Current accelerated qualification tests of photovoltaic (PV) modules mostly assist in avoiding premature failures but can neither duplicate changes occurring in the field nor predict useful product lifetime. Therefore, outdoor monitoring of field-deployed thin-film PV modules was undertaken at FSEC with the goal of assessing their performance in hot and humid climate under high system-voltage operation. Significant and comparable degradation rate of −5.13±1.53% and −4.5±1.46% per year was found using PVUSA type regression analysis for the positive and negative strings, respectively of 40W glass-to-glass Cu-In-Ga-Se (CIGS) thin-film PV modules in the hot and humid climate of Florida. Using the current-voltage measurements, it was found that the performance degradation within the PV array was mainly due to a few (8% to 12%) modules that had substantially higher degradation. The remaining modules within the array continued to show reasonable performance (>96% of the rated power after ∼ four years).
With efficiency of PV devices approaching theoretical numbers and cost of PV coming down, the most essential factor
that would determine their large scale deployment is the reliability and durability of the PV modules and the balance of system components. The degradation mechanism and reliability issues in PV cells must be determined by the tests
carried out on field-deployed modules. With this goal outdoor testing of PV modules was undertaken. The outdoor
performance variation of commercially available single junction and triple junction a-Si:H PV module has been studied
in the hot and humid climate of Florida. After the initial Staebler-Wronski degradation, the a-Si:H PV modules typically
degrade during the winter time when the temperatures are low and recuperate due to the annealing that takes place
during the summer time. Due to this seasonal variation, the monthly data was considered in the multiples of 12.
Performance variation calculated from the monthly PTC power showed +1.89±0.58% and +2.49±0.6% for positive and negative array of single junction a-Si:H PV modules and +0.42±0.87% and + 0.56±0.89% for the positive and negative array of triple junction a-Si:H PV modules respectively. The annual energy yield was found to be 1300 kWh/kWp/year and 1425 kWh/kWp/year for the single junction and triple junction a-Si:H PV arrays respectively.
The best conversion efficiency of champion, small-area CIGSeS cells and mass-produced modules are 20.3% and
~12.0% respectively. Molybdenum back-contact layer is scribed with a laser (P1). Development of laser scribing for P2
and P3 scribes will reduce the dead area and improve the reliability. Development of a laser annealing technique can
minimize and passivate micro-non-uniformities and grain boundaries thus reducing carrier recombination. In-situ
characterization of the sample through a nonintrusive method such as reflectivity measurement during the laser
recrystallization process can enhance insight into laser-material interaction and the effect of material structures on the
photoelectric properties of solar cells. The improvement in efficiency achieved with laser processing can help to bring
down the cost.
The accelerated tests currently carried out on PV modules reduce the infant mortality as well as improve the production
techniques during the manufacture of PV modules. However, the accelerated tests do not completely duplicate the real
world operating conditions of PV modules. Hence it is essential to deploy PV modules in the field for extended periods
in order to estimate the degradation, if any, as well as to elucidate the degradation mechanisms. Moreover, PV modules
should be tested by specially designed tests in harsh climates. At Florida Solar Energy Center (FSEC) high-voltage bias
testing of PV modules was carried out in hot and humid climate with the individual modules biased at +/- 600 V. It was
observed that the leakage currents flowing from the PV circuit to the ground is directly proportional to the bias voltage.
PV systems with maximum voltage of 1000 V are installed in Europe and elsewhere which means higher leakage
currents will be produced in the PV modules. Based on this fact and the earlier observations, high voltage bias testing of
c-Si PV modules specially designed for high voltage operation was carried out in hot and humid climate with the
individual modules biased at +/-1500 V at FSEC and higher. This paper provides results of high voltage bias testing of
PV modules. The results indicate that the test can be considered as reliable metric in determination of the long term
performance of PV modules.
Current accelerated qualification tests of photovoltaic (PV) modules mostly assist in avoiding infant mortality but can
neither duplicate changes occurring in the field nor can predict useful lifetime. Therefore, outdoor monitoring of fielddeployed
thin-film PV modules was undertaken at FSEC with goals of assessing their performance in hot and humid
climate under high system voltage operation and to correlate the PV performance with the meteorological parameters.
Significant and comparable degradation rate of -5.13% and -4.5% per year was found by PV USA type regression
analysis for the positive and negative strings respectively of 40W glass-to-glass CIGS thin-film PV modules in the hot
and humid climate of Florida. With the current-voltage measurements it was found that the performance degradation
within the PV array was mainly due to a few (8-12%) modules having a substantially high degradation. The remaining
modules within the array continued to show reasonable performance (>96% of the rated power after ~ 4years).
Thinner CuIn<sub>1-x</sub>Ga<sub>x</sub>S<sub>2</sub> (CIGS2) solar cells are being prepared with an aim to reduce the consumption of indium and
gallium. Post-sulfurization annealing is being used to enhance the grain size in order to overcome the problem of very
small grains that tend to form in thinner films that are not desirable for device quality solar cells. Based on the fact that
gallium gradient that is typically found in CIGS and CIGS2 solar cells has beneficial effect on preventing back contact
recombination of minority carriers, an attempt to determine the optimum regime for post-sulfurization annealing is made
to derive the benefits from larger grains and gallium gradient. An initial set of experiments carried out at PV materials
laboratory at FSEC has shown encouraging results with cell efficiencies of 9-10% for thinner (1.2-1.6 μm) films.
At present the failure modes and mechanism of PV modules are not well understood. The current accelerated tests
cannot duplicate the various field failures. It is very important to continue to carry out accelerated testing of PV modules
in order to reduce the infant mortality of new technology PV modules as well as to improve the production techniques of
the PV modules. However, the accelerate tests need to be complemented with actual field deployment of PV modules
and specifically designed tests in real world conditions or preferably in harsh climates. In this work the inclusion of
outdoor monitoring of PV modules and high voltage bias testing of PV modules in real world climatic conditions in the
current best practices for PV module reliability testing is being proposed. One of the objectives of this paper is to show
the importance of carrying out continuous monitoring of field deployed PV modules as well as high voltage bias testing
of PV modules over an extended period of time.
Proc. SPIE. 7412, Reliability of Photovoltaic Cells, Modules, Components, and Systems II
KEYWORDS: Thin films, Solar energy, Solar cells, Reliability, Scanning electron microscopy, Thin film solar cells, Silicon solar cells, Molybdenum, Copper indium gallium selenide, Mechanical efficiency
One of the important issues involved while taking a process from lab environment to pilot plant scale is the yield of the
process. Mechanical scribing that is used for making integral interconnects in CIGSeS thin film solar cells can be used to
test the mechanical properties of the absorber film. Hence, it is necessary that the process delivers cohesive absorber
films that exhibit good adhesion to molybdenum back contact and high efficiencies that are amenable to mechanical
scribing. Optical and scanning electron microscopy can be used to study the effect of scribing on the absorber film and
the morphology of the scribe lines.
Current accelerated tests of photovoltaic (PV) modules mostly prevent infant mortality but cannot duplicate changes
occurring in the field nor can predict useful lifetime. Therefore, monitoring of field-deployed PV modules was
undertaken at FSEC with goals to assess their performance in hot and humid climate under high voltage operation and to
correlate the PV performance with the meteorological parameters. This paper presents performance analysis of U.S.
Company manufactured thin film a-Si:H PV modules that are encapsulated using flexible front sheets and framed to be
outdoor tested. Statistical data analysis of PV parameters along with meteorological parameters, monitored continuously,
is carried out on regular basis with PVUSA type regression analysis. Current-voltage (I-V) characteristic of module
arrays that are obtained periodically complement the continuous data monitoring. Moreover, effect of high voltage bias
and ambient parameters on leakage current in PV modules on individual modules is studied. Any degradation occurring
during initial 18 months could not be assessed due to data acquisition and hurricane problems. No significant
degradation was observed in the performance of PV modules during the subsequent 30-months. It is planned to continue
this study for a prolonged period so as to serve as basis for their long-term warranties.
Outdoor monitoring of glass to glass a-Si:H thin film photovoltaic (PV) modules by a US manufacturer is carried out in
the hot and humid climate of Florida. The modules are monitored under fixed load conditions. Moreover, periodic
current-voltage (I-V) characteristics are carried out to validate the performance of PV modules. The data was normalized
at standard conditions and compared over a three year period. Two of the modules are biased to ±600 V to study the
leakage current. An extensive study of this nature needs to be carried out for prolonged period to accurately predict the
useful lifetime of PV modules.