The performance and degradation rate of photovoltaic (PV) modules primarily depend on the technology type, module design and field operating conditions. The metastability is a known phenomenon in the CIGS (copper indium gallium diselenide) module technology and it depends on the light exposure and operating temperature. This work aims to understand the metastability influence on the performance of CIGS modules exposed outdoor at three different operating temperatures at a fixed insolation over three years. Two types of CIGS modules from two different manufacturers have been investigated in this study. The three different temperatures were achieved by placing three CIGS modules per manufacturer at three different airgaps on a south facing mock rooftop tilted at 20°. The airgaps were 3”, 1.5” and 0”, and the 0” airgap module was thermally insulated to obtain a higher operating temperature. Throughout the test period over three years, all the modules were maintained at maximum power point using a setup containing optimizers and power resistors. The performance characterizations were carried out before and after exposure using both outdoor natural sunlight and indoor solar simulator. The influence of superstrate type and installation height on the soiling loss have also been investigated.
NREL CIGS devices with up to 20% efficiency are prepared using a three-stage process for the CIGS layer with the last
step of an intrinsic ZnO and conductive ZnO:Al bilayer. This work outlines the efficiency and performance parameters
for these CIGs devices when this bilayer is replaced with indium zinc oxide (a-InZnO), an amorphous metal oxide. It is
well known that metal oxides can serve a variety of important functions in thin film photovoltaics such as transparent
electrical contacts (TCO's), antireflection coatings and chemical barriers. In the case of a-InZnO, we have reported on
the determination of the relative roles of metals and oxygen stoichiometries on the opto-electronic properties of a-InZnO
thin films as well as the stability of those films in damp heat. Since InZO has a tunable conductivity based on the
amount of oxygen introduced during deposition, it can be used as both the intrinsic and TCO layers. We were able to
establish preliminary metrics for an all InZnO bilayer whose performance was comparable to a common CIGs device.
We observed a large variation in the damp heat (DH) durability of many of the unencapsulated CIGS devices fabricated
at NREL. Some devices failed within the first few hours of DH exposure; others failed within a few hundred hours while
some lasted for 1000 h. The initial degradation often showed a 50% decrease in efficiency in the first few hundred hours;
The premature device failures often correspond to the degradation of the ZnO window layer, the peeling of molybdenum
(Mo) from the soda-lime glass (SLG), or both. Repeated J-V measurements lead to significant damage of the contact
pads, which provide additional path for moisture ingress. To better understand the onset of degradation and the cause of
initial decrease in performance and to minimize the damage caused to the contact pads, we designed an encapsulation
scheme to control the moisture ingress by laminating the CIGS devices with a combination of different backsheets
having different water vapor transmission rates. The encapsulation provided external contacts which solved the damage
caused to the pads. This approach facilitates a way to slow down DH-induced degradation of the CIGS device for a more
detailed study.
This paper presents our recent observations on variations in properties and damp heat (DH)-induced degradation
behavior for single-layer 2%Al-doped ZnO (AZO) and bilayer ZnO (BZO), which comprises 0.1-μm intrinsic ZnO (i-ZnO) and AZO, deposited on glass substrates using the same sputtering system and essentially identical deposition
conditions. BZO films with 0.12-μm AZO have been used on the National Renewable Energy Laboratory's (NREL's)
high-efficiency CuInGaSe2 (CIGS) solar cells for years. For the as-deposited BZO films, the most apparent variations
appeared in notable peak shift in transmittance and reflectance spectra and ZnO (002) peak intensity and peak position in
X-ray diffraction. Location of substrates placed on the substrate holder platform contributed partly to the variations. For
the DH-degraded AZO and BZO, earlier films became highly resistive, porous, and 10~20 X thicker and showed
flattened transmittance spectra caused by a loss of free-carrier absorption. However, recent DH-exposed AZO and BZO
films also became highly resistive but exhibited only small changes in transmittance spectra, while the columnar grain
structure and film thickness remained nearly unchanged without porous features, but with granular particles formed on
the surfaces that increased in size with lengthening DH exposure time.
Long-term performance reliability is essential for any photovoltaic module to become established in the PV market. Reliability is characterized based on many factors, one of the most important being the capability of the module to be resistant to moisture at elevated temperatures. This work continues our efforts to search for a high-performance and high-stability transparent conducting oxide (TCO) window layer for CuInGaSe2 (CIGS) devices. In this experimental study, we compared the optical, electrical, and structural stability of various TCOs deposited on glass, including single-layer Al-doped ZnO (AZO), bilayer intrinsic-/Al-doped ZnO (BZO), B-doped ZnO (ZnO:B), amorphous In2O3:SnO2 (ITO), and amorphous In2O3:ZnO (IZO). The samples were exposed to damp heat (DH) at 85°C and 85% relative humidity (RH) and were characterized periodically. The results showed that all ZnO-based TCOs are more sensitive to moisture with substantial electrical degradation and apparent optical changes than the ITO and IZO. The amorphous IZO showed peculiar behavior in electrical property, and exhibited structural change with the appearance of some finite crystallinity after DH >220 h. The results from this experimental series will assist in determining the best-performing TCO for CIGS solar cells.
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