Keithley vFinal DR 2/9/09 11:16 Page 39
MANUFACTURINGAWARENESS
Figure 4. Connections of 4200-SCS’s SMU to
Solar Cell
Making connections to the PV cell
A solar cell connected to the 4200-SCS’s SMU for
I-V measurements is shown in Figure 4. A four-wire
resistance that could otherwise affect the
measurement accuracy. With the four-wire method,
a voltage is sourced across the PV cell using one data using the Model 4200-SCS’s built-in Figure 5. Results of
pair of leads (Force HI and Force LO), and the mathematical analysis tool, the Formulator. For Calculated Parameters
voltage drop across the cell is measured across a convenience, the “CVU_Pvcell” project has the Shown in Sheet Tab
second set of leads (Sense HI and Sense LO). The common parameters already calculated and the
39
sense leads ensure that the voltage developed values automatically appear in the Sheet tab every
across the cell is the programmed output value time the test is executed. Figure 5 shows some of
www
and compensates for the lead resistance. the derived parameters in the Sheet tab. These
.solar
parameters include the short-circuit current (ISC),
Forward bias I-V measurements of the PV cell are the open circuit voltage (VOC), the maximum power
-pv-management.com
generated under controlled illumination. The SMU point (Pmax), the maximum cell current (I
max
), the
is set up to source a voltage sweep and measure maximum cell voltage (Vmax), and the fill factor (FF).
the resulting current. This forward bias sweep can
be accomplished using the “fwd-ivsweep” ITM. The Using the Formulator, the conversion efficiency (η)
user can adjust the sweep voltage to the desired can also be calculated if the power input to the cell
values. As illustrated in Figure 2, the voltage is known. The current density (J) can also be
source is swept from V1 = 0 to V2 = VOC. When the derived using the area of the cell. Figure 6 shows
Issue IV 2009
voltage source is 0 (V1 = 0), the current is equal to an actual I-V sweep of an illuminated silicon PV cell
the short-circuit current (ISC). When the voltage generated by the 4200-SCS using the “fwd-
source is an open circuit (V2 = VOC), then the ivsweep” ITM. Because the system’s SMUs can
current is equal to zero (I2 = 0). The parameters sink current, the curve can pass through the fourth
VOC and ISC can easily be derived from the sweep quadrant and allow power to be extracted from the
device (I

, V
+
). The Graph tab options support an
easy transition between graphically displaying data
on either a linear or a log scale. The series
resistance, (rs), can be determined from the
forward I-V sweep at two or more light intensities.
First, make I-V curves at two different intensities.
Knowing the magnitudes of the intensities is not
important. Measure the slope of this curve from the
far forward characteristics where the curve
becomes linear. The inverse of this slope yields the
series resistance:
Figure 6. I-V Sweep of Silicon PV Cell Generated
with the 4200-SMU
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