Potential Induced Degradation Research Articles

Effect of Photoconductive Properties of SiNx Passivation Film on Anti-PID Performance of Photovoltaic Cells and Modules

We investigated the impact of the photoconductive properties of crystalline silicon solar cells, having a SiNx passivation film, on potential induced degradation (PID) using voltage-corona (V-Q) and self-adjusting stead state (SASS) tests. The experimental results show that the conductivity of SiNx on the cell surface was gradually enhanced by the increase in refractive index, which effectively decreased the accumulated charge on the cell surface. Thus, changes in the conductivity of SiNx were found to be the cause of the different PID performance of the modules. The present work provides a theoretical basis for solving the PID problem of solar modules and power stations, and the exploration of the V-Q and SASS techniques provides a new, convenient method and corresponding basis for testing the PID performance of solar cells during industrial production. DOI: http://dx.doi.org/10.5755/j01.ms.24.2.17171

Comprehensive study of potential‐induced degradation in silicon heterojunction photovoltaic cell modules

AbstractAccelerated tests were used to study potential‐induced degradation (PID) in photovoltaic (PV) modules fabricated from silicon heterojunction (SHJ) solar cells containing tungsten‐doped indium oxide (IWO) transparent conductive films on both sides of the cells and a rear‐side emitter. A negative bias of −1000 V was applied to a module with respect to the cover glass surface in a chamber maintained at 85°C, which significantly reduced the cell's short‐circuit current density (Jsc) within several days. Based on dark current density‐voltage and external quantum efficiency measurements, the reduction in the Jsc was attributed to optical losses rather than carrier recombination. X‐ray absorption fine structure spectroscopy showed the formation of metallic indium (In) in the IWO layers of a degraded cell, which suggests that the root cause of the optical loss was a darkening of the front IWO layers caused by the precipitation of metallic In. In extremely severe PID tests, the SHJ PV modules exhibited not only a further reduction in the Jsc but also a moderate reduction in the open‐circuit voltage (Voc). These Jsc and Voc reductions were probably caused by sodium being introduced into the base region of the cells. A comparison of the PID test results of the SHJ PV modules with those of other types of PV modules indicates that SHJ PV modules have a relatively high resistance to PID. As a module with an ionomer encapsulant exhibited little degradation, their high resistances to PID may be further improved by using encapsulants with high electrical resistances.

A comparative investigation on in-situ and laboratory standard test of the potential induced degradation of crystalline silicon photovoltaic modules

Abstract Potential induced degradation (PID) is one of the genuinely critical concerns of a sustainable power generation from a PV system. Generally, the PID behavior of a PV module is tested in the laboratory according to the IEC standard before installation into a plant. On the other hand, an electroluminescence imaging is a reliable technique to identify the different types of PV cell defects which cause the degradation of the PV modules. The aim of this research is to investigate the PID behavior of similar PV modules in both the real on-site test and the laboratory standard test conditions. This will facilitate the outcome of the tangible indoor PID test results with more ease and reliability. It has been observed from the EL images of the on-site degraded PV module that a performance degradation happens due to different types of PV cell defects, such as, localized shunting, cracks, front contact grid interruptions, etc. The maximum power versus EL mean intensity shows a linear relationship which predicts the quantitative performance analysis of a PV module from an EL imaging process. The PID of a PV module has been found in a negative voltage stress condition in both the on-site and the laboratory tests. The shunt resistance gradually decreases as a consequence of the negative voltage stress only. The on-site degradation levels of the Pmax, Voc, Isc, and FF are 46.5, 7.15, 30.4, and 17.35% respectively after a duration of nearly 11 years of a negative voltage stress generated from a 240 V string size. In a laboratory PID test, the Pmax, Voc, Isc, and FF are degraded due to a negative voltage stress with a value of 6.83%, 1.9%, 1.5%, and 3.5% respectively.

Effect of negative potential on the extent of PID degradation in photovoltaic power plant in a real operation mode

This paper deals with Potential Induced Degradation (PID) of p-type monocrystalline PV modules (Evergreen) from a photovoltaic power plant that has been in operation mode for 7 years. Within the PV module affected by the PID degradation, the effect of the electric field on individual PV cells is studied. The distribution of the electric field was simulated by SolidWorks software. The results show a random distribution of affected PV cells not related to the size and distribution of the electric field intensity. Furthermore, the dependencies of negative voltage potential on the range of PID degradation of individual PV modules located in the negative pole of the PV string is made. From measured current voltage characteristics (measured at STC), it is evident that the value of negative voltage potential is not directly proportional to the PID occurrence. These results are supplemented by electroluminescence images which confirm this finding.

Multistage performance deterioration in n-type crystalline silicon photovoltaic modules undergoing potential-induced degradation

This study addresses the behavior of n-type front-emitter (FE) crystalline-silicon (c-Si) photovoltaic (PV) modules in potential-induced degradation (PID) tests with a long duration of up to 20 days. By PID tests where a negative bias of −1000 V was applied at 85 °C to 20 × 20-mm2-sized n-type FE c-Si PV cells in modules, the short-circuit current density (Jsc) and the open-circuit voltage (Voc) started to be decreased within 10 s, and strongly saturates within approximately 120 s, resulting in a reduction in the maximum output power (Pmax) and its saturation. After the saturation, all the parameters were almost unchanged until after 1 h. However, the fill factor (FF) then started to decrease and saturated again. After approximately 48 h, FF further decreased again, accompanied by a reduction in Voc. The first degradation is known to be due to an increase in the surface recombination of minority carriers by the accumulation of additional positive charges in the front Si nitride (SiNx) films. The second and third degradations may be due to significant increases in recombination in the space charge region. The enhancement in recombination in the space charge region may be due to additional defect levels of sodium (Na) introduced into the space charge region in the p–n junction. We also performed recovery tests by applying a positive bias of +1000 V. The module with the first degradation completely recovered its performance losses, and the module with the second degradation was almost completely recovered. On the contrary, the modules with the third degradation could not be recovered. These findings may improve the understanding of the reliability of n-type FE c-Si PV modules in large-scale PV systems.

Research on Potential Induced Degradation (PID) of PV Modules in Different Typical Climate Regions

Potential Induced Degradation (PID) is one of the most important factors effecting the performances of Photovoltaic (PV) modules and PV systems in recent years. In this paper the PID phenomena of the PV power plant in different typical climate regions were studied and some experimental PID simulations were carried out in order to find out the factors effecting the performance by PID. The results show that the typical PID phenomena are easy to occur in cells close to the border of the PV module. PID phenomena can appear in PV power plants under different climate conditions, but the effecting degrees on module performance are different depending on temperature, humidity and other parameters. We also find the maximum power would recover in some degree after positive-bias voltage duration.

Investigating PID shunting in polycrystalline silicon modules via multiscale, multitechnique characterization

AbstractWe investigated the potential‐induced degradation (PID) shunting mechanism in multicrystalline‐silicon photovoltaic modules by using a multiscale, multitechnique characterization approach. Both field‐stressed modules and laboratory‐stressed mini modules were studied. We used photoluminescence, electroluminescence, and dark lock‐in thermography imaging to identify degraded areas at the module scale. Small samples were then removed from degraded areas, laser marked, and imaged by scanning electron microscopy. We used simultaneous electron‐beam induced current imaging and focused ion beam milling to mark around PID shunts for chemical analysis by time‐of‐flight secondary‐ion mass spectrometry or to isolate individual shunt defects for transmission electron microscopy and atom‐probe tomography analysis. By spanning a range of 10 orders of magnitude in size, this approach enabled us to investigate the root‐cause mechanisms for PID shunting. We observed a direct correlation between recombination active shunts and sodium content. The sodium content in shunted areas peaks at the SiNX/Si interface and is consistently observed at a concentration of 0.1% to 2% in shunted areas. Analysis of samples subjected to PID recovery, either activated by electron beam or thermal effects only, reveals that recovery of isolated shunts correlates with diffusion of sodium out of the structural defects to the silicon surface. We observed the role of oxygen and chlorine in PID shunting and found that those species—although sometimes present in structural defects where PID shunting was observed—do not play a consistent role in PID shunting.

Imaging charge carriers in potential-induced degradation defects of c-Si solar cells by scanning capacitance microscopy

We report on nm-resolution imaging of charge-carrier distribution around local potential-induced degradation (PID) shunting defects using scanning capacitance microscopy. We imaged on cross sections of heavily field-degraded module areas, cored out and selected by mm-scale photoluminescence imaging. We found localized areas with abnormal carrier behavior induced by the PID defects: the apparent n-type carrier extends vertically into the absorber to ∼1–2 μm from the cell surface, and laterally in similar lengths; in defect-free areas, the n-type carrier extends ∼0.5 μm, which is consistent with the junction depth. For comparison, we also investigated areas of the same module exhibiting the least PID stress, and we found no such heavily damaged junction area. Instead, we found slightly abnormal carrier behavior, where the carrier-type inversion in the absorber did not occur, but the p-type carrier concentration changed slightly in a much smaller lateral length of ∼300 nm. These nano-electrical findings may indicate a possible mechanism that the existing extended defects, which may not be significantly harmful to cell performance, can be changed by PID to heavily damaged junction areas.

Mechanism of Na accumulation at extended defects in Si from first-principles

Sodium (Na) impurities in silicon solar cells are considered to play an important role in potential-induced degradation (PID), a significant cause of solar cell degradation and failure. Shorting due to Na accumulation at extended defects has been suggested as a culprit for PID. However, it is not clear how the extended defects are decorated by Na impurities. Using first-principles density functional theory calculations, we find that Na impurities segregate from the bulk into extended defects such as intrinsic stacking faults and Σ3 (111) grain boundaries. The energy barrier required for Na to escape from the extended defects is substantial and similar to the sum of the barrier energy in bulk Si (1.1–1.2 eV) and the segregation energy to the stacking fault (∼0.7 eV). Surprisingly, the migration barrier for Na diffusion within the extended defects is even higher than the energy barrier for escaping. The results suggest that the extended defects likely accumulate Na as the impurities segregate to the defects from the bulk, rather than because of migration through the extended defects.

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.

Effects of diffusion process on potential induced degradation of silicon solar cells.

Potential induced degradation (PID) has recently been identified as one of the most important degradation mechanisms for silicon solar cells. It is widely considered that PID is closely related with the manufacture and application period of solar modules. In this study, the effects of diffusion sheet resistance on PID were verified and explained by testing the emitter doping profile, the minority carrier lifetime, the emitter saturation current, the electrical performance of different cells, and the PID process. With increasing sheet resistance of cells, the depth and saturation current density of the emitter both decreased, and the cell efficiency increased, whereas the PID phenomenon became serious. It was found that higher sheet resistance or thinner P-N junction could lead to higher PID sensitivity. Therefore, more attention should be paid to PID phenomenon as the photovoltaic industry develops in the direction of high sheet resistance.

Investigation of Potential-Induced Degradation in n-PERT Bifacial Silicon Photovoltaic Modules with a Glass/Glass Structure

Potential-induced degradation (PID) in n-type passivated emitter, rear totally diffused (n-PERT) bifacial crystalline silicon photovoltaic modules with a glass/glass structure is investigated. From front-side measurements, a significant loss in the short-circuit current ( I sc) and a relatively smaller loss in the open-circuit voltage ( V oc) and fill factor (FF) are observed due to PID. A similar degradation behavior is observed from the rear side, except that there is negligible change in I sc. External quantum efficiency and photoluminescence measurements reveal that the losses in I sc and V oc are most likely due to an increase in the front surface recombination. FF loss analysis and two-diode model fitting demonstrate that the FF loss is mainly attributed to an increased recombination in the space charge regions. Moreover, n-PERT bifacial silicon modules also suffer from PID when they are stressed from the rear side. Furthermore, some ethylene-vinyl acetate and polyolefin, which show high PID-resistance to conventional p-type technologies, are found to be not as effective in preventing PID in n-PERT technologies. However, a PID-free n-PERT bifacial module design is possible with the application of the sodium-free glass. Finally, the progression of PID is heavily dependent on the bias voltage and stress temperature.

Analysis of PID Degraded Photovoltaic Power Plant

In this paper, the extent of PID (Potential Induced Degradation) impacts on a photovoltaic plant fitted with silicon-based modules is studied. In the first step, a PID degradation is diagnosed and confirmed by a thermal imager. Subsequently, the PV modules are uninstalled and transferred to an accredited workplace where are measured current-voltage characteristics. From these results, the maximum power - P MPP of PV modules is determined as a parameter indicating the extent of the PID degradation. The maximum dependence of the P MPP on the negative voltage potential concludes that the PID degradation range of the PV modules is not directly proportional to the negative voltage potential. These results are accompanied by the PV module electroluminescence measurement.

Quantitative analysis of relationship between leakage current and power loss of multi-crystalline silicon photovoltaic module during potential-induced degradation test

The relationship between the leakage current and the power loss of a multi-crystalline silicon photovoltaic module during potential-induced degradation (PID) tests was analyzed. Since the current flowing into cells through a cover glass and an ethylene–vinyl acetate encapsulant is highly related to Na+ ion migration, which is presumed to be the main cause of PID, a setup for accurately measuring the current was designed. PID tests were also conducted under different temperature and voltage conditions following the same setup. From the current measured during PID tests, the charge transferred onto the active cell area was estimated. It was found that there is a one-to-one relationship between the charge transferred onto the active cell area and the power loss of a module. This result suggests that the PID power loss is due to the amount of Na+ ions accumulated on the active cell area.

Performance variation of dark current density-voltage characteristics for PID-affected monocrystalline silicon solar modules from the field

Potential-induced degradation (PID) has caught considerable attention in recent years because of its detrimental influence on output power of solar module and energy yield of photovoltaic power plant. In this paper, performance variation of dark current density-voltage (dark J-V) characteristics for PID-affected monocrystalline silicon solar modules dismounted from photovoltaic power plant was investigated. By measuring dark J-V characteristics of the PID-affected modules, the deterioration trend of dark J-V characteristics for PID-affected modules was found for the first time. Based on diffusion theory of metal ions, a new field-assisted diffusion model of metal ions was built, and the deterioration of open-circuit voltage and increase of recombination current density were explained for solar modules affected by PID. The thermal characteristics of solar modules with different level of PID were revealed under forward and reverse bias. There is an obvious temperature difference between the substrings neighboring the frame and the middle substring within the PID-affected modules. The results obtained in this paper give out the reasonable explanation about mechanism of PID and the deterioration trend of dark J-V for PID-affected modules.

Investigation of Correlation between Field Performance and Indoor Acceleration Measurements of Potential Induced Degradation (PID) for c-Si PV Modules

Investigation of Correlation between Field Performance and Indoor Acceleration Measurements of Potential Induced Degradation (PID) for c-Si PV Modules

Direct observation of changes in the effective minority-carrier lifetime of SiNx-passivated n-type crystalline-silicon substrates caused by potential-induced degradation and recovery tests

We directly observed reductions in the effective minority-carrier lifetime (τeff) of n-type crystalline silicon (c-Si) substrates with silicon-nitride passivation films caused by potential-induced degradation (PID). We prepared PID-test samples by encapsulating the passivated substrates with standard photovoltaic-module encapsulation materials. After PID tests applying −1000V to the c-Si samples from the glass surface, the τeff was decreased, which probably pertains to Na introduced into the c-Si. After PID tests applying +1000V, the sample, on the other hand, showed a considerably rapid τeff reduction, probably associated with the surface polarization effect. We also performed recovery tests of predegraded samples, by applying a bias opposite to that used in a degradation test. The τeff of a sample predegraded by applying +1000V was rapidly completely recovered by applying −1000V, while those of predegraded by applying −1000V show only slight and insufficient τeff recovery.

Development of in-situ high-voltage and high-temperature stressing capability on atomic force microscopy platform

Reliability has become an increasingly important issue as photovoltaic technologies mature. However, researching reliability at the nanometer scale is in its infancy; in particular, in-situ studies have not been reported to date. Here, to investigate potential-induced degradation (PID) of solar cell modules, we have developed an in-situ stressing capability with applied high voltage (HV) and high temperature (HT) on an atomic force microscopy (AFM) platform. We designed a sample holder to simultaneously accommodate 1000-V HV and 200°C HT stressing. Three technical challenges have been overcome along with the development: thermal drift at HT, HV interference with measurement, and arc discharge caused by HV. We demonstrated no observable measurement artifact under the stress conditions. Based on our in-situ stressing AFM, Kelvin probe force microscopy potential imaging revealed the evolution of electrical potential across the junction along with the PID stressing time, which provides vital information to further study the PID mechanism.

Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20.

Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized only with expensive organic hole-transporting materials. We demonstrate PSCs that achieve stabilized efficiencies exceeding 20% with copper(I) thiocyanate (CuSCN) as the hole extraction layer. A fast solvent removal method enabled the creation of compact, highly conformal CuSCN layers that facilitate rapid carrier extraction and collection. The PSCs showed high thermal stability under long-term heating, although their operational stability was poor. This instability originated from potential-induced degradation of the CuSCN/Au contact. The addition of a conductive reduced graphene oxide spacer layer between CuSCN and gold allowed PSCs to retain >95% of their initial efficiency after aging at a maximum power point for 1000 hours under full solar intensity at 60°C. Under both continuous full-sun illumination and thermal stress, CuSCN-based devices surpassed the stability of spiro-OMeTAD-based PSCs.

Real-Time Thermoelectrical Model of PV Panels for Degradation Assessment

In this paper, we develop a new real-time thermoelectrical model of a photovoltaic (PV) panel that takes degradations into consideration. The degradation modes under study are the potential-induced degradation, the light-induced degradation, the ultraviolet light degradation, the moisture-induced degradation, and the cell cracks. The effect of each degradation mode on the equivalent circuit of PV cells is described by equations as a function of environmental conditions and time. The thermal behavior of the PV panels is also addressed by building a thermal model of PV panels taking into consideration the environmental and operational conditions as well as the internal heat sources. At the end, we present a complete thermoelectrical model that treats the degradations as additional events. To assess the degradation impact on the cells, the normalized efficiency is taken as the key performance index.