The significant features of a series of stabilization experiments conducted at the National Renewable Energy Laboratory (NREL) between May 2009 and the present are reported. These experiments evaluated a procedure to stabilize the measured performance of thin-film polycrystalline cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) thin-film photovoltaic (PV) modules. The current-voltage (I-V) characteristics of CdTe and CIGS thin-film PV devices and modules exhibit transitory changes in electrical performance after thermal exposure in the dark and/or bias and light exposures. We present the results of our case studies of module performance versus exposure: light soaked at 65°C; exposed in the dark under forward bias at 65°C; and, finally, longer-term outdoor exposure. We find that stabilization can be achieved to varying degrees using either light-soaking or dark-bias methods and that the existing IEC 61646 light-soaking interval may be appropriate for CdTe and CIGS modules with one caveat: it is likely that at least three exposure intervals are required for stabilization.
KEYWORDS: Copper indium gallium selenide, Thin films, Photovoltaics, Thin film solar cells, Solar cells, Thin film devices, Copper, Renewable energy, Solar energy, Cadmium
The significant features of a series of stabilization experiments conducted at the National Renewable Energy Laboratory
(NREL) between May 2009 and the present are reported. These experiments evaluated a procedure to stabilize the
measured performance of thin-film polycrystalline cadmium telluride (CdTe) and copper indium gallium diselenide
(CIGS) thin-film photovoltaic (PV) modules. The current-voltage (I-V) characteristics of CdTe and CIGS thin-film PV
devices and modules exhibit transitory changes in electrical performance after thermal exposure in the dark and/or bias
and light exposures. We present the results of our case studies of module performance versus exposure: light-soaked at
65°C; exposed in the dark under forward bias at 65°C; and, finally, longer-term outdoor exposure. We find that
stabilization can be achieved to varying degrees using either light-soaking or dark bias methods and that the existing IEC
61646 light-soaking interval may be appropriate for CdTe and CIGS modules with one caveat: it is likely that at least
three exposure intervals are required for stabilization.
The degradation in performance for eight photovoltaic (PV) modules stressed at high voltage (HV) is presented. Four
types of modulestandem-junction and triple-junction amorphous thin-film silicon, plus crystalline and polycrystalline
silicon moduleswere tested, with a pair of each biased at opposite polarities. They were deployed outdoors between
2001 and 2009 with their respective HV leakage currents through the module encapsulation continuously monitored with
a data acquisition system, along with air temperature and relative humidity. For the first 5 years, all modules were biased
continuously at fixed 600 VDC, day and night. In the last 2 years, the modules were step-bias stressed cyclically up and
down in voltage between 10 and 600 VDC, in steps of tens to hundreds of volts. This allowed characterization of leakage
current versus voltage under a large range of temperature and moisture conditions, facilitating determination of leakage
paths. An analysis of the degradation is presented, along with integrated leakage charge. In HV operation: the bulk
silicon modules degraded either insignificantly or at rates of 0.1%/yr higher than modules not biased at HV; for the thinfilm
silicon modules, the added loss rates are insignificant for one type, or 0.2%/yr-0.6%/yr larger for the other type.
Transient or hysteresis effects in polycrystalline thin film CdS/CdTe cells are a function of pre-measurement
voltage bias and whether Cu is introduced as an intentional dopant during back contact fabrication. When Cu is added,
the current-density (J) vs. voltage (V) measurements performed in a reverse-to-forward voltage direction will yield
higher open-circuit voltage (Voc), up to 10 mV, and smaller short-circuit current density (Jsc), by up to 2 mA/cm2,
relative to scanning voltage in a forward-to-reverse direction. The variation at the maximum power point, Pmax, is
however small. The resulting variation in FF can be as large as 3%. When Cu is not added, hysteresis in both Voc and
Jsc is negligible however Pmax hysteresis is considerably greater. This behavior corroborates observed changes in
depletion width, Wd, derived from capacitance (C) vs voltage (V) scans. Measured values of Wd are always smaller in
reverse-to-forward voltage scans, and conversely, larger in the forward-to-reverse voltage direction. Transient ion drift
(TID) measurements performed on Cu-containing cells do not show ionic behavior suggesting that capacitance transients
are more likely due to electronic capture-emission processes. J-V curve simulation using Pspice shows that increased
transient capacitance during light-soak stress at 100 °C correlates with increased space-charge recombination. Voltagedependent
collection however was not observed to increase with stress in these cells.
CdS/CdTe photovoltaic solar cells were made on two different transparent conducting oxide (TCO) structures in order to
identify differences in fabrication, performance, and reliability. In one set of cells, chemical vapor deposition (CVD)
was used to deposit a bi-layer TCO on Corning 7059 borosilicate glass consisting of a F-doped, conductive tin-oxide
(cSnO2) layer capped by an insulating (undoped), buffer (iSnO2) layer. In the other set, a more advanced bi-layer
structure consisting of sputtered cadmium stannate (Cd2SnO4; CTO) as the conducting layer and zinc stannate (Zn2SnO4;
ZTO) as the buffer layer was used. CTO/ZTO substrates yielded higher performance devices however performance
uniformity was worse due to possible strain effects associated with TCO layer fabrication. Cells using the SnO2-based
structure were only slightly lower in performance, but exhibited considerably greater performance uniformity. When
subjected to accelerated lifetime testing (ALT) at 85 - 100 °C under 1-sun illumination and open-circuit bias, more
degradation was observed in CdTe cells deposited on the CTO/ZTO substrates. Considerable C-V hysteresis, defined as
the depletion width difference between reverse and forward direction scans, was observed in all Cu-doped CdTe cells.
These same effects can also be observed in thin-film modules. Hysteresis was observed to increase with increasing
stress and degradation. The mechanism for hysteresis is discussed in terms of both an ionic-drift model and one
involving majority carrier emission in the space-charge region (SCR). The increased generation of hysteresis observed
in CdTe cells deposited on CTO/ZTO substrates suggests potential decomposition of these latter oxides when subjected
to stress testing.
Polycrystalline photovoltaic (PV) modules containing cadmium telluride (CdTe) or copper indium gallium diselenide
(CIGS) thin film materials can exhibit substantial transient or metastable current-voltage (I-V) characteristics depending
on prior exposure history. Transient I-V phenomena confound the accurate determination of module performance, their
reliability, and their measured temperature coefficients, which can introduce error in energy ratings models or servicelifetime
predictions. Indeed, for either of these two technologies, a unique performance metric may be illusory without
first specifying recent exposure or stateeven at standard test conditions. The current standard preconditioning
procedure for thin-film PV modules was designed for amorphous silicon (a-Si), and is likely inadequate for CdTe and
CIGS. For a-Si, the Staebler-Wronski effect is known to result from defects, created via breaking of weak silicon bonds
or light-activated trapping at the device junction, occurring rapidly upon light-exposure. For CdTe and CIGS devices,
there is less agreement on the causes of metastable behavior. The data suggests that either deep-trapping of charge
carriers, or the migration and/or electronic activation of copper may be responsible. Because these are quite disparate
mechanisms, we suspect that there may be a more practical preconditioning procedure that can be employed prior to
accurate performance testing for CdTe and CIGS modules. We devise a test plan to examine and compare the effects of
light soaking versus forward-biased dark exposure at elevated temperatures, as parallel strategies to determine a feasible
standard protocol for preconditioning and stabilizing these polycrystalline PV technologies, and report on the results of
our tests.
The long-term performance data of copper indium diselenide (CIS) and gallium-alloyed CIS (CIGS) photovoltaic (PV)
modules are investigated to assess the reliability of this technology. We study and report on numerous PV modules
acquired from two manufacturers (A and B), deployed at NREL's outdoor test facility (OTF) in various configurations in
the field: some are free standing, loaded with a fixed resistance and periodically tested indoors at STC; other modules are
connected to data acquisition systems with their performance continuously monitored. Performance is characterized
using current-voltage (I-V) measurements obtained either at standard test conditions or under real-time monitoring
conditions: the power parameters plus other factors relating to quality like diode quality factors or series resistance are
analyzed for changes against time. Using standard diode analysis to determine the sources of degradation indicates that
CIS modules can exhibit between moderate and negligible degradation, with the dominant loss mode being fill factor
declines along with decreases in open-circuit voltage, for illumination intensities near 1-sun. At lower intensities, current
losses can appear appreciable. The real-time performance data also indicate that fill factor loss is the primary degradation
mode, generally as a result of increases in series resistance.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.