Process for producing a photovoltaic element

Details Process for producing a photovoltaic element Process for producing a photovoltaic element

2001-06-01

2003-07-01

Quach, T. N. (Department: 2814)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S058000, C438S612000, C136S256000

Reexamination Certificate

active

06586270

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a photovoltaic element. More particularly, the present invention relates to a process for producing a photovoltaic element having an improved electrode structure.
2. Related Background Art
Solar cells in which photovoltaic elements are used are attracting attention because they are able to replace conventional electric power generation such as thermal and hydraulic power generation.
There are known a variety of solar cells, such crystalline series solar cells, amorphous series solar cells, and compound semiconductor series solar cells, which are under development or used in practice. Of these solar cells, an amorphous silicon solar cell has many advantages over a crystalline silicon solar cell despite the former is inferior to the latter in terms of the photoelectric conversion efficiency. That is, the amorphous silicon solar cell has a high light absorption coefficient, it works in the form of thin film, and it can be readily made have a large area. Therefore, it is the most promising type of solar cell.
As known well, the amorphous silicon solar cell is constructed of an electrically conductive substrate of stainless steel or the like and layers of back electrode, semiconductor, and light-receiving electrode which are sequentially formed on the substrate. The light-receiving electrode is formed of a transparent conductive oxide.
On the surface of the light-receiving electrode is arranged a collecting electrode comprising fine metal wires for collecting electric current. Being arranged on the light incident side, the collecting electrode casts a shadow on the light receiving face, thereby reducing the active area that contributes to power generation by the solar cell. For this reason, it is a common practice to make the current collecting electrode to be in a thin comb-shaped form. Therefore, the collecting electrode is necessary to be formed by a material with a low electrical resistance such that it has a thin, long form and a cross section which reduces electrical resistance.
Moreover, on the collecting electrode, a so-called bus-bar electrode is formed in order to collect current which is collected by the collecting electrode. The bus-bar electrode is formed of a metal which is thicker than the collecting electrode.
As an example of such an electrode, Japanese Laid-open Patent application No. Hei 8-236796 discloses a collecting electrode formed using metal wires. FIGS.
6
(
a
) and
6
(
b
) are schematic views illustrating an example of the structure of said collecting electrode. Particularly, FIG.
6
(
a
) is a schematic plan view of a photovoltaic element having said collecting electrode, and FIG.
6
(
b
) is a schematic cross-sectional view taken along the line VI-VI′ in FIG.
6
(
a
).
In FIGS.
6
(
a
) and
6
(
b
), reference numeral
601
indicates a photovoltaic element comprising a back electrode layer, a semiconductor layer, and a transparent electrode layer sequentially formed on a substrate of stainless steel. Reference numeral
602
indicates an etching line along which the transparent electrode layer is removed so as to prevent the photovoltaic element from being short-circuited at its edge. The etching line
602
surrounds an active area of the photovoltaic element which contributes to power generation. Reference numeral
603
indicates an insulating material
603
and reference numeral
604
a collecting electrode. The collecting electrode
604
comprises a metal wire
605
(50 to 300 &mgr;m in diameter) coated with an electrically conductive paste
606
or the like, which is press-bonded to the transparent electrode layer. The electrically conductive paste has a resistivity of 10
−1
to 10
2
&OHgr;cm so that it does not cause short-circuiting (which decreases output) in case of direct contact with pinholes in the surface of the photovoltaic element and it prevents metal migration. Reference numeral
607
indicates a bus-bar electrode for additional current collection, which serves to collect current collected by the collecting electrode
604
and output it outside the photovoltaic element.
The conventional solar cell having such structure as above mentioned has a photoelectric conversion efficiency of 8 to 10% in practice. There has been a remarkable improvement in the photoelectric conversion efficiency for solar cells. Particularly, there recently has developed a semiconductor film having an improvement in terms of the short-circuit current (Isc) and having a photoelectric conversion efficiency of more than 10%.
However, in the case where the photoelectric conversion efficiency and the quantity of current is increased, there is a problem such that the loss of generated electric power at the electrode through which current flows increases in proportion to the square of the quantity of current. In other words, even when a high-efficiency semiconductor should have been developed, there is a tendency in that the practical photoelectric conversion efficiency is considerably lowered on account of the loss that occurs at a high-resistance part when generated current is led to the external circuit. Therefore, it is necessary for the solar cell to consider an adequate current collecting type in accordance with the quantity of current generated.
The collecting electrode disclosed in the above-mentioned Japanese Laid-open Patent Application No. Hei 8-236796 is constructed such that the junction of the bus-bar electrode and the wire electrode is formed with a carbon paste (which has a comparatively high resistivity) and hence has a high resistance. The solar cell with such a collecting electrode does not secure the desired photoelectric conversion efficiency because the resistance loss at the junction increases as the quantity of current increases.
One possible way to address this problem is to form the junction of the bus-bar electrode and the wire electrode with an electrically conductive paste or the like having a low resistance, thereby reducing the resistance loss.
An example of the photovoltaic element based on such an idea is schematically shown in FIGS.
7
(
a
) and
7
(
b
). FIG.
7
(
a
) is a schematic plan view of the photovoltaic element and FIG.
7
(
b
) is a schematic cross-sectional view taken along the line VII-VII′ in FIG.
7
(
a
).
The configuration of the photovoltaic element shown in FIGS.
7
(
a
) and
7
(
b
) differs from that of the photovoltaic element shown in FIGS.
6
(
a
) and
6
(
b
) in that the carbon paste at the junction of the metal wire
605
and the bus-bar electrode
607
is replaced by an electrically conductive paste
701
(such as silver paste) having a low resistance. The silver paste whose resistivity is about one-thousandth of that of carbon paste greatly reduces the resistance loss and permits the photovoltaic element to have a desired photoelectric conversion efficiency.
The photovoltaic element shown in FIGS.
7
(
a
) and
7
(
b
) is liable to have such problems as will be described below with reference to FIGS.
8
(
a
) and
8
(
b
).
FIG.
8
(
a
) shows an appearance of the electrically conductive paste
701
with low-resistance which has been “dotted”. FIG.
8
(
b
) shows an appearance of the electrically conductive paste
701
which has been pressed and heat-cured under the metal bus-bar.
Any known dispenser may be used to make a circular dot of silver paste as shown in FIG.
8
(
a
). The round dot is pressed and heat-cured under the metal bus-bar
607
as shown in FIG.
8
(
b
).
The problem with dotting a electrically conductive paste on a metal wire is that the electrically conductive paste flows out along the metal wire when it is heated under pressure as shown in FIG.
8
(
b
). Eventually, the electrically conductive paste is forced out from the metal bus-bar
607
.
The electrically conductive paste which has been forced out from the metal bus-bar poses the following problems.
(1) The electrically conductive paste enters the active area surrounded by the etching line
602
. It may come i

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