1. Field of the Invention
The present invention relates to a photovoltaic element such as a solar cell, and more particularly to a stacked photovoltaic element in which a plurality of pin junction constituent elements are stacked.
2. Related Background Art
Technology relating to the thin film photovoltaic element is disclosed for example in the U.S. Pat. No. 4,064,521. However it does not teach at all the configuration capable of improving and stabilizing the photoelectric conversion efficiency.
Also, with regard to the configuration of the stacked photovoltaic element, the U.S. Pat. No. 5,298,086 discloses a method of not degrading the fill factor (FF) of the photovoltaic element. More specifically, the above-mentioned patent discloses a method of forming a thinner semiconductor layer of satisfactory film quality and controlling the current of the stacked photovoltaic element by the thin constituent element of satisfactory film quality.
However, the method disclosed in the U.S. Pat. No. 5,298,086 is not sufficient for maximizing the photoelectric conversion efficiency of the stacked photovoltaic element and maintaining stability for a long light irradiation time. In particular, this method is unable to achieve a high photoelectric conversion efficiency and sufficient stability in case the i-type semiconductor layer (hereinafter, referred to as xe2x80x9ci-type layerxe2x80x9d) of the first pin junction constituent element of the stacked photovoltaic element is composed of a microcrystalline semiconductor and the i-type layer of the second pin junction constituent element is composed of an amorphous semiconductor.
The object of the present invention is to provide a stacked photovoltaic element employing a microcrystalline semiconductor in the i-type layer of the first pin junction constituent element and an amorphous semiconductor in the i-type layer of the second pin junction constituent element, and having a high photoelectric conversion efficiency and a less variation in the photoelectric conversion efficiency for a long light irradiation time.
In order to attain the above-mentioned object, the stacked photovoltaic element of the present invention is formed by stacking on a supporting member, at least a pin junction constituent element having a microcrystalline semiconductor in an i-type layer and a pin junction constituent element having an amorphous semiconductor in an i-type layer, wherein the current is controlled by the pin junction constituent element having a microcrystalline semiconductor in the i-type layer.
In the above-mentioned configuration, the short circuit current of the pin junction constituent element having a microcrystalline semiconductor in the i-type layer is preferably set to a value smaller than that of the pin junction constituent element having an amorphous semiconductor in the i-type layer.
Also, the average crystal grain size of the i-type layer composed of a microcrystalline semiconductor is preferably within a range of 100 xc3x85 to 1000 xc3x85.
Also, the i-type layer composed of a microcrystalline semiconductor preferably has a columnar crystalline structure.
Furthermore, the band gap of the i-type layer composed of a microcrystalline semiconductor is preferably set so as to become wider toward the interfaces between a p-type semiconductor layer (hereinafter referred to as xe2x80x9cp-type layerxe2x80x9d) and the i-type layer and between an n-type semiconductor layer (hereinafter referred to as xe2x80x9cn-type layerxe2x80x9d) and the i-type layer.