Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2000-11-29
2003-02-18
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S258000, C136S261000, C257S064000, C257S458000, C438S097000, C438S057000, C427S585000, C427S588000
Reexamination Certificate
active
06521826
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film solar cell that has satisfactory photovoltaic conversion efficiency and a method for fabricating the thin solar cell.
The solar cell is attracting a great deal of attention as an alternative energy source substitute for fossil fuels such as petroleum, which are considered to be supplied less in future and have the problem of carbon dioxide emission as a cause of the global warming phenomenon.
The solar cell employs a pn junction in its photoelectric conversion layer for converting a light energy into an electric power, and silicon is generally most frequently employed as a semiconductor for constituting the pn junction. It is preferable to employ single crystal silicon in terms of photovoltaic conversion efficiency. However, the single crystal silicon has problems of material supply, areal increase, cost reduction and so on.
On the other hand, as a material advantageous for the achievement of areal increase and cost reduction, there is amorphous silicon. A thin film solar cell that employs this amorphous silicon as a photoelectric conversion layer has been put into practical use, however, its photovoltaic conversion efficiency is inferior to that of the single crystal silicon solar cell. Furthermore, the amorphous silicon causes a phenomenon called the Staebler-Wronski effect that the defect density in a film increases as light is applied, and therefore, the amorphous silicon solar cell is accompanied by the problem of deterioration with a lapse of time in terms of photovoltaic conversion efficiency.
Accordingly, in recent years, there have been conducted researches on the applications of polysilicon to the photoelectric conversion layer in order to provide a stabilized high photovoltaic conversion efficiency on the same level as that of the single crystal silicon solar cell and the areal increase and cost reduction on the same level as that of the amorphous silicon solar cell. In particular, a thin film polysilicon solar cell in which a thin film polysilicon is formed by means of a thin film forming technique by the chemical vapor deposition (CVD) method similar to that of the amorphous silicon is attracting a great deal of attention.
However, the current photovoltaic conversion efficiency of the thin film polysilicon solar cell fabricated by this method is merely on the same level as the photovoltaic conversion efficiency of the amorphous silicon solar cell. Several factors can be considered with regard to the low photovoltaic conversion efficiency, and one great factor is ascribed to the fact that the junction state at the interface between a doped layer and an intrinsic photoelectric conversion layer is not appropriately formed.
In the case of the aforementioned amorphous silicon solar cell, the state of the interface between a p-layer located on the incident light side and the intrinsic photoelectric conversion layer is particularly important. As a method for giving solution, the Japanese patent No. 2,846,639 discloses a method for providing a p/i interface layer in which carbon concentration is gradually varied between the p-layer constructed of a-SiC (amorphous silicon carbide) and the intrinsic photoelectric conversion layer constructed of a-Si (amorphous silicon). Japanese Patent Laid-Open Publication No. HEI 11-135814 discloses a method for setting a film forming rate of the intrinsic layer that has a thickness of several tens of nanometers and is put in contact with the p-layer slower than the film forming rate of the intrinsic layer to be subsequently formed. That is, these methods are the methods of providing an intermediate layer for improving the state of junction between the p-layer and the intrinsic layer.
Of course, applying the method of providing an intermediate layer to the thin film polysilicon solar cell contributes to the improvement of photovoltaic conversion efficiency. For example, Japanese Patent Laid-Open Publication No. HEI 11-135818 discloses a solar cell provided with a microcrystalline buffer layer that is formed by the plasma enhanced CVD method and provided between a p-type hydrogenated microcrystalline silicon layer and an intrinsic hydrogenated microcrystalline silicon layer. Damage of the p/i interface can be reduced by virtue of the existence of this microcrystalline buffer layer, and a open-circuit voltage and a fill factor value are improved to increase the photovoltaic conversion efficiency from 0.93% to 1.68%.
Generally, in the case of a polysilicon formed by the vapor deposition method such as the plasma enhanced CVD method, there is formed a phase of mixture including an amorphous component instead of the formation of a thin film that is completely made only of a crystal component. Then, in the amorphous component and a portion where the crystal component and the amorphous component adjoin each other, the bond state of silicon atoms is significantly disordered, and there is existing a great many uncombined hands, or the so-called dangling bond portions. The dangling bond forms a defect level in the forbidden band to consequently deteriorate the electric characteristics. Therefore, in the case of the thin film polysilicon solar cell, it is required to perform device design taking the state of existence of the crystal component and the amorphous component into due consideration. However, the method disclosed in Japanese Patent Laid-Open Publication No. HEI 11-135818 is no more than a method similar to the method for the solution of the aforementioned amorphous solar cell and is not regarded as a method that takes the existence of the crystal component into due consideration.
As a solar cell device design that takes the existence of the crystal component into consideration and has been disclosed so far, there are the methods disclosed in Japanese Patent Laid-Open Publication No. HEI 11-87742 and Japanese Patent Laid-Open Publication No. HEI 11-145498. The methods disclosed in these prior art reference documents are to obtain a photoelectric conversion layer that has a high crystallization ratio, a large crystal grain size and a firm crystal orientation property by providing an intrinsic amorphous silicon layer as a foundation layer of the intrinsic photoelectric conversion layer that includes a crystalline structure, controlling the crystallization ratio of the doped layer that includes a crystalline structure to be the foundation layer or taking similar measures. The structural characteristics of the thin film solar cells fabricated by these methods are as follows. In the case of the method disclosed in Japanese Patent Laid-Open Publication No. HEI 11-87742, an amorphous silicon layer is inserted between the doped layer that includes a crystalline structure and the intrinsic photoelectric conversion layer. In the case of the method disclosed in Japanese Patent Laid-Open Publication No. HEI 11-145498, the crystallization ratio of the doped layer, or the foundation layer of the photoelectric conversion layer is equal to or smaller than the crystallization ratio of the intrinsic photoelectric conversion layer.
However, the solar cell devices that take the existence of the crystal component into consideration and are disclosed in Japanese Patent Laid-Open Publication No. HEI 11-87742 and Japanese Patent Laid-Open Publication No. HEI 11-145498 have the problems as follows.
That is, the structures of the prior art solar cell devices are considered to be inappropriate for solar cells. The problems reside in the existence of a large amount of amorphous components in the doped layer itself or between the doped layer and the intrinsic photoelectric conversion layer. The problems owned by these structures are now described in detail below.
In a pin type thin film polysilicon solar cell constructed by successively stacking a p-type doped layer, an intrinsic photoelectric conversion layer and an n-type doped layer, separation of a pair of carriers (electron and hole) is performed by an internal electric field generated in the vicinity of a junction interface between the p-la
Diamond Alan
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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