FIELD OF INVENTION
The present invention relates to a jig used for light induced plating (LIP)
of silicon solar cells in the field of solar photovoltaics.
BACKGROUND OF THE OPERATION
In silicon solar cells, metallization of both front and back surface is
carefully optimized to ensure a low series resistance and a high shunt resistance,
which result in a high fill factor. However, often the fill factor enhancement is
restricted by high series resistance. The series resistance of a silicon solar cell is
mainly composed of three parts: sheet resistance of the emitter, conducting
resistance of the grid lines on the front surface and contact resistance between
the front electrode and the emitter.
Fill factor is defined as the ratio of the maximum obtainable power to the
product of the open circuit voltage and short circuit current of the solar cell. It is
a key parameter in evaluating the performance of solar cells. It is related to
shunt resistance and series resistance of solar cell and is related to squareness of
the current voltage characteristics of solar cells. Higher the fill factor more
efficient a solar cell is.
Light-induced-plating (LIP) is an effective method to reduce the grid line
resistance of the cell front contact based on the two-layer model. This was
initially proposed to make contact grids narrower and thicker to have reduced
shadowing while maintaining the same area of cross-section. Metallization of the
front surface of solar cells is generally obtained by using the screen printing of

Ag conductive pastes. However, screen printing is not fully able to meet the
requirements for narrower and thicker guidelines, as the paste flows aside during
the annealing process. This limits the aspect ratio to a relatively low level. In the
two layer model, first a narrow and thin layer is screen printed and fired as the
seed layer, which is thickened by subsequent light induced plating.
To the best of our search the following patents are available on this subject.

The patent in Reference No.l presents an apparatus and a method of
simultaneous electroplating and light induced plating on solar cell. Here the
apparatus including the jig is capable of performing electroplating and light
induced plating on solar cell at the same time and thus the design of the jig used
for holding solar cell is significantly different from the one discussed in the
present invention, in which only Light Induced Plating is performed on solar cell.

The patent in Reference No.2 describes a method and composition for
plating metal contacts on photovoltaic solar cells. Here the focus is on method of
plating and composition of chemistry used for plating photovoltaics cells.
However, this differs from the focus of the present invention that is the design of
a jig used for holding solar cell during Light Induced plating process.
The patent in Reference No.3 describes a system for Light Induced Plating
of solar cells involving the use of a patterned metal electrode with a plurality of
raised contact areas that are in electrical contact with the electropositive material
of the rear metallization of the photovoltaic solar cell. The present invention also
uses plurality of contacts at the rear surface of the solar cell, however, the
design of the contacts and the design of the jig is significantly different from the
invention discussed here.
Therefore, to the best of our search and knowledge, no such patent has
been filed on the exact subject of this invention.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a jig used for light
induced plating of silicon solar cells which enables minimum obstruction of light
used for light induced plating.
Another object of the invention is to propose a jig for light induced plating
of silicon solar cells which is capable of minimizing leakage of electrolyte to the
solar cell back surface.
A further object of the invention is to propose a jig for light induced
plating of silicon solar cells which is able to provide negative bias to the back
surface of solar cell at multiple points thus preventing dissolution of the back Al
metal layer.

A further object of the invention is to propose a material for jig body and
metal contacts that is compatible with plating chemistry.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 : Shows the schematic of a light induced plating set up according to
the invention.
Fig. 2a : Shows the schematic of the front view of the jig used for light
induced plating of solar cells with transparent latch according to the
invention.
Fig. 2b : Shows the schematic of the rear view of the jig used for light
induced plating according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
In the present invention, the metalized solar cell (7) is placed inside the
electrolyte with its front surface exposed to light. An absorbed photon creates an
electron-hole pair in the cell and the pair is separated by the built-in electric field
giving rise to negative polarity on the front contact and positive on the rear
contact. The negative potential on the front attracts the positively charged Ag
metal ions from plating bath and thereby thickens the existing seed metal layer
resulting in the plating of Ag on the front Ag grids. In order to protect the back
Al layer from being dissolved, an external voltage is applied between the Ag

anode and the Al rear contact. The external voltage forces the Ag anode to lose
electrons so that the back Al layer remains intact. A schematic of the Light
Induced Plating setup is as shown in Fig.l. The purpose of the jig (1) is to hold
the solar cell (7) in the electrolyte (3) with front surface facing light (2) allowing
maximum light to fall on the solar cell (7) and at the same time provide a
negative bias to Al on the back surface of the solar cell (7) and also as far as
possible, limit the contact area between electrolyte (3) and back Al to prevent it
from dissolution.
Bias is term used in context of polarity (positive or negative). For ex-
"giving negative bias to back contact of solar cell" means giving negative charge
or negative polarity to back contact of solar cell.
The schematic of the Light Induced Plating setup (L) is shown in figure 1.
The design of the jig (1) for holding solar cells (7) is critical since it has to
simultaneously serve the following purposes:
1. To hold the solar cell (7) in its position without offering any obstruction
between the light source (2) and the solar cell front contact.
2. No or minimum leakage of the electrolyte on to solar cell back surface.
3. Offer negative bias to the solar cell back contact so that the back Al from
the solar cell back contact does not dissolve in the electrolyte.
4. Compatibility of the material of the jig (1) with the chemistry used for
plating.
With the requirements as above, the design of the jig is shown in Figs.
2(a) & 2(b). The material of the jig (1) is polypropylene or PPE and the metal
contacts on the jig (1) are made of titanium (8). These materials, i.e. titanium
and PPE are chosen since these are compatible with high purity plating
chemistry.

In order to have clear path between the light source and the solar cell
front contact, a transparent latch (9) is used which give point pressure to the
solar cell (7) front surface in order to hold it against the vertical surface of the
jig. The latch is fixed to the jig at one side so as to give angular movement in the
vertical direction with little movement in the horizontal direction. The latch (9)
can be forced up a little bit while placing or removing the solar cell. The latch
material is transparent so that maximum light falls on the solar cell front surface.
In order to ensure minimum leakage of the electrolyte to the solar cell
back surface, the jig (1) is designed so that solar cell rests on the vertical surface
of the jig (1). The point contact on the front surface of the cell (7) due to the
latch (9) keeps the cell (7) tightly pressed against the jig surface. This ensures
minimum leakage of the electrolyte (3) to the solar cell (7) back surface.
In order to provide bias to the back contact of the solar cells (7), titanium
connectors are fixed at the back side of the jig (1) which pierces through the PPE
body (6) of the jig (1) at 6 points. As stated earlier, titanium and PPE are used in
the construction of the jig (1) keeping in mind their compability with high purity
plating chemistry. The back contact of the solar cell (7) is kept at a negative bias
just enough to prevent any dissolution of the back aluminum into the electrolyte
(3) or deposition of Ag on the rear. Value of the negative bias is chosen which is
closer to open circuit voltage of the cell. At a time two solar cells (7) can be
plated in one batch.
In addition to the above, the design of the jig allows easy removal of solar
cells (7) after plating with a minimum breakage. After plating and DI water rinse
the solar cell (7) is tightly held onto the jig (1) due to surface tension. Holes at
appropriate places are made so as to remove the solar cells (7) easily from the
jig (1) with a minimum breakage.

The benefits of this design are as follows:
• Minimum obstruction between the light source and solar cell front contact
ensuring uniform plating on front seed metallization of solar cell.
• Minimum leakage of the electrolyte on to back surface of solar cell thereby
limiting dissolution of back Al layer in the electrolyte or deposition of Ag
onto the rear.
• Easy mechanism providing bias to the back surface of solar cell at multiple
points.
• Compatibility of the jig material with the high purity plating chemistry.

We Claim:
1. A jig for light induced plating of silicon solar cell comprising;
a jig body (6) having titanium connectors fixed at the back side of the
piercing through the body (6) of the jig (1) at six points for providing bias to the
back contact of the solar cells;
a transparent latch (9) fixed to the jig (1) at one side for providing point
pressure to the solar cell front surface for holding it tightly against the vertical
surface of the jig (1) and for providing a clear path between the light source (2)
and the solar cell front contact wherein, hole at appropriate places are made on
the jig body so that solar cells are removed from the jig with a minimum
breakage.
2. The jig (1) as claimed in claim 1, wherein the latch (9) of the jig (1) is
transparent for allowing maximum light falls on the solar cell front surface.
3. The jig (1) as claimed in claim 1, wherein the jig is made of polypropylene
and the metal contacts on the jig are made of titanium.
4. The jig (1) as claimed in claim 1, wherein the said jig (1) enables
minimum leakage of electrolyte to the solar cell back surface.
5. The jig (1) as claimed in claim 1, wherein the jig provides negative bias to
solar cell back contact at multiple points.

ABSTRACT

The present invention relates to a jig for light induced plating of silicon
solar cell is made up of a jig body (6) having titanium connectors a jig for light
induced plating of silicon solar cells. A transparent latch (9) is fixed to the jig (1)
at one side for holding the solar cell tightly against the vertical surface of the jig
(1). Few holes are provided on the jig body at appropriate places for easy
removal of the solar cells without breaking.