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Keithley vFinal DR 2/9/09 11:16 Page 40
MANUFACTURINGAWARENESS
Figure 7. Slope below a picoamp. When making very sensitive low
Method Used to current measurements (nanoamps and smaller),
Calculate the Series use low noise cables and place the device in a
Resistance shielded enclosure to shield the device
electrostatically. This conductive shield is
connected to the Force LO terminal of the 4200-
SCS. The Force LO terminal connection can be
made from the outside shell of the triax
connectors, the black binding post on the ground
unit (GNDU), or from the Force LO triax connector
on the GNDU.
Issue IV 2009
One method for determining the shunt resistance
of the PV cell is from the slope of the reverse bias
I-V curve, as shown in Figure 8. From the linear
region of this curve, the shunt resistance can be
Using additional light intensities, this technique can calculated as:
be extended using multiple points located near the
-pv-management.com knee of the curves. As illustrated in Figure 7, a line
is generated from which the series resistance can
.solar
be calculated from the slope. An important
www
measurement feature of the system’s SMU as an
ammeter is that it has very low voltage burden. The An actual curve of a reverse-biased PV cell is
40
voltage burden is the voltage drop across the shown in Figure 10. This curve was generated
ammeter during the measurement. Most using the ITM “rev-ivsweep”. In this semi-log
conventional digital multimeters (DMMs) will have a graph, the absolute value of the current is plotted
voltage burden of at least 200mV at full scale. as a function of the reverse bias voltage that is on
Given that only millivolts may be sourced to the an inverted x-axis.
sample, this can cause large errors. The 4200-
SCS’s SMU never produces more than a few Capacitance measurements
hundred microvolts of voltage burden, or voltage In addition to determining the I-V characteristics of
drop, in the measurement circuit. a PV cell, capacitance-voltage measurements are
also useful in deriving particular parameters about
Reverse bias I-V measurements the device. Depending on the type of PV cell, the
The leakage current and shunt resistance (rsh) can AC capacitance can be used to derive such
be derived from the reverse bias I-V data. Typically, parameters as doping concentration and the built-
the test is performed in the dark. The voltage is in voltage of the junction. A capacitance-frequency
sourced from 0V to a voltage level where the sweep can be used to provide information about
device begins to break down. The resulting current the existence of traps in the depletion region. The
is measured and plotted as a function of the 4200-CVU, an optional capacitance voltage unit,
Figure 8. Typical voltage. Depending on the size of the cell, the can measure the capacitance as a function of an
Reverse-Bias leakage current can be as small as in the picoamp applied DC voltage (C-V), a function of frequency
Characteristics of a region. The Model 4200-SCS has a preamp option (C-f), or a function of time (C-t).
PV Cell that allows making accurate measurements well
To make a C-V measurement, a solar cell is
connected to the 4200-CVU as shown in Figure 9.
Like I-V measurements made with the SMU, the C-
V measurement also involves a four-wire
connection to compensate for lead resistance. The
HPOT/HCUR terminals are connected to the anode
and the LPOT/LCUR terminals are connected to
the cathode. This connects the high DC voltage
source terminal of the CVU to the anode.
Not shown in the diagram are the shields of the
coax cables. The shields from the coax cables
need to be connected together as close as
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