Nondestructive Performance Characterization Techniques for Module Reliability Page: 4 of 7
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Other parameters such as fill factor are relatively
insensitive to the error in setting the irradiance. Figure 2
shows the change in performance of 28 copper indium
1E-5 0 ' ' ' ' '
0 5 10 15 20 25.
Figure 3. Dark current versus voltage for a module
with a large change in power compared with a
module showing a smaller change in power.
gallium diselenide (CIGS) modules as a function of time
in the field. The change in fill factor, FF, is
characterized by an average of the samples of 66.7%
initially, with a standard deviation of 1.8%, and 60.3%
recently, with a much larger, standard deviation of 5.3%.
There was no significant change in the short-circuit
current, Ic, until recently, with a possible 5% decrease
for some modules in the last 6 months. The change in
the open-circuit voltage, VO, was negligible. To explain
the decrease in FF the other I-V parameters were
investigated. Figure 3 compares the dark I-V for a
module with a small change in FF and one with a larger
change. It appears that the fill factor changes cannot be
explained by shunting because the low-voltage region
actually decreases for both modules. It also appears that
there is no measurable change in the portion of the curve
where the log of the current is linear with voltage,
indicating no appreciable change in the diode quality
factor or dark current. The only significant difference is
the high-voltage series-resistance region. The light I-V
curves minus Ic show similar behavior with more noise
due to light-level fluctuations. The resistance at VOc is
linearly related to the series resistance in that the
intercept of a plot of the resistance at VO versus 1/Isc is
the series resistance . Figure 4 shows that there is a
large change in the series resistance from the initial
values and the present values and provides an
explanation for the drop in power and fill factor.
3. Quantum Efficiency
The quantum efficiency of a single cell in a module
can be measured and can provide insight as to why the
short-circuit changed. Figure 5 shows the quantum
efficiency of an exposed and unexposed mono-Si
module . The drop in the ultraviolet region can be
explained by EVA browning . However the drop in
the infrared region cannot be explained by EVA
transmission changes, but can be explained by a decrease
in the diffusion length.
No change in the modules 'dark current"
Increase in "shunt resistance probably due to
increase in sees resistance
resistance for the
. power changed less
Solid curve initial data
y' The top curve in red had less of a change in power
1/1/99 1/1/2000 1/1/2001 1/1/2002 1/1/2003
Figure 4. Changes in the resistance at VO for the modules
in Fig. 1, showing a significant increase with the lower fill
factors having the higher resistances.
200 300 400 500 600 700 800 900 1000 1100 1200
Figure 5. Changes in the quantum efficiency for a single
cell in an unexposed module (diamond) and after
exposure (circle) .
Modules in forward or reverse bias can exhibit hot-spot
heating. There are international test procedures to
determine if hot-spot heating is a problem [6,7]. Figure 6 is
an example of a hot-spot of a mono-Si cell in a short-
circuited module under one-sun illumination . Localized
heating can also occur in thin-film modules in forward bias
because of resistance losses at the scribe-line interconnect
region. Thin-film modules can be damaged by excessive
reverse bias (voltages>VOc), so testing is often carried out in
forward bias. Hot-spot analysis can be conducted with an
infrared camera or with temperature sensitive film in contact
with the module surface. The temperature-sensitive film is
inexpensive and sensitive to a few mm, but is translucent so
it is normally used in the dark with the module in small
reverse bias or in forward bias. Defects in a module related
to high-resistance paths, localized shunting defects, or
28 modules measured versus e
.5 time showing a apparent
3 increase in series resistance
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Emery, K. Nondestructive Performance Characterization Techniques for Module Reliability, article, May 1, 2003; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc1406911/m1/4/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.