Global warming has become a serious problem, and energy sources which will replace fossil fuels have been attracting increasing attention. In recent years, research and development on photoelectric conversion devices, which are also called solar cells, have become very active, and the market is rapidly expanding.
The photoelectric conversion devices are attractive power generation means which use inexhaustible sunlight as the energy source and which do not emit carbon dioxide at the time of power generation. However, there are problems under present conditions in that photoelectric conversion efficiency per unit area is not sufficient, that the amount of power generation is affected by the daylight hours, and the like. Accordingly, a long time of around 20 years is needed for recovery of the initial cost. These problems prevent the widespread use of the photoelectric conversion devices for residential use. Thus, an increase in efficiency and a reduction in cost of the photoelectric conversion devices are required.
The photoelectric conversion devices can be manufactured using a silicon-based material or a compound semiconductor material. Most of the photoelectric conversion devices sold in the market use a silicon-based material such as bulk silicon or thin film silicon. The bulk silicon photoelectric conversion device which is formed using a single crystal silicon wafer or a polycrystalline silicon wafer has relatively high conversion efficiency. However, a region which is actually utilized for photoelectric conversion is only a part of the silicon wafer in the thickness direction, and the other region only serves as a support which has conductivity. Further, loss of a material used as a cutting margin which is required in cutting out the silicon wafer from an ingot, necessity of a polishing step, and the like are also the factors that prevent a decrease in cost of the bulk silicon photoelectric conversion devices.
On the other hand, a thin film silicon photoelectric conversion device can be formed by forming a silicon thin film using a required amount of silicon by a plasma CVD method or the like. In addition, by a laser method, a screen printing method, or the like, higher integration of a thin film silicon photoelectric conversion device is easy, and the thin film silicon photoelectric conversion device can be manufactured at lower cost than the bulk silicon photoelectric conversion device. However, the thin film silicon photoelectric conversion device has a disadvantage in that it has a lower conversion efficiency than the bulk silicon photoelectric conversion device.
In order to improve the conversion efficiency of the thin film silicon photoelectric conversion device, a method in which silicon oxide is used instead of silicon for a p-type semiconductor layer serving as a window layer has been suggested (e.g., Patent Document 1). A thin film of a non-single-crystal silicon based p-type semiconductor has a high light absorption coefficient, so that it hinders light absorption by an i-type semiconductor layer. In Patent Document 1, silicon oxide, which has a larger band gap than silicon, is used for a p-type semiconductor layer so as to suppress light absorption by the window layer.
In addition, a structure in which an inversion layer which is formed by a field effect is used instead of a p-type semiconductor layer or an n-type semiconductor layer which serves as a window layer has been suggested. In such a structure, by forming a light-transmitting dielectric or conductor over an n-i or p-i structure, an n-i-p or p-i-n junction can be formed when an electric field is applied. This structure is for the purpose of reducing loss of light due to light absorption by the window layer as much as possible in order to increase a light-absorption efficiency of the i-type semiconductor layer.    [Patent Document 1] Japanese Published Patent Application No. H07-130661