Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2001-02-07
2003-04-01
Nguyen, Nam (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S256000, C438S071000, C438S072000
Reexamination Certificate
active
06541696
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solar cell and a fabrication method thereof, particularly a solar cell with a bypass function including the function of a bypass diode that protects the solar cell from reverse bias voltage generated when, for example, the solar cell module is shadowed, and a method of fabricating such a solar cell.
2. Description of the Background Art
A solar cell module has a plurality of solar cells combined in series or in parallel to obtain a predetermined output voltage and output current. In the event of some of the cells being shadowed, voltage generated by the other cells will be applied in the reverse direction.
When the reverse breakdown voltage of the shadowed cells is surpassed by the reverse bias voltage applied in the reverse direction, breakdown occurs at the relevant cells to result in a great flow of current. There is a possibility of short-circuit failure occurring in these cells. Eventually, the output property of the entire solar cell module is degraded.
In the case of a solar cell module for use in space, the shadow of a portion of the satellite or any structural member such as the antenna may fall on the solar cell module during control of the posture of the satellite. In the case of a solar cell module for terrestrial use, the cells may be shadowed by an adjacent building or by droppings of a bird or the like.
Consider the case where a shadow falls on the submodule of a solar cell module formed of a series of solar cells connected in parallel.
FIGS. 12A and 12B
show a structure of a conventional solar cell module.
In a shunt mode where both ends of a solar cell module M are substantially short-circuited as shown in
FIG. 12A
, power V
12
generated by a group of submodules
12
not shadowed is applied as a reverse bias voltage to the shadowed submodule
11
. Therefore, V
11
=−V
12
, where V
11
is the voltage of submodule
11
.
In the case where an external power source V
B
is connected to solar cell module M as shown in
FIG. 12B
, the voltage of submodule
11
becomes V
11
=V
B
−V
12
. Positive charge is applied to the N electrode of shadowed submodule
11
. When the reverse bias voltage thereof exceeds the reverse breakdown voltage of the solar cells forming submodule
11
, breakdown occurs at that solar cell, resulting in the possibility of short-circuit failure. Accordingly, the output property of the shadowed module
11
is degraded, which in turn degrades the entire solar cell module M.
In order to prevent the disadvantage caused by such reverse bias voltages, a bypass diode may be attached for each solar cell or for every unit of specific modules. Alternatively, the so-called diode integrated solar cell having a bypass diode integrated with the solar cell is used.
Furthermore, a solar cell added with the bypass diode function is known. An example of a structure of a conventional solar cell with the bypass diode function will be described here with reference to the drawings.
FIG. 13
is a perspective view of a high-efficiency bypass diode function added solar cell with a reflectionless surface construction.
Referring to
FIG. 13
, the conventional solar cell includes a silicon substrate
1
of a first conductivity type such as the P type, a region of a second conductivity type such as the N type formed at the light receiving plane of substrate
1
to efficiently collect carriers generated by light energy, a P
+
region
3
formed at the bottom plane of substrate
1
for the back surface field (BSF) effect, an island-like P
+
region
4
provided at a portion of the light receiving plane of substrate
1
for bypass, an N electrode
7
provided at the surface of the N type region to obtain generated electricity efficiently, an anti-reflection film
8
covering substantially the entire plane of the N type region except for an N electrode connection portion not shown to reduce the surface reflection of incident light, and a P electrode
6
covering substantially the entirety of the bottom plane of P
+
region
4
to reflect light of a long wavelength passing through the bottom plane and to produce the generated electricity. The solar cell also includes a gridded reflectionless surface construction
13
provided at the light receiving side to reduce surface reflection, formed of a grid configuration having a plurality of recesses in the shape of upside down pyramids, and an oxide film layer
9
(not shown) on the N+ diffusion layer to reduce recombination of carriers at the surface. An oxide film layer
5
is provided on P
+
diffusion layer
3
to reduce recombination of carriers at the bottom face of P type silicon substrate
1
. N
+
diffusion layer
2
and surface electrode
7
are connected via an opening not shown in the oxide film layer. P
+
diffusion layer
3
and back electrode
6
are connected via an opening in oxide film layer
5
.
In a solar cell of the above-described structure, reflectionless surface construction
13
formed at the light receiving side serves to multiple-reflect the incident light to increase the quantity of light arriving inside the solar cell. The generated power depends upon the formation thereof. Therefore, the setting of the method of forming this configuration is extremely critical. In the present solar cell, reflectionless surface construction
13
is formed all over except for the region where surface electrode
7
is formed and the region of the ends of the solar cell.
FIG. 14
is a top view of the solar cell of
FIG. 13
, showing a circular region
10
for the P
+
diffusion region.
Referring to
FIG. 14
, the size and shape of each grid unit are identical. Therefore, the interval between each grid unit of the reflectionless surface construction is all identical.
A method of fabricating such a conventional solar cell of the above structure will be described hereinafter.
FIGS. 15A-15G
and
FIGS. 16A-16E
are sectional views of the solar cell of
FIG. 13
to describe a fabrication method thereof.
The reflectionless surface construction is fabricated according to the steps of
FIGS. 15A-15G
.
Referring to
FIG. 15A
, a silicon substrate
1
of plane orientation (
100
) is prepared.
Referring to
FIG. 15B
, oxide film
9
is formed by thermal oxidation or CVD at the surface of silicon substrate
1
.
Referring to
FIG. 15C
, a resist
15
is applied on oxide film
9
.
Referring to
FIG. 15D
, a predetermined pattern of reflectionless surface construction
13
and the portion of the alignment mark not shown are exposed and developed at the light receiving side. As a result, the pattern to form the reflectionless surface construction and the pattern of the alignment mark are formed by means of resist
15
on oxide film
9
.
Referring to
FIG. 15E
, oxide film
9
of the unrequired region is removed by etching or the like. Then, resist
15
is removed. Thus, the pattern of the reflectionless surface construction by oxide film
9
and the pattern of the alignment mark not shown are provided on silicon substrate
1
. At this stage, the distance d between the grid units is determined.
Referring to
FIG. 15F
, etching is applied for a predetermined time using an etching solution of a predetermined temperature and concentration such as a high temperature alkaline solution. For silicon substrate
1
, the etching rate with respect to the chemical substance differs for each crystal plane. By means of anisotropic etching thereof, a fine reflectionless surface construction
13
can be formed. At this stage, the alignment mark region takes a recess configuration.
Referring to
FIG. 15G
, oxide film
9
is removed, completing the formation of reflectionless surface construction
13
and the alignment mark not shown at the light receiving side of silicon substrate
1
.
The structure of providing the bypass function to the solar cell of
FIG. 13
is achieved by the fabrication steps of
FIG. 16A-16E
corresponding to sectional views of the solar cell.
Referring to
FIG. 16A
, P type silicon
Mutschler Brian L
Nguyen Nam
Sharp Kabushiki Kaisha
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