Batteries: thermoelectric and photoelectric – Photoelectric – Panel or array
Reexamination Certificate
2002-01-08
2004-03-23
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Panel or array
C136S256000, C428S615000, C428S646000, C428S655000, C428S673000, C439S500000, C174S260000, C174S263000, C438S098000
Reexamination Certificate
active
06710239
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to solar cells and particularly to an improvement in solder coating of electrodes included in the same.
2. Description of the Related Art
An exemplary solar cell including electrodes conventionally coated with solder is schematically shown in cross section in FIG.
1
. In the figures of the present application, like portions are denoted by like reference numerals.
The
FIG. 1
solar cell includes an etched p-type silicon substrate
1
having a light receiving side with an n-type diffusion layer
2
. On n-type diffusion layer
2
an anti-reflection film
3
is provided to reduce surface reflectance. P-type silicon substrate
1
has a back surface provided with a back surface aluminum electrode
4
. Back surface aluminum electrode
4
and anti-reflection film
3
on the light receiving side are provided thereon with silver electrodes
5
and
6
coated with solder layers
7
.
Such a solar cell is fabricated by such a method as represented in a flow chart of FIG.
2
. More specifically, in a case of using a crystalline silicon substrate, p-type silicon substrate
1
is initially etched at step S
1
. At step S
2
, p-type silicon substrate
1
is provided on its light receiving side with n-type diffusion layer
2
and thereon is provided anti-reflection film
3
to reduce surface reflectance.
At step S
3
, p-type silicon substrate
1
has its back surface almost entirely screen-printed with aluminum paste. The printed aluminum paste is dried and fired in an oxidizing atmosphere to form back surface aluminum electrode
4
.
At steps S
4
and S
5
, silver paste is screen-printed on back surface aluminum electrode
4
and anti-reflection film
3
in patterns and then dried. The dried silver paste is fired in an oxidizing atmosphere to form silver electrodes
5
and
6
. That is, silver electrodes
5
and
6
can be formed by simultaneous baking (step S
6
).
At step S
7
, substrate
1
is immersed in an activator-containing flux at a normal temperature for several tens seconds to provide silver electrodes
5
and
6
with the flux. Then, substrate
1
is exposed to hot air and thus dried.
At step S
8
, substrate
1
is immersed in a 6:4 eutectic solder bath (of about 195° C.) containing 2 mass % silver for about one minute to coat silver electrodes
5
,
6
with solder layers
7
.
At step S
9
, substrate
1
is ultrasonically washed several times in normal or hot water and it is then rinsed with pure water and finally exposed to hot air and thus dried. A conventional solar cell is thus obtained.
FIG. 3
shows a solar cell string including a plurality of conventional solar cells thus fabricated and interconnected. In this conventional string, a solar cell
10
has a main surface electrode
11
coated with 6:4 solder and a plurality of such solar cells
10
are connected by interconnectors
12
coated with 6:4 solder. Such a string is fabricated in such a method as follows. Interconnector
12
including a copper core line coated with 6:4 eutectic solder is superposed on main electrode
11
of solar cell
10
and exposed to blowing hot air of about 400° C. to melt the solder. The solder is then cooled and thus solidifies to provide the connection. Such a connection process is repeated for the plurality of solar cells on their front and back sides to provide a cell string. The string thus completed is used to fabricate a solar cell module.
In recent years, lead harmful to human body causes issues from an environmental view point and thus various electronic devices free of lead are increasingly developed. Fabrication of solar cells free of lead is also demanded in the industry of interest.
In the past, however, a solar cell using lead-free solder has not been produced. For example, if a conventional 6:4 eutectic solder bath is replaced with a Sn bath to coat with Sn an electrode formed of fired silver paste, the silver contained in the electrode would be taken into the Sn bath and the electrode would disappear in some locations and the product would not function as a solar cell. This is probably attributed to the fact that Sn has a melting point of 231.9° C., about 50° C. higher than that (i.e., 183° C.) of 6:4 eutectic solder.
U.S. Pat. No. 5,320,272 discloses an example of lead-free solder, which, however, is used for semiconductor integrated circuits.
SUMMARY OF THE INVENTION
In view of the above-described prior art, an object of the present invention is to provide a solar cell having good output properties without causing lead pollution. Another object of the present invention is to provide an interconnector which does not cause lead pollution and then provide a reliable solar cell string connected by such interconnectors.
A solar cell according to the present invention is characterized in that it has electrodes coated with lead-free solder.
The electrode itself can be formed by baking matal paste. Furthermore, the electrode may be formed by metal vapor deposition, spattering, or plating.
The lead-free solder can preferably be Sn—Bi—Ag-based solder or Sn—Ag-based solder.
The electrode is preferably formed from matal paste containing powdery silver, powdery glass, an organic vehicle, an organic solvent, phosphorus oxide, and halide.
The solar cell's electrode receives flux including resin, a solvent, and a resin stabilizer, before it is coated with lead-free solder.
An interconnector for the solar cell according to the present invention is characterized in that it is coated with lead-free solder.
A solar cell string according to the present invention is characterized in that a plurality of the solar cells having the electrodes coated with the lead-free solder are interconnected by the interconnectors coated with the lead-free solder.
The lead-free solder used for the solar cell and that used for the interconnector can be identical in composition.
At least one of the lead-free solder for the solar cell and that for the interconnector can contain Bi preferably at 3 to 89 mass %.
At least one of the lead-free solder for the solar cell and that for the interconnector may contain Ag preferably at 3.5 to 4.5 mass %.
In the solar cell according to the present invention, the electrodes can be protected from mechanical shock and moisture in the ambient by coating the electrodes with the lead-free solder which does not cause lead pollution. Coating the electrodes with the lead-free solder facilitates formation of the solar cell string by interconnecting the plurality of the solar cells with the interconnectors.
FIG. 4
is a schematic cross section of an example of a solar cell according to the present invention. The
FIG. 4
solar cell is different from the
FIG. 1
conventional solar cell only in that electrodes are coated with different solder layers. More specifically, the present solar cell uses lead-free solder layers
8
, rather than conventional 6:4 eutectic solder layers.
The lead-free solder can be Sn—Bi—Ag-based solder or Sn—Ag-based solder. Sn—Bi—Ag-based solder and Sn—Ag-based solder each have a melting point lower than Sn. Herein, Sn—Bi—Ag-based solder contains no less than 0.1 mass % Ag. Sn—Ag-based solder also contains no less than 0.1 mass % Ag.
To carry out the solder dip process without causing problems, it is desirable to use a conventional dip temperature of about 195° C. and it is necessary that the dip temperature is no more than 225° C. which is a practical limit in view of solar cell characteristics and reliability. To have a melting point of no more than 225° C., Sn—Bi—Ag-based solder containing 0.1 mass % Ag should contain 5 to 88 mass % Bi and that containing 1.3 mass % Ag should contain 3 to 89 mass % Bi. To have a melting point of no more than 195° C., Sn—Bi—Ag-based solder containing 0.1 mass % Ag should contain 27 to 79 mass % Bi and that containing 1.8 mass % Ag should contain 35 to 60 mass % Bi. Thus, Sn—Bi—Ag-based solder containing 3 to 89 mass % Bi is preferable and that containing 35 to 60 mass % Bi is more preferable.
Similarly, Sn—Ag-based solder having a melting point of
Diamond Alan
Sharp Kabushiki Kaisha
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