JPH1065195A - Photovoltaic element - Google Patents

Abstract

(57) [Problem] To provide a photovoltaic element with high reliability and high conversion efficiency. SOLUTION: In a photovoltaic element having at least a back reflection layer composed of a metal layer and a transparent layer on a substrate, a semiconductor layer, and a transparent electrode, the metal layer has a magnesium concentration of 4%.
A photovoltaic element which is a copper-magnesium alloy of 0 at% or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photovoltaic device such as a solar cell which has high performance, has high reliability for long-term use, and can be mass-produced at low cost.

[0002]

2. Description of the Related Art As a future energy source for humankind,
Oil and coal, which is said to cause global warming due to carbon dioxide generated as a result of its use,
Moreover, it is problematic to rely entirely on nuclear power, which is not without radiation hazard even during normal operation. Since solar cells use the sun as an energy source and have very little effect on the global environment, further spread is expected. However, at present, there are some problems that hinder full-fledged diffusion.

Conventionally, monocrystalline or polycrystalline silicon has been often used for photovoltaic power generation. However, these solar cells require a large amount of energy and time to grow crystals, and require complicated processes thereafter, so that the mass production effect is hard to increase, and it has been difficult to provide them at low cost.

On the other hand, amorphous silicon (hereinafter a-S)
i)) and so-called thin-film semiconductor solar cells using compound semiconductors such as CdS.CuInSe ₂ have been actively researched and developed. In these solar cells, it is only necessary to form as many semiconductor layers as necessary on an inexpensive substrate such as glass or stainless steel, the manufacturing process is relatively simple, and there is a possibility that the cost can be reduced.

[0005] However, thin-film solar cells have not been used in earnest because they have lower conversion efficiency than crystalline silicon solar cells and have concerns about their reliability for long-term use. Therefore, various measures have been taken to improve the performance of the thin-film solar cell.

One of them is a back surface reflection layer for increasing the reflectivity of light on the surface of the substrate so that sunlight not absorbed by the thin film semiconductor layer is returned to the thin film semiconductor layer again to effectively use incident light. . Since the short wavelength component of the sunlight spectrum is already absorbed by the thin film semiconductor, it is sufficient that the reflectance is higher for light of a longer wavelength. Which wavelength or higher the reflectance should be high depends on the light absorption coefficient and the film thickness of the thin film semiconductor used.

When sunlight is incident on the transparent substrate from the substrate side, an electrode formed on the surface of the thin film semiconductor is formed of silver (A).
g) and a metal having high reflectivity such as copper (Cu). Here, for comparison of the reflectance of various metals, the reflectance of Ag, Al, Cu and Ni films formed at 2000 Å is shown in FIG. In the case where sunlight is incident from the surface of the thin film semiconductor layer, a semiconductor layer may be formed after a similar metal layer is formed on a substrate.

When a transparent layer having appropriate optical properties is interposed between the metal layer and the thin film semiconductor layer, the reflectance can be further increased by the multiple interference effect. The use of such a transparent layer is effective in increasing the reliability of the thin-film solar cell. Japanese Patent Publication No. 60-41878 discloses that the use of a transparent layer can prevent alloying between a semiconductor and a metal layer. U.S. Pat. No. 4,532,37
Patent Documents 2 and 4,598,306 disclose that use of a transparent layer having an appropriate resistance can prevent an excessive current from flowing between the electrodes even if a short circuit occurs in the semiconductor layer. There is a description.

Another method for improving the conversion efficiency of a thin-film solar cell is a method in which the interface of the solar cell with the front and / or back reflection layers has a fine uneven structure (texture structure). By adopting such a configuration, sunlight is scattered at the interface with the front surface and / or back surface reflection layer of the solar cell, further confined inside the semiconductor (light trapping effect), and can be effectively absorbed in the semiconductor. become.

When the substrate is transparent, the surface of a transparent electrode such as tin oxide (SnO ₂ ) on the substrate may be formed into a texture structure. In the case where sunlight enters from the surface of the thin film semiconductor, the surface of the metal layer used for the back reflection layer may have a texture structure. Solar Energy Materials (Solar Energy Materials)
ls) 20 (1990) pp99-110 shows that a texture structure for the backside reflective layer can be obtained by depositing Al while adjusting the substrate temperature and the deposition rate.

FIG. 3 shows an example of an increase in the absorption of incident light due to the use of the back reflection layer having such a texture structure. Here, the curve (a) shows the spectral sensitivity of an a-SiGe solar cell using smooth silver as the metal layer, and the curve (b) shows the spectral sensitivity when textured silver is used. According to FIG. 3, since the light near the wavelength of 800 nm is not effectively used in the a-SiGe semiconductor layer, to further increase the conversion efficiency, a back reflection layer having a high reflectance for the light near 800 nm is used. Good.

Referring again to FIG. 2, silver and copper are required to be used in the thin-film semiconductor of the present invention.
While aluminum has a high reflectance in the entire wavelength range of 0 nm, aluminum has a minimum value near a wavelength of 800 nm. Therefore 8
Silver and copper, which exhibit high reflectance at 00 nm, can be said to be metals having the reflectance most suitable for the metal layer.

Further, it is possible to combine the idea of the back surface reflection layer composed of two layers of the metal layer and the transparent layer with the idea of the texture structure. U.S. Pat. No. 4,419,533 discloses the idea of a backside reflective layer in which the surface of a metal layer has a textured structure and a transparent layer is formed thereon. Further, a transparent layer having a texture structure may be formed on a smooth metal layer. Such a combination is expected to significantly improve the conversion efficiency of the solar cell.

[0014]

The use of silver or copper having particularly excellent reflectance as the metal of the backside reflection layer is extremely advantageous for obtaining a solar cell having high conversion efficiency.
However, these metals, particularly silver, are known as metals that cause electrochemical migration.

Electrochemical migration (hereinafter referred to as migration) means that a metal such as a foil, a plating, or a paste comes into contact with an insulator having a large hygroscopic property or a strong hydrophilic property under a condition where a DC voltage is applied. When used in an environment with high humidity in a state, a metal grows in a dendritic or stain-like manner on the surface or inside of the insulator by electrolysis to form a conductive path.

Other conditions are required depending on the metal.
For example, when migration is experimentally generated, silver (Ag), copper (Cu), lead (Pb), and the like are generated under the conditions of distilled water and an electric field (Ag has a particularly high growth rate of dendritic crystals), Gold (Au), palladium (Pd), indium (In), etc. require the presence of further halogen ions.
Aluminum (Al), Nickel (Ni), Iron (Fe)
Etc. are reported to occur only under special conditions other than these.

[0017] Even for a solar cell that can be used in various environments, a short circuit between electrodes due to migration becomes a problem during long-term use. For example, consider a case where a solar cell actually used outdoors is exposed to a high-temperature and high-humidity environment. In general, since the output voltage of a solar cell alone is low, a plurality of submodules (modules of the above-described thin-film semiconductor solar cells) are connected in series and used. When such a solar cell is partially covered by fallen leaves or the like, the output current of the submodule in the covered portion is extremely small as compared with the other submodules, and the internal impedance is substantially increased. As a result, the output voltages of the other sub-modules are reversed.

In other words, the conditions of occurrence of migration such as application of high temperature, high humidity and application of reverse bias are realized, and a short circuit between the electrodes occurs, leading to destruction of the submodule. This is especially true when Ag / Cu having high reflectivity is used for the back reflection layer. On the other hand, when a metal having excellent migration resistance, such as Al or Ni, is substituted, high conversion efficiency cannot be expected because the reflectance is lower than that of Ag or Cu.

The present invention has been made in view of such circumstances, and by using an improved back reflection layer,
It is an object of the present invention to provide a highly reliable and highly efficient photovoltaic element at a low price.

[0020]

That is, the present invention relates to a photovoltaic device having at least a back reflection layer comprising a metal layer and a transparent layer, a semiconductor layer, and a transparent electrode on a substrate. The magnesium concentration is 40 at% or less, preferably 20 to 40 at%, more preferably 30 at%.
It is a photovoltaic element characterized by being a copper-magnesium alloy of up to 35 at%.

Further, in a photovoltaic device having at least a back reflection layer comprising a metal layer and a transparent layer on a substrate, a semiconductor layer, and a transparent electrode, the metal layer has a wavelength of 7 nm.
A photovoltaic device characterized by being a copper-magnesium alloy that exhibits a reflectance of 90% or more for light of 00 to 1000 nm and does not cause electrochemical migration.

Further, in a photovoltaic device having at least a back reflection layer composed of a metal layer and a transparent layer on a substrate, a semiconductor layer and a transparent electrode, the magnesium concentration of the metal layer is 40 at% or less, preferably 20 at% or less. ~ 40a
The photovoltaic device is characterized in that the photovoltaic device is made of a copper-magnesium alloy having a reflectivity of at least 90% for light having a wavelength of 700 to 1000 nm.

The metal layer of the present invention is preferably formed by sputtering using a mosaic target or an alloy target of copper and magnesium.

It is preferable that at least the back reflection layer has a texture structure.

Further, the semiconductor layer is preferably made of a thin film semiconductor.

According to the present invention, the following effects can be obtained by adopting such a structure, particularly by using such a metal layer.

(1) Since a Cu-Mg alloy does not grow dendritic crystals, short-circuiting inside a photovoltaic element used in a severe environment can be prevented, and reliability is improved.

(2) When Mg is added to Cu in a range of 40 at% or less, the reflectance for light having a wavelength of 700 to 1000 nm is as high as 90% or more. Conversion efficiency is improved.

(3) A homogeneous alloy film can be obtained, and a photovoltaic element having stable characteristics can be obtained. In addition, since inexpensive metal is used as a main material, mass production at low cost becomes possible.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an example of the structure of the photovoltaic element of the present invention.
Shown in

Reference numeral 101 denotes a substrate. The substrate 101 is preferably a conductive metal substrate. When a non-conductive substrate is used, a metal conductive layer is preferably deposited by, for example, a vacuum evaporation method or a sputtering method.

Reference numeral 102 denotes a metal layer of a Cu—Mg alloy;
Denotes a transparent layer, and these are collectively referred to as a back reflection layer. The transparent layer 103 is transparent to sunlight transmitted through the semiconductor layer 104. Further, it is preferable that it has an appropriate electric resistance and its surface has a texture structure as shown in FIG.

Reference numeral 104 denotes a semiconductor layer (thin film semiconductor junction). FIG. 1 illustrates an example in which a pin-type non-single-crystal photovoltaic element is used as the semiconductor layer 104; however, a tandem cell or triple cell structure in which a plurality of pin-type non-single-crystal optical semiconductor elements are stacked may be used. . Here, 105 is an n-type a-S
i and 106 are i-type a-Si, and 107 is p-type a-Si. When the semiconductor layer 104 is thin, the entire semiconductor layer 104 often has the same texture structure as the transparent layer 103 as shown in FIG.

On top of this, a transparent electrode 108 and a current collecting electrode 109
Is provided.

(Metal layer 102) First, an experiment for showing the effect of the present invention will be described. In all experiments, the DC magnetron sputtering apparatus shown in FIG. 4 was used for depositing the metal layer. However, in general, the metal layer was deposited by resistance heating, vacuum deposition using an electron beam, sputtering, or ion plating. , A CVD method or the like is used.

Reference numeral 401 denotes a deposition chamber, which can be evacuated by an exhaust pump (not shown). Inside this, argon (Ar) is supplied by a gas introduction tube 402 connected to a gas cylinder (not shown).
A predetermined flow rate of inert gas such as is introduced, the opening degree of the exhaust valve 403 is adjusted, and the pressure in the deposition chamber 401 becomes a predetermined pressure.

The substrate 404 has a heater 405 inside.
Are fixed to the surface of the anode 406 provided with the.
A target 407 is provided on the surface facing the anode 406.
Is provided with a cathode electrode 408 to which is fixed. Target 407 is typically 99.9-99.999 pure
% Of blocks of metal to be deposited. The cathode electrode 408 is connected to a DC power supply 409, and applies a high DC voltage from the power supply 409 to generate a plasma 410 between the anode 406 and the cathode 408. The metal atoms of the retarget 407 are deposited on the substrate 404 by the action of the plasma 410.

The deposition rate can be further increased by using a magnetron sputtering apparatus in which a magnet is provided inside the cathode 408 to increase the intensity of the plasma 410.

(Experiment 1) Mirror-polished 5 cm × 5 cm
Mg concentration of 6, 10, 20, 34, 4 on a stainless steel plate (SUS304) by DC magnetron sputtering.
A Cu-Mg alloy of 6, 51 at% was deposited at 1000 Å, and samples 1a, 1b, 1c, 1
d, 1e, and 1f. For comparison, a sample 1k was similarly prepared using pure Cu. Since these samples were deposited on a smooth substrate at room temperature at 40 Å / sec, their surfaces were smooth.

In order to further examine the temperature dependence, 1b was used except that the substrate temperature was set to 100, 200, 300, and 400 degrees.
Samples 1g, 1h, 1i, and 1j were prepared in the same manner as described above.
The surface of 1 g to 1 i had a fine texture structure of several thousand angstroms.

For producing 1k, a 99.999% Cu target was used, and for producing alloy samples 1a to 1j, a mosaic target of Cu and Mg was used.

If a 5 mm × 5 mm × 1 mm chip made of Mg is placed on a Cu target instead of a mosaic target to form a film, it is probably due to the difference in sputtering rate between the two metals, poor electrical contact of the chip, and the chip due to the chip. Mg is less likely to fly uniformly due to the influence of irregularities formed on the surface, and the composition tends to be uneven in the film thickness direction. The mosaic target is preferable because composition unevenness in the film thickness direction can be improved.

The mosaic target was 99.9% Mg
9 or 99.99% Cu
A 9.99% Cu target is processed, and a reliable electric contact and a smooth surface can be obtained. In addition, Mg pieces and Cu
The composition can be changed by changing the ratio of the pieces to be fitted. The alloy sample was analyzed with an X-ray energy dispersive analyzer (XMA) to confirm the composition.

For these 11 types of samples, a wavelength of 400
The reflectivity for light of 1200 nm was measured. As described above, it is important that the a-SiGe semiconductor layer 8
FIG. 5 shows the value of the reflectance at 00 nm as a representative value of the reflectance obtained for each composition. The reflectivity gradually decreased as the Mg concentration increased, but the Mg concentration was 40 at%.
The following 1a to 1d showed a high reflectance of 90% or more.
There is no influence by the change of the substrate temperature, and 1g to 1j are 9
It exhibited a high reflectance of 0% or more.

(Experiment 2) A Cu-Mg alloy was deposited in the same manner as in Experiment 1 except that a comb-shaped electrode mask having a spacing of 200 μm was placed on a 7059 non-alkali glass of 5 cm × 5 cm, and evaluation of dendritic crystal growth was performed. Experimental samples were prepared.

Mg concentration 6, 10, 20, 34, 46, 5
1 at% was formed at room temperature, and for Mg concentration of 10 at%, films were formed at 100, 200, 300, and 400 degrees, respectively, and samples 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2
h, 2i, and 2j. Pure Cu and pure Ag samples 2k and 2l for comparison were similarly formed at room temperature.

With respect to these twelve kinds of samples, pure water droplets were dropped on the prepared electrode gap of 200 μm, a voltage of 10 V was applied, and dendritic crystal growth was observed under a microscope.
With respect to 2a to 2c and 2g to 2j, slight dendritic crystal growth was observed, but no short circuit occurred between the electrodes.
No dendritic crystals were observed in 2d to 2f, but a few thousand positive electrodes were observed to be dissolved, and the Mg content was 40 at.
%, 2e and 2f, the positive electrode melted in about 5 seconds after the voltage was applied. In the samples 2k and 2l, dendrites were generated at the same time when the voltage was applied, and short-circuit occurred.

(Experiment 3) A Cu-Mg alloy metal layer having a Mg concentration of 10 at% was formed at room temperature as in Experiment 1, except that a Cu-Mg alloy target was used as the target. As with the mosaic target, a homogeneous film could be obtained.

After that, as a transparent layer, ZnO was
Angstroms are formed, and an n-type a-Si layer is formed by glow discharge decomposition using SiH ₄ and PH ₃ as source gases.
0 angstrom, i-type a using SiH ₄ as source gas
4000 Å, SiH ₄ , B
Using F ₃ and H ₂ as source gases, a 100-Å p-type microcrystalline (μc) Si layer was deposited to form a thin film semiconductor junction.

An indium tin oxide film (ITO film) was deposited thereon as a transparent electrode by a resistance heating evaporation method at 650 Å, and a current collecting electrode having a width of 300 μm was formed with an Ag paste to form a solar cell. The sample thus obtained is designated as 3a. Similarly, for comparison, samples 3b and 3 in which the metal layers are pure Cu, pure Al, and pure Ag
c and 3d were obtained.

The photocurrent Jsc of these samples was measured under a solar simulator of AM-1.5. Sample 3c was 19.1 mA / cm ² , whereas sample 3a was 20.7 mA / cm ² and 21.
A high current value approaching 3 mA / cm ² was obtained. 3b
Did not function as a solar cell.

(Experiment 4) As in Experiment 3, Mg concentration was 10,
20, 34, 43 at% Cu-Mg alloy and pure A
g, a pure Al metal layer was deposited on a stainless steel plate.
Similarly, it was made into a solar cell. Samples 4a and 4
b, 4c, 4d, 4e, and 4f.

A reverse voltage of 0.85 V at a humidity of 85% and an ambient temperature of 85.degree.
(High humidity reverse bias test), and RshD with time
The change in k (Rsh without light) was measured and compared. When the RshDk is reduced to 10 Ωcm ² or less, no open voltage is generated under low illuminance light, and the characteristics and reliability of the solar cell become problematic. Therefore, in the high humidity reverse bias test, RshDk ≧ 10 kΩcm ² is set as a passing criterion.

The sample 4e starts measuring RshD
k dropped sharply and fell below 10 kΩcm ² .
Sample 4f did not drop below 31 kΩcm ² even after more than 100 hours. Samples 4a, 4b and 4c were slightly R
Although shDk was reduced, 16 kΩcm ² or more was maintained at 100 hours, but 4d was 10 kΩcm ^2.
Has fallen slightly. When combined with the results of Experiment 2, it can be said that a Cu—Mg alloy film having a Mg concentration of 40 at% or less has excellent reliability.

From the above experimental results, it was found that the Mg concentration of the present invention was 4%.
0 at% or less, preferably 20 to 40 at%, more preferably 30 to 35 at% in the range of a Cu-Mg alloy, particularly a Cu-Mg alloy formed by sputtering using a mosaic target or an alloy target, as a metal layer of the back surface reflection layer. , A highly efficient and highly reliable photovoltaic element can be obtained.

(Transparent layer 103) As the transparent layer, ZnO
, In ₂ O ₃ , SnO ₂ , CdO, CdSnO ₄ , T
Oxides such as iO are often used (however, the composition ratios of the compounds shown here do not always match the actual conditions). Generally, the higher the light transmittance of the transparent layer, the better, but it is not necessary to be transparent for light in the wavelength range absorbed by the semiconductor layer.

It is preferable that the transparent layer has a resistance in order to suppress a current caused by a pinhole or the like. However, the effect of the series resistance loss due to this resistance on the conversion efficiency of the photovoltaic element must be within a negligible range. From such a viewpoint, the range of the resistance per unit area (1 cm ² ) is preferably 10 ⁻⁶ to 10 Ω, more preferably 10 ^{−5 to} 3 Ω.
Ω, most preferably 10 ^{-4 to} 1 Ω.

The thickness of the transparent layer is preferably as small as possible from the viewpoint of transparency. However, when a texture structure on the surface is to be obtained, the average thickness is required to be 1000 Å or more. In some cases, a higher film thickness is required from the viewpoint of reliability.

For deposition of the transparent layer, a vacuum deposition method using resistance heating or an electron beam, a sputtering method, an ion plating method, a CVD method, a spray coating method, or the like is used. Also in this case, the sputtering apparatus shown in FIG. 4 can be used. Note that in the case of an oxide, there are a case where the oxide itself is used as a target and a case where a metal (Zn, Sn, or the like) target is used. In the latter case, Ar
At the same time, it is necessary to flow oxygen (this is called a reactive sputtering method).

In order to control the specific resistance of the transparent layer, an appropriate impurity may be added. When it is desired to increase the specific resistance of the conductive oxide, it is preferable to add the conductive oxide to appropriately increase the resistance. For example, an acceptor-type impurity (eg, Cu for ZnO and A for SnO ₂₎ is added to the transparent layer which is an n-type semiconductor.
l)) can be added to increase the resistance. Also, the addition of impurities often increases the chemical resistance.
To add an impurity to the transparent film, a desired impurity may be added to an evaporation source or a target. In particular, in a sputtering method, a small piece of a material containing an impurity may be usually placed on a target.

(Texture Structure of Back Reflection Layer) The back reflection layer comprising at least a metal layer and a transparent layer of the present invention preferably has a texture structure so that light is confined.

The reason why light confinement occurs is considered to be light scattering in the metal layer due to the texture structure of the metal layer. When the surface of the semiconductor layer has a texture structure similar to that of the transparent layer, light is easily scattered due to the phase difference of light, and the effect of the light trap is high. However, a fine texture structure such as the surface of the samples 1g to 1j in Experiment 1 has no effect of increasing diffuse reflection. In such a case, it is possible to increase the irregular reflectance of the metal layer by, for example, lowering the film formation rate or increasing the film thickness. The light confinement also takes place by using a textured structure, so that the incident light can be used effectively.

(Substrate 101) Various metals are used for the substrate. Among them, stainless steel plate, zinc steel plate,
Aluminum plates, copper plates, and the like are suitable because they are relatively inexpensive. These metal plates may be cut into a certain shape and used, or may be used in a long sheet-like form depending on the plate thickness. In this case, since it can be wound in a coil shape, it is suitable for continuous production, and storage and transportation are easy. Depending on the application, a crystal substrate such as silicon or a glass or ceramic plate may be used. The surface of the substrate may be polished, but may be used as it is when the finish is good, such as a stainless steel plate subjected to a bright annealing treatment.

(Semiconductor Layer 104) Semiconductor materials particularly preferably used for the photovoltaic device of the present invention include a-S
i: H (abbreviation of hydrogenated amorphous silicon), a-Si:
Amorphous semiconductor material such as F, a-Si: H: F, μC-
Si: H (abbreviation for hydrogenated microcrystalline silicon), μC-S
Microcrystalline semiconductor materials such as i: F, μC-Si: H: F, and the like.

The semiconductor layer can perform valence electron control and forbidden band width control. Specifically, a raw material compound containing an element serving as a valence electron controlling or forbidden band width controlling agent when forming a semiconductor layer is used alone or mixed with a raw material gas or a diluting gas for forming a deposited film to form a film forming space. You just need to introduce it.

The semiconductor layer is controlled by valence electrons.
At least a portion is doped p-type and n-type to form at least one set of pin junctions.

As a method of forming a semiconductor layer, a microwave plasma CVD method, an RF plasma CVD method,
VD method, thermal CVD method, various CVD methods such as MOCVD method, or various evaporation methods such as EB evaporation, MBE, ion plating, ion beam method, sputtering method,
It is formed by a spray method, a printing method, or the like. As a method adopted industrially, a plasma CVD method in which a raw material gas is decomposed by plasma and deposited on a substrate is preferably used. In addition, as the reaction device, a batch type device, a continuous film forming device, or the like can be used as desired.

Hereinafter, a semiconductor layer using a silicon-based non-single-crystal semiconductor material particularly suitable for the photovoltaic device of the present invention will be described in more detail.

(1) i-type semiconductor layer (intrinsic semiconductor layer) In a photovoltaic device using a silicon-based non-single-crystal semiconductor material, the i-type layer used for the pin junction generates and transports carriers to irradiation light. It is an important layer. As the i-type layer, a slightly p-type or slightly n-type layer can be used.

As described above, the silicon-based non-single-crystal semiconductor material contains a hydrogen atom (H, D) or a halogen atom (X), which has an important function. Hydrogen atoms (H, D) or halogen atoms (X) contained in the i-type layer work to compensate for dangling bonds in the i-type layer, and the mobility of carriers in the i-type layer. And product life. P-type layer / i-type layer, n
Works to compensate the interface state of each interface of the mold layer / i-type layer,
This has the effect of improving the photovoltaic power, photocurrent and photoresponsiveness of the photovoltaic element.

The optimum content of hydrogen atoms and / or halogen atoms contained in the i-type layer is 1 to 40 at%. In particular, those in which a large amount of hydrogen atoms and / or halogen atoms are distributed on each interface side of the p-type layer / i-type layer and the n-type layer / i-type layer are mentioned as preferred distribution modes. The content of hydrogen atoms and / or halogen atoms in the above is preferably in a range of 1.1 to 2 times the content in the bulk. Further, it is preferable that the content of hydrogen atoms and / or halogen atoms is changed in accordance with the content of silicon atoms.

Amorphous silicon and microcrystalline silicon are a-Si:
H, a-Si: F, a-Si: H: F, μC-Si:
H, μC-Si: F, μC-Si: H: F, etc.

Further, i suitable for the photovoltaic device of the present invention
The characteristics of the type semiconductor layer include the hydrogen atom content (C _H )
Is 1.0 to 25.0%, AM1.5, 100 mW / c.
pseudo-sunlight irradiation of a light conductivity of the m ² (σp) is 1.0
× 10 ⁻⁷ S / cm or more, dark conductivity (σd) is 1.0 ×
10 ^-9 S / cm or less, Urbach energy by the constant photocurrent method (CPM) is 55 me
V or less, and those having a localized level density of 10 ¹⁷ / cm ³ or less are suitably used.

(2) Doping layer (p-type semiconductor layer or n-type semiconductor layer) The doping layer (p-type semiconductor layer or n-type semiconductor layer)
This is an important layer that affects the characteristics of the photovoltaic device of the present invention.

As an amorphous material (denoted as a-) or a microcrystalline material (denoted as μc-) of the doping layer, for example, a-Si: H, a-Si: HX, a-Si
C: H, a-SiC: HX, a-SiGe: H, a-S
iGe: HX, a-SiGeC: H, a-SiGeC:
HX, a-SiO: H, a-SiO: HX, a-Si
N: H, a-SiN: HX, a-SiON: H, a-S
iON: HX, a-SiOCN: H, a-SiOCN:
HX, μc-Si: H, μc-Si: HX, μc-Si
C: H, μc-SiC: HX, μc-SiO: H, μc
—SiO: HX, μc-SiN: H, μc-SiN: H
X, μc-SiGeC: H, μc-SiGeC: HX,
μc-SiON: H, μc-SiON: HX, μc-S
iOCN: H, μc-SiOCN: HX, etc., p-type valence electron controlling agents (Group III atoms B, Al, Ga,
In, Tl) and a material in which an n-type valence electron controlling agent (Group V atom P, As, Sb, Bi) in the periodic table is added at a high concentration.

As the polycrystalline material (denoted as poly-), for example, poly-Si: H, poly-Si: H
X, poly-SiC: H, poly-SiC: HX,
poly-SiO: H, poly-SiO: HX, po
ly-SiN: H, poly-SiN: HX, poly
-SiGeC: H, poly-SiGeC: HX, po
ly-SiON: H, poly-SiON: HX, po
ly-SiOCN: H, poly-SiOCN: HX,
poly-Si, poly-SiC, poly-Si
O, poly-SiN, etc., p-type valence electron control agents (Group III atoms B, Al, Ga, In, Tl) in the periodic table and n-type valence electron control agents (Group V atoms P in the periodic table) , As, S
b, Bi) at a high concentration.

In particular, the p-type layer or the n-type layer on the light incident side includes:
A crystalline semiconductor layer with little light absorption or an amorphous semiconductor layer with a wide band gap is suitable.

Further, hydrogen atoms (H, D) or halogen atoms contained in the p-type layer or the n-type layer work to compensate for dangling bonds of the p-type layer or the n-type layer, and To improve the doping efficiency. A suitable amount of hydrogen atoms or halogen atoms added to the p-type layer or the n-type layer is preferably 0.1% to 50%, more preferably 1% to 40%. When the p-type layer or the n-type layer is crystalline, hydrogen atoms or halogen atoms are 0.1%
-10% is mentioned as the optimum amount.

The p-type layer and the n-type layer of the photovoltaic element preferably have an activation energy of 0.2 eV or less, and most preferably have an activation energy of 0.1 eV or less. The non-resistance is preferably 100 Ωcm or less, and most preferably 1 Ωcm or less. Further, the layer thickness of the p-type layer and the n-type layer is 1 to
50 nm is preferred, and 3 to 10 nm is optimal.

(3) Method of Forming Semiconductor Layer In order to form a suitable silicon-based non-single-crystal semiconductor material as the semiconductor layer of the photovoltaic device of the present invention, a preferable manufacturing method is plasma CVD using high frequency. Is the law. The high frequency is preferably 3 MHz to 3 GHz.

As a source gas suitable for depositing a silicon-based non-single-crystal semiconductor layer suitable for the photovoltaic device of the present invention, a gasizable compound containing silicon atoms can be given.

Specific examples of the gasizable compound containing a silicon atom include a chain or cyclic silane compound. Specifically, for example, SiH ₄ , Si ₂ H ₆ , Si
F ₄ , SiFH ₃ , SiF ₂ H ₂ , SiF ₃ H, Si ₃ H ₈ ,
SiD ₄ , SiHD ₃ , SiH ₂ D ₂ , SiH ₃ D, SiF
D ₃ , SiF ₂ D ₂ , Si ₂ D ₃ H ₃ , (SiF ₂ ) ₅ , (Si
F ₂ ) ₆ , (SiF ₂ ) ₄ , Si ₂ F ₆ , Si ₃ F ₈ , Si ₂ H ₂
F ₄ , Si ₂ H ₃ F ₃ , SiCl ₄ , (SiCl ₂ ) ₅ , Si
Br ₄ , (SiBr ₂ ) ₅ , Si ₂ Cl ₆ , SiHCl ₃ , S
Examples thereof include those in a gas state or those which can be easily gasified, such as iH ₂ Br ₂ , SiH ₂ Cl ₂ , and Si ₂ Cl ₃ F ₃ .

Examples of the substance introduced into the p-type layer or the n-type layer for controlling valence electrons include Group III atoms and Group V atoms in the periodic table.

As a starting material for effectively introducing a group III atom, specifically, for introducing a boron atom, B ₂ H ₆ , B ₄ H ₁₀ , B ₅ H ₉ , B ₅ H ₁₁ , B
Borohydrides such as ₆ H ₁₀ , B ₆ H ₁₂ , B ₆ H ₁₄ , BF ₃ , B
And boron halide such as Cl ₃ . In addition, AlCl ₃ , GaCl ₃ , InCl ₃ , T
1Cl _{3 and the} like can also be mentioned. Particularly, B ₂ H ₆ and BF ₃ are suitable.

The starting material for introducing a group V atom can be effectively used as a starting material for introducing a phosphorus atom.
₃ , hydrogenated phosphorus such as P ₂ H ₄ , PH ₄ I, PF ₃ , PF ₅ , PC
and phosphorus halides such as l ₃ , PCl ₅ , PBr ₃ , PBr ₅ and PI ₃ . In addition, AsH ₃ , AsF ₃ , AsC
l ₃ , AsBr ₃ , AsF ₅ , SbH ₃ , SbF ₃ , Sb
_{F 5, SbCl 3, SbCl 5} , BiH 3, BiCl 3, B
iBr _{3 and the} like can also be mentioned. Particularly, PH ₃ and PF ₃ are suitable.

The compound capable of being gasified is H ₂ , H
The gas may be appropriately diluted with a gas such as e, Ne, Ar, Xe, or Kr and introduced into the deposition chamber.

In particular, microcrystalline or polycrystalline semiconductors and a-S
When depositing a layer having a small light absorption or a wide band gap such as iC: H, it is preferable to dilute the source gas with hydrogen gas and to introduce a relatively high frequency power.

(Transparent Electrode 108) In the present invention, the transparent electrode 108 is an electrode on the light incident side that transmits light, and also functions as an anti-reflection film by optimizing its film thickness. The transparent electrode 108 is required to have a high transmittance in a wavelength region where the semiconductor layer can absorb light and a low resistivity. Preferably, the transmittance at 550 nm is at least 80%, more preferably at least 85%. Also, the resistivity is preferably
5 × 10 ^-3 Ωcm or less, more preferably, 1 × 10 ^-3 Ω
cm or less.

The materials include In ₂ O ₃ , SnO ₂ ,
ITO (In ₂ O ₃ + SnO ₂ ), ZnO, CdO, Cd ₂
SnO ₄ , TiO ₂ , Ta ₂ O ₅ , Bi ₂ O ₃ , MoO ₃ , N
A conductive oxide such as a _x WO ₃ or a mixture thereof is preferably used. Further, an element (dopant) that changes the conductivity may be added to these compounds.

As the element (dopant) for changing the conductivity, for example, when the transparent electrode 108 is ZnO,
l, In, B, Ga, Si, F, etc., In the case of In ₂ O ₃ , Sn, F, Te, Ti, Sb, Pb, etc., and in the case of SnO ₂ , F, Sb, P, As, In, T
1, Te, W, Cl, Br, I and the like are preferably used.

Further, as a method for forming the transparent electrode 108, an evaporation method, a CVD method, a spray method, a spin-on method, a dipping method and the like are suitably used.

(Current Collecting Electrode 109) In the present invention, if the resistivity of the transparent electrode 108 cannot be made sufficiently low, the current collecting electrode 109 is formed on a part of the transparent electrode 108 as necessary, It acts to lower the rate and lower the series resistance of the photovoltaic element. Examples of the material include metals such as gold, silver, copper, aluminum, nickel, iron, chromium, molybdenum, tungsten, titanium, cobalt, tantalum, niobium, and zirconium; alloys such as stainless steel; and powdered metals. Paste and the like.
Then, the shape is formed in a branch shape so as not to block incident light to the semiconductor layer as much as possible.

Further, in the entire area of the photovoltaic element,
The area occupied by the collecting electrode is preferably 15% or less, more preferably 10% or less, and most preferably 5% or less.

Further, a pattern of the collecting electrode is formed by using a mask, and the forming method includes a vapor deposition method, a sputtering method,
A plating method, a printing method, or the like is used.

When a photovoltaic device (module or panel) having a desired output voltage and output current is manufactured using the photovoltaic device of the present invention, the photovoltaic devices of the present invention are connected in series or in series. They are connected in parallel, protective layers are formed on the front and back surfaces, and output output electrodes and the like are attached. When the photovoltaic elements of the present invention are connected in series, a diode for preventing backflow may be incorporated.

[0097]

The present invention will be described in more detail with reference to the following examples.

(Example 1) A pin-type a-Si photovoltaic element having the structure shown in the schematic sectional view of FIG. 1 was produced. A transparent layer (ZnO layer) 103 of 10,000 angstroms was deposited on the sample 1d (Mg 34 at%) produced in Experiment 1 at a substrate temperature of 350 ° C. using a ZnO target. The surface of the transparent layer 103 has a texture structure.

Subsequently, the substrate 1001 on which the back reflection layer composed of the metal layer 102 and the transparent layer 103 was formed was set in a commercially available capacitively coupled high-frequency CVD apparatus (CHJ-3030 manufactured by ULVAC, Inc.) shown in FIG. Exhaust pump 1009
, A rough evacuation operation and a high evacuation operation were performed through the exhaust pipe of the reaction vessel 1004. At this time, the surface temperature of the substrate is 350
° C was controlled by a temperature control mechanism.

At the time when the exhaust was sufficiently performed, 300 sccm of SiH ₄ , ₄ sccm of SiF ₄ , P
H ₃ / H ₂ (1% H ₂ dilution) 55 sccm, H ₂ 40 scc
m, the opening of the throttle valve was adjusted, the internal pressure of the reaction vessel was maintained at 1 Torr, and when the pressure was stabilized, 200 W of power was immediately supplied from the high frequency power supply. The plasma was maintained for 5 minutes. Thereby, the n-type a
-An Si layer 105 was formed on the transparent layer 103.

After exhausting again, this time, 300 sccm of SiH ₄ , ₄ sccm of SiF ₄ , and H ₂ 4
0 sccm was introduced, the opening of the throttle valve was adjusted, and the internal pressure of the reaction vessel 1004 was maintained at 1 Torr.
And the plasma was maintained for 60 minutes. Thus, an i-type a-Si layer 106 was formed on the n-type a-Si layer 105.

After exhausting again, this time, SiH ₄ 50 sccm, BF ₃ / H ₂ (1% H ₂ dilution) 5
0 sccm and H ₂ 500 sccm were introduced, the opening of the throttle valve was adjusted, and the internal pressure of the reaction vessel 1004 was set to 1
When the pressure was stabilized and the pressure was stabilized, 300 W of power was immediately supplied from the high frequency power supply. Plasma is 2
Lasted for minutes. As a result, a p-type μc-Si layer 107 was formed on the i-type a-Si layer 106.

Next, the sample was taken out of the high-frequency CVD apparatus, and ITO was deposited by a resistance heating vacuum evaporation apparatus. Then, a paste containing an iron chloride solution was printed, and a desired transparent electrode 10 was formed.
8 were formed. Further, an Ag paste was screen-printed to form a current collecting electrode 109, thereby completing a thin-film semiconductor solar cell.

In this way, ten samples were prepared, and AM-
When Jsc was measured under a light of 1.5, pure A was measured.
On average, a current value 6.1% higher than that of the 1-metal layer solar cell was obtained. When these ten solar cells were subjected to a high humidity reverse bias test, RshDk did not fall below 19 kΩ.

(Example 2) Using the apparatus shown in FIG. 7, a back reflection layer was continuously formed.

Here, a cleaned stainless sheet roll 1103 having a width of 350 mm, a thickness of 0.2 mm, and a length of 500 m is set in the substrate delivery chamber 1101. From here, the stainless steel sheet 1102 is placed in the Al layer deposition chamber 110.
4. It is sent to the substrate winding chamber 1113 via the metal layer deposition chamber 1107 and the transparent layer deposition chamber 1110. Sheet 11
02 denotes a substrate heater 1105, 110 in each deposition chamber.
8 and 1111 can be heated to a desired temperature.

The stainless sheet 1102 has a purity of 99.
At a substrate temperature of 400 ° C., an Al layer having a texture structure is deposited by a magnetron sputtering method in an Al layer deposition chamber 1004 in which a 99% Al target 1006 is installed.

Thereafter, the purity of the metal layer deposition chamber 1007 is adjusted to 9
With a 9.99% Cu-Mg alloy target 1109, D
A metal layer (Cu—Mg alloy layer: Mg concentration: 15 at%) is deposited by C magnetron sputtering at 1000 Å without increasing the temperature of the substrate.

Target 111 of Transparent Layer Deposition Chamber 1111
2 is ZnO having a purity of 99.99%, and a transparent layer (ZnO layer) is continuously formed by DC magnetron sputtering to 10,000.
Angstrom is deposited.

The a-Si / a- layer having the structure shown in FIG.
A SiGe tandem solar cell was formed.

Here, 1201 is a substrate, 1202 is Al
Layer 1203 is a metal layer, 1204 is a transparent layer, 1205 is a bottom cell, and 1209 is a top cell. 12 more
06, 1210 are n-type a-Si layers, 1208, 1212
Is a p-type μc-Si, 1207 is an i-type a-SiGe layer, 1
211 is an i-type a-Si layer. These thin film semiconductor layers were continuously manufactured using a roll-to-roll type film forming apparatus as described in US Pat. No. 4,492,181.

Reference numeral 1213 denotes a transparent electrode which was deposited by a sputtering apparatus similar to the apparatus shown in FIG. Reference numeral 1214 denotes a current collecting electrode. After patterning the transparent electrode 1213 and forming the current collecting electrode 1214, the sheet 1102 was cut.

In this way, all the processes were continuously processed, and the effect of mass production could be improved.

With this method, 100 samples were prepared and AM-
When Jsc was measured under a light of 1.5, a current value 5.3% higher than that of the solar cell having the pure Al metal layer on average was obtained. Also, in the high humidity reverse bias test, RshD
k did not drop below 23 kΩ.

Example 3 A metal layer and a transparent layer of 20 at% Cu-Mg were deposited using the apparatus shown in FIG. 7 by using the same stainless steel as in Example 2 except that the surface was textured. Each layer was deposited in the same manner as in Example 2 except that the Al layer deposition chamber 1104 was not used for depositing the metal layer.

Thereafter, a photovoltaic element was formed using the roll-to-roll type photovoltaic element forming apparatus shown in FIG. 9 under the photovoltaic element forming conditions shown in Table 1.

A sheet-like substrate (sheet width 35 cm) was set in a load chamber 5010 for introducing a sheet-like substrate 5401. The sheet-like substrate 5401 is placed in all deposition chambers 5020 and 503
0, 5040, 5050, 5060, 5070, 508
0, 5090, 5100, 5110, 5120, 513
0, 5140 and through all gas gates 5201 to 5214, a sheet winding jig 540 of the unloading chamber 5150.
2 was connected.

Each of the deposition chambers 5020, 5030, 5040,
5050, 5060, 5070, 5080, 5090,
5100, 5110, 5120, 5130, and 5140 were evacuated to 10 ⁻³ Torr or less using an exhaust device (not shown).

Mixing device 502 for forming each deposited film
6, 5036, 5046, 5056, 5066, 507
6, 5086, 5096, 5106, 5116, 512
6, 5136, and 5146, hydrogen gas is supplied to each deposition chamber 502.
0, 5030, 5040, 5050, 5060, 507
0, 5080, 5090, 5100, 5110, 512
0, 5130, and 5140.

Hydrogen gas was supplied to each of the gas gates 5201 to 5214 from each of the gate gas supply devices 5301 to 5314. In this embodiment, the gas gates 5201 to 5214 are used.
Since the interval of passing through the sheet-like substrate 5401 was set to 1 mm, the hydrogen gas flowed at 1000 sccm.

The substrate 5 was heated by the substrate heating heater of each deposition apparatus.
401 was heated to the substrate temperature shown in Table 1. When the substrate temperature is stabilized, each of the deposition chambers 5020, 5030, 5040, 50
50, 5060, 5070, 5080, 5090, 51
00, 5110, 5120, 5130, and 5140 are supplied to the respective deposition chambers 5020, 5030,
040, 5050, 5060, 5070, 5080, 5
090, 5100, 5110, 5120, 5130, 5
The raw material gas deposited at 140 was switched to the raw material gas shown in Table 1.

When the switching of the source gas is completed, the opening / closing degree of the exhaust valve of each exhaust device is adjusted to adjust the deposition chambers 5020, 50
030, 5040, 5050, 5060, 5070, 5
080, 5090, 5100, 5110, 5120, 5
130 and 5140 were adjusted to the degree of vacuum shown in Table 1, and the conveyance of the sheet-like substrate 5401 was started.

When the degree of vacuum is stabilized, each deposition chamber 5020,
5030, 5040, 5050, 5060, 5070,
5080, 5090, 5100, 5110, 5120,
5130 and 5140 show the following RF for plasma generation.
Power and MW power were supplied.

As described above, a photovoltaic element in which three pin structures were stacked on the sheet-like substrate 100m was formed.

In this way, 100 samples were prepared, and the AM
Jsc was measured with a solar simulator of -1.5. The average value was 6.4% higher than the current value obtained in the solar cell using the pure Al metal layer, and there was no problem in the high humidity reverse bias test.

[0126]

[Table 1]

[0127]

According to the present invention, a back reflection layer having high reflectivity and excellent in migration resistance can be obtained, and as a result, a photovoltaic element having high reliability and high conversion efficiency can be obtained. In addition, since copper and magnesium, which are the main materials of the back reflection layer, are inexpensive, mass production at low cost becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view illustrating an example of the configuration of a photovoltaic element of the present invention.

FIG. 2 is a graph showing the reflectance of various metals.

FIG. 3 is a graph showing an increase in the absorption of incident light due to the use of a textured backside reflective layer.

FIG. 4 is a schematic diagram of a DC magnetron sputtering apparatus.

FIG. 5 is a graph showing the relationship between the Mg concentration and the reflectance at 800 nm.

FIG. 6 is a schematic diagram of a capacitively coupled high-frequency CVD apparatus.

FIG. 7 is a schematic view of a continuous type backside reflection layer forming apparatus.

FIG. 8 is a schematic sectional view showing another example of the configuration of the photovoltaic element of the present invention.

FIG. 9 is a schematic view of a roll-to-roll type photovoltaic element forming apparatus.

Claims (6)

[Claims] 1. A photovoltaic device having at least a back reflection layer comprising a metal layer and a transparent layer, a semiconductor layer, and a transparent electrode on a substrate, wherein the metal layer has a magnesium concentration of 40 at% or less. A photovoltaic element characterized by being a magnesium alloy. 2. A photovoltaic device having at least a back reflection layer comprising a metal layer and a transparent layer, a semiconductor layer, and a transparent electrode on a substrate, wherein the metal layer has a wavelength of 700 nm.
Exhibiting a reflectance of 90% or more for light of ~ 1000 nm,
A photovoltaic device comprising a copper-magnesium alloy that does not cause electrochemical migration. 3. A photovoltaic device having at least a back reflection layer comprising a metal layer and a transparent layer, a semiconductor layer, and a transparent electrode on a substrate, wherein the magnesium concentration of the metal layer is 40 at% or less. Reflectance is 700 ~
A photovoltaic device comprising a copper-magnesium alloy that accounts for 90% or more of 1000 nm light. 4. The photovoltaic device according to claim 1, wherein the metal layer is formed by sputtering using a mosaic target or an alloy target of copper and magnesium. 5. The photovoltaic device according to claim 1, wherein at least the back reflection layer has a texture structure. 6. The photovoltaic device according to claim 1, wherein the semiconductor layer is made of a thin film semiconductor.