Si Photovoltaic Modules Research Articles

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.

Engineering Cells, Membranes and Light Harvesting/Photosystem II Super-Complexes in Bio-Photoelectrochemical Cells

Photosynthetic organisms, membranes and complexes are attractive starting materials for solar energy conversion (SEC). Our overall goal is to develop methods to perform SEC using these materials in simple, inexpensive and a fashion that will be non-polluting and will not compete with the growth of food materials. I will describe here how the remarkable photocatalytic activity of the photosynthetic apparatus can provide overall water splitting with oxygen and hydrogen production in Bio-Photo-Electro-Chemical (BPEC) cells via the simplest and cleanest of processes wither in the absence or presence of added electron transport molecules. With plant thylakoids, electrons are shuttled by FeCN to a transparent FTO electrode, yielding a photocurrent density of 0.5 mA·cm-2. Hydrogen evolution occurs at the cathode at a bias as low as 0.8 V. A tandem cell comprising the BPEC cell with the thylakoid membranes and a Si photovoltaic module achieves overall water splitting with solar to hydrogen conversion efficiency of 0.3% (Pinhassi et al. Nature Communications 2016). With cyanobacterial cells, following a brief treatment that does not induce loss of cell viability, electrons from both the respiratory and photosynthetic systems are transferred directly to a graphite electrode, utilizing endogenous electron carriers (Saper et al. Nature Communications 2018). The current produced can be used for hydrogen production at low additional bias for significantly longer durations than the plant thylakoids.Another route is to isolate the photosynthetic complexes and chemically connect them to the electrochemical cell. One of the issues with this strategy is the small number of light absorbing chromophores connected to the complex, lowering the photochemical efficiency. Connection of isolated light harvesting complexes can help to overcome this deficiency. Photosystem II (PSII) is the only enzyme that catalyzes light-induced water oxidation being the basis for its application as a biophotoanode in various bio-photovoltaics and photo-bioelectrochemical cells. However, the absorption spectrum of PSII limits the quantum efficiency in the range of visible light, due to a gap in the green absorption region of chlorophylls (500 - 650 nm). To overcome this limitation, we have stabilized the interaction between PSII and Phycobilisomes (PBSs) – the cyanobacterial light harvesting complex, in vitro. The PBS of different cyanobacteria were analyzed for their ability to transfer energy to Thermosynechococcus elongatus (Te) PSII by fluorescence spill-over and photocurrent action spectra. Integration of the PBS-PSII super-complexes within an Os-complex-modified hydrogel on macro-porous indium tin oxide electrodes (MP-ITO) resulted in notably improved, wavelength dependent, incident photon-to-electron conversion efficiencies (IPCE). IPCE values in the green gap were doubled from 3 % to 6 % compared to PSII electrodes without PBS and a maximum IPCE up to 10.9 % at 670 nm was achieved. Figure 1

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.

Optimization of Self-Cleaning Thin-Film Layer for Photovoltaic Applications.

To improve the productivity of a photovoltaic (PV) module, TiO₂ thin films of different thicknesses were applied as a self-cleaning layer on soda-lime glass and a Si PV module by spray-coating a TiO₂ solution. The structural, optical, and wettability characteristics of the TiO₂ thin films were investigated with respect to the thickness. Thermogravimetric-differential thermal analysis, X-ray diffraction, field-emission scanning electron microscopy, contact-angle analysis, ultraviolet-visible spectroscopy, atomic force microscopy, Fourier transform infrared spectroscopy, and a solar simulator were used to analyze the prepared TiO₂ thin films. The optimal thickness was determined to be 100 nm. The TiO₂ thin film exhibited a self-cleaning ability even after post-annealing at 250 °C. After the self-cleaning ability was confirmed, the TiO₂ thin film was applied to the PV module.

Outdoor Performance Test of Bifacial n-Type Silicon Photovoltaic Modules

The outdoor performance of n-type bifacial Si photovoltaic (PV) modules and string systems was evaluated for two different albedo (ground reflection) conditions, i.e., 21% and 79%. Both monofacial and bifacial silicon PV modules were prepared using n-type bifacial Si passivated emitter rear totally diffused cells with multi-wire busbar incorporated with a white and transparent back-sheet, respectively. In the first set of tests, the power production of the bifacial PV string system was compared with the monofacial PV string system installed on a grey concrete floor with an albedo of ~21% for approximately one year (June 2016–May 2017). In the second test, the gain of the bifacial PV string system installed on the white membrane floor with an albedo of ~79% was evaluated for approximately ten months (November 2016–August 2017). During the second test, the power production by an equivalent monofacial module installed on a horizontal solar tracker was also monitored. The gain was estimated by comparing the energy yield of the bifacial PV module with that of the monofacial module. For the 1.5 kW PV string systems with a 30° tilt angle to the south and 21% ground albedo, the year-wide average bifacial gain was determined to be 10.5%. An increase of the ground albedo to 79% improved the bifacial gain to 33.3%. During the same period, the horizontal single-axis tracker yielded an energy gain of 15.8%.

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.

Current Characteristics Estimation of Si PV Modules Based on Artificial Neural Network Modeling.

In the photovoltaic (PV) field, the outdoor evaluation of a PV system is quite complex, due to the variations of temperature and irradiance. In fact, the diagnosis of the PV modules is extremely required in order to maintain the optimum performance. In this paper, an artificial neural network (ANN) is proposed to build and train the model, and evaluate the PV module performance by mean bias error, mean square error and the regression analysis. We take temperature, irradiance and a specific voltage for input, and a specific current value for output, repeat several times in order to obtain an I-V curve. The main feature lies to the data-driven black-box method, with the ignorance of any analytical equations and hence the conventional five parameters (serial resistance, shunt resistance, non-ideal factor, reverse saturation current, and photon current). The ANN is able to predict the I-V curves of the Si PV module at arbitrary irradiance and temperature. Finally, the proposed algorithm has proved to be valid in terms of comparison with the testing dataset.

Reversible Efficiency Variation of Tandem Amorphous/Microcrystalline Si Photovoltaic Modules in Outdoor Operation

The Staebler-Wronski effect in amorphous silicon based photovoltaic devices is responsible for degradation of their power conversion efficiency, within approximately the first one thousand hours of light soaking. Several experimental studies led to highlight the performance instability phenomena for the mentioned devices, underling that recovery and improvement of such performance are observable, by subjecting such devices (both of single-junction and tandem types) to DC reverse bias stresses under illumination, or to operation in the Maximum Power Point (MPP) under variable conditions of temperature and illumination. In this work, we present and discuss the results of novel recent outdoor tests on stabilized specimens (i.e., exposed to 1000 h extended light soaking, before our tests) of tandem amorphous/microcrystalline Si (a-Si/µc-Si) photovoltaic (PV) minimodules operating in their MPP, by analyzing the causes of the performance instability effects, systematically observed on a daily scale. During the mentioned tests, we have monitored the solar cell operating temperature and the incident solar spectrum at various times in different days to verify the effect of cell temperature and solar spectrum changes on the cell performances. The experimental results show a clear correlation between performance improvements of the photovoltaic modules and their thermal history during the outdoor tests, proving the interplay between defect build-up at a lower temperature and defect annealing at a higher temperature, taking place in the solar cells operated in MPP during conventional outdoor operation.

Reversible Efficiency Variation of Tandem Amorphous/Microcrystalline Si Photovoltaic Modules in Outdoor Operation

Reversible Efficiency Variation of Tandem Amorphous/Microcrystalline Si Photovoltaic Modules in Outdoor Operation

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.

Similarity of potential-induced degradation in superstrate-type thin-film CdTe and Si photovoltaic modules

Degradation phenomena under high-voltage stress, referred to as potential-induced degradation (PID) in general, were studied in superstrate-type thin-film photovoltaic (PV) modules, such as CdTe and thin-film Si PV modules. Both deterioration in the PV performance and delamination of the transparent conducting oxide (TCO) layer on the glass substrate were observed by the PID test for both PV modules, although a change in the PV parameters during the PID test with negative voltage application was somewhat different between them. It was also found for both PV modules that recovery from PID is accomplished by positive voltage application and that quick and drastic deterioration in all the PV parameters occurs by the second negative voltage application after recovery. It is suggested that the common origin of PID for superstrate-type thin-film PV modules is damage and delamination of the TCO layer.

Thermal Residual Stress Analysis of Soldering and Lamination Processes for Fabrication of Crystalline Silicon Photovoltaic Modules

In this study, we developed a finite element model to assess the residual stress in the soldering and lamination processes during the fabrication of crystalline silicon (Si) photovoltaic (PV) modules. We found that Si wafers experience maximum thermo-mechanical stress during the soldering process. Then, the Si solar cells experience pressure during the process of lamination of each layer of the PV module. Thus, it is important to decrease the residual stress during soldering of thin Si wafers. The residual stress is affected by the number of busbars, Si wafer thickness, and solder type. Firstly, as the number of busbars increases from two to twelve, the maximum principal stress increases by almost a factor of three (~100 MPa). Such a high first principal stress can cause mechanical failure in some Si wafers. Secondly, thermal warpage increases immediately after the soldering process when the thickness of the Si wafers decreases. Therefore, the number and width of the busbars should be considered in order to avoid mechanical failure. Finally, the residual stress can be reduced by using low melting point solder. The results obtained in this study can be applied to avoid mechanical failure in PV modules employing thin Si wafers.

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.

Electrochemical mechanisms of leakage-current-enhanced delamination and corrosion in Si photovoltaic modules

This paper analyzes the mechanisms for corrosion and delamination observed in Si photovoltaic modules subjected to high temperature and humidity with a negative-ground bias testing. Based on the thermodynamic data, the ionic component of the leakage current causes reduction reactions of water on the cathodic metallization, producing hydrogen gas and hydroxide ions. Ag fingers are strong catalysts for this reduction reaction compared to other materials used in solar cells and can be the initiation sites of corrosion and delamination caused by the electrochemical reactions. The produced hydrogen gas accumulates inside the module potentially producing a high gas pressure that can promote delamination, which is often preferentially initiated on metallization where the adhesion strength is lower [1–3]. The local basicity near the metal surface increases due to the hydroxide ion generation. Thus, the environment inside the encapsulant can be alkaline despite the presence of acetic acid decomposition products from the encapsulant. Corrosion of materials such as Si used in solar cells occurs and the extent depends on their corrosion resistance to the alkaline solution. This suggests that corrosion and delamination are interactive and promote the formation and propagation of one another.

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.

Evaluation of the performance and degradation of crystalline silicon-based photovoltaic modules in the Saharan environment

The aim of this paper is to present three years of an evaluation of the performance and degradation rate of three different crystalline silicon-based photovoltaic (PV) modules in the Saharan environment. The PV modules are: mc-Si (multi-crystalline), c_Si (mono-crystalline, back contacted) and HiT (heterojunction with intrinsic thin-layer); they are installed in Saida which is located at the proximity of Algeria’s Sahara. Two methods were used to calculate the degradation rate; the effective peak power of the PV modules and the temperature corrected performance ratio. It was found that the HIT technology performs worse than the other technologies with the highest degradation rate, ranging from −1.53%/year to −1.92%/year. The mc_Si PV and c_Si PV module technologies present a lower degradation rate than the HIT technology in the range of −0.74%/year to −0.83%/year and −0.58%/year to −0.79%/year respectively.