Crystalline Si Photovoltaic Modules Research Articles

Delay of the potential-induced degradation of n-type crystalline silicon photovoltaic modules by the prior application of reverse bias

We investigated the influence of the pre-application of reverse bias on the potential-induced degradation (PID) of n-type front-emitter (n-FE) crystalline Si (c-Si) photovoltaic modules. Applying a prior positive reverse bias to n-FE cells delays charge-accumulation-type PID (PID-1), decreases in short-circuit current density (J sc) and open-circuit voltage (V oc). The prior positive bias accumulation may accumulate negative charges in the SiN x , which leads to an increase in a duration for the positive charge accumulation by the PID test applying a negative bias. We also found that sufficiently long prior positive bias application results in the delay of PID-2, a fill factor reduction by the incursion of Na ions into the depletion region of the p–n junction of n-FE cells.

Acceleration of potential-induced degradation in crystalline Si photovoltaic modules after a lightning impulse strike

The potential-induced degradation (PID) is one of the significant issues in realizing low-cost electricity from photovoltaic (PV) power generation plants. In this paper, we have investigated PID in crystalline Si (c-Si) PV modules with conventional p-type multicrystalline Si solar cells after the application of lightning impulse strikes. Lightning impulses with a voltage of −40 kV were applied to the module between the shorted electrodes of the c-Si cell and the mimic aluminum frame. It is confirmed that no degradation in the electrical characteristics of the c-Si cell occurs by applying the impulse only. We have found that the PID of c-Si PV modules was accelerated by applying the impulses between a c-Si cell and a metal frame. The acceleration of PID in the module applied with a lightning impulse might be caused by the migration of Na+ ions easily toward the c-Si cell owing to damage to the ethylene-vinyl acetate encapsulant by impulses.

Potential-induced degradation of n-type front-emitter crystalline silicon photovoltaic modules — Comparison between indoor and outdoor test results

We perform indoor and outdoor potential-induced degradation (PID) tests for n-type front-emitter (n-FE) crystalline Si (c-Si) photovoltaic (PV) modules and compare their results. The indoor/outdoor acceleration factor for the polarization-type PID (PID-p) of n-FE PV modules is found to be ∼1000 at an indoor test temperature of 85 °C. We observe the progression of the PID-p of n-FE PV modules in rainy days, while their performance is recovered in sunny days. This may be because of the formation of a water film on a module surface acting as a conductive film, resulting in a larger electric field near the n-FE cells in rainy days. The recovery of the PV performance of n-FE modules in sunny days can partly explain the large acceleration factor for the indoor PID test against the outdoor test. • We performed indoor and outdoor PID tests for n-FE PV modules. • n-FE PV modules show polarization-type PID (PID-p) both indoors and outdoors. • The acceleration factor for the PID-p is ∼1000 at an indoor test temperature of 85 °C. • We observe the recovery of PID-p for the outdoor modules in sunny days. • The recovery may be one of the reasons for the large acceleration factor.

Influence of light irradiation on the charge-accumulation-type potential-induced degradation of n-type front-emitter crystalline Si photovoltaic modules

We investigated the influence of light irradiation on the charge-accumulation-type potential-induced degradation (PID) of n-type front-emitter (n-FE) crystalline silicon (c-Si) photovoltaic (PV) modules. A PID test under one-sun irradiation leads to faster reductions of short-circuit current–density (J sc) and open-circuit voltage (V oc) compared to the case of a PID test in the dark. This indicates that light irradiation accelerates the charge-accumulation-type PID of the n-FE PV modules. The J sc and V oc reductions become slower under irradiation without ultraviolet (UV) light, showing almost the same time dependence as the PID test in the dark. The acceleration of PID by the addition of UV light may be explained by the excitation of electrons at K0 centers in silicon nitride (SiN x ) and their faster drift to the surface by the electric field applied to SiN x .

Influence of light illumination on the potential-induced degradation of n-type interdigitated back-contact crystalline Si photovoltaic modules

We investigate the potential-induced degradation (PID) of n-type interdigitated back-contact (IBC) crystalline Si (c-Si) photovoltaic (PV) modules under a negative bias stress and the influence of light illumination on the PID. IBC PV modules show PID characterized by a reduction in the short-circuit current density (J sc) and open-circuit voltage (V oc) under negative bias stress, while no fill factor (FF) reduction is observed. The degradation may originate from the introduction of sodium (Na) into c-Si and the resulting enhancement of carrier recombination on the surfaces of the IBC cells. Light illumination of 1 sun during the negative bias PID test results in less severe reductions of J sc and V oc. A reduction in the electric field on the surface Si nitride (SiN x ) film, due to carrier generation in the SiN x and the resulting increase in its conductivity, is a possible explanation for the mitigation of the Na-related PID.

Influence of hygrothermal stress on potential-induced degradation for homojunction and heterojunction crystalline Si photovoltaic modules

The influences of both high-voltage stress and hygrothermal stress were studied for homojunction and heterojunction crystalline Si photovoltaic (PV) modules. In order to separately access the influence of these stresses, these PV modules were subjected to sequential tests with hygrothermal stress and high-voltage stress for various stress durations of each test. It was found that for p-type homojunction crystalline Si PV modules hygrothermal stress applied in advance considerably enhances potential-induced degradation (PID) by high-voltage stress. High-voltage stress applied in advance also accelerates finger-electrode degradation by hygrothermal stress. It was also clarified that hygrothermal stress for short duration applied in advance considerably enhances PID by high-voltage stress for n-type heterojunction crystalline Si PV modules. The possible mechanisms for these accelerated degradation phenomena by the combined stresses are presented.

Influence of shadow on shunt-type potential-induced degradation for crystalline Si photovoltaic modules exposed outdoors

Potential-induced degradation (PID) was observed for crystalline Si photovoltaic (PV) modules set at a PID test system constructed outdoors. It was clearly shown that partial shadow and periodical water spray during daytime accelerate PID generation. PID easily occurred just underneath the shielding plate. It was also found that when a large part of one cell in the PV module was shadowed, PID is accelerated; however, when a small part of one cell in the PV module was shadowed or when the weak light is uniformly irradiated, less PID acceleration is observed. Simulation was also carried out for discussing the influence of light irradiation. These findings suggest that UV irradiation during the PID indoor test is essential for the exact estimation of the acceleration factor of the indoor PID test.

Reliability Study of c-Si PV Module Mounted on a Concrete Slab by Thermal Cycling Using Electroluminescence Scanning: Application in Future Solar Roadways

Several tests were conducted to ratify the reliability and durability of the solar photovoltaic (PV) devices before deployment in the real field (non-ideal conditions). In the real field, the temperature of the PV modules was varied during the day and night. Nowadays, people have been bearing in mind the deployment of PV modules on concrete roads to make use of the space accessible on roads. In this regard, a comparative study on the failure and degradation behaviors of crystalline Si PV modules with and without a concrete slab was executed via a thermal cycling stress test. The impact of the concrete slab on the performance degradation of PV modules was evaluated. Electroluminescence (EL) results showed that the defect due to thermal cycling (TC) stress was reduced in the PV module with a concrete slab. The power loss due to the thermal cycling was reduced by approximately 1% using a concrete slab for 200 cycles. The Rsh value was reduced to approximately 91% and 71% after thermal cycling of 200 cycles for reference PV modules, respectively. The value of I0 was increased to approximately 3.1 and 2.9 times the initial value for the PV modules without and with concrete, respectively.

Universal explanation for degradation by charge accumulation in crystalline Si photovoltaic modules with application of high voltage

It was experimentally found that surface recombination due to charge accumulation, called polarization-type potential-induced degradation (PID), occurs by applying high voltage in n-type crystalline Si photovoltaic (PV) modules. By contrast, polarization-type PID has not been observed yet in p-type crystalline Si PV modules. We investigated the effect of differences in anti-reflection coating (ARC) and the conduction type of the substrate used as a base for PV cells on PID. PID was examined for PV modules using p-type and n-type crystalline Si PV cells with a SiNx or SiNx/SiO2 stacked ARC layer. The results indicate that PID owing to charge accumulation occurs even for p-type crystalline Si PV modules by applying high positive voltage. Furthermore, we found that polarization-type PID due to charge accumulation in ARC, leading to surface recombination, is due not to the conduction type of the substrate but to the ARC structure.

Effect of additives in electrode paste of p-type crystalline Si solar cells on potential-induced degradation

Sodium (Na) and lithium (Li) in the silver (Ag) paste cause the potential-induced degradation (PID), while the PID of p-type crystalline silicon (Si) photovoltaic modules is caused by Na in the front cover glass. Some Ag pastes contain these elements to control the firing properties in solar cells fabrication. In order to eliminate the effect of Na and other elements in the front cover glass on PID, PID tests for crystalline Si photovoltaic modules without front cover glass and using cells with various electrode pastes of controlled additive contents were performed. When the Ag paste with Na is used, the shunt resistance decreases and the PID occurs. This phenomenon is similar to that induced by Na in the front cover glass, but, it requires the shorter time duration as compared with the case of Na in the front cover glass. In addition, it is found that Li in the paste also causes the PID equivalent to that by Na in the paste.

Conductive paste assisted interconnection for environmentally benign lead-free ribbons in c-Si PV modules

Crystalline Si (c-Si) photovoltaic (PV) modules with lead-free (LF) ribbons have drawn much attention for harvesting solar light using an environmentally friendly method. However, the high temperature required for processing the LF solder coated ribbons hinders their widespread use because of the increased thermomechanical stress. Hence, in this work, we proposed a versatile interconnection method employing low-temperature processable Sn57.6Bi0.4Ag-incorporated conductive paste (CP). By introducing CP to the PV module, the cells and LF ribbons can be assembled below 150 °C regardless of the solder coating of ribbon, diminishing the thermomechanical stress. Additionally, the introduction of CP makes it possible to connect c-Si PV cells using a pure copper strip without the solder. Analysis of the adhesion strength shows that the CP-assisted interconnection guarantees a strong mechanical bond (>2 N/mm) between the cell and the ribbon without increasing ohmic losses. In addition, the interconnection withstands thermomechanical stress and long-term exposure to humidity and high temperature. Therefore, CP assisted interconnection, as suggested in this work, should facilitate easier and smoother transition to LF ribbons in the PV industry.

Origin of Na causing potential-induced degradation for p-type crystalline Si photovoltaic modules

This paper presented whether Na ion in the front cover glass is absolute root cause of potential-induced degradation (PID) for p-type crystalline Si photovoltaic (PV) modules or not. P-type monocrystalline Si PV modules with and without the front cover glass, and with and without intentional Na contamination were subjected to PID test. Even without the front cover glass, a decrease in shunt resistance which is a characteristic feature of PID for p-type crystalline Si PV modules has been observed. Intentional Na incorporation on the cell or encapsulant also brings about remarkable PID. These results indicated that Na ion in the front cover glass is not a necessary condition for PID. Furthermore, PID occurs regardless of origin of Na ions. Relationship between PID and leakage current will be also discussed.

Potential-induced degradation of n-type crystalline Si photovoltaic modules in practical outdoor systems

Potential-induced degradation (PID) of full-size photovoltaic (PV) modules based on n-type monocrystalline Si solar cells was observed in practical outdoor PV systems in the short term. The maximum power output of the module decreased by about 14% after 12 days of outdoor exposure with application of about −115 V. In contrast, no degradation was observed after +115 V application in the outdoor system. These results indicate that the PID of the n-type Si PV module is easily induced by applying a lower negative potential that does not induce the PID of p-type Si PV modules in outdoor systems. Results suggest that water on the top of the modules during/after a rain, which increases the leakage current between the Al frame and the Si solar cells, is the predominant factor causing the PID of n-type Si PV modules exposed outdoors, rather than temperature and humidity.

Effect of light irradiation during potential-induced degradation tests for p-type crystalline Si photovoltaic modules

Light irradiation during a potential-induced degradation (PID) test for p-type crystalline Si photovoltaic modules was studied under various test conditions. PID progress was clearly delayed by light irradiation. Such effects by light irradiation strongly depended on the wavelength. Clear effects on PID delay were observed in the case of light irradiation with a UV component below approximately 400 nm. On the other hand, almost no effects were observed in the case of light irradiation without a UV component. Delay of PID progress was also observed even under high-humidity condition. It was also found that PID progresses in a partially shadowed region even under UV light irradiation, which means that a partial shadow easily brings about PID for PV modules exposed outdoors.

Lamination-interface-dependent deacetylation of ethylene vinyl acetate encapsulant in crystalline Si photovoltaic modules evaluated by positron annihilation lifetime spectroscopy

The generation of acetic acid from the ethylene vinyl acetate (EVA) encapsulant in crystalline Si photovoltaic (PV) modules was investigated by free-volume analysis with positron annihilation lifetime spectroscopy (PALS). The reduction in size of a free-volume hole attributable to deacetylation was clearly observed near the surface of the EVA encapsulant for a module aged in a damp heat (DH) test following UV irradiation (hereafter referred to as the post-UV DH test). Deacetylation in the post-UV DH test was considerable compared with that in a normal DH test. The depth profile of the size of a free-volume hole in a sample exposed outdoors was revealed to be very similar to that obtained in the post-UV DH test. This strongly implies that deacetylation was promoted near the interface between EVA and other components in the accelerated degradation test as well as by outdoor exposure.

Guiding principle for crystalline Si photovoltaic modules with high tolerance to acetic acid

A guiding principle for highly reliable crystalline Si photovoltaic modules, especially those with high tolerance to acetic acid generated by hydrolysis reaction between water vapor and an ethylene-vinyl acetate (EVA) encapsulant, is proposed. Degradation behavior evaluated by the damp heat test strongly depends on Ag finger electrodes and also EVA encapsulants. The acetic acid concentration in EVA on the glass side directly determines the degradation behavior. The most important factor for high tolerance is the type of Ag finger electrode materials when using an EVA encapsulant. Photovoltaic modules using newly developed crystalline Si cells with improved Ag finger electrode materials keep their maximum power of 80% of the initial value even after the damp heat test at 85 °C and 85% relative humidity for 10000 h. The pattern of dark regions in electroluminescence images is also discussed on the basis of the dynamics of acetic acid in the modules.

Durable crystalline Si photovoltaic modules based on silicone-sheet encapsulants

Crystalline Si photovoltaic (PV) modules were fabricated with sheets of poly(dimethylsiloxane) (silicone) as an encapsulant. The long-term durability of the silicone-encapsulated PV modules was experimentally investigated. The silicone-based modules enhanced the long-term durability against potential-induced degradation (PID) and a damp-heat (DH) condition at 85 °C with 85% relative humidity (RH). In addition, we designed and fabricated substrate-type Si PV modules based on the silicone encapsulant and an Al-alloy plate as the substratum, which demonstrated high impact resistance and high incombustible performance. The high chemical stability, high volume resistivity, rubber-like elasticity, and incombustibility of the silicone encapsulant resulted in the high durability of the modules. Our results indicate that silicone is an attractive encapsulation material, as it improves the long-term durability of crystalline Si PV modules.

Potential-induced degradation of thin-film Si photovoltaic modules

Potential-induced degradation (PID) of thin-film Si photovoltaic (PV) modules was investigated. The characteristics of PID phenomena of thin-film Si PV modules are markedly different from those of crystalline Si PV modules. Not only performance loss but also linear-shape and spot-shape delamination was observed after negative voltage application. Recovery from PID was also observed after positive voltage application. However, rapid progression of PID was found after the second negative voltage application after recovery from the initial PID. The root cause of PID of thin-film Si PV modules is thought to be the delamination between a transparent conductive oxide film and a glass substrate. Such degradation accompanied by delamination was also observed in thin-film Si PV modules exposed outside for about 5 years.

Microscopic aspects of potential-induced degradation phenomena and their recovery processes for p-type crystalline Si photovoltaic modules

Processes for potential-induced degradation (PID) and recovery phenomena were characterized using p-type multicrystalline Si photovoltaic modules and by PID test method using Al plate. Very severe PID phenomena accompanied with a drastic reduction in both open-circuit voltage and shunt resistance were observed within only several hours. It was found that PID phenomena are strongly accelerated at higher temperature and under higher negative-voltage application, on the other hand, PID phenomena do not necessarily require high humidity in this test method using Al plate. Na diffusion from the cover glass to the Si cell was observed after PID test. Recovery process from PID was also observed by applying positive voltage. However, complete recovery of photovoltaic performances was observed at room temperature in the dark without positive-voltage application for test modules with PID although recovery process requires a few hundred days.

Sequential and combined acceleration tests for crystalline Si photovoltaic modules

The sequential combination test for photovoltaic modules is effective for accelerating degradation to shorten the test time and for reproducing degradation phenomena observed in modules exposed outdoors for a long time. The damp-heat (DH) test, thermal-cycle (TC) test, humidity-freeze (HF) test or dynamic mechanical load (DML) test is combined for the test modules. It was confirmed that chemical corrosion degradation or physical mechanical degradation is reproduced by the combination of the above tests. Cracks on the back sheet and delamination, often observed upon outdoor exposure, were well reproduced by the combination of DH and TC tests and TC and HF tests, respectively. Sequential DH and TC tests and DML and TC tests accelerated the degradation. These sequential tests are expected to be effective in reducing the required time of indoor testing for ensuring long-term reliability.