Tadanori Tanahashi

The electrical insulation resistance of photovoltaic (PV) modules plays an important role in maintaining the operational integrity of PV systems deployed in fields. This is because the breakdown of the electrical insulation of PV modules directly contributes to performance loss and electrical safety risks. To gain insight into the characteristics of the electrical leakage circuit between the PV cells and the metal frame of the PV modules as a substantial reflection of the behavior of the electrical insulation resistance in the PV modules, an analysis technique using ac impedance spectroscopy was applied to PV modules installed in a field. We identify the existence of an ionic diffusion element (Warburg impedance) in the circuit in addition to the elements consisting of the conventional parallel resistor–capacitor model including a series resistor. Thus, an ac equivalent circuit model, including these elements, was established for the electrical leakage circuit of the PV module. Given the distinct dependencies of these elements on weather conditions, we propose a direct coupling of the resistive elements to the surface condition of the PV module. However, the Warburg impedance and capacitance are also linked to the conditions of the internal components within the PV module. Furthermore, by verifying the interrelationships among the ac impedance parameters in the ac equivalent circuit model, we can predict these values accurately in both dry and wet weather conditions. These findings would offer valuable perspectives for addressing concerns related to the decreased performance and potential safety risks of PV modules installed in various environments.

Various cell crack modes (with or without electrically inactive cell areas) can be induced in crystalline silicon photovoltaic (PV) cells within a PV module through natural thermomechanical stressors such as strong winds, heavy snow, and large hailstones. Although degradation in the performance of PV modules by cell cracks has been reported occasionally, the mode-dependent evolutions in the electrical signatures of cracks have not yet been elucidated. In this study, we propose that the reduction of the time constant in the AC impedance spectra, which is caused by the elevation of minority-carrier recombination in the p–n junction of a PV cell, is a ubiquitous signature of cracked PV cells encapsulated in a commercially available PV module. Several other characteristics derived from the illuminated current-voltage (I–V) and dark I–V data significantly evolved only in PV cells with inactive cell areas. We also propose that the evaluation by carrier recombination is a crucial diagnostic technique for detecting all crack modes, including microcracks, in wafer-based PV modules.

To identify the common degradation mechanism(s) between field-aged photovoltaic (PV) modules and PV cells/modules exposed to artificial corrosive stress, the ac impedance associated with the corrosion in the front electrodes was analyzed in the individual PV cells laminated within the PV modules, fielded for 21 years. By evaluating the ac impedance together with the PV performance for these PV cells, we found that the short-circuit current (I _sc ) remarkably decreased, depending on the expression and elevation of the ac impedance relevant to the front-electrode corrosion. This I _sc -reduction can be explained by the formation of rectifying contacts to restrict the current conduction between the front electrodes and the associated silicon wafer. Since the I _sc -reduction was primarily observed in the PV cells with extensive loss of fill factor, we suggest that the power-loss is induced by the gap formation underneath the front electrodes, followed by the emergence of rectifying contacts, even in the PV cells/modules degraded in field. The ac impedance parameters of the PV cells/modules could be a valuable predictor of the service lifetime of PV modules, as our findings in this study mean the equivalency of the failure attained by extended damp-heat testing with that observed in the field.

To analyze the dynamic behavior of Na ions in the p–n junction of crystalline Si photovoltaic (PV) cells during potential-induced degradation (PID), a bias voltage was applied between the electrodes of the testing PV modules. During the PID test with a negative high voltage stress (HVS), PID was suppressed by the simultaneous loading of a forward bias voltage, although that of a reverse bias voltage accelerated the progression of PID. However, in the recovery stage after the PID test, the opposite loading effects of the bias voltage with a positive HVS were found. In other words, the extent of recovery from PID was suppressed or enhanced by forward or reverse bias loading, respectively. Similar effects of bias loading were observed even under the recovery conditions without the positive HVS. The effect of Na ions within a depletion layer on the evolution of PID will be discussed for the interpretation of these results.

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.

The failures induced by thermomechanical fatigue within crystalline silicon photovoltaic modules are a common issue that can occur in any climate. In order to understand these failures, we confirmed the effects of compressive or tensile stresses (which were cyclically loaded on photovoltaic cells and cell interconnect ribbons) at subzero, moderate, and high temperatures. We found that cell cracks were induced predominantly at low temperatures, irrespective of the compression or tension applied to the cells, although the orientation of cell cracks was dependent on the stress applied. The fracture of cell interconnect ribbons was caused by cyclical compressive stress at moderate and high temperatures, and this failure was promoted by the elevation of temperature. On the basis of these results, the causes of these failures are comprehensively discussed in relation to the viscoelasticity of the encapsulant.

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.

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.

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.