Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1997-09-16
2001-01-23
Wojciechowicz, Edward (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S064000, C257S436000
Reexamination Certificate
active
06177711
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonmonocrystalline photoelectric conversion element having an improved lower conductive layer surface configuration, i-layer crystal structure, and doped layer structure.
2. Related Background Art
Increase in photoelectric conversion efficiency and improvement in optical degradation have been studied heretofore for the photoelectric conversion elements incorporating the pin junction of a nonmonocrystalline semiconductor.
It is known that increasing a concentration of dopant in a doped layer decreases activation energy of the doped layer, thereby increasing the built-in potential of the pin junction and the open-circuit voltage of the element.
It is also known that use of a microcrystalline material for the i-type semiconductor layer improves optical degradation.
It is reported that a solar cell using microcrystalline silicon (&mgr; c-Si) achieved a photoelectric conversion efficiency of 4.6% using plasma enhanced CVD using VHF (70 MHz) and that the solar cell demonstrated no optical degradation at all, as seen in J. Meier, A. Shah, “INTRINSIC MICROCRYSTALLINE (&mgr; c-Si:H)-A PROMISING NEW THIN FILM SOLAR CELL MATERIAL,” IEEE WCPEC; 1994 Hawaii, p.409. Further, a stacked solar cell was fabricated by combining of amorphous silicon with microcrystalline silicon and achieved an initial photoelectric conversion efficiency of 9.1%.
It is also known that a transparent, conductive layer is interposed between the substrate or metal layer and the semiconductor layers. This prevents elements in the metal layer from diffusing or migrating into the semiconductor layers, thus preventing the photoelectric conversion element from shunting. Further, it has a moderate resistance and prevents the semiconductor layers from short-circuiting due to a defect such as a pinhole. In addition, the transparent, conductive layer is provided with an uneven surface, thereby increasing irregular reflection of incident light and reflected light to lengthen optical pathlengths in the semiconductor layers.
With the above-stated solar cell using the microcrystalline silicon based material, however, the photoelectric conversion efficiency thereof is still too low, 4.6%, to be of practical use.
With the stacked solar cell of a-Si/&mgr; c-Si, the initial photoelectric conversion efficiency is as high as 9.1%, but it suffers from great optical degradation of the a-Si layer on the light incidence side. Further, the thickness of the &mgr; c-Si layer is thick, 3.6 &mgr;m, and the rate of deposition is slow, 1.2 Å/sec. Thus, the time necessary for layer formation is approximately eight hours; this poses another problem in that the time for layer formation does not reach an industrially practical level.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a photoelectric conversion element having a substrate, a lower conductive layer, a first doped layer, an ilayer, a second doped layer, and an upper conductive layer, wherein a surface of the lower conductive layer has an uneven configuration, the i-layer contains prismatic crystalline grains, and longitudinal directions of the prismatic crystalline grains are inclined with respect to a direction of a normal line to the substrate. According to an embodiment of the present invention, as a numerical definition, a percentage of an overall volume of prismatic crystalline grains, each having a below-defined angle of 20° or less, is 70% or more with respect to an overall volume of the i-layer; the angle is defined as an angle between a straight line A passing a prismatic crystalline grain and being parallel to a longitudinal direction thereof and a straight line B passing the prismatic crystalline grain A out of straight lines taking shortest courses between interface
1
between the first doped layer and the i-layer and interface
2
between the second doped layer and the i-layer.
Further, according to another embodiment, the photoelectric conversion element is characterized in that a third doped layer, a second i-layer, and a fourth doped layer are interposed between the second doped layer and the upper conductive layer, the second i-layer has an amorphous silicon based semiconductor, and a thickness of the second i-layer is in the range of 0.1 &mgr;m to 0.4 &mgr;m both inclusive.
According to the present invention, the photoelectric conversion element is characterized in that the first doped layer and/or the third doped layer is of a stacked structure comprising a layer of a microcrystalline silicon based semiconductor material and a layer of an amorphous silicon based semiconductor material and in that the layer of the microcrystalline silicon based semiconductor material is in contact with the i-layer.
The photoelectric conversion element of the present invention is improved in characteristics including photoelectric conversion efficiency, open-circuit voltage, short-circuit photocurrent, low-illuminance open-circuit voltage, and leak current of the photoelectric conversion element. In addition, durability of the element is enhanced in outdoor exposure tests, in mechanical strength tests, and upon long-term irradiation with light. Further, the cost of a photoelectric conversion element can be decreased greatly.
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Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Wojciechowicz Edward
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