As a measure against global warming, solar photovoltaic power generation has become popular in the world. For the solar photovoltaic power generation, a photoelectric conversion device (also called a solar cell) in which light energy is converted into electrical energy by using photoelectric characteristics of a semiconductor is applied in many cases, as compared to the case of utilizing solar heat.
Production of photoelectric conversion devices tends to increase year after year. For example, the total production of solar cells in the world in 2005 was 1,759 MW, which is a drastic increase of 147% as compared to that in the previous fiscal year. Photoelectric conversion devices which have become popular worldwide use a crystalline semiconductor; photoelectric conversion devices using a single crystal silicon substrate or a polycrystalline silicon substrate constitute a large part of the production.
The thickness of a crystal-type photoelectric conversion device using silicon, which is needed to absorb sunlight, is only about 10 μm. Nevertheless, a single crystal silicon substrate or a polycrystalline silicon substrate manufactured as a product has a thickness of about 200 to 300 μm. That is, the photoelectric conversion device using a single crystal semiconductor substrate or a polycrystalline semiconductor substrate has a thickness ten or more times as large as the thickness needed for photoelectric conversion, and thus the single crystal silicon substrate or the polycrystalline silicon substrate is far from being effectively utilized wholly. Speaking of extremes, most part of the single crystal silicon substrate or the polycrystalline silicon substrate functions only as a structural body for keeping the shape of the photoelectric conversion device.
With the increase of production of photoelectric conversion devices year after year, short of supply of polycrystalline silicon, which is the material of a silicon substrate, and resulting rise of cost of the same have become problems of the industry. The production of polycrystalline silicon is expected to be about 36,000 tons in 2007; in contrast, 25,000 tons or more of polycrystalline silicon is needed for semiconductor (LSI) and 20,000 tons or more of polycrystalline silicon is needed for solar cells, which means polycrystalline silicon seems to be in short of supply by about 10,000 tons. This short of supply is predicted to continue.
There are a variety of structures of photoelectric conversion devices. In addition to a photoelectric conversion device having a typical structure in which an n-type or a p-type diffusion layer is formed in a single crystal silicon substrate or a polycrystalline silicon substrate, a stacked photoelectric conversion device in which different kinds of unit cells, i.e., a unit cell formed of a single crystal semiconductor and a unit cell formed of an amorphous semiconductor, are combined is known (see Examined Patent Application Publication No. H6-044638). The photoelectric conversion devices are the same in that a single crystal silicon substrate or a polycrystalline silicon substrate is used. Here, as another mode of a photovoltaic device using a single crystal semiconductor substrate, a photovoltaic device using a single crystal semiconductor layer formed into a slice is given. For example, Patent Document 4 (Patent Document 4: Japanese Published Patent Application No. H10-335683) discloses a tandem solar cell in which hydrogen ions are implanted into a single crystal silicon substrate, a single crystal silicon layer which is separated from the single crystal silicon substrate in a layer shape is disposed over a support substrate in order to lower the cost and save resources while maintaining high conversion efficiency. In this tandem solar cell, a single crystal semiconductor layer and a substrate are bonded to each other with a conductive paste.
On the other hand, photoelectric conversion devices using a crystalline silicon thin film have also been developed. For example, a method for manufacturing a silicon thin-film solar cell in which a crystalline silicon film is deposited over a substrate by a plasma CVD method using a VHF of 27 MHz or higher which has been pulse modulated is described (see Japanese Published Patent Application No. 2005-50905). Further, a technique for controlling plasma process condition to optimize dopant concentration in crystal grains and crystal grain boundaries when a polycrystalline silicon thin film is formed by a plasma CVD method over a special electrode called a texture electrode which has minute unevenness on its surface is disclosed (see Japanese Published Patent Application No. 2004-14958). However, a crystalline thin-film silicon solar cell is still inferior to a single crystal silicon solar cell in crystal quality and photoelectric conversion characteristic. Moreover, a crystalline silicon film needs to be deposited to a thickness of 1 μm or more by a CVD method, which leads to a problem of low productivity.