Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2000-06-07
2002-03-12
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S257000, C136S259000, C136S251000, C257S432000, C257S434000, C257S435000, C257S461000, C438S065000, C438S089000, C438S098000, C438S087000
Reexamination Certificate
active
06355875
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a covered solar cell, as well as a manufacturing method thereof, which enables protection from low energy protons that cause deterioration of electrical characteristics.
In a space covered solar cell to be used as power supply for artificial satellites (shown in FIG.
8
), an about 50 &mgr;m-1 mm thick cover glass
2
is bonded on top of a solar cell
1
with silicon adhesive
3
. Radiations of various energies are flying across the cosmic space. The solar cell
1
, when receiving such radiation impinging thereon, suffers crystal defects so that its photoelectric conversion capability is deteriorated. In particular, protons of low energy, upon collision against an object, are absorbed by the very surface without reaching the interior of the object. However, the solar cell
1
has a PN junction at as shallow a portion of its surface as 0.1 &mgr;m-0.3 &mgr;m, thus resulting in a large deterioration due to collisions of low energy protons.
For this reason, as described above, the about 50 &mgr;m-1 mm thick cover glass
2
is bonded on the surface of the space solar cell with the silicon adhesive
3
. With the thusly bonded cover glass
2
, low energy protons are absorbed by the cover glass
2
, and do not reach the solar cell
1
. In this way, radiation deterioration of the solar cell
1
is partly prevented by the cover glass
2
. In the case of ordinary glasses, since the glass is colored by radiation, cerium (Ce) is added to the glass to prevent the coloring due to radiation.
Surface electrodes
4
of the solar cell
1
are linearly formed with spacings of 0.5 mm- a few mm so as to efficiently take out electric current from a PN junction portion
5
, and so designed as to gather at a current takeout portion (not shown). The surface electrodes
4
are formed of silver or other metals having a low resistivity so that the electrical resistance becomes low. There is a further demand for forming the surface electrodes
4
thick to thereby increase the cross section and decrease the electrical resistance. Whereas the surface electrodes
4
are generally formed by vacuum deposition or the like, forming the electrodes thick for lower electrical resistance would take longer time. Thus, a method of increasing the thickness of the surface electrodes
4
by plating is conceivable.
However, the surface electrodes
4
, if formed by plating, would be upsized not only in the direction of thickness but also in the direction of width, so that solar light incident on the solar cell
1
would be shielded. On this account, in order to form the surface electrodes
4
thick only in the direction of thickness by plating, there arises a need of patterning with a resist having a thickness larger than the plating thickness. Further, since plating a P-type silicon substrate
6
directly with silver would result in insufficient adhesion strength, it is necessary that after the patterning of a metallic material such as titanium (Ti) into the electrode shape, another patterning for plating be performed so as to surround the patterned metallic material.
One manufacturing method for solar cells is a hydrogen ion peeling process capable of obtaining silicon wafers which are thin and uniform in film thickness (Japanese Patent Laid-Open Publication HEI 10-93122). In this hydrogen ion peeling process, hydrogen ions are implanted into a polysilicon ingot or wafer, and a second substrate is bonded to the implantation-side surface of the ingot or wafer. Then, by performing appropriate heat treatment, a silicon substrate on the second substrate side is peeled in a small thickness. Then, by using this thin-film silicon, a power-saving type covered solar cell is manufactured.
These conventional solar cells, however, have the following problems. In the case of the covered solar cell shown in
FIG. 8
, silicon adhesive is used as the adhesive
3
for bonding the cover glass
2
as described above. This adhesive
3
is a very expensive refined resin in response to the requirement that out gas be emitted under high temperature and high vacuum in the space be less in amount. This leads to a problem of increased cost. Also, the adhesive
3
, although having a property of being soft at normal temperature, yet exhibits an abrupt property change at lower temperatures below −80° C. as compared with silicon and glass. This causes the solar cell
1
and the cover glass
2
to undergo large thermal stress under very low temperature environments. As a result, such faults as damage of the solar cell
1
and the cover glass
2
or peeling of the adhesive
3
are more likely to occur, as a further problem.
The adhesive
3
may overflow to side faces of the solar cell
1
or to the top of the cover glass
2
during the work of bonding the cover glass
2
. Since the silicon adhesive
3
, when irradiated with ultraviolet rays, would be deteriorated, it is necessary to remove adhesive
3
a
that has stuck to the surface of the cover glass
2
or flowed over the side faces as described above. However, the work of removing the adhesive
3
a
may often cause breaks of the very thin cover glass
2
, which is as thin as 50 &mgr;m-1 mm, or the solar cell
1
, thus requiring handling with great precision.
The alignment at the side wall between the solar cell
1
and the cover glass
2
needs to be performed with a dimensional tolerance of, normally, 0.2 mm or less according to the requirements that the solar cell
1
not be exposed and that the cover glass
2
not largely protrude out of the solar cell
1
. Besides, when the surface electrodes
4
are formed by plating as described above, the alignment between the ground metal such as titanium (Ti) and the plating patterning for preventing lateral expansion of the surface electrodes
4
needs to be done with high precision. However, this alignment between the ground metal and the plating patterning takes long time for manufacture, making it hard to mass produce the space covered solar cell, as a disadvantage.
Furthermore, in the covered solar cell manufacturing method by the hydrogen ion peeling process, because of a small film thickness of the silicon substrate in the resulting covered solar cell, it is necessary to previously bond a second substrate thereto for mechanical reinforcement. This second substrate is given by an electrically conductive metal material, or an insulating material having optical transmittance to at least part of the solar light, for example, by a glass plate or aluminum plate or the like. In such a case, with a glass plate used as the second substrate, the second substrate is exploited as the cover glass during the formation of the covered solar cell. As a result, during the bonding of the second substrate to the silicon wafer, there may occur problems similar to those in the case of bonding the cover glass to the solar cell. Also, use of aluminum or the like as the second substrate leads to a problem that the cost would be increased proportionally to the second substrate.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a covered solar cell, as well as a manufacturing method thereof, which eliminates the need for the removal of overflowed adhesive and high precision alignment, and which involves less distortion by heating even under iterative environmental changes in the earth and the space.
In order to achieve the above object, an aspect of the present invention provides a covered solar cell in which a transparent glass layer is formed directly on a surface of a solar cell.
With this constitution, a glass plate for radiation protection is formed directly on the surface of the solar cell without the aid of adhesion. Therefore, the need of an expensive adhesive for bonding the glass is eliminated, the work of removing any overflowed adhesive is eliminated, and alignment faults between the solar cell and the glass plate are eliminated, by which a cost reduction is achieved. Further, thermal stresses on the solar cell and the glass plate due to property changes of the
LandOfFree
Covered solar cell and manufacturing method thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Covered solar cell and manufacturing method thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Covered solar cell and manufacturing method thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2843816