JP2003298077A - Solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell whose electric characteristics such as power generation efficiency or the like are appropriate and that can be manufactured in high productivity. <P>SOLUTION: A silicon layer containing carbon formed by a hot wire CVD is formed on a single crystal or polycrystal silicon layer. It is preferable that there exist fine columnar crystals in the silicon layer containing the carbon, and the silicon layer containing the carbon has a high conductivity. In addition, a high hydrogen containing layer is preferably provided on a boundary between the silicon layer containing the carbon and the single crystal or polycrystal silicon layer. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell, and more particularly to a so-called heterojunction type solar cell in which semiconductors having two different band gaps are joined.

[0002]

2. Description of the Related Art The general structure of a solar cell is one in which amorphous silicon is doped with impurities to form a PN junction. This type of solar cell has high mass productivity and can be manufactured at a relatively low manufacturing cost, but has a problem of relatively low power generation efficiency. On the other hand, a single crystal or polycrystalline silicon substrate is doped with impurities to form P
A solar cell having an N-junction is known. According to the solar cell having such a structure, the thickness of the substrate forming the solar cell becomes thicker, and the productivity is lower than that of the amorphous silicon solar cell described above, but the power generation efficiency is high.

Further, a so-called heterojunction type solar cell in which semiconductors having two different band gaps are joined is known. In this solar cell, for example, higher power generation efficiency can be expected by forming a junction by combining silicon and a semiconductor film such as SiC having a band gap wider than that of silicon. As a method of manufacturing such a heterojunction solar cell, a plasma CVD method or an ECR (Electron Cyclotron Res
Onance) CVD method or the like for depositing materials having different band gaps is known. However, these methods for manufacturing a heterojunction solar cell have problems in terms of production efficiency and manufacturing cost due to a low film forming rate, an extremely expensive apparatus, and the like.

[0004]

The present invention has been made in view of the above circumstances, and provides a solar cell which has good electrical characteristics such as power generation efficiency and which can be manufactured with good productivity. The purpose is to

[0005]

A solar cell of the present invention is characterized in that a silicon-containing silicon layer formed by hot wire CVD is formed on a single crystal or polycrystalline silicon layer.

Here, the silicon layer containing carbon includes
It is preferable that fine columnar crystals of silicon are present. Further, it is preferable that the silicon layer containing carbon has high conductivity. Further, it is preferable to provide a high hydrogen content layer at the interface between the silicon layer containing carbon and the single crystal or polycrystalline silicon layer.

According to the present invention described above, since the silicon-containing silicon layer formed by hot wire CVD is formed on the single crystal or polycrystalline silicon layer, a heterojunction solar cell can be easily formed. . Since this solar cell is a heterojunction type solar cell, it has high open-circuit voltage and short-circuit current characteristics and good electrical characteristics such as high power generation efficiency. Further, since the silicon layer containing carbon is formed by hot wire CVD, it is possible to form a solar cell having a good film quality with relatively high productivity.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a solar cell according to a first embodiment of the present invention. In this solar cell, a silicon layer 12 containing carbon formed by hot wire CVD is deposited on a monocrystalline or polycrystalline silicon substrate 11. Thereby, a heterojunction solar cell is formed. Here, the single crystal or polycrystalline silicon substrate 11 has, for example, a thickness of 150 μm.
It is preferably formed from the following dendritic web (ribbon crystal). Such a single crystal or polycrystalline silicon substrate grows a thin band-shaped (sheet-shaped) crystal by pulling a seed crystal from a crucible obtained by melting a silicon raw material adjusted to a predetermined temperature along a crystal axis of a predetermined orientation. Can be produced. That is, a long crystal can be continuously pulled up by sandwiching this crystal between endless belts and pulling it up. Then, a thin band-shaped crystal is cut into an appropriate size to obtain a rectangular sheet-shaped single crystal or polycrystalline silicon substrate having a thickness of 150 μm or less. For more details, see Japanese Patent Application No. 2000-2 of the applicant.
See 75315.

The silicon substrate 11 is a single crystal silicon substrate having a plane orientation of <100>, is N-type doped, and has a resistivity of 2 Ωcm as an example. A silicon layer 12 containing carbon formed by hot wire CVD is formed on a silicon substrate 11. The silicon layer 12 contains amorphous (non-crystalline) silicon carbide (a-SiC) and microcrystalline silicon (μc-Si), and is a heterostructure conductive film in which a-SiC and microcrystalline Si are mixed. .
The thickness of the carbon-containing silicon layer 12 is about 1 to 50 nm, and the opposite conductivity type (P type) to the silicon substrate 11 is doped with impurities. Therefore, a PN junction is formed between the silicon substrate 11 and the silicon layer 12, and a power generation operation as a solar cell is performed.

As shown in FIG. 3, the silicon layer 12 containing carbon has microcrystalline silicon columns 12b which are fine columnar crystals of silicon. This microcrystalline silicon pillar 1
2b is a silicon layer containing carbon in an amorphous state (S
It is a columnar crystal body existing in the iC layer) 12 and perpendicular to the film formation surface. This microcrystalline silicon column has a diameter of about 1 to 50 nm, and 10 to 20 nm is typical. The microcrystalline silicon column 12b exhibits high conductivity with respect to the surrounding SiC layer in the amorphous state. The content of the microcrystalline silicon pillar can be adjusted to about 0.1% to 80%. Therefore, the conductivity can be adjusted by this content rate.

Further, the carbon content in the carbon-containing silicon layer 12 can be adjusted in a wide range, for example, about 0.1% to 70%. The carbon content has a great influence on the conductivity.

As described above, the content rate of the microcrystalline silicon pillars 15 and the content rate of carbon can be controlled, and the concentration of the doping material can be adjusted. Therefore, the conductivity of the silicon layer 12 and effective A wide band gap can be adjusted. For example, a P-type silicon layer 12
In the case of, the conductivity is 5 × 10 ^{−6 to} 5 × 10 ⁰ (S /
cm), and an effective band gap of 1.7 to 2.6
It can be adjusted to about eV. As a result, in the heterojunction solar cell, high conductivity can be obtained in the silicon layer 12, and electrical characteristics such as power generation efficiency, short circuit current, open circuit voltage, etc. can be improved.

In the hot wire CVD method, as shown in FIG. 3 (a), a gas supply system device 2 is provided in a vacuum container capable of depressurizing.
1, a high temperature hot wire 22, a substrate 23 to be film-formed, a substrate holder 24 and the like. Here, the high temperature hot wire 22 is made of a wire material such as tungsten or molybdenum.
It is heated to 00 to 2400 ° C. to activate a target gas to form a vapor phase growth film on the surface of the substrate 23. What is generally known as catalytic CVD and the basic structure of the apparatus are It is the same. Note that the hot wire is a general term for heat rays, and its shape is a two-dimensional shape such as a surface shape or
It can have a dimensional shape. On the other hand, FIG.
The plasma CVD apparatus shown in (b) as a comparative example supplies a high-frequency voltage from the high-frequency power source 27 between the electrodes 25 and 26 to make the gas molecules 28 to be film-formed into plasma and vibrate in the arrow direction in the figure. , By this, the substrate 2 to be film-formed
3 is to form a vapor growth film on the surface of No. 3.

The hot wire CV shown in FIG. 3 (a)
Compared to the plasma CVD method shown in FIG. 3B, the D method has almost no plasma damage to the film formation target substrate, the structure of the device is simple and the cost is low, and the scale-up is easy. There is a feature of. In particular, the film formation rate is relatively high, for example, about 5 to 10 nm / sec, and a film of high quality can be obtained in a relatively short time by forming an amorphous silicon film or a microcrystalline silicon film.

The lower side of the silicon layer 12 is a high hydrogen P layer and the upper side is a P layer. Gas ratio to form a high hydrogen P layer is, for _{example, CH 4: 0.9, SiH 4} : 0.1, H 2: 9, B 2 H
₆ : 0.002, the hot wire temperature is 2100 ° C., and the substrate temperature is 150 ° C. The reaction pressure is 10 Pa and the film thickness is 5 nm. The hydrogen concentration in the film of the high hydrogen concentration P layer is 6 at% or more.

[0017] The P layer 12 on the upper side of the high hydrogen P layer 12a is formed, gas _{ratio, CH 4: 0.1, SiH 4} : 0.1, H 2: 1, B 2 H
₆ : 0.005, the hot wire temperature is 2100 ° C., and the substrate temperature is 250 ° C. The reaction pressure at this time is 1
It is 0 Pa and the film thickness is 15 nm.

A high-concentration silicon layer (N ⁺ layer) 11a is similarly formed on the back surface of the silicon substrate 11 by a hot wire CV.
It is formed by the D method. At this time, the gas ratios are SiH ₄ : 0.1, H ₂ : 9, PH ₃ : 0.005, the hot wire temperature is 1800 ° C., and the substrate temperature is 200 ° C. The reaction pressure is 10 Pa and the film thickness is 20 nm. Silicon layer containing carbon 1
An antireflection film 15 is formed on the upper part of 2, and the antireflection film 15 is made of an ITO film and has a thickness of about 0.05 μm. Further, a vapor deposition film of Al is used for the surface electrode and has a thickness of about 3 μm. Similarly, for the back side electrode, A
The vapor deposition film No. 1 is used, and the thickness is about 5 μm.

FIG. 2 shows a solar cell according to the second embodiment of the present invention. The structure is similar to that of the first embodiment, except that the single crystal or polycrystalline silicon substrate 11 is P-type, and the carbon-containing silicon layer 12 is N-type impurity-doped. PH ₃ is used as a doping gas for the silicon layer 12, and has a conductivity of 5 × 10 5.
It is possible to adjust to ⁻⁶ to 8 × 10 ¹ (S / cm). When an N type substrate is used as the silicon substrate 11, a Ti / Pd / Ag layer is used as the back electrode.

In the production of these solar cells, SiH ₄ , Si ₂ H _6, etc. are used as the silicon-based source gas for forming the carbon-containing silicon layer 12, and CH ₄ , C ₂ H ₄ , C ₂ H ₂ or the like is used, and PH ₃ , B ₂ H ₆ or the like is used as a doping gas for doping N-type or P-type impurities. These source gases are activated by the catalytic decomposition action of the hot wire (hot wire), and the a-SiC / μc-Si pillar heterostructure conductive film is formed on the silicon substrate 11. As a result, the carbon-containing silicon layer 12 has a wide bandgap of about 2 eV,
A heterojunction is formed.

When a heterojunction is formed by using, for example, a single crystal or polycrystal silicon substrate as a film formation target substrate, a large number of interface states are generally formed near the surface of the silicon substrate. Therefore, when depositing a thin film by the hot wire CVD method, first, the thin film 1 of the high hydrogen concentration layer is formed.
By forming 2a, it is possible to effectively passivate the substrate and its surface by hydrogen without damaging the silicon substrate 11 and reduce the defect levels of the bulk and the interface. The hydrogen concentration at this time is 6 to 24a.
t (atomic)% is suitable, and 8 at (atomic)% or more is preferable.

Next, a preferable composition ratio of the raw material gas will be described. SiH ₄ as a source gas containing silicon
The case where CH ₄ is used as a source gas containing carbon, PH ₃ or B ₂ H ₆ is used as an impurity doping gas, and H ₂ gas is used as a diluent gas will be described. The ratio of the carbon-based gas in the raw material gas is preferably CH ₄ / (SiH ₄ + CH ₄ ) = 0.01 to 95%. As the dilution ratio of hydrogen _{gas, H 2 / (SiH 4 +} CH 4) = 0~50 ( Note: 0 to 5,000% in terms of the%) is preferably about. Further, the ratio of the P-type doping gas to the source gas is preferably B ₂ H ₆ / (SiH ₄ + CH ₄ ) = 0.001 to 1%. The ratio of the N-type doping gas to the raw material gas is preferably PH ₃ / (SiH ₄ + CH ₄ ) = 0.001 to 1%.

The operating conditions of this hot wire CVD apparatus are as follows. Deposition pressure: 10 ⁻¹ to 10 ² Pa Hot wire temperature (tungsten or molybdenum wire): 1750 to 2400 ° C. Substrate temperature: 150 to 500 ° C. Substrate-hot wire distance: 1 to 10 cm Deposition rate: 0.1 -10 nm / sec

The characteristics of the silicon layer formed by the hot wire CVD method under the above conditions are as follows. First, the electrical conductivity in the parallel direction of the film formation surface of the high hydrogen concentration P layer (hydrogen concentration> 8 at%) is 5 × 10 ^{−6 to} 5 × 10 ⁰ (S / c
m), and the electric conductivity of the P layer in the direction parallel to the film formation surface is 5 ×
It is 10 ⁻⁶ to 8 × 10 ¹ (S / cm). The carbon concentration in the film is 0.1 to 60 at%, but the carbon concentration is 4
Microcrystalline silicon columns are precipitated at 2 at% or less. The diameter of the precipitated microcrystalline silicon columns is 1 to 50 nm,
The crystallization rate measured by Raman spectroscopy is 60 to 90%. The hydrogen concentration in the film is 2 to 24 at%, and the band gap optically measured is 1.9 to 2.6 (eV). However, this band gap has a carbon content of 30 to
This is for the silicon layer 12 in the case of about 40%.

In the solar cell of the above-mentioned embodiment, the conversion efficiency is increased from 10.8% to 12.2% as compared with the conventional example.
Short-circuit current density is 27.8 (mA / cm ² ) to 30.1
(MA / cm ² ), the open circuit voltage is improved from 0.54 (mV) to 0.55 (mV).

In the heterostructure type solar cell,
It is also possible to join semiconductor layers of the same conductivity type and use a junction (high-low junction) due to a difference in bandgap size. In addition, a by the above-mentioned hot wire CVD method
The -SiC / [mu] c-Si pillar heterostructure conductive film can be used for forming devices such as TFTs of display devices other than solar cells.

Although one embodiment of the present invention has been described so far, it goes without saying that the present invention is not limited to the above-described embodiment and may be implemented in various different forms within the scope of the technical idea thereof.

[0028]

As described above, according to the present invention,
It is possible to provide a heterojunction solar cell having improved electrical characteristics such as open circuit voltage and short circuit current.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a solar cell according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration example of a solar cell according to a second embodiment of the present invention.

FIG. 3 is a diagram for explaining a silicon microcrystalline column.

4A is a diagram showing a hot wire CVD method, and FIG. 4B is a diagram showing a plasma CVD method.

[Explanation of symbols]

11 Silicon Substrate 11a N ⁺ Layer 11b P ⁺ Layer 12 Carbon-containing Silicon Layer 12a High Hydrogen Content Layer 13 Back Electrode 15 Antireflection Film / Transparent Electrode 16 Front Electrode

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Claims (4)

[Claims] 1. A solar cell, wherein a silicon-containing silicon layer formed by hot wire CVD is formed on a single crystal or polycrystalline silicon layer. 2. The solar cell according to claim 1, wherein fine columnar crystals of silicon are present in the carbon-containing silicon layer. 3. The solar cell according to claim 1, wherein the carbon-containing silicon layer has high conductivity. 4. The solar cell according to claim 1, wherein a high hydrogen content layer is provided at the interface between the silicon layer containing carbon and the single crystal or polycrystalline silicon layer.