Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
1999-03-02
2002-11-19
Chaudhuri, Olik (Department: 2814)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C118S718000
Reexamination Certificate
active
06482668
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing a non-single crystal semiconductor type photovoltaic device by a roll-to-roll system.
2. Related Background Art
Photovoltaic devices which are photoelectric conversion devices that convert sunlight into electric energy are put into wide use as public-purpose power sources for low-power supply such as in electronic calculators and wrist watches. Photovoltaic devices also attract notice as possible future substitute power generation means for petroleum fuel such as oil and coal. Photovoltaic devices utilize photovoltaic force attributable to, e.g., p-n junction of semiconductor devices. Semiconductors such as silicon absorb sunlight to produce photocarriers of electrons and holes by the aid of photon energy, and the photocarriers are taken out by differences in chemical potential at the p-n junction region.
In order to bring photovoltaic devices into practical use as electric power sources, it is important to achieve cost reduction and large-area devices, and various studies is conducted thereon. Researches are made on materials such as low-cost materials and materials with high photoelectric conversion efficiency. Such materials for photovoltaic devices may include tetrahedral type amorphous semiconductors such as amorphous silicon, amorphous silicon germanium and amorphous silicon carbide, and compound semiconductors of Groups II-VI such as CdS and Cu
2
S and those of Groups III-V such as GaAlAs. In particular, thin-film photovoltaic devices in which amorphous semiconductors are used in photovoltaic layers have advantages that they can provide films having larger area than single-crystal photovoltaic devices, can be formed in a small layer thickness and can be deposited on any desired substrate material; thus they are regarded as promising.
However, in order to put such amorphous semiconductor type photovoltaic devices into practical use as electric power sources, it has been a subject for study to improve photoelectric conversion efficiency and improve reliability.
As a means for improving the photoelectric conversion efficiency of the photovoltaic devices making use of amorphous semiconductors, various methods are available. For example, with regard to a photovoltaic device that utilizes a p-i-n type semiconductor junction, a p-type semiconductor layer, an i-type semiconductor layer, an n-type semiconductor layer, a transparent electrode and a back surface electrode which constitute the device must be improved in characteristics for each layer.
As another method for improving photoelectric conversion efficiency of photovoltaic devices, U.S. Pat. No. 2,949,498 discloses the use of what is called a stacked cell, in which photovoltaic devices having a certain unit device structure are superposed in plurality. This stacked cell makes use of p-n junction crystal semiconductors. Its concept is common to both amorphous and crystalline and is to make sunlight spectra absorb efficiently through photovoltaic devices having different band gaps and make open-circuit voltage (Voc) higher so that electricity generation efficiency can be improved.
In the stacked cell, constituent devices having different band gaps are superposed in plurality, and sunlight rays are absorbed efficiently at every part of their spectra so that photoelectric conversion efficiency can be improved. The cell is so designed that what is called the bottom layer positioned beneath what is called the top layer has a narrower band gap than the band gap of the top layer positioned on the light-incident side of the superposed constituent devices.
Meanwhile, Y. Hamakawa, H. Okamoto and Y. Nitta report what is called a cascade type cell, in which amorphous silicon layers having the same band gaps are superposed in multi-layer in such a way that no insulating layer is provided between photovoltaic devices so that the open-circuit voltage (Voc) of the whole device can be made higher. This is a method in which unit devices made of amorphous silicon materials having the same band gaps are superposed.
In the case of such stacked cells, too, like the case of single-layer cells (single cells), in order to improve photoelectric conversion efficiency, characteristics must be improved for each layer of the p-type semiconductor layer, i-type semiconductor layer, n-type semiconductor layer, transparent electrode and back electrode which constitute the photovoltaic device.
For example, in the case of the photoactivation layer, i-type semiconductor layer, it is very important to make band-gap internal levels (localized levels) as low as possible to improve transport performance of photocarriers.
With regard to what is called doped layers such as the p-type semiconductor layer and n-type semiconductor layer, it is first required that their activated acceptors or donors are in high density and can be activated at a small energy. This makes diffusion potential (built-in potential) large when a p-i-n type junction is formed and enhances the open-circuit voltage (Voc) of the photovoltaic device, bringing about an improvement in photoelectric conversion efficiency.
It is second required that the doped layers, which basically do not contribute to the generation of photocurrent, do not obstruct, as far as possible, the light entering the photocurrent-generating i-type semiconductor layer. Accordingly, in order to make the doped layers absorb less light, it is important to make their optical band gaps wide and to form them in small layer thickness.
Materials for doped layers having such characteristics include, e.g., Group IV semiconductor materials such as Si, SiC, SiN and SiO, and those having amorphous or microcrystalline form have been studied. In particular, Group IV semiconductor alloy materials having a wide optical band gap have been considered preferable because of their small absorption coefficient, and microcrystalline or polycrystalline semiconductor materials are preferred, because of their small absorption coefficient and small activation energy.
However, significant lowering of carrier transport performance and fill factor (FF) has occurred which is ascribable to lattice matching and junction interfacial levels between the i-type semiconductor layer and the microcrystalline or polycrystalline p-type semiconductor layer, and its improvement has been a subject for study.
Methods for solving such problems are under study. As an example thereof, U.S. Pat. Nos. 4,254,429 and No. 4,377,723 disclose a method in which what is called a buffer layer(s) is/are provided at the junction interface(s) between the p-type semiconductor layer and/or n-type semiconductor layer and the i-type semiconductor layer. At the junction interface between the p-type semiconductor layer or n-type semiconductor layer and the i-type semiconductor layer, the former being formed of amorphous silicon and the latter being formed of amorphous silicon germanium, many midgap levels are produced because of differences in lattice constant. Hence, they serve as the center of recombination at the junction interface to make the lifetime of carriers short. Such a buffer layer is formed so that by the use of the buffer layer the band-gap internal levels can be reduced and the carrier transport performance is not damaged, thereby bringing about an improvement in characteristics.
Now, as a process for producing photovoltaic devices by forming semiconductor functional deposited films continuously on a substrate, a process is known in which independent film-forming chambers for forming all kinds of semiconductor layers are provided. The respective film-forming chambers are connected through gate valves by a load-lock system, and the substrate is moved successively to the respective film-forming chambers to form thereon all kinds of semiconductors.
As a process which can improve mass productivity greatly, U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD (chemical vapor deposition) process employing a roll-to-roll system. According to this process, a continuous belt-lik
Hori Tadashi
Kanai Masahiro
Ohtoshi Hirokazu
Okada Naoto
Chaudhuri Olik
Wille Douglas A.
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