FIELD OF INVENTION
The present invention generally relates to an improved system for making
contacts to solar cells for measuring their current-voltage (I-V) characteristics.
In particular, it relates to an arrangement for measurement of I-V characteristics
of industrial solar cells employing four probe contacting scheme on the front side
of the cell and creating vacuum for holding solar cells on a vacuum chuck for
making reliable back contact.
The details of the measurement setup and functioning of the electronic load,
light source, etc. used for the measurement are parts of Patent Application No.
329/KOL/2008 filed on 22.2.2008 by the same applicant.
The arrangement developed in the present invention can be adapted for use in R
& D laboratories and production facilities for testing of solar cells in a convenient,
accurate and reliable manner. The arrangement of the present invention is novel,
as was evident from several patent searches made.

BACKGROUND OF THE INVENTION
Solar cells are essentially large area diodes having certain added features that
enable generation of electric power on being shone with visible light. The
efficiency of solar cells, a direct measure of its ability to convert incident light
into electricity, is of paramount importance in most of the applications and
therefore, needs to be measured as accurately as possible. This is obtained from
a measurement of the current-voltage I-V characteristics of the device under
light. For this, solar cell under test needs to be held by vacuum on a suitable
metallic chuck for low resistance, reliable contact from the rear of solar cells.
Also, low resistance, reliable electrical contacts are required to be made from the
front side to complete the circuit. For accurate measurement separate contacts
have to be made for current and voltage measurements. The probes used should
also ensure minimum shadowing on the solar cell. Furthermore, solar cells being
delicate and fragile, making and breaking of the contacts on solar cells, as also
removal of the cell after the test, should be done with care, so as to avoid
breakage as well as damage to the cell.

SUMMARY OF THE INVENTION
The main object of the present invention is to provide a system for making low
resistance reliable electrical contacts from the rear and front of a solar cell with a
four probe contacting scheme for accurate and repeatable measurement of I-V
characteristics of solar cells.
One more object of the invention is to provide measurement probes that ensure
minimum shadowing on the solar cell.
Yet another object of the invention is to ensure that making and breaking of the
contacts on solar cells during testing and removal of the cell after the test is
done with care so as to avoid breakage/damage to the cells.
These and other objects of the invention are achieved by the system of the
present invention.
For reliable and accurate testing of the solar cells the requirements to be met
are:
- a steady source of light,
- holding the test cell firmly on a vacuum chuck and making
firm electrical contacts on both bus bars of the cell with probes, and
- an accurate electronic load

A temperature controlled nickel coated vacuum chuck is provided' using
circulating water from a water bath.
For enhanced accuracy, the contact to the cell needs to be made with four
spring-loaded probes - two for current and two for voltage measurements. In
the present invention vacuum in the chuck is created by Venturi effect.
The present invention dispenses with the need for a vacuum pump and the
arrangement allows gentle release of the cells on completion of the test. The
same source of compressed air is used both for creation of vacuum by Venturi
effect and for controlling the probe-holder movement.
For creation of vacuum, a pneumatic circuit is designed which allows turning xon'
or 'off' the vacuum at the chuck through operation of a one-way solenoid valve
and at the same time allows upward and downward movement of the probe-
holders through operation of a two-way solenoid valve.
An arrangement for four-probe measurement on the solar cell is adapted using
two probe-holders (one for each bus bar). The probe-holders are designed in
such a way that the current collection from each bus bar is efficient, that each
probe-holder carries both current and voltage probes independently and causes
minimum shadowing on the solar cell.

The current and voltage probes are fixed on a single probe holder in proximity
and at a fixed distance. The voltage probe is isolated from the current probe
with the help of a Teflon sheath.
An electronic load can be connected to the outputs of the solar cell for
measurement of its I-V characteristics.
Thus, in one preferred embodiment the present invention provides an improved
system for making contacts to solar cells for measuring their I-V characteristics
said system comprising: a pneumatic circuit for creation of vacuum in a vacuum
chuck for holding the solar test cell firmly and for making back contact; and a
four-probe contacting arrangement on the front side of the cell for measurement
on the solar cell; said four probe arrangement is designed such that the current
collection from each bus bar is efficient while measuring the I-V characteristics of
the solar cell.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The invention will now be described with the help of the accompanying drawings
where:
Fig. 1 shows a schematic arrangement of 4-probe I-V
measurement circuit for solar cells.

Fig. 2 shows a schematic arrangement of the complete set up for
measurement of solar cell I-V characteristics.
Fig. 3 shows side view and top view of the probe-holder.
Fig. 4 shows a schematic arrangement of the vacuum hold
for solar cells and manipulation of test probes.
Fig.5a shows light I-V characteristic of a typical industrial solar cell.
Fig.5b shows dark log J-V characteristic of a typical industrial solar cell.
DETAILED DESCRIPTION OF THE INVENTION
The basic circuit for I-V measurement of solar cells is shown in Fig. 1 and a
complete arrangement for the I-V measurement of solar cells is shown in Fig. 2.
It can be seen from these figures that the circuit for current measurement is
separate from that of the voltage measurement. This is done in order to
eliminate the effect of voltage drop (Ohmic) at the contacts. A vacuum chuck 21
is provided for holding the solar cell S under test. The chuck 21 can be a nickel-
plated metallic chuck 21 for providing the contact at the rear of solar cells S.

As shown in Figure 3, the present invention comprises i) a probe-holder 30
designed to hold the spring-loaded current probe 10 and voltage probe 12 and
cause minimum shadowing on solar cells and ii) a pneumatic circuit meant for
vacuum holding solar cells S as well as for operating the probe-holders 30 up
and down with a single source of compressed air. These features are described
in detail in the following paragraphs.
The probe-holder 30 is designed for firmly holding the spring-loaded, gold-coated
probes 10,12 used for contacting the solar cell under test. The side view and the
top view of one such probe-holder 30 is shown in Fig. 3. The basic requirement
is to hold the spring-loaded probes firmly, cover the entire length of the bus bar
uniformly, isolate the voltage probe from the current probe and reduce
shadowing. The first objective is met by drilling holes of diameter close to the
diameter of the probes through the probe-holder and push fitting the probes.
Out of the fourteen probes, thirteen are shorted from the top with a tin coated
copper strip 31. The fourteenth probe is isolated from the body of the holder by
drilling slightly wider hole and push fitting the probe through a Teflon lining 32.
These features are brought out clearly in Fig. 3a. Thus effectively there is one
current probe 10 and one voltage probe 12 fitted in each probe-holder 30.

Also, the probe-holder is designed in such a way that the shadowing caused on
solar cells is minimum. For this, the width of the probe-holder is reduced at
places where there is no boring for the probes, without sacrificing on its
mechanical strength. This is clearly shown in Fig. 3b.
The pneumatic circuit used for vacuum generation is provided with a single
source of compressed air as shown in Fig. 4. The compressed air source 41 is
bifurcated into two branches at a brass T-joint 42, one for controlling the probe-
holder movement and the other for creating vacuum by Venturi effect for holding
the solar cell under test without the use of a vacuum pump.
The pneumatic circuit is also provided with a two-way solenoid valve 43 for up /
down movement of probe-holders 31.
The first branch of the compressed air source 41 has two-way solenoid valve 43
with two outputs connected to the two inlets of a two-way pneumatic cylinder
45, one responsible for the upward movement and the other for the downward
movement of the cylinder piston. The assembly containing the probe-holders 31
is connected securely to the piston rod 46 and moves up and down with it. With

the flow-control valves 44, 44' fully open, there is a minimum air pressure
required for smooth movement of the piston rod. The solenoid valve 43 is
operated with an Up/Down toggle switch 43'. For "on" position, the piston rod 46
along with the probe-holder assembly 31 moves upward and for "off" position,
the movement is downward. The speed-control valves 44, 44' fitted before each
inlet regulate the speed of piston movement and thereby the speed of movement
of the probes. Controlling speed of the probe-holders is important as a high
downward speed may break the delicate and fragile solar cell under test and a
high upward speed may cause mechanical disturbance to the setup.
With the minimum airflow set as per the requirement described above, the air
from the second branch is used for vacuum generation in a chuck 21 using a
vacuum generator 47 (Venturi effect) that is connected to a solenoid valve 48
(Fig. 4). Operating the solenoid valve 48 with a toggle switch 49 makes or breaks
the vacuum at the chuck 21. Opening the solenoid valve 48 allows air to pass
through it and produce vacuum at the chuck. Closing the solenoid valve 48 cuts
the airflow through the solenoid valve and breaks the vacuum. However, even at
the open position, the solenoid valve allows only a limited airflow through it
(depending on its orifice size) and the vacuum produced at the chuck is
inadequate. However, the probe-holder movement is satisfactory under this
condition.

Generation of adequate vacuum at the chuck is possible under this condition if
some more air is allowed to flow through the vacuum generator 47. This is done
by simply piercing a small hole 47' on the tube connecting the vacuum generator
47 to the solenoid valve 48 (Fig. 4). As more air rushes out through the tube, the
vacuum level at the chuck improves and holds the solar cell firmly on the chuck
making satisfactory back contact. Also, when the solenoid is closed, a small
amount of air flows through the holes in the chuck 21. The airflow through the
vacuum generator 47 can be decreased by operating the solenoid, thereby
facilitating gentle lifting of the solar cell S under test and its removal from the
test arena on competition of the test.
Typical result of I-V measurement on industrial solar cells under illumination and
in dark made with the arrangement described above is shown respectively in
Figs. 5a and 5b. The results have been crosschecked with other sources and
found to be all right proving adequacy of the setup described under the present
invention.

WE CLAIM
1. An improved system for making contacts to solar cells for measuring their
I-V characteristics, said system comprising:
- a pneumatic circuit for creation of vacuum in a vacuum chuck for holding
the solar test cell firmly and for making back contact; and
- a four-probe contacting arrangement on the front side of the cell for
measurement on the solar cell;
said four probe arrangement is designed such that the current collection from
each bus bar is efficient while measuring the I-V characteristics of the solar
cell.
2. The system as claimed in claim 1, wherein said pneumatic circuit is
provided with a single compressed air inlet for vacuum generation at the
chuck by Venturi effect without the need for a vacuum pump.
3. The system as claimed in claim 1, wherein said pneumatic circuit is
provided with a two-way solenoid valve for up / down movement of probe
holders respectively for contacting the cell on the front side.

4. The system as claimed in claim 3, wherein the airflow through the vacuum
generator can be increased by piercing a hole on the tube at the outlet of
the vacuum generator for providing adequate vacuum at the chuck for
holding the solar cell.
5. The system as claimed in claim 3, wherein the airflow through the vacuum
generator can be decreased by operating a solenoid, thereby facilitating
gentle lifting of the solar cell for removal on completition of the
measurements.
6. The system as claimed in claim 1, wherein the probe-holders are provided
with spring loaded current and voltage probes fixed on a single probe-
holder in proximity at a fixed distance.
7. The system as claimed in claim 5, wherein the voltage probe is isolated
from the current probe with the help of a Teflon sheath.
8. The system as claimed in the preceding claims, wherein the width of the
probe-holder is reduced at places where there is no boring for the probes,
so that the shadowing caused on the solar cells is minimum.

9. An improved system for making contacts to solar cells for measuring their
I-V characteristics, substantially as herein described and illustrated in the
figures of the accompanying drawings.

The main object of the present invention is to provide a system for making low resistance reliable electrical contacts from the rear and front of a solar cell with a four probe contacting scheme for accurate and repeatable measurement of I-V characteristics of solar cells. One more object of the invention is to provide measurement probes that ensure minimum shadowing on the solar cell. Yet another object of the invention is to ensure that making and breaking of the contacts on solar cells during testing and removal of the cell after the test is
done with care so as to avoid breakage/damage to the cells. These and other objects of the invention are achieved by the system of the
present invention. For reliable and accurate testing of the solar cells the requirements to be met are:
- a steady source of light,
- holding the test cell firmly on a vacuum chuck and making firm electrical contacts on both bus bars of the cell with probes, and
- an accurate electronic load
A temperature controlled nickel coated vacuum chuck is provided' using circulating water from a water bath. For enhanced accuracy, the contact to the cell needs to be made with four
spring-loaded probes - two for current and two for voltage measurements. In the present invention vacuum in the chuck is created by Venturi effect. The present invention dispenses with the need for a vacuum pump and the
arrangement allows gentle release of the cells on completion of the test. The same source of compressed air is used both for creation of vacuum by Venturi effect and for controlling the probe-holder movement. For creation of vacuum, a pneumatic circuit is designed which allows turning xon' or 'off' the vacuum at the chuck through operation of a one-way solenoid valve
and at the same time allows upward and downward movement of the probe-holders through operation of a two-way solenoid valve. An arrangement for four-probe measurement on the solar cell is adapted using two probe-holders (one for each bus bar). The probe-holders are designed in
such a way that the current collection from each bus bar is efficient, that each probe-holder carries both current and voltage probes independently and causes minimum shadowing on the solar cell. The current and voltage probes are fixed on a single probe holder in proximity
and at a fixed distance. The voltage probe is isolated from the current probe with the help of a Teflon sheath. An electronic load can be connected to the outputs of the solar cell for
measurement of its I-V characteristics.