Electrical characterization of photovoltaic materials and solar cells
V
Reverse Bias
r
sh
Linear re
gion used to estimate
∆I
)
Reverse Bias
∆V
∆V
Reverse Bias
r
S
=
∆I
r
ent (mA
∆V
C
u
r
r
Reverse Bias
sh
≈ log I
∆I
Reverse Bias
Reverse Bias
Figure 9. Typical reverse-biased characteristics of a PV cell
Voltage (V)
Figure 8. Slope method used to calculate the series resistance
Forward-biased I-V measurements the fourth quadrant
Forward-biased I-V measurements of and allows power
the solar cell are made under controlled to be extracted
illumination. The SMU is set up to from the device (I–,
output a voltage sweep and measure the V+). If the current
resulting current. This forward bias sweep output spans
can be performed using the “fwd-ivsweep” several decades as
ITM, which allows adjusting the sweep a function of the
voltage to the desired values. As previously applied voltage, it
illustrated in Figure 3, the voltage source may be desirable to
is swept from V
1
= 0 to V
2
= V
OC
. When generate a semilog
the voltage source is 0 (V
1
= 0), the current plot of I vs. V. The
is equal to the source-circuit current (I
Figure 10. Reverse-biased I-V measurement of silicon solar cell using the Model
1
= Graph tab options
4200-SMU
I
SC
). When the voltage source is an open support an easy
circuit (V
2
= V
OC
), then the current is equal transition between
to zero (I
2
= 0). The parameters, V
OC
and displaying data SMUs is their very low voltage burden. The
I
SC
, can easily be derived from the sweep graphically on either a linear or a log scale. voltage burden is the voltage drop across
data using the Model 4200-SCS’s built-in If desired, the graph settings functions the ammeter during the measurement.
mathematical analysis tool, the Formulator. make it easy to create an inverted version Most conventional digital multimeters
For convenience, the SolarCell project has of the graph about the voltage axis. Simply (DMMs) will have a voltage burden of at
the commonly derived parameters already go to the Graph Settings tab, select Axis least 200 mV at full scale. Given that only
calculated, so the values automatically Properties, select the Y1 Axis tab, and click millivolts may be sourced to the sample
appear in the Sheet tab every time the on the Invert checkbox. The inverse of the in solar cell testing, this can cause large
test is executed. Figure 5 shows some of graph will appear as shown in Figure 7. errors. The Model 4200-SCS’s SMUs
the derived parameters in the Sheet tab. The series resistance (r
s
) can be don’t produce more than a few hundred
These parameters include the short-circuit determined from the forward I-V sweep at microvolts of voltage burden, or voltage
current (I
SC
), the open circuit voltage two or more light intensities. First, make drop, in the measurement circuit.
(V
OC
), the
max
imum power point (P
max
), the I-V curves at two different intensities
max
imum cell current (I
max
), the
max
imum cell (the magnitudes of the intensities are not Reverse-biased I-V measurements
voltage (V
max
), and the fill factor (FF). important). Measure the slope of this curve The leakage current and shunt resistance
Th user can easily add other formulas from the far forward characteristics where (r
sh
) can be derived from the reverse-biased
depending on the required parameters that the curve becomes linear. The inverse of I-V data. Typically, the test is performed in
need to be determined. this slope yields the series resistance: the dark. The voltage is sourced from 0V
Using the Formulator, the conversion to a voltage level where the device begins
efficiency (η) can also be calculated if to break down. The resulting current is
the user knows the power input to the measured and plotted as a function of
cell and inputs the formula. The current the voltage. Depending on the size of the
density (J) can also be derived by using the By using additional light intensities, this cell, the leakage current can be as small
Formulator and inputting the area of the technique can be extended using multiple as picoamps. The Model 4200-SCS has a
cell. points located near the knee of the curves. preamp option that allows making accurate
Figure 6 shows an actual I-V sweep of As illustrated in Figure 8, a line is generated measurements well below a picoamp.
an illuminated silicon PV cell generated from which the series resistance can be When making very sensitive low current
with the Model 4200-SCS using the “fwd- calculated from the slope. measurements (nanoamps or less), use
ivsweep” ITM. Because the system’s SMUs When considered as ammeters, one low noise cables and place the device in a
can sink current, the curve passes through important feature of the Model 4200-SCS’s shielded enclosure to shield it
www.globalsolartechnology.com
Global Solar Technology – October/September 2009 – 33
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