H8, i (intrinsic) type a-SiC: H9, n type μc
-Si: H10 are sequentially formed.

It is said that the photo-deterioration of the a-Si solar cell is caused by the effect of decreasing the conductivity due to light irradiation (Staebra-Wronski effect). The cause of this effect is not clear due to various theories, but the basic idea is that carriers (electrons or holes) excited by light irradiation in the a-Si film are recombined. At this time, it is said that new recombination centers are formed, which recombine the carriers again, and as a result, the conductivity may decrease due to light irradiation. As a method of reducing photodegradation in an a-Si solar cell, the electric field strength inside the solar cell is increased by reducing the film thickness of the i (intrinsic) type layer, and the photoexcited electrons and holes are converted into the i-type film. There is known a method of suppressing the increase of recombination centers by decreasing the frequency of recombination within the inside. However, it is extremely time-consuming to confirm the relationship between the film thickness of the i-type layer and photodegradation by long-term light irradiation, and therefore the relationship between the film thickness of the i-type layer and photodegradation is not clear. There wasn't.

Therefore, the inventors first conducted an experimental study to find out how much the photo-deterioration is improved by the film thickness of the i-type layer in the relation between the light incident amount and the photo-deterioration. This will be described with reference to the evaluation device shown in FIG. As an evaluation device, a film thickness of 20 n obtained by forming TCO 13 on the glass substrate 12 and adding 0.2% of boron by plasma CVD method.
m p-type a-SiC: H14, 50 nm i (intrinsic) type a-
SiC: H15, 30 nm n-type μc-with 1% phosphorus added
Si: H16 was sequentially formed, and an Al electrode 17 was formed thereon. Single-layer a-Si solar cells each having one pin photoelectric conversion element unit having a structure similar to this, in which only the film thickness of i-type a-SiC: H15 was changed to 100,200,400 nm were prepared. First, the initial energy conversion efficiency of these was measured under an illuminance of 100 mW / cm ² under simulated sunlight. Next, the temperature of these samples is kept under simulated sunlight at an annual average temperature of 40 ° C., and the solar radiation amount of 5000 MJ / m ² corresponding to one year of the annual irradiation energy is applied to measure the energy conversion efficiency. The change (hereinafter, referred to as photodegradation rate) was evaluated. FIG. 3 is a result showing the above, and is a characteristic showing the relationship between the initial energy conversion efficiency (%) and the photodegradation rate (after one year) (%) when the film thickness (nm) of the i-type layer is changed. It is a figure. From this result, it was found that the photodegradation rate decreased as the thickness of the i-type layer decreased. This light deterioration rate is 40
In order to set the thickness to 20% or less, which is half of the%, it is appropriate to set the film thickness of the i-type layer to 100 nm or less. However, since the current generated by the solar cell is proportional to the film thickness of the i-type layer, if the i-type layer is too thin, a sufficient photocurrent cannot be obtained. That is, the lower limit of the film thickness of the i-type layer is 30 nm, which does not cause a sudden decrease in output. That is, it was found that the film thickness of the i-type layer of the first pin photoelectric conversion element unit constituting the laminated a-Si solar cell is preferably 30 to 100 nm.

On the other hand, the photodegradation of the laminated a-Si solar cell is caused by the photodegradation of both the first pin photoelectric conversion element unit and the second pin photoelectric conversion element unit. That is, the first p
Even if the photo-deterioration is suppressed by making the film thickness of the i-type layer in the in-photoelectric conversion element unit smaller than in the past, even if the photo-deterioration in the second pin photoelectric conversion element unit is large, even in the laminated a-Si solar cell Large photodegradation occurs. Here, the photodegradation of the second pin photoelectric conversion element unit means that the light partially absorbed by the first pin photoelectric conversion element unit enters the second pin photoelectric conversion element unit and enters the second pin photoelectric conversion element unit. This occurs because electron-hole pairs excited in the unit recombine to form a new recombination level. If the film thickness of the i-type layer in the second pin photoelectric conversion element unit is also sufficiently thin, photodegradation can be suppressed. But,
Similar to the case where the maximum output is obtained when the current is the same in the dry cells connected in series, the laminated type a-Si solar cell has the first pin photoelectric conversion element unit and the second pin photoelectric conversion element unit. The maximum output cannot be obtained unless the currents of are the same. Also, the first pin photoelectric conversion element unit and the second pin photoelectric conversion element unit
Even if the current is the same as that of the photoelectric conversion element unit, if a high resistance layer is formed in the middle of the same, a large electrical loss occurs and the output decreases. The laminated a-Si solar cell has a second pi
n-photoelectric conversion element unit p-type layer has high resistance a-SiC: H
Therefore, a large amount of boron was added to reduce the resistance of the p-type layer, and the effect of reducing the resistance was investigated. Figure 4
FIG. 3 is a schematic diagram showing the configuration of the evaluation device. As an evaluation device, after forming a TCO 21 on a glass substrate 20, an n-type μc-Si: H22 and a p-type a-SiC: H are formed.
23 is formed, and the Al electrode 24 and the p-type a are formed on the TCO 21.
An Al electrode 25 was formed on -SiC: H23. Boron is added to p-type a-SiC: H23 having such a structure in an amount of 0.15 to 2%.
A-Si consisting of one pin photoelectric conversion element unit added
A device for evaluation, which is a solar cell, was prepared and prepared. These evaluation devices were irradiated with pseudo sunlight having an illuminance of 100 mW / cm ² . FIG. 5 shows the current (mA) -voltage (V) characteristics when the boron addition amount (%) was changed. When the amount of added boron is less than 0.6%, current flows in the direction opposite to the direction of flow of the a-Si solar cell, but when the amount of added boron is 0.6% or more, it becomes almost zero, so the amount of added boron is 0.6% or more. Is desirable. Also, the resistance becomes smaller by increasing the added amount of boron, but if the added amount of boron is excessive, a-SiC: H
Since the optical gap becomes narrower and the light transmission amount is reduced, the maximum addition amount is up to about 3%. That is, the amount of boron added to a-SiC: H in the p-type layer of the second pin photoelectric conversion element unit constituting the laminated a-Si solar cell is 0.6 to
It turns out that 1.5% is desirable.

Taking the above experimental results into consideration, the laminated a-Si solar cell 100 shown in FIG. 1 was manufactured. The film thickness of the i-type a-Si: H6 of the first pin photoelectric conversion element unit 3 is set to 80 nm, and the i-type a-Si: H of the second pin photoelectric conversion element unit 4 is set to 80 nm.
A sample having a film thickness of 9 of 200 to 600 nm was prepared. The p and n-type layers of the first pin photoelectric conversion element unit 3 and the second pin photoelectric conversion element unit 4 were made of the above-mentioned materials and film thicknesses. Also, the p-type a-Si of the second pin photoelectric conversion element unit 4 is used.
0.7% of boron was added to C: H8. The initial energy conversion efficiency and the photodegradation rate of these samples were evaluated by the methods described above. FIG. 6 shows the second p-type of the stacked a-Si solar cell 100.
in photoelectric conversion element unit 4 i-type a-Si: H9 film thickness (n
It is a characteristic view which showed the relationship between the initial energy conversion efficiency (%) when changing m), and a photodegradation rate (1 year after) (%).
That is, it is shown that the laminated a-Si solar cell 100 of the present invention can suppress the photodegradation rate to about half with a higher initial energy conversion efficiency than the conventional solar cell composed of one pin photoelectric conversion element unit.

Next, a laminated a-Si solar cell 100 was manufactured with the following film thickness. The film thickness of the i-type a-Si: H6 of the first pin photoelectric conversion element unit 3 is 80 nm,
The film thickness of the i-type a-Si: H9 of the second pin photoelectric conversion element unit 4 was 400 nm, and the material and film thickness of the other layers were the same as those described above. Then, a sample was prepared in which 0.015% (conventional boron concentration) to 2% of boron was added to the p-type a-SiC: H8 of the second pin photoelectric conversion element unit 4. Then, the p-type a-Si of the second pin photoelectric conversion element unit 4 is used.
An experiment was conducted to examine how much the optical gap is changed by adding boron to C: H8. FIG. 7 is a characteristic diagram showing the above results and showing the relationship between the boron addition amount (%) and the optical gap (eV). It is shown that the change in the optical gap becomes small when the amount of boron added is about 2%.
This is because the absorption coefficient increases as the amount of boron added increases from the conventional amount. That is, the addition of boron to the p-type a-SiC: H8 of the second pin photoelectric conversion element unit 4 also contributes to the suppression of the photodegradation of the second pin photoelectric conversion element unit 4. FIG. 8 shows the p-type a-SiC: H8 of the second pin photoelectric conversion element unit 4 of the laminated a-Si solar cell 100.
FIG. 6 is a characteristic diagram showing the relationship between the initial energy conversion efficiency (%) and the photodegradation rate (after one year) (%) when the amount (%) of boron added to is changed. The photodegradation rate becomes the minimum when the amount of boron added is about 1.0%. On the other hand, the initial energy conversion efficiency decreases when the amount of boron added exceeds 1.5%. This is the p-type a-SiC of the second pin photoelectric conversion element unit 4:
This is due to the action of H8 absorbing the light incident on the i-type a-Si: H9. From these results, it is understood that the addition of boron to the p-type a-SiC: H8 of the second pin photoelectric conversion element unit 4 also contributes to the suppression of photodegradation.

[0013]

As described above, according to the present invention, two pin photoelectric conversion element units are directly joined by ohmic contact, and the thickness of the i (intrinsic) type semiconductor of the pin photoelectric conversion element units is set to a predetermined value. Therefore, it is possible to obtain a highly reliable laminated a-Si solar cell with improved energy conversion efficiency and less photodegradation. Further, in the laminated a-Si solar cell of the present invention, photodegradation can be suppressed to about half of that in the conventional case, and the area can be reduced by that amount, so that there is an effect that a large cost reduction can be achieved.

[Brief description of drawings]

FIG. 1 is a laminated a-Si according to a specific embodiment of the present invention.
It is the schematic diagram which showed the structure of the solar cell.

FIG. 2 is a single layer a composed of one pin photoelectric conversion element unit.
-Si is a schematic view of a solar cell.

3 is a characteristic diagram showing a relationship between an initial energy conversion efficiency and a photodegradation rate when the film thickness of an i-type layer in the single-layer a-Si solar cell of FIG. 2 is changed.

FIG. 4 is a first pin photoelectric conversion element unit and a second pin.
It is a schematic diagram of the evaluation device manufactured in order to investigate the current-voltage characteristic in the junction part with a photoelectric conversion element unit.

5 is a characteristic diagram showing current-voltage characteristics when the amount of boron added to the p-type layer of the evaluation device of FIG. 4 is changed.

FIG. 6 shows the relationship between the initial energy conversion efficiency and the photodegradation rate when the film thickness of the i-type layer of the second pin photoelectric conversion element unit in the laminated a-Si solar cell according to the present invention is changed. FIG.

FIG. 7 is a characteristic diagram showing the relationship between the amount of boron added to the p-type layer of the second pin photoelectric conversion element unit and the optical gap in the laminated a-Si solar cell according to the present invention.

FIG. 8 shows initial energy conversion efficiency and photodegradation rate when the amount of boron added to the p-type layer of the second pin photoelectric conversion element unit in the laminated a-Si solar cell according to the present invention is changed. FIG.

[Explanation of symbols]

1-Glass Substrate 2-Transparent Conductive Film (TCO) 3-First Pin Photoelectric Conversion Element Unit 4-Second Pi
n photoelectric conversion element unit 5-p type a-SiC: H 6-i type a-Si: H 7-
n-type μc-Si: H 8-p-type a-SiC: H 9-i-type a-Si: H 10
-N type μc-Si: H 11-metal electrode 100-multilayered a-Si solar cell

Claims (1)

[Claims] 1. A p-type, i
A stacked solar cell comprising two pin photoelectric conversion element units in which (intrinsic) -type and n-type amorphous semiconductor films are sequentially formed, wherein a p-type layer or an n-type photoelectric conversion element unit facing each other is provided between the two pin photoelectric conversion element units. The type layer is heavily doped with impurities and directly bonded by ohmic contact, and the film thickness of the i (intrinsic) type semiconductor of the two pin photoelectric conversion element units viewed from the light incident direction side is 30 to 100 nm and 200 to 600 nm in order. In the range, the laminated solar cell is formed so that carriers excited in the two pin photoelectric conversion element units are made equal to each other.