Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2003-02-11
2004-12-14
Hiteshew, Felisa (Department: 1765)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C117S013000, C117S019000, C117S020000, C117S937000
Reexamination Certificate
active
06830740
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing a solar cell utilizing a silicon single crystal wafer useful as a material of solar cell and a solar cell.
BACKGROUND ART
When a silicon single crystal is used as a material for producing a solar cell, reduction of the production cost as well as improvement of the conversion efficiency have constituted serious problems.
Hereafter, the technical background of use of a silicon single crystal as a material for solar cells will be explained.
Characteristics of solar cells will be explained first based on type of a substrate material constituting a solar cell. Solar cells are roughly classified based on the type of the substrate material into three types, i.e., “silicon crystal type solar cells”, “amorphous silicon type solar cells” and “compound semiconductor type solar cells”, and the silicon crystal type solar cells further include “single crystal type solar cells” and “polycrystal type solar cells”. Among these, the solar cells showing high conversion efficiency, which is the most important characteristic as a solar cell, are the “compound semiconductor type solar cells”, and the conversion efficiency thereof reaches almost 25%. However, as for the compound semiconductor type solar cells, it is extremely difficult to produce compound semiconductors used as the materials thereof, thus they have a problem for becoming popular for general use in respect of the production cost of solar cell substrates, and use thereof has been limited.
The term “conversion efficiency” used herein means a value representing “a ratio of energy, which can be converted into electric energy by a solar cell taken out of the solar cell, to energy of light irradiated on the solar cell” and represented in percentage (%) (it is also called photoelectric conversion efficiency).
Solar cells showing high conversion efficiency in the next place to the compound semiconductor type solar cells are silicon single crystal type solar cells. Since they show power generation efficiency of 20% order, which is close to that of the compound semiconductor solar cells, and substrates for those solar cells may be relatively easily obtained, they constitute the mainstream of the solar cells for wide general use. Furthermore, the silicon polycrystal type solar cells and amorphous silicon type solar cells are also practically used because of low production cost of the solar cell substrate materials therefor, although the conversion efficiencies thereof are inferior to those of the aforementioned two types of solar cells, i.e., about 5 to 15%.
A general method for producing a silicon single crystal type solar cell will be briefly explained hereafter. First, a cylindrical ingot of silicon single crystal is produced by the Czochralski method (referred to as the CZ method or the Czochralski method hereafter) or the floating zone melting method (referred to as the FZ method or the floating zone method hereafter) in order to obtain a silicon wafer serving as a substrate of a solar cell. Further, this ingot is sliced into, for example, a thin wafer having a thickness of about 300 &mgr;m, and the wafer is etched for the surface with a chemical solution to remove mechanical damages on the surface to obtain a wafer (substrate) used as a solar cell. This wafer is subjected to a diffusion treatment for impurity (dopant) to form a pn-junction on one side of the wafer, electrodes are attached on the both sides, and an antireflection film for reducing light energy loss by light reflection is finally attached on the sunlight incidence side surface to complete a solar cell.
Although demands for solar cells are recently increasing as one of clean energy sources with a background of environmental problems, the higher energy cost compared with general commercial powers has constituted an obstacle of wide use thereof. In order to reduce the cost of silicon crystal solar cells, it is necessary to further increase the conversion efficiency as well as to reduce the production cost of substrates. For this reason, the cost of substrate material has been reduced by using cone portions, tail portions of single crystal ingots and so forth, which can not be made into products or are not suitable for electronic use for producing so-called semiconductor devices, as raw materials of substrates of single crystal type solar cells. However, such supply of raw materials is unreliable, and the amount thereof is also limited. Therefore, considering expansion of the demands for silicon single crystal type solar cells in future, it will be difficult to stably produce solar cell substrates in a required amount by such a method.
Moreover, in solar cells, it is important to produce a solar cell of larger area in order to obtain a larger electric current. As a method of obtaining a silicon wafer having a large diameter used as a substrate material for producing a solar cell of large area, the CZ method is suitable, which enables easy production of a silicon single crystal having a large diameter and provides superior strength of a produced single crystal. Therefore, the CZ method constitutes the mainstream of the production of silicon crystals for solar cells.
Further, if a silicon wafer serving as a substrate material of a single crystal type solar cell does not have a substrate lifetime (referred to as “lifetime” or abbreviated as LT hereafter), which is one of the characteristics thereof, of 10 &mgr;s or more, it cannot be used as a solar cell substrate. Furthermore, in order to obtain a solar cell of high conversion efficiency, it is required that the substrate lifetime should be preferably 200 &mgr;s or more.
However, as for a single crystal produced by the CZ method, which is the mainstream of the current methods for producing single crystal ingots, if the solar cell is irradiated with a strong light when the single crystal is processed into a solar cell, lifetime of the solar cell substrate is reduced, and photodegradation is caused. Therefore, sufficient conversion efficiency cannot be obtained, and improvement is desired also for performance of solar cells.
It is known that the cause of the reduction of the lifetime and the photodegradation upon irradiation of strong light on a solar cell produced by using such a CZ method silicon single crystal is an influence of boron and oxygen present in the single crystal substrate. Currently, the conductivity type of wafers used as solar cells is mainly p-type, and boron is usually added to p-type wafers as a dopant. Although a single crystal ingot used as the material of the wafer may be produced by either the CZ method (including the magnetic field applied CZ method, also referred to as the MCZ method hereinafter) or the FZ method, the FZ method suffers from higher production cost for single crystal ingots compared with the CZ method, and in addition, a silicon single crystal having a large diameter is more easily produced by the CZ method as described above. Therefore, at present, single crystals are mainly produced by the CZ method, which enables production of single crystals having a large diameter at a relatively low cost.
However, a crystal produced by the CZ method suffers from a problem that it contains oxygen at a high concentration, and thus the lifetime characteristic is affected by boron and oxygen in a p-type CZ-method silicon single crystal to cause photodegradation.
In order to solve such a problem, the applicants of the present application proposed use of Ga (gallium) instead of B (boron) as a p-type doping agent in a previous application (PCT/00/02850). By using Ga as a dopant as described above, it became possible to prevent reduction of the lifetime due to the influence of B and oxygen.
However, in spite of the elimination of the influences of B and oxygen by use of Ga as the dopant, the lifetime might be reduced and characteristics of solar cells fluctuated among produced solar cells. Such fluctuation of characteristics invited decrease of production yield of solar cells and decrease of the conversion efficiency as the whole solar c
Abe Takao
Oki Konomu
Hiteshew Felisa
Hogan & Hartson LLP
Shin-Etsu Handotai & Co., Ltd.
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