JPH034570A - Manufacture of amorphous solar cell - Google Patents

Abstract

PURPOSE:To make an impurity layer low in resistance by a method wherein a microcrystallized layer is formed through a plasma CVD method under conditions such as high hydrogen dilution and a low discharge power. CONSTITUTION:A PIN-type solar cell is composed of a transparent board 1, a transparent electrode 2, a hetero P-layer 3, a microcrystallized P-layer 4, an I-layer 5, an N-layer 6, and a metal electrode 7. A mixed gas of silane gas, methane, and B2H6 is introduced at a pressure of 0.1Torr into a reaction chamber in which the glass substrate 1 is placed, and a plasma discharge is carried out with a power of 0.03W/cm<2> to deposit the hetero P-layer 3. Then, a mixed gas of silane and B2H6 (2% by volume) which is diluted with hydrogen 150 times in volume is introduced, and a discharge is carried out under a pressure of 1.0Torr with a power of 0.03W/cm<2> to form the amorphous Si layer 4 which contains microcrystals on the hetero P-layer 3, which is made to serve as a window-side impurity layer 160Angstrom thick as a whole.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing a solar cell using an amorphous semiconductor. More specifically, the present invention relates to a method for manufacturing an amorphous semiconductor solar cell with improved photoelectric conversion efficiency.

(Prior art) In recent years, solar cells using amorphous semiconductors have attracted attention because they are cheaper to manufacture than single-crystalline solar cells, and are being actively developed as a promising solar cell. . Various structures have already been proposed for this solar cell, but among these, the intrinsic layer (i-layer)
The p-1-n type or the n-1-p type (hereinafter abbreviated as the pin type in this specification), which is sandwiched between impurity layers (p or n layer), is known to achieve high efficiency. It is being

Conventionally, when these pin-type amorphous silicon semiconductors function as solar cells, pTfl or n-layer amorphous silicon absorbs light and generates carriers (hereinafter referred to as photogenerated carriers). ) has a short lifespan,
Photogenerated carriers that serve as photocurrents are known to occur in the i-layer.

However, in the case of pin-type solar cells, the iN is sandwiched between the second layer and the n layer, so light passes through the second layer or the n layer.
incident on the layer. For this reason, it is preferable that the two-layer or n-layer be made of a material with a wide optical gap and low light absorption.However, conventional n-type and p-type amorphous silicon is Since the light absorption coefficient was comparable to or larger than that of silicon, light absorption loss in the n-layer or two layers was one of the causes of lowering photoelectric conversion efficiency.

Further, a pin type amorphous silicon solar cell has a structure such as glass/transparent electrode/(pin or ntp)/metal electrode, transparent electrode/(pin or n1p)/metal, and light is incident from the transparent electrode.

Therefore, light loss due to light reflection from solar cells also prevents an increase in photoelectric conversion efficiency. In this case, the transparent electrode and two layers or n
Although it is desirable to control the refractive index of the bilayer and n-layer to minimize light reflection at layer interfaces, the refractive index of a conventional bilayer or nJi, e.g. at 1500 nm, is lower than that of the i-layer. Same refractive index as amorphous silicon of 3.4-3
．． 6, which is a large difference from the refractive index of the transparent electrode of about 2.0, which causes reflection loss.

Therefore, in order to improve the above drawbacks, an amorphous silicon (a-3iC:H) material containing carbon and hydrogen as impurities (Japanese Patent Laid-Open No. 57-126
175 etc.) and amorphous hydrogenated silicon containing microcrystals (μ
It is proposed to use c-3i:H).

However, although a-3iC;H can suppress light absorption and improve photoelectric conversion efficiency from that aspect, it is necessary to provide a buffer layer between the i-layer and the i-layer in order to increase the open circuit voltage and fill factor. there were.

On the other hand, in the case of μc-3t:H, there was a drawback that the compatibility with the transparent electrode was not particularly sufficient.

As a result of intensive research based on conventional knowledge, the present inventors have determined that the window-side impurity layer is divided into an amorphous silicon layer containing microcrystals (microcrystalline layer) and an amorphous silicon N layer containing certain elements (heterolayer). It has been found that the performance of an amorphous semiconductor solar cell can be improved by forming a multilayer structure of ), and this has already been proposed (Japanese Patent Application Laid-Open No. 60-207319).

However, even if the invention related to a stacked solar cell is simply applied, the open-circuit voltage and fill factor decrease significantly, making it difficult to achieve the desired conversion efficiency.

(Problems to be Solved by the Invention) As a result of further study, the inventors of the present invention adopted a substrate having a transparent electrode with a textured structure, and the microcrystalline layer of the window-side impurity layer with high hydrogen dilution and low discharge power. By forming the solar cell using the plasma CVD method, we succeeded in obtaining a solar cell with high conversion efficiency without reducing the open circuit voltage or fill factor.

Therefore, a first object of the present invention is to provide a method for manufacturing a pin-type amorphous solar cell with high conversion efficiency.

A second object of the present invention is to provide a method for producing an impurity layer with a wide optical gap and low optical loss, which is effective as a window-side impurity layer of a pin-type amorphous solar cell.

A third object of the present invention is to provide a method for manufacturing an impurity layer that has low electrical resistance and is effective as a window-side impurity layer of an amorphous semiconductor solar cell.

(Means for Solving the Problems) The above-mentioned aspects of the present invention are such that a transparent electrode and an amorphous semiconductor layer with a textured structure are grown, and the window-side impurity layer is an amorphous silicon layer containing microcrystals (microcrystalline A method for manufacturing a solar cell having a multilayer structure of a layer) and an amorphous silicon carbide layer (heterolayer), the microcrystalline layer being formed by a plasma CVD method under high hydrogen dilution and low discharge power conditions. This was achieved by a method for manufacturing amorphous semiconductor solar cells with characteristics.

Although it is usually preferable to use glass as the transparent electrode substrate used in the present invention, plastics and other transparent materials can also be used.

The surface of the substrate is usually flat, but in order to increase the light absorption efficiency, it is preferable to use a substrate with an uneven surface of about 0, oi to 3 μm, and the shape of the unevenness is hemispherical or pyramidal. or any intermediate form thereof may be used.

The transparent electrode substrate with a texture structure used in the present invention can be produced by forming an ITO film or SnO film unevenly on a flat glass plate to make the surface uneven, or by welding a flat glass plate and finely powdered glass. It can be obtained by a method of manufacturing a glass substrate with unevenness, a method of partially etching a transparent conductive film, etc. In addition, the inventors proposed,
A method of bringing a material with an uneven surface into contact with glass and transferring the unevenness to the glass surface (Japanese Patent Application No. 63-209823)
Alternatively, it can also be manufactured by a method such as a method in which a raw material gas consisting of organic aluminum and a diluent gas is prepared and a thin aluminum film having a textured structure is formed on a glass substrate by plasma CVD (Japanese Patent Application No. 1-31946). can do.

Methods for providing transparent electrodes such as ITO or SnO on a substrate include electron beam evaporation, sputtering, and CV
An appropriate method can be selected from known methods such as the D method, the spray method, and the CMD method.

When a combination of an amorphous silicon carbide layer (hetero layer) and an amorphous silicon layer containing microcrystals (microcrystallized layer) is selected as the multilayer window-side impurity layer according to the present invention, a particularly good pin type A solar cell element can be obtained. In this case, the amorphous silicon containing microcrystals can contain hydrogen and/or halogen atoms.

As the halogen atoms, fluorine and chlorine are preferred, and fluorine atoms are particularly preferred. Such an amorphous silicon thin film containing microcrystals can be formed by plasma CVD.

As the raw material gas, silicon-containing compounds containing hydrogen atoms, such as silane and disilane, are used.

The raw material gas is diluted with hydrogen and supplied to the plasma CVD apparatus. The amount of hydrogen used is 20 to 1% of the raw material gas.
．． 000 times.

If it is less than 20 times, good microcrystals cannot be formed,
If it is 1,000 times or more, the film formation rate is too slow to be practical.

Discharge power is 0. The power should be low, below IW/cd.

If it exceeds 0.1 W/cd, the surface of the transparent electrode will be reduced and damaged by the hydrogen plasma, which is not preferable.

On the other hand, 0. Since the film forming efficiency decreases below OLW/ci, especially 0.01 to 0. It is preferable to set the discharge power to IW/cd.

The substrate temperature is in the range of 100 to 350 °C, especially 150 to 2
A range of 50'C is preferred. If the temperature is below 100°C, the substrate temperature (180
There is a risk of thermal deterioration at temperatures between 35 and 250℃.
At 0'C or higher, the previously formed two-layer amorphous Si
This is not preferable since it may cause thermal deterioration of the C layer.

The pressure during discharge is in the range of 500 to 1,500 mTorr,
Especially 800-1. A range of 300 mTorr is preferred. Outside the above range, dark electrical conductivity decreases, which is not preferable.

The main feature of the present invention is that a microcrystalline layer is formed under the conditions of high hydrogen dilution and low iut force as described above.

According to this method, the electrical conductivity can be reduced to 1.0% while keeping the optical absorption coefficient small. 5 x 10-'~5 x 10-'Scm
- It is possible to obtain a microcrystalline layer having excellent performance. By using high hydrogen dilution conditions, the growth surface of the microcrystalline layer is coated with hydrogen, increasing the surface diffusion coefficient of reactive species, preventing the bonding of silicon atoms to unstable sites, and hydrogen radicals. It is thought that by separating the silicon atoms bonded to unstable sites (etching reaction), a film can be grown using only the silicon atoms bonded to stable sites, and microcrystals can be created.

In addition, by using low discharge power conditions, thin (approximately 50 Å)
(Below) The presence of the hetero layer alone can not only suppress damage to the transparent electrode caused by hydrogen plasma, but also reduce the effects of temperature, pressure, etc. on film quality.

The hetero layer is formed by a plasma decomposition method that uses silicon compounds and carbon compounds containing hydrogen atoms, such as silane or its derivatives, as raw gases and discharges hydrogen (H2) as a carrier as necessary to create a plasma atmosphere. .

It is particularly preferable that the window-side impurity layer has two layers, and dopants can be easily introduced into the amorphous silicon semiconductor by a known method.

The physical properties of amorphous silicon semiconductor thin films largely depend on the type and amount of impurities contained in the thin film. for example,
The presence of carbon atoms contained in the amorphous silicon semiconductor causes the structure of silicon carbide (~6.0eV) with a large band gap to partially exist, so the structure of the amorphous silicon (a-3t) semiconductor is The bandgap is widened, which allows the absorption coefficient to be reduced. Furthermore,
The same applies to the refractive index, and silicon carbide (refractive index 2°4
By partially having the structure 8), the refractive index of the a-3i semiconductor can be controlled.

On the other hand, hydrogen contained as an impurity in the amorphous silicon semiconductor thin film eliminates the dangling bonds of silicon atoms that occur when forming the amorphous silicon thin film, and reduces the density of localized levels in the silicon thin film. Therefore, it is significant for controlling the electrical resistance of the semiconductor device according to the present invention.

The total thickness of the multilayer window-side impurity layer in the present invention is 10
Try to exceed 0 people. If it is less than ioo Å, the open circuit voltage will not improve, which is not preferable. A particularly preferable thickness is 120 to 400 people.

Among these, the thickness of the p-type machine crystallized layer is preferably 60 to 340, particularly preferably 80 to 300. 60Å
If it is less than 340 Å or more than 340 Å, the open circuit voltage and conversion efficiency will decrease, which is not preferable.

On such a multilayer window side impurity layer, after forming IJg by a known method, nM or pm is further formed and bonded to an electrode to form a ptn type or ni.
A p-type solar cell element can be obtained. In this case, as the impurity layer on the electrode side other than the window side, a-3t:Hji with normal impurity control or an amorphous hydrogenated silicon layer containing microcrystals (μc-3i:H), etc. can be used. .

Methods for sequentially depositing each amorphous silicon film on a transparent electrode include plasma CVD, sputtering, thermal CVD, and optical CVD.
It can be appropriately selected from known amorphous film manufacturing methods such as VD.

Next, the raw material gas when implementing the present invention by plasma CVD method will be described.

As a silicon source that can be used in the present invention, a silane represented by the formula -a S inHt-m *1 or a silane represented by the general formula S i Hll-3X5-1 (X represents a halogen atom) Any one can be selected from halogenated silanes. These may be used alone or in combination of two or more.

The carbon source is a hydrocarbon (e.g. methane, ethane, etc.) that has sufficient vapor pressure to perform plasma CVD at room temperature.
You can arbitrarily choose from among them.

The ratio of the silicon-containing compound forming the reaction gas and the raw material gas for impurities can be changed as appropriate depending on the desired physical property values and reaction conditions, but for example, if the input power for plasma generation is about 0°02W. /cd, the reaction pressure is about Q, 1 Torr, the substrate temperature is about 250, and the C1 gas flow rate is about 1103 CC. You can get results.

In the present invention, when imparting p-type or n-type properties to the silicon thin film, it is necessary to further add a dopant gas within the above film forming conditions. The P-type dopant gas is an element of Group 1 of the Periodic Table of Elements or a compound thereof, and is preferably a hydride, particularly boron hydride (B, H,). The n-type dopant is an element of Group V of the Periodic Table of Elements or a compound thereof, and is preferably a hydride, particularly phosphine (P)Is) or arsine (AsHs). The mixing ratio of these dopant gases depends on the plasma conditions. Although it is not always constant, good results can be obtained if the amount is about 0.1 to 1% by volume based on the silicon-containing compound.

A typical configuration example of a solar cell obtained by the present invention is shown in FIG. 1. In FIG. Two crystallized layers, 5 is an i layer, 6 is an n layer, and 7 is a metal electrode on the back surface. Each of these components has a function similar to that of a so-called pin type element.

The thickness of iN can be arbitrarily selected between 300 and 4,000, but according to the present invention, even a thin film of 600 Å or less will not cause a decrease in open circuit voltage or fill factor.

(Effects of the Invention) The solar cell obtained by the present invention has a light trapping effect due to the textured structure of the upper transparent electrode where light absorption by the window-side impurity layer is small, and the conversion efficiency is higher than that of conventional pin type elements. The present invention is significant because the open circuit voltage and fill factor do not decrease.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto.

(Example) A solar cell having the structure shown in FIG. 1 was manufactured using a plasma CVD method.

First, silane gas (S i Ha ) was placed in a reaction chamber equipped with a glass substrate (manufactured by Asahi Glass) with a haze rate of 5% and a textured transparent electrode (film thickness: 8.000 μm).
A mixed gas of 10 S CCM, 15 SCCM of methane, and about 0.3% by volume of Bz Hb with respect to silane gas was added at a pressure in the reaction chamber of about 0.3% by volume. ITorr was installed. Next, plasma discharge was started with an input power of 0.03 W/ld, and about 70% of
Deposited two layers (3) of human hetero.

Next, after evacuating the reaction chamber, silane gas and a mixed gas of 0.2% by volume of BtHi to the silane gas were diluted 150 times with hydrogen gas and introduced, and the pressure inside the reaction chamber was reduced to l.
, 0 Torr. Under the conditions of input power 0.03W/cd and substrate temperature 230''C, approximately
An amorphous silicon layer (4) containing 0 microcrystals was formed, resulting in a total of 160 window-side impurity layers.

On the impurity layer thus formed, an i-layer and an n-layer were further formed by a conventional method, and metal electrodes were attached to obtain a sample (D) of the present invention.

The conditions for forming the microcrystalline layer were set to H,/SiH, = 50
(A) is the sample when the dilution rate of hydrogen is reduced as shown in (A), and (B) is the sample when the discharge power is increased to 0.16 W/cffl. Instead of (4), the sample where 100 people formed the buffer layer under the condition of S i H, /CH, = 4/1 was (
C) and the results of comparison are shown in Table 1.

Table 1 The results demonstrate that the solar cell obtained by the manufacturing method of the present invention has extremely high photoelectric conversion efficiency.

[Brief explanation of the drawing]

FIG. 1 shows a typical configuration example of a solar cell manufactured according to the present invention. In the figure, symbol l is a transparent substrate, 2 is a transparent electrode,
3 is hetero 2 layers, 4 is microcrystalline 2 layers, 5 is 1 layer, 6 is n
Layer 7 is the metal electrode on the back side.

Claims (1)

[Claims] It has a textured transparent electrode and an amorphous semiconductor layer, and the window-side impurity layer has a multilayer structure of an amorphous silicon layer containing microcrystals (microcrystalline layer) and an amorphous silicon carbide layer (heterolayer). A method for manufacturing a solar cell, wherein the microcrystalline layer is subjected to plasma CVD under conditions of high hydrogen dilution and low discharge power.
A method for manufacturing an amorphous solar cell, characterized in that it is formed by method D.