JPH034569A - Amorphous solar cell - Google Patents

Abstract

PURPOSE:To improve a solar cell of this design in conversion efficiency and to make an impurity layer wide in optical gap and small in optical loss by a method wherein an impurity layer on a window side is formed into a multilayered structure composed of an amorphous Si layer which contains microcrystals and an amorphous SiC layer and prescribed in thickness. CONSTITUTION:A PIN-type amorphous solar cell is composed of a transparent board 1 of glass or the like, 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 formed on the rear side. An impurity layer on a window side is formed into a multilayered structure composed of the amorphous SiC layer 3 and the amorphous Si layer 4 which contains microcrystals and as thick as 120Angstrom or above, whereby the solar cell can be excellent in characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a solar cell using an amorphous semiconductor. More specifically, the present invention relates to 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 (iN)
The p-1-n type or n-1-p layer (hereinafter abbreviated as 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, carriers (
It is known that photogenerated carriers that serve as photocurrent are generated in the i-layer because of their short lifetime (hereinafter referred to as photogenerated carriers). However, in the case of pin-type solar cells, the i-layer is sandwiched between the p-layer and the n-layer, so
Light enters the i-layer through the p-layer or n-layer. For this reason,
The p-layer or n-layer is preferably a material with a wide optical gap and low light absorption, but conventional n-type and p-type amorphous silicon have a light absorption coefficient of iN, which is the same as that of amorphous silicon. As a result, light absorption loss in the n-layer or p-layer 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 n1p)/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 the p-layer or n-layer
Although it is desirable to control the refractive index of the p-layer and n-layer to minimize light reflection at layer interfaces, the refractive index of a conventional p-layer or n-layer at, for example, 1500 nm is i The same refractive index as the amorphous silicon layer 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-mentioned drawbacks, the transparent electrodes and substrates have a textured structure, and amorphous silicon (a-3iC:H) containing carbon and hydrogen as impurities is used as the material for the two-layer or n-layer. Materials (Japanese Unexamined Patent Publication No. 57-12617
5 etc.) and amorphous hydrogenated silicon containing microcrystals (μc-
3i:H) is proposed.

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 it and the i-layer in order to increase the open circuit voltage and fill factor. Ta.

On the other hand, in the case of μc-3i: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 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 have decided to: ■ adopt a substrate having a transparent electrode with a textured structure, and ■ increase the thickness of the window-side impurity layer to a thickness exceeding 120 layers. By doing so, 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 pin type amorphous solar cell with high conversion efficiency.

A second object of the present invention is to provide 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 an impurity layer with low electrical resistance that is effective as a window-side impurity layer of an amorphous semiconductor solar cell.

(Means for Solving the Problems) The above aspects of the present invention are a pin type solar cell having a textured transparent electrode and an amorphous semiconductor layer,
A film in which the window side impurity layer of the solar cell has a multilayer structure of an amorphous silicon layer containing microcrystals (microcrystalline layer) and an amorphous silicon carbide layer (heterolayer), and the window side impurity layer has more than 120 layers. This was achieved by using an amorphous semiconductor solar cell characterized by its thickness.

As the transparent electrode substrate used in the present invention, it is usually preferable to use glass, but other transparent materials such as plastics 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 0.1 to 3 μm. In particular, the shape of the unevenness may be hemispherical, pyramidal, or an intermediate shape thereof.

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 a transparent electrode such as ITO or SnOz on a substrate include electron beam evaporation, sputtering,
An appropriate method can be selected from known methods such as a CVD method, a spray method, and a CMD method.

When a combination of an amorphous silicon carbide layer and an amorphous silicon layer containing microcrystals is selected as the multilayer window-side impurity layer according to the present invention, a particularly good pin-type solar cell element can be obtained. In this case, the amorphous silicon containing microcrystals can contain hydrogen and/or halogen atoms. As the halogen atom, fluorine and chlorine are preferred, with fluorine atom being particularly preferred.

A method for manufacturing an amorphous silicon thin film containing such microcrystals is described, for example, in Japanese Patent Laid-Open No. 59-39713.

As described above, the other amorphous silicon heterolayer forming a multilayer structure, which is formed on the amorphous silicon layer containing microcrystals, contains hydrogen atoms and carbon atoms.

In order to produce a silicon thin film containing hydrogen atoms and carbon atoms as impurities by the plasma decomposition method, a silicon compound containing hydrogen atoms such as silane or its derivatives is used as a raw material gas, or hydrogen (H2) is used as a carrier. All that is required is to generate a plasma atmosphere by discharging.Furthermore, a silicon carbon thin film containing oxygen atoms as an impurity can also be produced.Similarly, various dopants can be introduced into an amorphous silicon semiconductor.

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 100 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.

If it is less than 60 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, one layer is formed by a known method, and then an n layer or two layers are further formed, and by bonding with an electrode, a pin type or nip type is formed.
type solar cell element can be obtained. In this case, as the impurity layer on the electrode side other than the window side, use a-3i:T (1g) or an amorphous hydrogenated silicon layer containing microcrystals (μc-3i:H), etc., which has undergone normal impurity control. I can do it.

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. That is, for example, when using the plasma CVD method, a predetermined reaction gas is introduced into a vacuum reaction tank in which a transparent substrate on which a transparent electrode is deposited is installed, the internal pressure is set to a predetermined value, and then high-frequency discharge and low-frequency discharge are applied. It can be manufactured by forming plasma using any method, such as various discharge methods such as direct current discharge.

Apart from sputtering, when CVD is employed, a reactive gas is required as described above. Among these CVD methods, plasma CVD is particularly effective in terms of reaction efficiency.
It is preferable to adopt method D.

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

As silicon sources that can be used in the present invention, silanes represented by the general formula S in H,T,
or general formula 5iHo~'jX4~. It can be arbitrarily selected from among the halogenated silanes represented by (X represents a halogen atom). These may be used alone or in combination of two or more.

The carbon source can be arbitrarily selected from hydrocarbons (eg, methane, ethane, etc.) that have a vapor pressure sufficient to carry out the CVD method at room temperature.

In particular, when carbon atoms and oxygen atoms are simultaneously contained as impurities, one or two selected from carbon oxide, carbon dioxide gas, and oxygen-containing organic compounds (e.g., alcohols, ethers, carboxylic acids, ketones, etc.) It is preferable to use the above.

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. /c (1. When the reaction pressure is set to about 0.ITorr, the substrate temperature is set to about 250°C, and the gas flow rate is set to about 1103CC, a good mixing ratio of the impurity element is set to 1 to 2 times that of the silicon-containing compound. You can get results.

In the present invention, when imparting n-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 n-type dopant gas is an element of group Ⅰ of the periodic table of elements or a compound thereof, preferably a hydride, particularly boron hydride (BzHh). The n-type dopant is an element of group V of the periodic table of elements or a compound thereof, preferably a hydride, especially phosphine (PH3) or arsine (AsH,). Although the mixing ratio of these dopant gases is not constant depending on the plasma conditions, good results can be obtained if the mixing ratio is about 0.1 to 1% by volume based on the silicon-containing compound.

A typical structural example of the solar cell of the present invention is shown in FIG. In the figure, numeral 1 is a transparent substrate such as glass; 2 is a transparent substrate such as glass;
3 is a transparent electrode, 3 is a hetero two layer, 4 is a microcrystalline two layer, 5 is an i layer, 6 is an 0 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 the i-layer 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 of the present invention has a light confinement effect due to the textured structure of the upper transparent electrode with little light absorption by the window-side impurity layer, and has a higher conversion efficiency and lower open voltage than conventional pin type elements. The present invention is significant because the fill factor does 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 (SiH,
Approximately 0 for CM, methane 15SCCM and silane gas
．． A mixed gas of 3% by volume of BzHb was introduced into the reaction chamber at a pressure of approximately 0. Then, plasma discharge was started with input power of 10W, and the substrate temperature was 250°.
Approximately 20 heterobilayers (3) were deposited on the transparent electrode at C.

Next, after evacuating the reaction chamber, silane gas and a mixed gas of 0.2% by volume of B and H6 with respect to the silane gas were diluted 50 times with hydrogen gas and introduced, and the pressure inside the reaction chamber was adjusted to Q,
It was set to 3 Torr. Input power 20W, board temperature 250
Under the conditions of 'C, an amorphous silicon layer (4) containing about 130 microcrystals was formed on the above-mentioned hetero 2 layer, resulting in a total of 150 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.

A comparative sample in which the i-layer and n-layer were exactly the same as this sample, and the window-side impurity layer was a single layer of μc-3i:H (
A), a comparative sample (B) with a single layer of a-3iC:H,
Table 1 shows the results of comparison with a comparative sample (C) using a flat substrate.

This result demonstrated that the solar cell obtained by the present invention has extremely high photoelectric conversion efficiency.

[Brief explanation of the drawing]

FIG. 1 shows a typical configuration example of the solar cell of the present invention. In FIG. The layer 6 is nN, and 7 is a metal electrode on the back side.

Claims (1)

[Claims] A pin type solar cell having a transparent electrode with a textured structure and an amorphous semiconductor layer, wherein the window side impurity layer of the solar cell is an amorphous silicon layer containing microcrystals (microcrystalline layer) and amorphous silicon carbide. An amorphous solar cell characterized in that it has a multilayer structure of layers (heterolayers), and the window-side impurity layer has a film thickness exceeding 120 Å.