Semiconductor device manufacturing: process – Including control responsive to sensed condition – Electrical characteristic sensed
Reexamination Certificate
1999-06-10
2001-05-08
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Including control responsive to sensed condition
Electrical characteristic sensed
C438S004000, C438S010000, C438S012000, C438S013000, C136S243000, C136S258000, C136S290000
Reexamination Certificate
active
06228662
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the fabrication of solar cells, and in particular, to a method of removing short circuits in solar cell elements during manufacturing.
2. Description of the Related Art
As shown in
FIG. 1
, a thin film solar cell
10
comprises a plurality of solar cell elements
5
a,
5
b and
5
c formed on an insulating substrate
1
. Each solar cell element
5
comprises a first electrode
2
formed on one face of substrate
1
in a preset pattern; a semiconductor layer
3
for performing photoelectric conversion which is formed on the surface of the first electrode
2
and a second electrode
4
formed on the surface of the photoelectric-conversion semiconductor layer
3
. The semiconductor layer
3
may consist of a non-crystalline semiconductor. The plurality of solar cell elements
5
a,
5
b and
5
c are connected in series by connecting the first electrode
2
a of solar cell element
5
a to the second electrode
4
b of neighboring solar cell element
5
b, and the first electrode
2
b of solar cell element
5
b to the second electrode
4
c of neighboring solar cell element
5
c.
When a glass substrate, a transparent resin substrate or the like is used as the insulating substrate
1
of the solar cell, a transparent electrode material such as ITO (Indium Tin Oxide, indium oxide mixed with tin oxide) or the like is used as the first electrode
2
, and a metal electrode material is used as the second electrode
4
. When a nontransparent material is used as the insulating substrate
1
, a metallic electrode material is used as the first electrode
2
, and a transparent electrode material is used as the second electrode
4
.
In the case where the semiconductor layer
3
is a non-crystalline silicon base semiconductor, non-crystalline silicon consisting of alloy of silicon and carbon or other metal such as germanium, tin, etc. may be used, as well as non-crystalline silicon, hydrogenated non-crystalline silicon, hydrogenated non-crystalline silicon carbide, or non-crystalline silicon nitride. Furthermore, these non-crystalline or polycrystalline semiconductor materials may be used in the form of pin-type, nip-type, ni-type, pn-type, MIS-type, heterojunction-type, homojunction-type, Schottky barrier-type, or a combination of the above. Also, the semiconductor layer may be formed by using not only a silicon base but also a Cd base, GaAs base, InP base, etc.
When, for example, a short-circuit occurs between the first electrode
2
b and the second electrode
4
b of solar cell element
5
b by a pinhole formed in the photoelectric-conversion semiconductor layer
3
b during manufacturing, it is well known to remove the short-circuited section or to insulate it by oxidation
When a short-circuited section to be removed is between the first electrode
2
b on the substrate side of the solar cell element
5
b and the second electrode
4
b on the back side of the photoelectric-conversion semiconductor layer
3
b, probe electrodes
6
a and
6
b contacting electrodes
4
b and
4
c, respectively, are used. A DC voltage or voltage of a square-wave pulse, not exceeding the reverse limit voltage (reverse breakdown voltage), is applied in the reverse direction (0 V side), as shown in
FIG. 2
, between the first electrode
2
b and the second electrode
4
b that sandwich the photoelectric-conversion semiconductor layer
3
b. The electric current is concentrated at the short-circuited section which generates Joule heat. Oxidation of metal occurs due to the generated Joule heat. The oxidation of metal insulates the short-circuited section. Another method of removing the short circuit involves removing the short-circuited section by dissipation of the metal.
However, a solar cell is equivalent to a diode. Thus, when the voltage is applied in the reverse direction between the electrode
2
and the electrode
4
, the solar cell element
5
consisting of the first electrode
2
, the photoelectric-conversion semiconductor layer
3
and the second electrode
4
functions as a capacitor, and charges accumulate across the capacitor. As a result, when the DC voltage is applied between electrodes
2
and
4
, charges remain between electrodes
2
and
4
even after the applied voltage has been removed abruptly. The voltage generated by these charges may cause electrical breakdown in weak sections of the photoelectric-conversion semiconductor layer
3
other than the locations where the faults occur.
To avoid such accumulated charges and the high voltage generated by such charges, the voltage of the square-wave pulse applied to the electrodes to induce oxidation (
FIG. 2
) is typically restrained to levels much lower than the reverse limit voltage. Typically, a 4 V voltage pulse is applied in the reverse direction. However, such a low-voltage pulse often does not generate sufficient Joule energy, and consequently, not all of the short-circuited sections can be removed or oxidized by the generated Joule heat. At the same time, some breakdown of the non-short-circuited normal sections by discharge of the accumulated charges still occurs at certain sections where the limit voltage of the semiconductor is low.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method for removing short-circuited sections in a thin film solar cell element that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method that allows reliable removal of short-circuited sections by a voltage source applied in the reverse direction between the electrodes without damaging non-short-circuited sections.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a method of removing short-circuited sections in a thin film solar cell element comprising applying a pseudo-alternating voltage between the first and second electrodes of the solar cell element. This voltage induces an alternating current which discharges the accumulated charges in the solar cell element, thereby protecting it from high voltages generated by accumulated charges.
The voltage of the pseudo-alternating voltage changes periodically with time. The waveform of the pseudo-alternating voltage may be a sinusoidal wave, a half-wave sinusoidal wave, a sawtooth wave, a square wave, etc. The peak voltage in the reverse direction is up to the reverse breakdown voltage of the solar cell element, and the waveform may either contain a small forward component or no forward component. The period of the pseudo-alternating voltage matches the time constant of the solar cell element determined by the capacitance and reverse resistance of the solar cell element.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
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Hayashi Katsuhiko
Kondo Masataka
Bowers Charles
Hogan & Hartson LLP.
Kaneka Corporation
Sarkar Asok K.
LandOfFree
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