Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-11-28
2002-02-19
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S940000
Reexamination Certificate
active
06348362
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an integrated photovoltaic device, particularly relates to a manufacturing method of a photovoltaic device provided with a transparent electrode of zinc oxide.
2. Description of Prior Art
A photovoltaic device such as a solar cell can directly convert sunlight into electricity and has been commercialized as a new energy source. Such the photovoltaic device has been formed of crystalline semiconductor material such as single crystalline silicon and polycrystalline silicon, compound semiconductor material such as GaAs, InP or the like, and amorphous semiconductor material such as amorphous silicon and amorphous silicon germanium or the like. The photovoltaic device using the amorphous semiconductor material is manufactured at a low temperature as compared with other semiconductor material, can increase a size easily, and can easily be integrated on a substrate.
FIG. 1
is a structural cross sectional view of an integrated photovoltaic device using the amorphous semiconductor material.
Explanation is made on the integrated photovoltaic device by referring to FIG.
1
.
A substrate
1
is formed of translucent and insulating material such as glass, plastic, or the like. A plurality of first electrodes
2
are arranged on a surface of the substrate
1
. The first electrode
2
is formed of tin oxide (SnO
2
) and has a rough plane for scattering light incident from the substrate
1
on the surface. This rough plane is generally referred as a texture plane.
A photovoltaic conversion layer
3
is formed of amorphous semiconductor material, and generally includes a p-type amorphous silicon carbide film of approximately 100 Å, an intrinsic amorphous silicon film of approximately 4000 Å, and an n-type amorphous silicon film of approximately 200 Å laminated in this order from a side of the first electrode
2
. A second electrode
4
is formed of a highly reflective metal film such as Ag, Al or the like.
A lamination body of the first electrode
2
, the photovoltaic conversion layer
3
, and the second electrode
4
is a unit cell
10
, and adjacent unit cells
10
are electrically connected in series by electrically linking the first electrode
2
of the one unit cell
10
and the second electrode
4
of the other unit cell
10
.
FIGS. 2A-2E
are structural cross sectional views of each of processes for illustrating manufacturing processes of the conventional photovoltaic device.
As shown in
FIG. 2A
, a transparent electrode film
21
of tin oxide (SnO
2
) having a texture plane is formed on a surface of the substrate
1
. A predetermined part of the transparent electrode film
21
is eliminated by laser beam irradiation and is divided into a plurality of the first electrodes
2
, as shown in FIG.
2
B.
An amorphous semiconductor film
31
having pin junction inside is formed on the substrate so as to cover over the first electrodes
2
as shown in FIG.
2
C.
Then, a predetermined part of the amorphous semiconductor film
31
is eliminated by laser beam irradiation and is divided into a plurality of the photovoltaic conversion layers
3
as shown in FIG.
2
D.
A metal film
4
is formed on the substrate
1
so as to cover the photovoltaic conversion layers
3
as shown in FIG.
2
E. Then, a predetermined part of the metal film
41
is eliminated by laser beam irradiation and is divided into a plurality of the second electrodes
4
to produce the photovoltaic device as shown in FIG.
1
.
The material for forming the first electrode as the transparent electrode is conventionally SnO
2
. However, formation of SnO
2
requires a temperature higher than approximately 500° C., resulting in an increase in manufacture cost. And a substrate of less heat resistance such as plastic can not be usable and selection of substrate material is limited.
In conjunction with this, zinc oxide as material for the first electrode has been considered. The first electrode of zinc oxide can be prepared at a low temperature of not higher than 200° C. by sputtering, resulting in reduction of manufacture cost. In addition, selection of the substrate material is not limited as tightly as in the conventional case.
SUMMARY OF THE INVENTION
The applicants of this invention have examined and found that it is more difficult to electrically separate the adjacent first electrodes of zinc oxide in eliminating the predetermined part of the zinc oxide film by laser beam irradiation and dividing into a plurality of the first electrodes as compared with the first electrode of a SnO
2
film. Thus, leak current is likely to occur between adjacent unit cells of the photovoltaic device comprising the first electrodes of zinc oxide and the photovoltaic conversion characteristics are degraded.
A cause of the above problem has been examined. An expected cause of the problem is explained by referring to schematic cross sectional views of
FIGS. 3A-3B
. Elements having the same functions as in
FIG. 1
have the same reference numerals.
When laser beam intensity for irradiating to the zinc oxide film is great, the temperature of the zinc oxide film increases and doping material of Al, Mg, Ga or the like for reducing a resistance value included in the zinc oxide film is scattered in the substrate
1
to form the diffusion region
1
A on a surface of the substrate as shown in FIG.
3
A. And leak current is generated between the adjacent first electrodes
2
,
2
through the diffusion region
1
A.
On the other hand, when laser beam intensity is small so as to suppress formation of the diffusion region
1
A, the residual
21
A of the zinc oxide film
21
is generated as shown in FIG.
3
B and leak current occurs between the adjacent first electrodes
2
through the residual
21
A.
When the first electrode is formed of zinc oxide and intensity of laser beams to be irradiated is great, doping material of Al, Mg, Ga or the like included in the zinc oxide is scattered and the diffusion region is formed on a surface of the substrate. When the intensity of the laser beams is small, residual of zinc oxide is generated. In these cases, it is expected that leak current occurs between the adjacent first electrodes through the diffusion region and the residual, and photovoltaic conversion characteristic are degraded.
In recent years, a thin film semiconductor device using a glass substrate has become large. A solar cell device which is used in the outside is particularly required to have mechanical strength. In conjunction with this, only attachment of a reinforced glass which requires complicated processes and is difficult to reduce production cost, and a reinforcement process, as a post-process, of a glass substrate with a transparent electrode which is difficult to have sufficient strength and film characteristics have been considered. In this case, when a temperature of the reinforced glass increases higher than 500° C. of a glass melting point after the reinforcement process, an effect of reinforcement and, as a result, the strength may be degraded. Although a SnO
2
film is commonly used material, this film can have sufficient characteristics only when formed at higher than 500° C. Therefore, the reinforced glass can not be used as a substrate. Thus, a method for reinforcing a glass substrate after mounting a transparent electrode has been examined.
In processing the transparent electrode by energy beams, the energy beams should be applied so that the temperature increases momentarily up to approximately 2000° C. of a melting point. Therefore, heat from irradiation energy transmits to a side of the glass when the energy beams are irradiated in order to divide the transparent electrode into a plurality of regions even though the transparent electrode of good characteristics are formed at lower than 500° C. As a result, the temperature increases up to higher than 500° C. locally, which causes fine cracks and irregular degradation of strength of the part of the glass to degrade the strength of the entire glass.
This invention was made to solve the existin
Maruyama Eiji
Sasaki Manabu
Sayama Katsunobu
Arent Fox Kintner Plotkin & Kahn
Elms Richard
Smith Brad
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