Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2001-06-01
2004-03-16
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C438S098000, C438S083000, C438S612000, C257S431000
Reexamination Certificate
active
06706961
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photovoltaic device (or a photovoltaic element) such as a solar cell and a process for the production thereof. More particularly, the present invention relates to a photovoltaic device (a photovoltaic element) having an electrode structure with improved durability and a process for the production thereof.
2. Related Background Art
In recent years, power generation means utilizing various kinds of natural energies have received public attention from the viewpoint of environmental protection. In particular, attention has been focused on a sunlight power generation system which generates electric power by irradiating sunlight to photovoltaic elements (or solar cells) without causing pollution.
However, such sunlight power generation system still have a subject necessary to be solved in order to make use thereof to be widespread such that the initial cost required for establishing a sunlight power generation system using photovoltaic elements (solar cells) is relatively high because photovoltaic elements are costly.
In order to overcome this subject, various kinds of photovoltaic elements (solar cells) and processes for the production thereof have been proposed up till now.
In particular, there is a proposal to reduce the cost required for establishing a sunlight power generation system by making each photovoltaic element (solar cell) used therein have a large light receiving area. That is, the output voltage of one photovoltaic element (solar cell) is as low as several volts and because of this, in order to achieve a high output voltage, it is necessitated that a plurality of photovoltaic elements (solar cells) are serialized. There are known several methods for serializing a plurality of photovoltaic elements (solar cells) in order to make it possible to reduce the cost of the sunlight power generation system. One of these methods is to diminish the number of photovoltaic elements (solar cells) serialized by enlarging the light receiving area of each photovoltaic element. Particularly, this method is intended to reduce the cost of the sunlight power generation system by enlarging the light receiving area of each of the photovoltaic elements used therein as large as possible to diminish the number of the photovoltaic elements to be serialized as well as the production process of the sunlight power generation system is simplified.
A feature of such large area photovoltaic element (solar cell) is to have an electrode structure on the light incident side in that a metallic wire is used.
FIG. 8
is a schematic view illustrating an example of a photovoltaic element (a solar cell) having a relatively large light receiving area. In
FIG. 8
, reference numeral
810
indicates a collecting electrode (or a grid electrode) comprising a plurality of metallic wires,
811
a bus bar electrode,
807
a photovoltaic layer,
808
a transparent electrode layer, and
812
a backside electrode.
In the photovoltaic element shown in
FIG. 8
, a current flows by way of a path connecting the backside electrode, photovoltaic layer, transparent electrode layer, collecting electrode, and bus bar electrode. In
FIG. 8
, the current flowing from the transparent electrode via the collecting electrode to the bus bar electrode is indicated by an arrow. As shown in
FIG. 8
, the current convergently flows from the transparent electrode to the collecting electrode, followed by flowing in the bus bar electrode. It is preferred for the metallic wires as the collecting electrode to be as thinner as possible so as to prevent light incident to the photovoltaic element from being shielded by the collecting electrode.
It is easily understood from
FIG. 8
that as the light receiving area of the photovoltaic element is enlarged, each of the metallic wires as the collecting electrode is made to be longer accordingly, where the quantity of current flown therein is increased. As the quantity of current flown in the collecting electrode is increased, the Joule loss of the collecting electrode is increased, and because of this, it is necessary that the electrical resistance of the collecting electrode is reduced. In the case of a photovoltaic element having a size of 10 cm square, in general, a printed electrode obtained by printing a conductive resin on the transparent electrode layer and forming a low melting point metal such as a solder thereon by a reflow process is effective to use as the collecting electrode. However, in the case of a large area photovoltaic element whose size is larger than the above photovoltaic element, when such printed electrode is used as the collecting electrode, the electrical resistance of the collecting electrode unavoidably becomes higher to increase the Joule loss. In order avoid this situation, there is considered a manner to thicken the printed electrode.
However, this manner is not effective for the following reason. It is difficult to thicken the printed electrode in the longitudinal direction and therefore, the remaining solution is to thicken the printed electrode in the width direction. This entails a problem in that the printed electrode thickened in the width direction shields incident light, where the quantity of power generated is lowered. Accordingly, for a large area photovoltaic element, it is necessitated to use a collecting electrode comprising a plurality of metallic wires having a low electrical resistance which less shields the incident light. By the way, such metallic wire is thin in the width direction but is thicker than the printed electrode in the longitudinal direction. Therefore, the use of the metallic wire makes it possible to realize a collecting electrode which is thin and is low in electrical resistance.
U.S. Pat. No. 4,260,429 discloses a photovoltaic element in which a metallic wire is used the collecting electrode.
Besides, in order to establish an electrical connection of a collecting electrode comprising a metallic wire and a bus bar electrode in an electrode structure of a photovoltaic element, there are know a method of joining the metallic wire with the bus bar electrode by direct welding or fusing, and a method of joining the metallic wire with the bus bar electrode through a low melting metal represented by a solder or through a conductive resin.
Now, in the electrode structure of a photovoltaic element as disclosed in U.S. Pat. No. 4,260,429, the electrical connection portion between the metallic wire and the bus bar electrode is very important factor in view of the following factors.
1. The electrical connection portion is necessary to be low in electrical resistance. As above described, a current convergently flows in the collecting electrode, where a large current flows also in a portion of the electrical connection portion which located at the leading end of the collecting electrode. To sufficiently suppress the Joule loss generated by the flow of such a large current, the electrical resistance of the electrical connection portion is necessitated to be sufficiently low.
2. The electrical connection portion is necessary to be high in stress resistance. A photovoltaic element (a solar cell) is used in an outdoor environment. In this, although the photovoltaic element is used by configuring as a solar cell module, a relatively large stress is applied to the photovoltaic element itself due to wind or snow cover. The stress applied to the collecting electrode is liable to be concentrated at the electrical connection portion thereof with the bus bar electrode. Accordingly, the stress resistance of the electrical connection portion is necessitated to be sufficiently large.
3. The electrical connection portion is necessary to have sufficient durability so that the electrical connection portion is maintained in a stable state over a long period of time without deteriorating its low electrical resistance and large stress resistance.
Now, when calculation is made on the basis of the present power cost, it will take about ten and several years to compensate the initial
Murakami Tsutomu
Shimizu Koichi
Tsuzuki Kouji
Canon Kabushiki Kaisha
Diamond Alan
Fitzpatrick, Cella, Harper & Scnito
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