Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2002-03-15
2003-11-25
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S249000, C136S255000, C136S261000, C257S064000, C257S066000, C257S053000, C257S461000, C257S458000, C438S096000, C438S097000
Reexamination Certificate
active
06653554
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thin film polycrystalline solar cells and methods of forming them.
2. Related Background Art
The thin film polycrystalline Si solar cells have polycrystalline Si films formed by film formation steps such as CVD, epitaxial growth, etc., can be produced at a lower production cost than bulk crystal solar cells produced by forming a semiconductor junction in a wafer, are expected to achieve a higher photoelectric conversion efficiency than a-Si solar cells, and are promising candidates as next generation solar cells. Typical structures of the conventional thin film polycrystalline Si solar cells include those of the pn junction as shown in
FIG. 15
or of the pin junction as shown in FIG.
16
.
In
FIG. 15
, numeral
81
designates a substrate also serving as a support, and
82
an electroconductive metal film that also acts as a light reflecting layer. Numeral
83
denotes a polycrystalline Si thin film semiconductor layer doped in a high concentration with an impurity of a conductivity type, which is laid in order to establish good electrical contact between the metal layer
82
and a semiconductor layer
84
. Numeral
84
represents a polycrystalline Si thin film semiconductor layer, which is normally doped with a slight amount of an impurity of the same conductivity type as that of the layer
83
. Inside this layer
84
a potential distribution is made on the basis of contact with a layer
85
, and thus the layer
84
acts as a photocharge generating layer. Numeral
85
indicates a thin film semiconductor layer doped in a high concentration with an impurity of the opposite conductivity type to that of the layers
83
and
84
. Numeral
87
denotes an antireflection layer for preventing reflection of light, which is provided for taking in light efficiently. Numeral
86
stands for collecting electrodes for extraction of electric current.
When the solar cells are constructed using films of polycrystalline Si of small crystal grain sizes, the pin structure as shown in
FIG. 16
is employed in order to flow the electric current by drift. Numeral
91
designates a substrate also serving as a support, and
92
an electroconductive metal film also acting as a light reflecting layer. Numeral
93
denotes a polycrystalline Si thin film semiconductor layer doped with an impurity of a conductivity type. Numeral
94
represents an intrinsic, polycrystalline Si thin film semiconductor layer.
Numeral
95
represents a thin film semiconductor layer doped with an impurity of the opposite conductivity type to that of the layer
93
. An electric field is established in the intrinsic semiconductor layer
94
interposed between the layer
93
and the layer
95
, and the charge generated in the layer
94
flows along the electric field. Numeral
97
indicates an antireflection layer for preventing reflection of light, which is provided for taking in light efficiently. Numeral
96
stands for collecting electrodes for extraction of electric current.
The solar cells of such structures can be produced without necessity for slicing and polishing steps, different from the bulk crystal Si solar cells, and thus the production cost can be lower. Since they can also be produced on the substrate of glass, metal, or the like, it is also feasible to perform continuous production. For this reason, they can also be used in the stack structure with the a-Si solar cells and, therefore, the polycrystalline Si thin film semiconductor layers are promising materials as a long-wavelength light absorbing and photocharge generating layer. The reason is that the a-SiGe film also used similarly as a long-wavelength light absorbing and photocharge generating layer has to be made of the high cost source material of GeH
4
gas.
The polycrystalline Si solar cells of the structure of
FIG. 15
or
FIG. 16
were actually produced to evaluate their characteristics and it was shown that short-circuit currents and fill factors were greatly different among samples. None of the samples demonstrated good short-circuit current and fill factor characteristics. To yield solar cells with good characteristics about the both short-circuit current and fill factor was thus a significant subject in the research on the thin film polycrystalline Si solar cells.
SUMMARY OF THE INVENTION
An object of the present invention is to provide thin film polycrystalline Si solar cells with good characteristics about both the short-circuit current and fill factor.
According to a first aspect of the present invention, there is provided a thin film polycrystalline solar cell comprising a substrate; a first semiconductor layer provided on the substrate and comprised of Si highly doped with a conductivity-type controlling impurity; a second semiconductor layer provided on the first semiconductor layer and comprised of polycrystalline Si slightly doped with a conductivity-type controlling impurity of the same conductivity type as that of the first semiconductor layer; and a third semiconductor layer provided on the second semiconductor layer and highly doped with a conductivity-type controlling impurity of a conductivity type opposite to that of the impurities for the doping of the first and the second semiconductor layers, wherein crystal grains grown from crystal nuclei generated in the first semiconductor layer are continuously grown to form the first and the second semiconductor layers, are also horizontally grown to contact neighboring crystal grains, and are perpendicularly grown to form an interface with the third semiconductor layer.
According to a second aspect of the present invention, there is provided a thin film polycrystalline solar cell comprising a substrate; a first semiconductor layer provided on the substrate and comprised of Si doped with a conductivity-type controlling impurity; a second semiconductor layer provided on the first semiconductor layer and comprised of Si of an intrinsic conductivity type; and a third semiconductor layer provided on the second semiconductor layer and doped with a conductivity-type controlling impurity of a conductivity type opposite to that of the impurity for the doping of the first semiconductor layer, wherein crystal grains grown from crystal nuclei generated in the first semiconductor layer are continuously grown to form the first and the second semiconductor layers, are also horizontally grown to contact neighboring crystal grains, and are perpendicularly grown to form an interface with the third semiconductor layer.
The first aspect as described above includes a solar cell having the structure of n
+
−
/p
+
or p
+
/p
−
+
, and the second aspect includes a solar cell having the structure of n/i/p or p/i
.
A method of forming a thin film polycrystalline solar cell according to the present invention is a method of forming a thin film polycrystalline solar cell by stacking on a substrate a first semiconductor layer of a thin film comprised of Si highly doped with a conductivity-type controlling impurity, stacking thereon a second semiconductor layer of a thin film comprised of polycrystalline Si slightly doped with a conductivity-type controlling impurity of the same conductivity type as that of the first semiconductor layer, and further stacking thereon a third semiconductor layer of a thin film highly doped with a conductivity-type controlling impurity of a conductivity type opposite to that of the impurities for the doping of the first and the second semiconductor layers, thereby forming a solar cell with a semiconductor junction structure of n
+
−
/p
+
or p
+
/p
−
+
, the method comprising: repeatedly carrying out film deposition and plasma processing to form the first semiconductor layer; then growing crystal grains from crystal nuclei generated in the first semiconductor layer in a direction perpendicular to the substrate and also growing the crystal grains in a horizontal direction until the crystal grains contact neighboring crystal grai
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