Keithley vFinal DR 2/9/09 11:16 Page 41
MANUFACTURINGAWARENESS
possible to the solar cell. Connecting the shields
together is necessary for obtaining the highest
accuracy because it reduces the effects of the
inductance in the measurement circuit. This is
especially important for capacitance
measurements made at the higher test
frequencies. Given that the capacitance of the cell
is directly related to the area of the device, it may
be necessary to reduce the area, if possible, to
avoid capacitances that may be too high to
measure. Also, setting the 4200-CVU to measure
capacitance at a lower test frequency (10kHz) Figure 10. Actual Reverse Bias Figure 11. C-V Sweep of Silicon
and/or lower AC drive voltage will allow making Measurement of Silicon PV Cell Solar Cell
higher capacitance measurements.
Sweeping
C-V measurements can be made either forward-
biased or reverse-biased. However, when the cell is
forward-biased, the applied DC voltage must be
limited; otherwise, the conductance may get too
high. The maximum DC current cannot be greater
than 10mA; otherwise, the DC voltage output will
not be at the desired level. Figure 11 illustrates a
C-V curve of a silicon solar cell generated by the
41
4200-CVU using the “cvsweep” ITM. This test was
performed in the dark while the cell was reverse- Figure 12. 1/C2 vs. Voltage of a Figure 13. C-f Sweep of Solar Cell
www
biased. Instead of plotting dC/dV, it is sometimes Silicon Solar Cell
.solar
desirable to view the data as 1/C
2
vs. V. The doping
density (N) can be derived from the slope of this
-pv-management.com
curve because N is related to the capacitance by:
Issue IV 2009
where: N(a) = the doping density (1/cm
3
)
q = the electron charge
Es = semiconductor permittivity
(1.034 x 10
–12
F/cm for silicon)
A = area (cm
2
)
C = measured capacitance (F)
V = applied DC voltage (V)
The built-in voltage of the cell junction can be
derived from the intersection of the 1/C
2
curve and
the horizontal axis. This plot should be a fairly
straight line. An actual curve taken with the 4200- generated by using the “cfsweep” ITM. The user
CVU is shown in Figure 12. The “Linear Line Fits” can adjust the range of sweep frequency as well as Figure 9. Connecting
graph option can be used to derive both the the bias voltage. Measuring the electrical the 4200-CVU to a
doping density (N) and the built-in voltage on the characteristics of a solar cell is critical for Solar Cell
x-axis. The doping density is calculated as a determining the output performance and efficiency.
function of voltage in the Formulator and appears The 4200-SCS simplifies cell testing by automating
in the ITM. The user must input the Area of the the I-V and C-V measurements and provides
device in the Constants area of the Formulator. The graphics and analysis capability. The 4200-SCS
4200-CVU can also measure capacitance as a can be used to make resistivity measurements on
function of frequency. The curve in Figure 13 was the materials used for the PV cells.
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