JPH0249474A - Solar cells and their manufacturing method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a solar cell and a manufacturing method thereof, and particularly to an inexpensive and high quality solar cell and a manufacturing method thereof.

(Prior art) Conventionally, a crystalline silicon thin film solar cell having a PN junction is
Usually, it is produced by epitaxially growing silicon on a single crystal or polycrystalline silicon substrate in the vapor phase or liquid phase, and when using single crystal silicon as the substrate,
A substrate cut to a thickness of about 0.5 m is used. However, this processing is not easy, which has the disadvantage of increasing the cost of the solar cell.

Therefore, polycrystalline silicon, which can be manufactured more cheaply than epitaxial silicon, is attracting attention.

A polycrystalline silicon thin film is a state in which a large number of crystalline silicon particles of several hundred to several micrometers are aggregated.
Manufactured by VD method.

For example, in JP-A-58-172217, 400-9
It is disclosed that a polycrystalline silicon thin film is formed on a substrate by supplying a hesilane compound to a reaction chamber at 00° C. and performing thermal CVD.

However, the above method not only requires a large amount of thermal energy to heat the reactant gas with poor conduction efficiency to a high temperature, but also has the problem of poor thin film productivity because heating and cooling takes too much time and cost. , 400~700°
When temperature conditions around C were used, there was a drawback that the film forming rate was slow and impractical.

Furthermore, since polycrystalline silicon thin films are generally formed on substrates made of materials other than crystalline silicon, such as alumina or graphite, impurities such as aluminum and nickel contained in the substrate may be newly formed when exposed to high temperatures of 700°C or higher. It is known that it diffuses into the formed silicon crystal and deteriorates the characteristics of the deposited thin film ([Latest LSI
Process Technology J, P2O3, published in 1982, written by Kazuo Maeda, Kogyo Kenkyukai Akebono).

(Problems to be Solved by the Invention) As a result of intensive study to solve the above drawbacks of the conventional technology, the present inventors discovered that the 5i
In addition to using Ha, St, and H as film-forming gases, 5tFi,
By using S1mF & or Ft as an etching gas and hydrogen and an inert gas as a diluting gas, it is possible to achieve good crystal growth of silicon at a low substrate temperature using the plasma CVD method, resulting in a polycrystalline film with a sufficient thickness for a solar cell. The present invention was achieved by discovering that a silicon thin film can be easily obtained.

Therefore, a first objective of the present invention is to provide a high quality solar cell using a low grade substrate.

A second object of the present invention is to provide a method for manufacturing a high-quality solar cell by forming a polycrystalline thin film of silicon on a relatively low-temperature substrate.

Furthermore, a third object of the present invention is to provide a tandem solar cell in which a photoelectric conversion layer is further laminated on a solar cell having at least one silicon crystal thin film.

(Means for Solving the Problem) The above aspects of the present invention provide a solar cell having a silicon thin film photoelectric conversion layer consisting of a P layer and an N layer on a silicon crystal substrate, wherein the substrate contains a considerable amount of impurities. A solar cell characterized in that it is a polycrystalline silicon thin film containing a silicon polycrystalline substrate, and a polycrystalline silicon thin film having a small amount of the PFI and N layers (both the layers on the substrate side have a specific resistance of 10-IΩ·1 or more, and This was achieved by a suitable solar cell manufacturing method.

The substrate used in the solar cell of the present invention is polycrystalline silicon and contains a considerable amount of impurities as described later.

Polycrystalline silicon substrates usually contain 2.0, Cr, and Fe.
, Mn, Tl, V and other impurities, especially Aj
! , for Fe and O, the content is 0.5 at%
It may range to a certain extent. Therefore, when forming a silicon thin film on a polycrystalline silicon substrate by thermal CVD, impurities contained in the substrate as described above diffuse into the silicon thin film formed on the substrate, and this diffusion Therefore, a high-quality silicon thin film cannot be obtained in the conventional method in which the substrate temperature is set to about i,00° C. or higher.

For example, Al and F, which are contained in the largest amount as impurities,
The diffusion coefficient of e is as shown in the table below.

However, in the present invention, the substrate temperature is
``C or less, the contamination of the above-mentioned impurities can be reduced by more than three orders of magnitude compared to the conventional method.In this way, in the present invention, a low-quality substrate with relatively many impurities can be reduced. Since a high-quality silicon thin film can be formed even when using this method, manufacturing costs can be reduced.

In the present invention, a silicone polycrystalline substrate containing a considerable amount of impurities means that aluminum is IPPm or more and iron is iPPm or more.
ppm or more, meaning a silicon polycrystalline substrate containing oxygen of 10 ppm or more.

However, if the amount of impurities is too large, it is not possible to obtain a solar cell with sufficient performance even if the substrate temperature is 300°C or less. is 0.2 atomic% or less,
It is preferable to use a silicon polycrystalline substrate containing 0.3 atomic % or less of oxygen.

The specific resistance of the polycrystalline silicon provided as a single layer or N layers on the substrate is 10-1Ω・on the base plate, and preferably 1Ω・
It is 1 or more. In addition, the particle size of the crystal is 9 layers 1 m in diameter.
It is preferable that the polycrystal has a cross-sectional ratio of 80% or more, and in particular, the polycrystal has a diameter of 0.3 mm to 10 mm.
It is preferably 5% or more.

In the present invention, in order to form the polycrystalline silicon thin film constituting one layer or NJIL on the substrate, SiH and/or Is are used as the film-forming gas, and SiF is used as the etching gas. ,,S1g F,
and fluorine gas is used as the main component.

The etching gas is about 0.1 to 100% of the film forming gas.
The amount of hydrogen gas used is about 500 times or less that of the film-forming gas.

If the etching gas is less than 0.1 times the film forming gas, the substrate surface cannot always be kept in the best condition so that silicon crystal growth tends to occur.

On the other hand, if the hydrogen gas is more than 100 times the etching rate, the etching rate becomes too high and it is difficult to grow a silicon crystal thin film, and if the hydrogen gas is more than 500 times the film-forming gas, the concentration of silicon atoms becomes low and the silicon crystal thin film becomes difficult to grow. This is not preferable because the formation speed of the crystal thin film becomes too slow.

Further, the etching gas can be used after being diluted with an inert gas such as He or Ar. The dilution ratio with the inert gas depends on the amount of film-forming gas and hydrogen gas used, but is preferably about 0.1 times to 50 times.

Next, about 0.0% that satisfies the above conditions. ITorr~15T
orr pressure of reaction gas, power density 0°1~5 W
/ cm”, it is turned into plasma by the high frequency of about 10
A silicon crystal thin film is formed on a substrate maintained at a constant temperature between 0°C and 300°C, preferably between 150°C and 250°C.

If the substrate temperature is lower than 100° C., the silicon crystal growth rate will be extremely slow, which is not realistic.

When the substrate temperature is higher than 300° C., the quality of the silicon crystal thin film deteriorates because hydrogen adsorption on the substrate surface becomes insufficient.

Further, the power density and the pressure of the reactant gas for generating plasma affect the interlocking energy of atoms and ions arriving at the substrate, and therefore the quality of the silicon crystal thin film.

Power density is 0. If it is less than IW/cm'', the pressure of the reaction gas must be sufficiently lowered, resulting in a slow film formation rate, and if it is more than 5 W/cm'', the quality of the silicon crystal thin film cannot be maintained at a high level, which is undesirable.

If the pressure of the reaction gas is lower than 0.1 Torr, the silicon crystal growth rate is slow (and if it is higher than 15 Torr, the discharge state will not be stable, which is not preferable).

In the present invention, approximately 0°5To which satisfies the above conditions is used.
The reaction gas at a pressure of r r ~ 10 Torr is turned into plasma by a discharge with a power density of 0.3 ~ 2.5 W/cm'', and a silicon crystal thin film is formed on a substrate maintained at approximately 100° C. ~ 300° C. .

As is well known, it is necessary for solar cells to have a PN junction near the light-receiving surface. Therefore, in the present invention, a dopant is also used to form a PN junction.

When making the dopant p-type, the dopant should be selected from the periodic table of elements.
In the present invention, these dopant elements are mixed in the source gas as a dopant gas in the form of a single vapor or a gaseous compound. Examples of these dopant gases include Bl H&, PH, and PFs.

The dopant gas is mixed in a gas ratio of 10-5% to 1% by volume with respect to the raw material containing silicon atoms.

In the present invention, both the P layer and the NN containing a dopant can be formed in the same manner as when the dopant is not included, but either the Pi or the N layer (the first layer,
In other words, after forming the substrate-side layer by crystal growth, amorphous silicon or an amorphous silicon layer containing microcrystals can be formed as the NWI or PJI (second N) formed thereon. The film thickness of the 1st N is 1 μm
~50 μm, particularly preferably in the range of 5 μm ~ 20 am, and the 271st film thickness is 0.5 μm. 01 μm to 5 am, especially 0.01 μm to 5 am.
It is preferable to set it as 05 micrometers - 1 micrometer.

Amorphous silicon can be formed by plasma CVD.

Substrate temperature is 100℃~300℃, reaction pressure is 50mT
It is adjusted to a range of o r r to l OTo r r.

In the case of P-type amorphous silicon, amorphous silicon carbide mixed with carbon as an impurity is suitable.

Amorphous silicon carbide is formed using a raw material gas in which silane, disilane, etc. are mixed with a hydrocarbon compound (C+-C3) such as methane as an impurity.

Amorphous silicon containing microcrystals is also plasma C
It can be formed by a VD method.

In this case, the substrate temperature is adjusted to a range of 150''C to 400°C, and the reaction pressure is adjusted to a range of 100mTorr to 100Torr.

After providing the PN or NP junction on the substrate, a transparent conductive film or a metal comb electrode or both can be formed. As the transparent conductive film, tin oxide or ITo (indium tin oxide) is used.

The film thickness is preferably about 100 to 1,000 or less.

When a P layer is provided as a second layer on the substrate, Pl
Since i functions as a conductive film, there is no need to form a transparent conductive film.

The solar pond with the PN junction produced in this way has an optical gap of about 1 to 1.5 eV, but in addition to this,
Furthermore, a tandem solar cell can be constructed by forming an amorphous silicon thin film using a known method. In the plasma CVD method described above, a pin junction of amorphous silicon is performed at a substrate temperature of 100 to 300°C and a reaction gas pressure of 10 mTorr to lOTorr.
, , can be formed by adopting conditions of a power density of 0.01 to 0.05 W/cm''.

The film thickness is, for example, pl = 100 to 300 people, 1 layer: l, 0
00-5. ooo people, 3rd layer: estimated to be 100 to 400 people. Note that one layer usually does not contain a dopant, but it can also be formed so that the dopant concentration changes in a gradient manner.

The solar cell formed as described above may be of either a two-terminal type or a four-terminal type, and the optical gap of this tandem solar cell is about 1.5 to 2. OeV, and since the optical gaps of both solar cells are different, it is possible to obtain a tandem structure solar cell with improved conversion efficiency.

(Effects of the Invention) As detailed above, according to the present invention, high-quality solar cells can be obtained even by using low-grade substrates, so the manufacturing cost of solar cells can be significantly reduced. However, along with this cost reduction, the applications of solar cells can be greatly expanded.

The present invention will be described in more detail below with reference to Examples, but the present invention is not limited thereto.

Example 1 Metallic grade silicon was processed in a crucible to produce a polycrystalline wafer, which was treated with an acid and used as a substrate. The substrate is N type and contains 40 ppm of Af and 240 ppm of O as impurities.
It contained ppm.

Next, a reaction gas SiH4:5iFn:Ht 1:50
jlOO (101005 CC in total was adjusted so that the pressure of the reaction gas was 2 TOr r.This reaction gas was heated to a power density of 1.5 using a 13.56 MHz high frequency power source.
A 10.un thick polycrystalline silicon thin film was grown on the substrate which was plasmatized at 200° C. and heated to 200° C. This silicon layer can be designated as an N-layer and acts as a photoactive layer. .

Next, BgHa is added to the above reaction gas, and (Bg Ha
/S i Ha -1%), and polycrystalline silicon Pli was provided on the N- layer (film thickness -0, 1 layer m).

The photoelectric conversion efficiency of the solar cell obtained in this way is A
Under M −1 and 100 mW/d, it was 5.1%.

Example 2 After growing polycrystalline silicon to a thickness of 10 am on a substrate in the same manner as in Example 1, amorphous silicon carbide (a-3iC(B)) with an optical band gap Eopt-2, OeV was grown. :H) 150 people were set up as Pli by normal plasma CVD method. The conditions for forming Pwl are as follows.

Reaction gas: CH, /SiH4-1/1, dopant:
Bg Ha (0.1% by volume relative to S i Ha) Reaction gas flow rate: 80SCCM Power density: 0.03W/cii Substrate temperature: 250°C, pressure = 300mTorr This allows the The photoelectric conversion efficiency was 5.3%.

Example 3 A film with a thickness of 0° and 34 pm was applied on the solar cell produced in Example 2.
amorphous silicon (optical bandgap 1,7
eV) were deposited in the order of nip.

The photoelectric conversion efficiency of this tandem solar cell is AM −1
: Under 100 mW/aJ, it was 5.6%*n
The manufacturing conditions for each of the s'i and p layers are as follows.

(150 people): Same as in Example 2 (3,000 people): Reaction gas: 5iHa- Reaction gas flow rate: 20SCCM Power density: 0.02W/cm Substrate temperature: 250°C Pressure: 100mTorr 1 layer (200 people) ): Reactive gas: SiH4 Dopant: PH5 (0.1% to S I Ha) Reactive gas flow rate: 803 CCM Power density: 0.03 W/d Substrate temperature: 250°C Pressure crab 1,000 mTorr 2 layers 1i Comparative example 1 A solar cell was obtained in exactly the same manner as in Example 1 except that the substrate temperature was 1,000°C. It was confirmed that the photoelectric conversion efficiency of the obtained solar cell was about 1%, which was far below the conversion efficiency of the solar cell obtained by the present invention.

Comparative Example 2゜311(a:)it-t:1 as reaction gas
A solar cell was obtained in exactly the same manner as in Example 1 except that 00 was used. The specific resistance of the N layer was 10-tΩ·bi, and the conversion efficiency of the solar cell was 2.3%.

Patent applicant: Toa Fuel Industries Co., Ltd.

Claims (1)

[Scope of Claims] 1) A solar cell having a silicon thin film photoelectric conversion layer consisting of a P layer and an N layer on a silicon crystal substrate, the substrate being a silicon polycrystalline substrate containing a considerable amount of impurities, At least the layer on the substrate side of the P layer and the N layer is 10
^-^ A solar cell characterized by being a polycrystalline silicon thin film having a specific resistance of 1 Ω·cm or more. 2) A tandem solar cell characterized in that an amorphous silicon photoelectric conversion layer is further laminated on the upper photoelectric conversion layer constituting the solar cell according to claim 1. 3) In a solar cell manufacturing method in which a silicon thin film photoelectric conversion layer consisting of a P layer and an N layer is formed on a substrate by a plasma CVD method, at least as a layer on the substrate side of the P layer and the N layer, SiH_4 and Si_2H_6, at least one film-forming gas selected from SiF_4
, Si_2F_6, and an etching gas mainly composed of at least one selected from the group consisting of fluorine gas at a ratio of 1:0.1 to 1:100 to form a reaction gas, and heated to about 100 to 300°C. A method for manufacturing a solar cell characterized by forming a polycrystalline silicon thin film on a maintained substrate.