Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2001-01-18
2004-11-09
Norton, Nadine G. (Department: 1765)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C423S348000, C117S019000, C117S015000, C117S041000
Reexamination Certificate
active
06815605
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a silicon single crystal produced by Czochralski method (hereinafter occasionally referred to as “Czochralski method”, “CZ method”, or a pulling method), which is especially useful for material of solar cell, a method for producing it and a silicon single crystal solar cell produced using it.
2. Background Art
First, characteristics of a solar cell will be explained, in relation to a substrate material constituting the solar cell. The solar cell can be roughly classified on the basis of material of the substrate, into three types of silicon crystal solar cell, amorphous silicon solar cell, and compound semiconductor solar cell. The silicon crystal solar cell can be further classified into single crystal solar cell and polycrystal solar cell. The solar cell having the highest conversion efficiency that is the most important characteristics as a solar cell is the compound semiconductor solar cell among them, of which conversion efficiency is almost 25%. However, it is difficult to produce a compound semiconductor that is material of the compound semiconductor solar cell, and the compound semiconductor solar cell has a problem of cost for production, so that it cannot be generally and widely used. Accordingly, it can be used only for a limited purpose.
In the following description, the word “Conversion efficiency” means “the rate of energy that can be taken out by being converted to electric energy with a solar cell”, that is represented as a percentage (%).
The solar cell having the highest conversion efficiency except the compound semiconductor solar cell is silicon single crystal solar cell, of which power generation efficiency is about 20% that is close to conversion efficiency of the compound semiconductor solar cell. The substrate for the silicon crystal solar cell can be prepared relatively easily. Accordingly, it is a major type of solar cell used generally. Furthermore, silicon polycrystal solar cell and amorphous silicon solar cell or the like have also been used practically, since the material of the substrate for them can be produced at low cost, although conversion efficiency of them is about 5 to 15%, which is lower than that of the above-mentioned two types of solar cell.
Secondly, a general method for producing a silicon single crystal solar cell will be explained below. First, in order to make silicon wafer to be a substrate of a solar cell, a columnar silicon single crystal ingot is produced according to Czochralski method or a floating zone melting method (hereinafter occasionally referred to as FZ method, Floating zone method). Then the ingot is sliced to give a thin wafer having a thickness of, for example about 300 &mgr;m, and a mechanical damage on the surface of the wafer is removed by etching with chemical to provide a wafer (substrate) that is to be a solar cell. PN junction is formed on one side of the wafer by a diffusion treatment of impurity (dopant), and thereafter electrode is formed on both surface of the wafer, and finally an antireflection coating film is formed on the surface which gets sunbeam in order to reduce loss of optical energy due to reflection of light, and thereby a solar cell is produced.
Nowadays, a demand for a solar cell as one of clean energy is increased to solve the environmental problems. However, its energy cost is higher than common commercial electric power, which is an obstacle to prevalence thereof. It is necessary for cost reduction of silicon crystal solar cell to decrease production cost of the substrate, and improve conversion efficiency. Accordingly, for reducing the cost of substrate materials, a cone part, a tail part and the like have been used as raw materials, which could not be used for electronic purpose such as production of semiconductor devices or which could not be useful product of single crystal ingot. However, acquisition of such a raw material is unstable, and the amount is limited. Accordingly, in light of increase of a demand on silicon single crystal solar cell, it is difficult to produce a necessary amount of solar cell substrates stably by such a method.
It is also important in the solar cell industry to produce a solar cell having wider area in order to obtain more electric current. CZ method is suitable for producing a silicon wafer with a large diameter which can be substrate materials for production of a solar cell having wider area, since a silicon single crystal having a large diameter can be easily produced according to CZ method, and the produced silicon single crystal is excellent in strength. Accordingly, a silicon single crystal for a solar cell is mainly produced according to CZ method.
A silicon wafer can be used as material for substrate of single crystal solar cell, only where its substrate lifetime (hereinafter occasionally referred to as lifetime, LT), that is one of characteristics thereof, is more than 10 &mgr;s. Furthermore, the lifetime is preferably 200 &mgr;s or more in order to provide a solar cell having a high conversion efficiency.
However, concerning a single crystal produced by CZ method being at present a main method for producing a single crystal ingot, when it is radiated with a strong light after it is processed to be a solar cell, the lifetime of the solar cell substrate is lowered resulting in photo-degradation, so that it is required to be improved also as for performance of the solar cell.
It is known that boron and oxygen existing in the single crystal substrate cause lowering of life time and photo-degradation that is occurred when a solar cell produced using the silicon single crystal produced by CZ method is irradiated with strong light. A conductive type of the wafer that is presently used as a solar cell is mainly P type, and the P type wafer is generally doped with boron as a dopant. Although the single crystal ingot that is material for the wafer can be produced according to CZ method (including MCZ (hereinafter occasionally referred to as Magnetic field applied CZ method) or FZ method, production cost in FZ method is higher than in CZ method, and the silicon single crystal having a large diameter is produced more easily according to CZ method as described above. Accordingly, it is presently produced according to CZ method wherein a single crystal having a large diameter can be produced at relatively low cost.
However, oxygen exists at high concentration in the crystal produced according to a CZ method, and thus there is a problem that boron and oxygen in P type silicon single crystal produced according to CZ method may affect the lifetime characteristic of the solar cell substrate and may cause photo-degradation.
DISCLOSURE OF THE INVENTION
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide a silicon single crystal and a silicon single crystal wafer for producing a solar cell having very high conversion efficiency of optical energy that is not suffered from photo-degradation, even though it has high oxygen concentration, and also provide a method for producing them.
To achieve the above object, the present invention provides a silicon single crystal doped with Gallium wherein resistivity is 5&OHgr;.cm to 0.1&OHgr;.cm.
The present invention also provides a silicon single crystal doped with Ga wherein concentration of Ga in the crystal is 5×10
17
atoms/cm
3
to 3×10
15
atoms/cm
3
.
Although the substrate of the solar cell is desired to be a substrate having low resistivity and long lifetime, in the substrate wafer having extremely low resistivity, the lifetime of minority carrier becomes shorter due to Auger recombination, resulting in lowering of conversion efficiency. Accordingly, the amount of Gallium contained in the silicon single crystal of the present invention is preferably such an amount that the resistivity is 0.1&OHgr;.cm or more, more preferably 0.2&OHgr;.cm or more. Alternatively, concentration of Gallium is preferably 5×10
17
atoms/cm
3
or less.
In the fo
Abe Takao
Hirasawa Teruhiko
Igarashi Tetsuya
Tokunaga Katsushi
Yamaguchi Masafumi
Anderson Matthew
Norton Nadine G.
Oliff & Berridg,e PLC
Shin-Etsu Handotai & Co., Ltd.
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