JP2001267598A - Stacked solar cell - Google Patents

Abstract

(57) [Problem] To provide a stacked solar cell excellent in photoelectric conversion efficiency and radiation resistance. SOLUTION: In a stacked solar cell in which a plurality of photoelectric conversion cells each having a PN junction are stacked, a photoelectric conversion cell (lowest photoelectric conversion cell) located farthest from a light receiving surface is a single crystal or a polycrystalline cell. A stacked solar cell and a photoelectric conversion cell having a PN junction formed using a crystalline silicon semiconductor substrate, wherein the PN junction is present on the semiconductor substrate opposite to the light receiving surface. An interface has a conductive intermediate layer, and the lowermost photoelectric conversion cell has a light incident side surface made of single-crystal, polycrystalline, or microcrystalline silicon of the first conductivity type, and the light incident side surface contains hydrogen. Including first or second
Conductive amorphous or microcrystalline Si _1-x C _x (0 ≦ x
The above-mentioned object is attained by a stacked solar cell characterized by being in contact with an intermediate layer consisting of ≦ 0.2).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stacked solar cell comprising a plurality of photoelectric conversion cells stacked,
In particular, it relates to a silicon laminated solar cell having high efficiency and high radiation resistance. The stacked solar cell of the present invention is suitable for a solar cell for space, and can also be applied to a terrestrial solar cell and a photosensor.

[0002]

2. Description of the Related Art A photoelectric conversion device that directly converts sunlight into electricity by utilizing the internal photoelectric effect of a semiconductor or the like is called a solar cell. Currently, various solar cells such as a single-crystal silicon solar cell having a single junction, a polycrystalline silicon solar cell, and an amorphous silicon solar cell are used according to various uses on the ground. In space, in order to prevent the photoelectric conversion efficiency of the solar cell from deteriorating due to irradiation of radiation (electrons and protons), II such as GaAs / Ge and InGaP / GaAs / Ge, which have excellent radiation resistance.
Some space solar cells using group IV compounds have been put into practical use, and research and development are also being conducted in parallel. As space solar cells, single-crystal silicon solar cells are still frequently used due to their advantages such as low cost and light weight, and among them, those using a P-type silicon substrate because of their high radiation resistance are often used.

However, in order to further reduce the cost, it is necessary to further increase the efficiency. As the method, for example, it is considered to reduce the weight by using thin film silicon and to stack the thin film silicon. For example, Neuchatel University in Switzerland has announced a two-layer cascade-connected photoelectric conversion device (hereinafter referred to as a tandem solar cell) in which two photoelectric conversion cells are stacked as a method for achieving high efficiency using thin film silicon. (2nd World Conference on Photovoltaics held in Vienna, June 1999). In this solar cell, an upper amorphous silicon photoelectric conversion cell made of a high energy band gap semiconductor material and a lower polycrystalline thin film thin film photoelectric conversion cell made of a low energy band gap semiconductor material are directly used to effectively use incident light. It has a connected two-terminal tandem cell structure. In other words, in this solar cell, the upper photoelectric conversion cell collects light on the short wavelength side, and the lower photoelectric conversion cell collects light on the long wavelength side, so that incident light can be effectively used from short to long wavelengths. Thus, the photoelectric conversion efficiency is to be improved.

In this announcement, (PIN /
Two types of tandem suns having a junction type structure A and a junction type structure B (PN: sunlight enters from the P side) and (NIP / NP: sunlight enters from the N side). A cell is shown, wherein the tandem solar cell of structure A has a photoelectric conversion efficiency of 10.7%, and the tandem solar cell of structure B has 9.3%
It has a photoelectric conversion efficiency of In each of the solar cells, the PN junction of the lower photoelectric conversion cell is formed on the light incident side of the semiconductor substrate.

A solar cell having a PN structure in which amorphous silicon is deposited on an N-type crystalline silicon substrate by a method different from the conventional thermal diffusion method has been disclosed by Sanyo Electric Co., Ltd. Fourth High Efficiency Solar Cell Workshop 1994, Japanese Patent Application Laid-Open No. Hei 7-27553). This solar cell has a single junction structure, but is called a heterojunction solar cell (a-Si / c-Si) because the semiconductor on the light incident side and the semiconductor on the substrate side are made of different materials. .

[0006]

The tandem solar cell of structure A proposed by Neuchatel University has a structure B (N-type) in which the photoelectric conversion efficiency of the upper photoelectric conversion cell (PIN, P-side incident) is higher. In contrast, the lower photoelectric conversion cell (PN) has a lower photoelectric conversion efficiency than that of the structure B (NP) (it is a main cause of this). Is
It is considered that the quality of the polycrystalline thin film used for the lower photoelectric conversion cell was low.) As a result, the photoelectric conversion characteristics are limited, and the overall photoelectric conversion efficiency (10.7%) is higher than that of a single-junction solar cell using only a polycrystalline substrate (17%).
%). Therefore, in such a tandem-type solar cell, there is a limited situation that good photoelectric conversion efficiency cannot be obtained unless the upper photoelectric conversion cell is a PIN junction type (P side incidence). Even so, satisfactory photoelectric conversion efficiency has not been obtained.

On the other hand, since a heterojunction solar cell manufactured by Sanyo Electric Co., Ltd. uses an N-type crystalline silicon substrate, it is predicted that when used in space, the photoelectric conversion efficiency will be reduced by irradiation of electrons and protons. Is done. When an N-type crystalline silicon substrate is used, a passivation film is required to reduce surface recombination. In such a single-junction amorphous solar cell, the amorphous layer on the incident side needs to be P-type because of its semiconductor characteristics. For this reason, the substrate side is necessarily made of N-type crystalline silicon, the radiation resistance is not sufficient, and practical application as a space solar cell is difficult. In view of the current situation of conventional solar cells as described above, the present inventor uses a single-crystal or polycrystalline silicon substrate as a semiconductor substrate, and places the PN junction of the bottom photoelectric conversion cell on the side opposite to the light incident side of the semiconductor substrate. The present inventors have found that the formation of a laminated solar cell having more excellent photoelectric conversion efficiency and radiation resistance can provide the present invention, and have completed the present invention.

[0008]

Thus, according to the present invention, in a stacked solar cell in which a plurality of photoelectric conversion cells each having a PN junction are stacked, the photoelectric conversion cell located farthest from the light-receiving surface is located. A stacked solar cell having a PN junction formed using a single crystal or polycrystalline silicon semiconductor substrate, and wherein the PN junction is present on the semiconductor substrate opposite to the light receiving surface. Provided. Further, according to the present invention, in a stacked solar cell in which a plurality of photoelectric conversion cells each having a PN junction are stacked, the stacked solar cell has a conductive intermediate layer at an interface of each stacked photoelectric conversion cell, and has a light-receiving surface. The photoelectric conversion cell at the farthest position is the first
A light incident side surface made of conductive single crystal, polycrystalline or microcrystalline silicon, and the light incident side surface is a first or second conductive type amorphous or microcrystalline Si _1-x containing hydrogen.
A stacked solar cell is provided which is in contact with an intermediate layer made of C _x (0 ≦ x ≦ 0.2).

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A stacked solar cell according to one embodiment of the present invention is formed by stacking a plurality of photoelectric conversion cells having a PN junction, and is mainly located at a position farthest from a light receiving surface. (Hereinafter, referred to as a lowermost photoelectric conversion cell) has a PN junction formed using a single crystal or polycrystalline silicon semiconductor substrate, and the PN junction is located on the side opposite to the light receiving surface. It is characterized by being present on a semiconductor substrate. The photoelectric conversion cell in the present invention is formed by stacking a semiconductor substrate and a semiconductor layer, by stacking a plurality of semiconductor layers, or by a semiconductor substrate or a semiconductor layer. The PN junction is formed by stacking a semiconductor substrate and a semiconductor layer having different conductivity types, by stacking a plurality of semiconductor layers having different conductivity types, or by forming a P-type and an N-type on the semiconductor substrate or the semiconductor layer. It is constituted by doping impurities.

The material constituting the semiconductor layer is not particularly limited as long as it is generally used for a solar cell, and examples thereof include silicon, silicon germanium, silicon carbon, GaAs, CdS, CdSe, and CdTe. These crystal systems may be single crystal, polycrystal, microcrystal, non-single crystal, amorphous or semi-amorphous. Examples of the impurity atoms that determine the conductivity type of the semiconductor layer include boron, aluminum, gallium, and indium in the case of a P-type, and include phosphorus and arsenic in the case of an N-type. Note that as a semiconductor layer, an I-type semiconductor layer can be used in addition to a P-type or N-type semiconductor layer to which impurities are doped.

Here, as the material constituting the I-type semiconductor layer, any of the above-mentioned semiconductor materials can be used, and among them, silicon germanium and amorphous silicon having a small amount of hydrogen are preferable. The thickness of the semiconductor layer is not particularly limited, but is preferably as thin as possible in order to reduce the characteristic deterioration phenomenon (the so-called Steb-Shironsky effect) due to light irradiation and to increase the efficiency.
It is preferably in the range of 0 to 500 nm. All possible combinations can be applied to the configuration of the conductivity type of the photoelectric conversion cell in the present invention. For example, (P-
N) junction type, (NP) junction type, (PIN) junction type, (NIP) junction type, (PPN) junction type,
(P-N-N) junction type, (N-P-N) junction type, (N-
NP) Any type of junction type may be used. Above all, photoelectric conversion cells other than the bottommost photoelectric conversion cell, in that high efficiency can be easily achieved and radiation resistance is good,
It is a (PIN) junction type from the light incident side, and the lowermost photoelectric conversion cell is a (PPN) junction type or a (PNN) junction type from the light incident side. preferable. In addition, according to such a configuration, the conventional technology and equipment can be used in manufacturing the lowermost photoelectric conversion cell, which is advantageous in terms of the manufacturing process and cost.

Each of the stacked photoelectric conversion cells is preferably composed of a semiconductor layer or a semiconductor substrate having a band gap of a different energy band, and the band gap is determined from the photoelectric conversion cell closest to the light receiving surface. It is preferable that the temperature decreases toward the farthest photoelectric conversion cell. The lowermost photoelectric conversion cell according to the present invention has a PN junction on a single-crystal or polycrystalline silicon semiconductor substrate opposite to the light-receiving surface. Here, the “semiconductor substrate on the side opposite to the light receiving surface”
Means on the surface of the semiconductor substrate or in the semiconductor substrate near the surface. The single crystal or polycrystalline silicon semiconductor substrate in the present invention is doped with impurities in advance. As the impurity atoms that determine the conductivity type of the semiconductor substrate, the same as those used for the semiconductor layer can be used. The conductivity type of the semiconductor substrate is preferably a P-type when used as a space solar cell because the resulting laminated solar cell has excellent radiation resistance. The thickness of the single crystal or polycrystalline silicon semiconductor substrate is not particularly limited, but is preferably as thin as possible for weight reduction and high efficiency, for example, preferably in the range of 50 to 200 μm.

For example, when the obtained laminated solar cell is used as a solar cell for space, the single-crystal or polycrystalline silicon semiconductor substrate is a P-type, and 50 to 100 μm.
Preferably, it has a thickness of m. Next, another embodiment of the present invention is a stacked solar cell in which a plurality of photoelectric conversion cells each having a PN junction are stacked, and a conductive intermediate layer is mainly provided at the interface of each stacked photoelectric conversion cell. The lowermost photoelectric conversion cell has a light-incident side surface made of single-crystal or polycrystalline silicon of the first conductivity type, and the light-incident side surface has a first or second conductivity type amorphous or fine particle containing hydrogen. It is in contact with an intermediate layer made of crystalline Si _1-x C _x (0 ≦ x ≦ 0.2).

Here, "first or second conductivity type" means different conductivity types, and may be either P-type or N-type. That is, if the first conductivity type is P
When the first conductivity type is N-type, the second conductivity type is P-type when the first conductivity type is N-type. The conductive intermediate layer in the present invention may be a single layer or a plurality of layers.
The intermediate layer is made of Si _1-x C _x (0 ≦ x ≦ 1.0), and its crystalline system may be either amorphous or microcrystalline. As the impurity atoms that determine the conductivity type of the intermediate layer, the same atoms as those used for the semiconductor layer can be used. Although the concentration of the impurity is not particularly limited, for example, 5 × 10 ¹⁹ to
It is about 5 × 10 ²¹ cm ⁻³ .

The thickness of the intermediate layer is not particularly limited, but is, for example, about 50 to 150 nm. The intermediate layer is not affected by the junction type of the photoelectric conversion cells. For example, photoelectric conversion cells other than the lowermost photoelectric conversion cell are (P-I-N) junction types,
In addition, even in a stacked solar cell in which the lowermost photoelectric conversion cell is a (PN) junction type, photoelectric conversion cells other than the lowermost photoelectric conversion cell are (NIP) junction type and the lowermost photoelectric conversion The same applies to the case where the cell is of the (NP) junction type. The light incident side surface in the present invention is made of first conductivity type single crystal or polycrystalline silicon. As the impurity atoms that determine the conductivity type of the light incident side surface, the same as those used for the semiconductor layer can be used. The thickness of the light incident side surface is not particularly limited, but is, for example, about 0.2 to 0.5 μm.

In the present invention, the intermediate layer in contact with the light incident side surface is made of first or second conductivity type amorphous or microcrystalline Si _1-x C _x (0 ≦ x ≦ 0.2) containing hydrogen. It may be a single-layer film or a multi-layer film. That is, in the formula, x
Has a single value, a multi-layer film when x has a value of 2 or more, and a graded film when x has a continuous value. Above all, since the stress inside the intermediate layer is reduced, it is preferable that this intermediate layer is a graded layer film. The content of hydrogen is not particularly limited, but is preferably, for example, about 10 to 25%. As the impurity atoms that determine the conductivity type of the intermediate layer in contact with the light incident side surface, the same atoms as those used for the semiconductor layer can be used. The concentration of the impurity is preferably as low as possible because the intermediate layer in contact with the light incident side surface functions as a passivation film, and is, for example, about 5 × 10 ^{15 to} 5 × 10 ¹⁶ .

The thickness of the intermediate layer that is in contact with the light incident side surface is determined by the fact that this intermediate layer functions as a passivation film.
It is preferably as thin as possible, for example, about 5 to 30 nm. When the intermediate layer in contact with the light incident side surface is made of, for example, P-type amorphous Si _1-x C _x (0 ≦ x ≦ 0.2) containing hydrogen with an impurity concentration of 2 × 10 ¹⁶ and a film thickness of about 5 to 10 nm. Is preferable because the impurity concentration is low and the film thickness is small, so that an electric field can be introduced to the light incident side surface of the photoelectric conversion cell made of single crystal or polycrystalline silicon existing under the intermediate layer. Accordingly, the surface passivation effect that the minority carrier reduces the rejoining speed at the interface can be expected, which is preferable because the photoelectric conversion efficiency is improved. This is apparent from the fact that the effect of significantly reducing the surface re-joining speed has been experimentally confirmed by the heterojunction composed of amorphous silicon and single-crystal silicon (fourth example).
Times high efficiency solar cell workshop, P61,199
4).

The intermediate layer in contact with the light incident side surface basically functions as a passivation film. In order to add the function as an antireflection film of the photoelectric conversion cell and the function as a protective film for electron and proton radiation to the function as the passivation film, hydrogen and nitrogen are contained on the intermediate layer in contact with the light incident side surface. Amorphous or microcrystalline Si _1-x C _{x of} the first or second conductivity type (0 ≦ x ≦ 0.1)
Can be laminated. The hydrogen content in the second intermediate layer is not particularly limited, but is preferably, for example, 10 to 25%. The nitrogen content is not particularly limited, but is preferably, for example, about 5 to 30%.

As the impurity atoms that determine the conductivity type of the second intermediate layer, the same as those used for the semiconductor layer can be used. The impurity concentration is preferably as low as possible in order for the second intermediate layer to function as a passivation film, and is, for example, about 5 × 10 ^{15 to} 5 × 10 ¹⁶ . The thickness of the second intermediate layer is not particularly limited. However, in order for the intermediate layer to function as an antireflection film of a photoelectric conversion cell and as a protective film for electron and proton radiation, for example, 60 to 40
It is preferably about 0 nm. The second intermediate layer preferably has a structure close to a diamond structure. Above all, hydrogen is more than 10 ²¹ cm ⁻³ (1: 1) because it has better radiation resistance to electrons and protons than crystalline silicon.
0% or more) and has a band gap of 1.45 to 3.5.
Amorphous carbon (aC) containing hydrogen and nitrogen of the first or second conductivity type of eV is preferable. The intermediate layer made of this material has a random arrangement of atoms, but a diamond structure with a short-range order (diamondlik) in which the coordination number of atoms and the closest interatomic distance are quite close to the values of the corresponding crystal system.
e carbon). Therefore, a film thickness of about 60 to 1400 nm shows a refractive index of about 1.6 to 1.9,
It is preferable because it can effectively function as a radiation-resistant protective film and an antireflection film for the lower cell.

Further, for example, the first or second layer containing hydrogen and nitrogen is formed on an intermediate layer made of amorphous Si _1-x C _x (0 ≦ x ≦ 0.2) of the first or second conductivity type containing hydrogen. When a second intermediate layer made of two-conductivity type amorphous carbon (a-C) is laminated, both layers have different lattice constants.
An amorphous Si _1-x C _x (0.2 <x <1.0) of the first or second conductivity type containing hydrogen between both layers, wherein x
It is preferable to form a graded layer film having continuous values. Thereby, the stress inside the intermediate layer can be reduced, and a higher quality intermediate layer can be obtained. In another aspect of the present invention, the PN junction of the lowermost photoelectric conversion cell may be formed on the light receiving surface side of the semiconductor substrate or on the side opposite to the light receiving surface. The details of the photoelectric conversion cell, the semiconductor layer, and the semiconductor substrate in the present invention are the same as those described above.

The single crystal or polycrystalline silicon semiconductor substrate, the semiconductor layer and the intermediate layer can be formed by, for example, an evaporation method, a CVD method, a plasma CVD method, a sputtering method, or the like. Among them, the plasma CVD method is preferable because the manufacturing cost can be reduced.
It is desirable that the photoelectric conversion cell be formed in a large area and a uniform thin film. By realizing this, further mass production and cost reduction can be achieved in the future. An upper electrode and a lower electrode are respectively formed above and below the photoelectric conversion cells stacked by such a method by, for example, a vapor deposition method, a sputtering method, or the like, and the stacked solar cell of the present invention is manufactured. The upper electrode and the lower electrode that can be used in the present invention include, for example, those formed of a metal layer, a metal oxide layer, or a laminate of a metal layer and a metal oxide layer. Here, the metal layer is, for example, gold, silver, aluminum,
It is made of a material such as copper, nickel, and titanium. The metal oxide layer is made of, for example, a material such as indium tin oxide (ITO), zinc oxide (ZnO), and tin oxide (SnO).

[0022]

The present invention will be described in detail based on the following embodiments, but the present invention is not limited thereto. Embodiment 1 FIG. 1 shows a schematic cross-sectional view of a tandem solar cell according to one embodiment of the present invention. This tandem solar cell includes an upper electrode 10, an upper photoelectric conversion cell 14, an intermediate layer 13, a lower photoelectric conversion cell (a lowermost photoelectric conversion cell in the present invention) 12, and a lower electrode 1. The upper electrode 10 has a thickness of 107 nm and is made of transparent conductive indium tin oxide (ITO). Upper photoelectric conversion cell 14
Are a P-type amorphous silicon layer (a-Si) 9 containing 5 to 10 nm thick hydrogen, an I-type amorphous silicon layer (a-Si) 8 containing about 300 nm thick hydrogen, and a high energy band gap (1. A hydrogen-containing N-type amorphous silicon layer (a-Si) 7 having a thickness of 20 nm and a thickness of 70-1.85 eV) is formed.
-N) Construct a junction type.

The intermediate layer 13 includes the first intermediate layer 5 and the second intermediate layer 5.
It is composed of an intermediate layer 6 as a layer. The first intermediate layer 5 has a thickness
Is 5 to 10 nm and the impurity concentration is 2 × 10¹⁶And N-type
Energy band higher than amorphous silicon layer 7
An electrode containing hydrogen having a gap (1.9 to 2.0 eV)
Morphas silicon carbon alloy (a-Si _1-xC_x)
(0 ≦ x ≦ 0.2). The first middle layer is impure
Low concentration of substances and functions as a surface passivation film
You. The second intermediate layer 6 has a thickness of 1000 to 1400.
nm, hydrogen content is 10^{twenty one}cm^-3(10% or more),
Band gap (1.45-3.5 eV), refractive index
1.6 to 1.9, a structure close to the diamond structure
Amorphous carbon containing hydrogen and nitrogen (a-
C). This second intermediate layer 6 is made
It functions as a protective film and an antireflection film for the lower cell.

The lower photoelectric conversion cell 12 has a thickness of about 0.3 μm.
m-type P-type crystalline silicon layer (c-Si) 4, having a thickness of 50 to
100 μm, crystal plane (100), resistivity 10-50
It is composed of a P-type crystal silicon substrate (c-Si) 3 having Ω · cm and a band gap of 1.12 eV and an N-type crystal silicon layer (c-Si) 2 having a thickness of 0.4 μm. A P ⁺ / P (High-Low junction) junction type is formed, and a (PN) junction type is formed on the side opposite to the light incident side of the P-type crystal silicon substrate 3 from the light incident side. The lower electrode 1 has a thickness of 1.5 μm and is made of a three-layer metal composed of Ti / Pd / Ag. This tandem solar cell
It has a PIN / P ⁺ -PN ⁺ junction type in which light enters from the P side. An N-type amorphous silicon layer 7 and a second intermediate layer 6 are formed in a connection tunnel junction between the upper photoelectric conversion cell 14 and the lower photoelectric conversion cell 12.

According to such a tandem solar cell,
Since two photoelectric conversion cells composed of semiconductor layers or semiconductor substrates having different band gaps are stacked, light in a short wavelength region (for example, about 300 to 700 nm) of the incident light is absorbed by the upper photoelectric conversion cell 14, Light in a long wavelength region (for example, about 700 to 1200 nm) passes through the upper photoelectric conversion cell 14 and the intermediate layer 13 and is absorbed by the lower photoelectric conversion cell 12. As a result, each of the upper and lower photoelectric conversion cells is converted into electricity, so that the output voltage is twice as large as that of a single junction cell, and the photocurrent can be reduced. As a result, it is possible to easily reduce the series resistance loss, which has been technically considered to be technically difficult, and to improve the photoelectric conversion efficiency. Further, in such a tandem solar cell, since the PN junction of the lower photoelectric conversion cell 12 is formed on the side opposite to the light incident side of the P-type crystalline silicon substrate 3, the radiation resistance can be improved, It is suitable as a tandem solar cell for space. Hereinafter, a manufacturing process of the tandem solar cell will be described with reference to FIG.

<Manufacture of Lower Photoelectric Conversion Cell 12> The P-type crystalline silicon substrate 3 is made alkaline (NH ₄ OH: H ₂ O ₂ :
H ₂ O = 1: 1: 5) and acidic (HCl: H ₂ O ₂ : H ₂ O)
= 1: 1: 5) for 30 minutes at 80 ° C. Next, a P-type crystalline silicon layer 4 is formed on the P-type crystalline silicon substrate 3 by a thermal diffusion method using boron as a dopant in a mixed atmosphere of oxygen and nitrogen. Then
Unnecessary P-type crystal silicon layers adhering to the side and back sides of the P-type crystal silicon substrate 3 are made of concentrated HNO ₃ and HF (50%).
Is etched with a mixed solution (3: 1) of the above to expose the P-type crystalline silicon substrate 3. Next, the P-type crystal silicon layer 4
Is protected with a silicon oxide coating solution, dried under a nitrogen atmosphere, and an N-type crystalline silicon layer 2 is formed on the side of the P-type crystalline silicon substrate 3 opposite to the light incident side by a thermal diffusion method using phosphorus as a dopant. After removing silicon oxide with a solution of HF: H ₂ O = 1: 10, the lower electrode 1 is deposited on the N-type crystalline silicon layer 2 by a vacuum deposition method. Next, annealing is performed in a nitrogen atmosphere to form a P ⁺ -P-N ⁺ junction type lower photoelectric conversion cell 12.

<Manufacture of Intermediate Layer> Parallel Plate Plasma CV
D apparatus, H ₂ -diluted SiH ₄ , CH
_{Using 4} gases, B ₂ H ₆ as a dopant, a power supply frequency of 13.56 mHz, a pressure of 0.5 Torr, and a substrate temperature of 20
Under the condition of 0 to 400 ° C., the first intermediate layer 5 is formed on the P-type crystalline silicon layer 4. At this time, the band gap of the first intermediate layer can be easily controlled within the range of 1.9 to 2.0 eV by changing the flow ratio of the source gas. Next, using a mixed gas atmosphere of CH ₄ , H ₂ , and N ₂ at a pressure of 0.8 Torr and using B ₂ H ₆ as a dopant,
A second intermediate layer 6 is formed on the first intermediate layer 5. At this time, the refractive index of the second intermediate layer 6 is changed to 1.6 to 1.9 by changing the nitrogen gas flow ratio of the mixed gas.
Can be controlled within the range. FIG. 3 shows the dependency between the nitrogen concentration of the mixed gas at the time of film formation and the refractive index and extinction coefficient of the second intermediate layer 6.

<Manufacture of Upper Photoelectric Conversion Cell> Plasma CV
According to the D method, SiH ₄ (silane) or H ₂
(High purity hydrogen), B ₂ H ₆ (diborane) as dopant
Or a substrate temperature of 150 to 1 using PH ₃ (phosphine).
About 80 ° C., pressure of a film forming chamber of 0.2 to 0.75 Torr,
Energy supplied to plasma 10 to 30 mW / cm
² conditions, by a second layer of N-type amorphous silicon layer 7 on the intermediate layer 6, I-type amorphous silicon layer 8 and the P-type amorphous silicon layer 9 are formed in this order, the upper photoelectric conversion cell 14 Form.

<Manufacture of Solar Cell> An upper electrode 10 is formed by depositing ITO on the P-type amorphous silicon layer 9.
Then, by sputtering and electron beam evaporation,
An Ag surface collecting electrode 11 is formed on the upper electrode 10. Through the above steps, a tandem solar cell is manufactured. Fig. 2 shows the energy diagram of this tandem solar cell.
Shown in The energy band of the upper photoelectric conversion cell 14 is
The electric field generated by the P-type amorphous silicon layer 9 and the N-type amorphous silicon layer 7 gives an energy band of the I-type amorphous silicon layer 8. Carriers generated between the P-type amorphous silicon layer 9 and the N-type amorphous silicon layer 7 are accelerated, and current collection efficiency can be improved.
The tandem solar cell (area 1 cm ² ) of Embodiment 1 is
1.48V open circuit voltage, 19.3A / c short circuit current
m ² , fill factor of 76.0%, photoelectric conversion efficiency (intrinsic efficiency) of 16.0% (AM0: 135.3 mW / c)
m ² ), the open-circuit voltage, the short-circuit current, and the photoelectric conversion efficiency were all improved as compared with the conventional one.

[Embodiment 2] FIG. 4 is a schematic sectional view of a terrestrial solar cell according to an embodiment of the present invention. This solar cell has a film thickness of 200 instead of the P-type crystalline silicon substrate 3.
Inexpensive N-type polycrystalline silicon substrate (c-S
i) Amorphous carbon (a-) containing hydrogen and nitrogen having a refractive index of 1.9 and a film thickness of 60 nm using (17, (resistivity 0.1 to 0.8 Ω · cm, band gap 1.12 eV)) C: H: N) Second middle layer 2
0, P-type crystalline silicon layer 18 having a thickness of 0.2 to 0.5 μm
(C-Si) and an amorphous silicon carbon alloy (a-Si _1-x C _x ) containing hydrogen having a thickness of 5 to 8 nm (0 ≦ x
≦ 0.2), except that the first intermediate layer 19 is formed.

<Manufacture of lower photoelectric conversion cell> N-type polycrystalline silicon
The recon substrate 17 was washed with the same solution as in the first embodiment.
Then, P-type crystalline silicon is placed on N-type polycrystalline silicon substrate 17.
The layer 18 is formed. Side view of N-type polycrystalline silicon substrate 17
And unnecessary P-type crystalline silicon layer deposited on the back side
NO _ThreeWith a mixed solution (3: 1) of HF and HF (50%)
To expose the N-type polycrystalline silicon substrate 17. P
The type crystal silicon layer 18 is protected with a silicon oxide coating solution.
You. Next, an N-type polycrystalline silicon substrate is formed by a thermal diffusion method.
An N-type crystalline silicon layer 16 is provided on the side opposite to the light incident side on
Form. HF: H_TwoO = 1: 10 solution with silicon oxide
After the removal of the lower electrode, the lower electrode
15 is deposited. Next, annealing is performed in a nitrogen atmosphere.
By that, P⁺-N-N⁺Junction type lower photoelectric conversion cell 2
6 is formed.

<Manufacture of Intermediate Layer> In the same manner as in the first embodiment, the first intermediate layer 19 is formed on the P-type crystalline silicon layer 18.
The second intermediate layer 20 is formed in this order. <Production of Upper Photoelectric Conversion Cell> An N-type amorphous silicon layer (a-Si) 21 containing hydrogen and an I-type amorphous silicon containing hydrogen were formed on the second intermediate layer 20 under the same film forming conditions as in the first embodiment. Silicon layer (a-Si) 22, P containing hydrogen
The upper photoelectric conversion cell 28 is formed by forming the type amorphous silicon layers (a-Si) 23 in this order. <Manufacture of Solar Cell> An upper electrode 24 is formed on a P-type amorphous silicon layer 23 in the same manner as in the first embodiment. Next, an Ag surface collecting electrode 25 is formed on the upper electrode 24 by an electron gun evaporation method, and (P−I−N / P ⁺ −N−).
Manufacture N ⁺ ) junction type tandem solar cells for ground use.
FIG. 5 shows an energy diagram corresponding to the obtained tandem solar cell.

The tandem solar cell of the second embodiment (area 4
cm ² ), the open circuit voltage is 1.43 V and the short circuit current is 15.
5 A / cm ² , fill factor 73.0%, photoelectric conversion efficiency (intrinsic efficiency) 16.2% (AM 1.5: 100)
mW / cm ² ), which significantly improved the photoelectric conversion efficiency as compared with the conventional one. FIG. 6 shows the IV characteristics of the photoelectric conversion efficiency.
Shown in As shown in FIG. 6, the photo-current density (photo-curve)
rent density) (mA / cm ² ), the voltage V is 0.75
It can be seen that the voltage starts decreasing around the point where the voltage exceeds V, and sharply decreases around the point where the voltage exceeds 1.25 V.

[0034]

According to the stacked solar cell of the present invention, the lowermost photoelectric conversion cell has a single crystal or polycrystalline silicon semiconductor substrate, and the PN junction of the lowermost photoelectric conversion cell is opposite to the light receiving surface. By being present on the semiconductor substrate, a stacked solar cell having more excellent photoelectric conversion efficiency and radiation resistance can be provided. In addition, an intermediate layer is provided at the interface of each photoelectric conversion cell,
The bottom photoelectric conversion cell has a light incident side surface made of single-crystal, polycrystalline or microcrystalline silicon of the first conductivity type, and the light incident side surface has a first or second conductivity type amorphous or hydrogen containing hydrogen. By contacting the intermediate layer made of microcrystalline Si _1-x C _x (0 ≦ x ≦ 0.2), a stacked solar cell having more excellent photoelectric conversion efficiency can be provided at low cost.

The present invention is based on the following facts: 1) The phenomenon that a PN junction formed on the light incident side of a semiconductor substrate moves to the side opposite to the light incident side after performing electron beam irradiation.
(Appl.Phys.lett., 70, 21 (1996) SJ Taylor, Ming-Ju Yan
g et.al “Type Conversion in Irradiated Silicon Dio
des ”). 2) The dependence of the minority carrier diffusion length of the crystalline silicon material on the amount of electron beam irradiation is expressed by the following equation:

(Equation 1) (Where L _O and L are the diffusion length before and after irradiation, and φ is 1 MeV
Dose of an electron beam having an energy, K _L is represented by representing the damage factor), further damage coefficient of P-type and N-type crystalline silicon substrate, the following formula: K against P-type substrate _L (P-Si) = 9.7 × 10 -19 p 0.524 [cm -3] (2) K with respect to n-type substrate _{L (n-Si) = 5.4} × 10 -25 n [Cm ^-3 ] (3) That is, since the damage coefficient of the diffusion length of the minority carrier is smaller in the P-type crystalline silicon substrate than in the N-type crystalline silicon substrate, the radiation resistance is relatively higher in the P-type crystalline silicon substrate than in the N-type crystalline silicon substrate. Be superior. These facts are considered to be one of the grounds for obtaining the effects of the present invention. Still further, according to the present invention, it is possible to provide a next-generation laminated solar cell for space which is excellent in high efficiency, low cost and radiation resistance and is lightweight (Embodiment 1). In addition, it is possible to provide a stacked solar cell for terrestrial use with high efficiency using an inexpensive polycrystalline substrate (Embodiment 2).

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a solar cell for space which is one embodiment of the present invention.

FIG. 2 is a diagram showing an energy diagram of a solar cell for space.

FIG. 3 is a graph showing the dependency between the nitrogen concentration in a film formation atmosphere and the refractive index and extinction coefficient of an amorphous silicon carbon alloy containing hydrogen and nitrogen.

FIG. 4 is a schematic sectional view of a terrestrial solar cell according to an embodiment of the present invention.

FIG. 5 is a diagram showing an energy diagram of a terrestrial solar cell.

FIG. 6 is a diagram showing IV characteristics of a terrestrial solar cell.

[Explanation of symbols]

1, 15 Lower electrode 2, 16 N-type crystal silicon layer (c-Si) 3 P-type crystal silicon substrate (c-Si) 4, 18 P-type crystal silicon layer (c-Si) 5, 19 First layer Intermediate layers 6, 20 Second intermediate layer 7, 21 N-type amorphous silicon layer containing hydrogen (a-Si) 8, 22 I-type amorphous silicon layer containing hydrogen (a-Si) 9, 23 Contains hydrogen P-type amorphous silicon layer (a-Si) 10, 24 Upper electrode 11, 25 Ag surface collecting electrode 12, 26 Lower photoelectric conversion cell 13, 27 Intermediate layer 14, 28 Upper photoelectric conversion cell 17 N-type polycrystalline silicon substrate (c -Si)

Claims (4)

[Claims] 1. A stacked solar cell in which a plurality of photoelectric conversion cells each having a PN junction are stacked, wherein a photoelectric conversion cell located farthest from a light receiving surface uses a single crystal or polycrystalline silicon semiconductor substrate. A stacked solar cell, having a PN junction configured as described above, wherein the PN junction is present on the semiconductor substrate opposite to the light receiving surface. 2. A stacked solar cell in which a plurality of photoelectric conversion cells having a PN junction are stacked, a conductive intermediate layer is provided at an interface between the stacked photoelectric conversion cells, and is farthest from the light receiving surface. The photoelectric conversion cell located at the position has a light incident side surface made of single crystal, polycrystalline or microcrystalline silicon of the first conductivity type, and the light incident side surface has a first or second conductivity type containing hydrogen. Amorphous or microcrystalline Si _1-x C _x (0
≦ x ≦ 0.2), which is in contact with the intermediate layer. 3. An intermediate layer made of amorphous or microcrystalline Si _1-x C _x (0 ≦ x ≦ 0.2) of the first or second conductivity type containing hydrogen, the first containing hydrogen and nitrogen. Or a second conductivity type amorphous or microcrystalline Si _1-x C _x (0 ≦ x ≦
3. The laminated solar cell according to claim 2, wherein a second intermediate layer made of 1.0) is laminated. 4. A photoelectric conversion cell located farthest from a light receiving surface has a PN junction formed by using a single crystal or polycrystalline silicon semiconductor substrate, and the PN junction is formed.
The stacked solar cell according to claim 2, wherein the bonding portion exists on the semiconductor substrate opposite to the light receiving surface.