Keithley vFinal DR 2/9/09 11:16 Page 38
MANUFACTURINGAWARENESS
power output to the power input to the cell:
where:
Pmax = the maximum power output
Pin = the power input to the cell defined as the total
Issue IV 2009
Figure 2. Typical forward bias I-V characteristics radiant energy incident on the surface of the cell.
of a PV cell These described parameters of the solar cell can
be determined through electrical characterisation
Several factors determine the efficiency of the solar of the device.
cell, including the maximum power point (Pmax), the
energy conversion efficiency (η), and the fill factor The 4200-SCS
(FF). These points are illustrated in Figure 2, which We have chosen to use the Keithley 4200-SCS
-pv-management.com shows a typical forward bias I-V curve of an semiconductor characterisation system to
illuminated PV cell. The maximum power point demonstrate capacity in such measurement. The
.solar
(Pmax) is the product of the maximum cell current 4200-SCS is a measurement system that includes
www
(Imax) and voltage (Vmax) where the power output of instruments for both I-V and C-V measurements, as
the cell is greatest. This point is located at the well as software, graphics, and mathematical
38
“knee” of the curve. analysis capability. The software includes tests for
making I-V and C-V measurements specifically on
The fill factor is a measure of how far the I-V solar cells and deriving common PV cell
characteristics of an actual PV cell differ from those parameters from the test data. We look at how to
of an ideal cell. The fill factor is defined as: use the Model 4200-SCS to make electrical
measurements.
To simplify testing, a project has been created for
the 4200-SCS that makes both I-V and C-V
measurements on a solar cell and also extracts
common measurement parameters such as
Another parameter is the conversion efficiency (η), maximum power, short-circuit current, open-circuit
which is defined as the ratio of the maximum voltage, etc. The project is called “CVU_Pvcell”
and is included with all 4200-SCS systems running
KITE version 7.0 or later. A screen shot of the
project is shown in Figure 3. This project has five
tests, called ITMs (Interactive Test Modules), that
perform a forward bias I-V sweep (fwd-ivsweep),
reverse bias I-V sweep (rev-ivsweep), C-V sweep
(cvsweep), 1/C
2
vs. V plot (C-2vsV) and C-f sweep
(cfsweep).
Many device parameters can be determined from
current-voltage (I-V) measurements of the solar
cell. The I-V characteristics are measured using
one of the Model 4200-SCS’s Source Measure
Units (SMUs), which can source and measure both
current and voltage. Two types of SMUs are
available: one which can source/sink up to 100mA,
and another, that can source/sink up to 1A. If the
output current of the cell exceeds these current
levels, then the output current may have to be
reduced. One way of reducing the output is to
Figure 3. Screen Shot of PV Cell Project for the 4200 reduce the area of the cell.
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