Search

Combined-accelerated stress testing (C-AST) is developed to establish the durability of photovoltaic (PV) products, including for degradation modes that are not a priori known or examined in standardized tests. C-AST aims to comprehensively represent the sample, stress factors, and their combinations using levels at the statistical tails of the natural environment. Acceleration factors for relevant climate sequences within the C-AST cycle with respect to the Florida USA climate are estimated for selected degradation mechanisms. It is found that for degradation of the outer backsheet polymer layer, the acceleration factor of the tropical climate sequence (the longest of the climate sequences) is f (T, G) = 17.3 with ultraviolet photodegradation; for polyethylene terephthalate hydrolysis (backsheets), f (T, RH) = 426; for electrochemical corrosion (PV cell), f (I) = 14.1; and for PbSn solder fatigue f (ΔT, r (T)) = 17.3. Here, T is the module temperature, G is the broadband spectrum irradiance on the plane of array of the module, RH is the relative humidity on the module surface, I is the leakage current through the module packaging, and r(T), the number of temperature reversals. The methods discussed herein are generally applicable for evaluating acceleration factors in other accelerated test methods.

Previously undiscovered failure modes in photovoltaic (PV) modules continue to emerge in field installations despite passing protocols for design qualification and quality assurance. Failure to detect these modes prior to widespread use could be attributed to the limitations of present-day standard accelerated stress tests (ASTs), which are primarily designed to identify known degradation or failure modes at the time of development by applying simultaneous or sequential stress factors (usually two at most). Here, we introduce an accelerated testing method known as the combined-accelerated stress test (C-AST), which simultaneously combines multiple stress factors of the natural environment. Simultaneous combination of multiple stress factors allows for improved identification of failure modes with better ability to detect modes not known a priori. A demonstration experiment was conducted that reproduced the field-observed cracking of polyamide- (PA-) and polyvinylidene fluoride (PVDF)–based backsheet films, a failure mode that was not detected by current design qualification and quality assurance testing requirements. In this work, a two-phase testing protocol was implemented. The first cycle (“Tropical”) is a predominantly high-humidity and high-temperature test designed to replicate harsh tropical climates. The second cycle (“Multi-season”) was designed to replicate drier and more temperate conditions found in continental or desert climates. Testing was conducted on 2 × 2-cell crystalline-silicon cell miniature modules constructed with both ultraviolet (UV)–transmitting and UV-blocking encapsulants. Cracking failures were observed within a cumulative 120 days of the Tropical condition for one of the PA-based backsheets and after 84 days of Tropical cycle followed by 42 days of the Multi-season cycle for the PVDF-based backsheet, which are both consistent with failures seen in fielded modules. In addition to backsheet cracking, degradation modes were observed including solder/interconnect fatigue, various light-induced degradation modes, backsheet delamination, discoloration, corrosion, and cell cracking. The ability to simultaneously apply multiple stress factors may allow many of the test sequences within the standardized design qualification procedure to be performed using a single test setup.

To examine the mechanisms of degradation owing to liberated organic acid from polymer materials in crystalline silicon (c-Si) photovoltaic (PV) modules, we attempt to characterize the degraded site in a PV cell exposed to acetic acid vapor by means of ac impedance spectroscopy. The location was electrically determined at the interface between the front electrodes and emitter of a silicon wafer. Concerning this interface, two distinct electric characteristics were identified as follows: 1) the contact in this interface can rectify the current in the same direction as the p–n junction of a p-type PV cell, and 2) any linear relationship in the Mott–Schottky plot was not confirmed in the capacitance component placed in this interface, unlike in the case of that located in the p–n junction. From these characteristics, the inclusion of a layer with a voltage-independent capacitance in this interface, an inhomogeneous depth profile of impurities within the near-surface of the n+-Si layer of this contact, and/or the existence of peculiar surface/interface states in this interface were deduced. It is concluded that a crucial electrical feature involved in the degradation of c-Si PV cells/modules under acidic conditions is verified with regard to these characteristics.

To assess the contribution of the thermomechanical stress on the long-term reliability of photovoltaic (PV) modules, we applied extended thermal cycling tests up to 600 cycles to 13 models of commercially available PV modules. The extensive testing, using 5–10 PV modules for each model, revealed that the levels of power loss, induced by thermal fatigue during this extended testing, differed in each model of the PV module. Degradations by solder bond failure and bypass diode failure, which were observed in a few models of PV modules, were most likely to result from incorrect soldering that may be completely avoidable through appropriate implementation of a quality management system. Therefore, we suggest that, in most of the latest PV modules, their durability to thermal fatigue was established, as evidenced by the extended thermal cycling test with 600 cycles. However, solder bond failure due to inappropriate manufacturing process quality, which is not identifiable by the conventional thermal cycling test (200 cycles), would be detected by this extended test in a few models. Based on these suggestions, we discuss the evaluation procedures of these failures caused by the quality-control and design issues of PV modules.

The “Framing” (local discoloration along cell edges) was induced by a simple sequential accelerated stress test (consisting of hygrothermal- and UV-stressors) applied to the PV modules with high OTR (oxygen transmission rate) backsheet, irrespective of the inclusion of UV-absorber in poly(ethylene-co-vinyl acetate) (EVA) encapsulant. UV-fluorescence imaging of the PV modules suggests that the spatially-inhomogeneous degradation of EVA material under UV-irradiating conditions is correlated to this “Framing” indicating an underlying common mechanism. These findings would contribute to the development of test procedures to broadly mimic the actual failures observed in fielded PV.

The mechanism of current-collection failure resulting from corrosion of front electrodes is investigated by AC impedance measurement of PV cells and modules degraded by acetic acid. In addition to our previous findings which first incidence of power-loss (with fill factor reduction) is caused by the gap-formation underneath front electrodes, we suggest in this study that the following power-loss with reduction of short-circuit current $(I_{\text{sc}})$ is due to the formation of rectifying contact between silver bulk in front electrodes and emitter in silicon wafer. These suggestions assist further understanding of degradation observed in PV modules deployed in fields.

For the current collection failure induced by the acidic corrosion in finger electrodes on photovoltaic (PV) cells, the formation and evolution of gaps underneath the electrodes were able to be found by the monitoring of AC impedance signals and I-V characteristics of PV cells. The similar degradation behavior was also observed in PV modules under damp heat test. Then, the degradation mechanisms of crystalline silicon PV module under hygrothermal conditions are discussed, to contribute the lifetime prediction of PV modules installed in fields.

Origin of degradation in output power for crystalline Si photovoltaic modules by hygrothermal stresses was studied. The direct evidence that pn junction is hardly damaged and the origin is damage in Ag finger electrodes was obtained through the experiments composed of removal of degraded finger electrodes and reconstruction of electrodes. It was suggested that electrode is final defense for reliability of photovoltaic modules under hygrothermal stresses. The most important issue for highly reliable photovoltaic modules may be the development of electrode materials with toughness against acetic acid.

Although fibroblast growth factor 1 (FGF-1) (formerly known as acidic FGF) but not FGF-2 (or basic FGF), has been suggested to play a pathophysiological role in liver regeneration, its clinical application has been restricted by its limited mitogenecity and heparin dependence. We report here that a chimeric human FGF protein, FGF-C(1211), is a heparin-independent potent mitogen for liver parenchymal cells both in vitro and in vivo. In the presence of serum and physiological concentration of insulin, FGF-C(1211) by itself induces as much as 55% of hepatocytes in culture to proliferate and up to 88% when used in combination with hepatocyte growth factor or epidermal growth factor. Furthermore, hepatocytes nuclear labeling in vivo with bromodeoxyuridine was markedly enhanced when FGF-C(1211) was injected intravenously into carbon tetrachloride-administered mice. Because FGF-C(1211) is heparin independent and nonantigenic, it has potential for clinical and preclinical applications.

We examined the effects of hyper-hygrothermal stresses with or without air on the degradation of crystalline silicon (c-Si) photovoltaic (PV) modules, to shorten the required duration of a conventional hygrothermal-stress test [i.e., the “damp heat (DH) stress test”, which is conducted at 85 °C/85% relative humidity for 1,000 h]. Interestingly, the encapsulant within a PV module becomes discolored under the air-included hygrothermal conditions achieved using DH stress test equipment and an air-included highly accelerated stress test (air-HAST) apparatus, but not under the air-excluded hygrothermal conditions realized using a highly accelerated stress test (HAST) machine. In contrast, the reduction in the output power of the PV module is accelerated irrespective of air inclusion in hyper-hygrothermal test atmosphere. From these findings, we conclude that the required duration of the DH stress test will at least be significantly shortened using air-HAST, but not HAST.

We propose a dew condensation sensor which combines surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) to measure both refractive index change and mass loading caused by dew condensation simultaneously. In order to excite SPR and enhance water vapor sorption, a periodic silver nanostructure is fabricated on an AT-cut quartz crystal oscillator by template deposition. A self-assembled membrane (SAM) which consists of polystyrene spheres with the diameter of 202 nm was used as the template, and silver thin film with the thickness of 45 nm was deposited on the SAM by vacuum evaporation. Sensitivities of the sensor for detection of dew condensation were evaluated as the shifts of the SPR extinction peak wavelength and the resonant frequency of quartz crystal. The sensor is cooled down with the chilling rate of -0.5°C/min in the environment-controlled chamber with relative humidity and the temperature of 43.2%RH and 25.0°C, respectively. The proposed hybrid sensor was able to measure both the wavelength shifts of SPR and the additional mass caused by dew condensation simultaneously. Furthermore, the QCM response of the sensor achieved the sensitivity higher than the under detection limit (3 μg/cm2) of conventional optical detection method such as chilled mirror surface dew point hygrometer.

We examined the sequential effects of salt-mist stress followed by high-system-voltage stress on the power loss of crystalline silicon photovoltaic (PV) modules to determine whether a crucial failure as potential-induced degradation (PID) is accelerated by material-property changes caused by the long-term effects of a less harmful stress such as salt-mist spraying. Degradation profiles confirmed in this study show that PID is accelerated by certain types of salt-mist preconditioning. For the acceleration of PID, the contribution of sodium ions liberated from the front glass of the PV module seems to be excluded. Therefore, we consider that the sodium ions penetrating into the PV modules from the ambient environment may also cause degradation according to the proposed mechanisms of PID, as the sodium ions existing in the front glass cause PID. Furthermore, this type of degradation may indicate the wear-out phenomenon after a long-term exposure in the field (especially near the coast).

We propose a micro-dew condensation sensor equipped with a quartz tuning fork (QTF) coated with a polystyrene sphere self-assembled monolayer (PSSAM) for dew condensation prevention. Polystyrene spheres with diameters of 202 and 356 nm were used. A micro-dew condensation sensing experiment was performed at 25°C and 45% relative humidity. The inflection point of the frequency shift curve for the QTF sensor coated with a PSSAM shifted to a higher temperature than that of a bare QTF. This suggests that the modified sensor can serve as a sensor used to prevent dew condensation.

We reported previously that the potency of heparin-binding fibroblast growth factor-1 (FGF-1) as a mitogen for rat hepatocytes in primary culture is as high as that of epidermal growth factor (EGF) and hepatocyte growth factor. To gain insight into the pathophysiological significance of FGF-1 in hepatocyte growth, we analyzed the cooperative mitogenicity of FGF-1 and EGF. Results from a nuclear labeling assay using [3H]thymidine suggest that most hepatocytes in primary culture consist of two cell populations that differ in response to FGF-1; one is an FGF-1-responsive cell population, and the other is an EGF-responsive (but not FGF-1-responsive) cell population. On the other hand, autoradiographic analysis of 125I-FGF-1 binding demonstrated that high affinity FGF receptors were homogeneously distributed on the surface of all hepatocytes. Cross-linking 125I-FGF-1 to the nonstimulated hepatocyte surface indicated that the high affinity FGF receptors comprise two FGF receptors that differ in molecular mass (128 and 79 kDa). Furthermore, the 79-kDa receptor was preferentially down-regulated when the hepatocytes were stimulated with EGF or hepatocyte growth factor. These data suggest that the abundant expression of the 79-kDa FGF receptor on some populations of hepatocytes is involved in their lack of response to FGF-1. The 128- and 79-kDa FGF receptors were assigned as FGFR2 using an antibody specific to the ectodomain of FGFR2, whereas the 79-kDa receptor was not reactive to the antibody against the carboxyl terminus of FGFR2. This 79-kDa FGF receptor was not tyrosine-phosphorylated in response to FGF-1 stimulation, while the 128-kDa FGF receptor was recognized by anti-phosphotyrosine antibody under the same conditions. Also, the heterodimer of 79- and 128-kDa FGF receptors was less tyrosine-phosphorylated than the homodimer of 128-kDa FGF receptors. These data suggest that the 79-kDa FGF receptor inhibits the function of the 128-kDa FGF receptor through their heterodimerization. Thus, we surmise that the difference in response to FGF-1 between the cell populations of normal rat hepatocytes was caused by the different levels of the 79-kDa FGF receptor in each cell population.

We have found that in the primary culture of rat hepatocytes, the potent mitogenic activity of native FGF-1 is independent of heparin. The well-established characteristic of FGF-1 as a heparin-dependent mitogen is confirmed by human umbilical vein endothelial cells using the same preparation of FGF-1. Cross-linking experiments reveal that binding of FGF-1 to the hepatocyte cell surface receptors can be accomplished in the absence of exogenous heparin, in contrast to human endothelial cells for which it remains as a limiting factor. For both cell types, however, it is demonstrated that either endogenous or exogenous heparan sulfate/heparin moieties are essential for FGF-1 to establish receptor binding and mitogen action. Thus, the results suggest that hepatocytes harbor cell surface heparan sulfate moieties that are fully capable of utilizing FGF-1 in the environment. These results raise the possibility that FGF-1 is of differential potency for different cell types according to the nature and/or quantity of cell surface heparan sulfate moieties in vivo.

To accelerate the degradation with thermal fatigue in crystalline silicon photovoltaic modules, the modules were exposed to dry thermal stress with rapid thermal cycling, and module impedance was monitored in situ during this testing. The spikelike increase in module impedance at a temperature-alteration point was observed in the early stage of this rapid thermal cycling. The pattern of increase in module impedance proceeded step-by-step, from the early stage, via the double-spikelike pattern at two temperature-alteration points (the middle stage), and finally to the successive increases in module impedance in the high-temperature period (the late stage). The nondestructive analyses suggest that the interconnector failures without the defects of photovoltaic cells occurred. From these results, it is suggested that the pattern of increase in module impedance is related to the interconnection degradation of modules, and that the rapid thermal cycling with in situ monitoring of module impedance would be a useful procedure for the earlier detection of interconnection failures in photovoltaic modules.

To address the origin of the elevated series-resistance (R s ), which is a primary cause of corrosive degradation observed in field-aged photovoltaic (PV) modules, we evaluated the electrical characteristics of PV cells corroded with acetic acid vapor. The increase in R s was detected at the interface between the front electrodes and the associated silicon wafer, where is identical to the location of a novel impedance emerging by the corrosion. These results suggest that the corrosive degradation is virtually governed by the evolution of electrical characteristics in front surface, but not that in rear surface of PV cells.