Mitigating Potential-Induced Degradation (PID) Using SiO2 ARC Layer
Potential-induced degradation (PID) of photovoltaic (PV) cells is one of the most severe types of degradation, where the output power losses in solar cells may even exceed 30%. In this article, we present the development of a suitable anti-reflection coating (ARC) structure of solar cells to mitigate the PID effect using a SiO2 ARC layer. Our PID testing experiments show that the proposed ARC layer can improve the durability and reliability of the solar cell, where the maximum drop in efficiency was equal to 0.69% after 96 h of PID testing using an applied voltage of 1000 V and temperature setting at 85 °C. In addition, we observed that the maximum losses in the current density are equal to 0.8 mA/cm 2. compared with 4.5 mA/cm 2 current density loss without using the SiO2 ARC layer.
Introduction
Photovoltaic (PV) energy conversion based on crystalline silicon (c-Si) solar cells is one of the significant technological pillars of the tremendous success of the PV industry in the last decade. Confirmed terrestrial PV module efficiencies of c-Si are above 24%, and for multi-crystalline silicon solar cells are as close as 20% [1].
Improving the efficiency of solar cells has been a perpetually challenging task, since there is a strong interaction between the different recombination losses, including intrinsic and extrinsic recombination loss with current density at maximum power point (MPP) as a function of the surface recombination at the rear side [2]. In addition, the anti-reflection coating (ARC) structures of the solar cell play an essential role in shaping the maximum efficiency of the cell; an incorrect ARC structure can lead to a significant drop in the current density of more than 3.5 mA/cm 2 [3].
To keep the efficiency of solar cells at its highest levels, the reliability and stability must be carefully checked. To facilitate this inspection, potential-induced degradation (PID) testing is strongly advised [4]. The PID test measures the leak current by applying a high voltage (normally 1000 V and above, according to IEC 62804 standard) in a high-temperature and high-humidity environment. After the full duration of the test—96 h—the degradation of the PV module, or on a small scale as a standalone solar cell, can be measured by comparing the current-voltage (I-V) curve before PID testing begins and after the PID test is fully complete [5]. In addition, the electroluminescence (EL) imaging can be captured prior to and after the PID test to visualise the actual collision on the surface structure after PID testing is done. EL imaging can also facilitate the overview of the cracks and defects of the cell, which is immeasurably valuable [6].
Current research expresses significant interest in mitigating the PID problems of solar cells. This is for three main reasons. Firstly, the PID degrades the output power of the solar cells. Secondly, every solar cell affected by PID reduces the total string voltage [7]. Thirdly, it is difficult to detect this problem quickly when overall PV installations are up and running [8] because degradation of PV output power is not only due to the impact of PID but also the existence of faulty bypass diodes [9], faulty PV modules [10,11,12] and hotspots [13].
There are numerous ARC structures now available in the literature. However, a limited number of these structures have had PID testing to check their reliability, stability and degradation, to ensure that the reported efficiencies are accurate. N-type solar cell structures, including BiSoN, MoSon and pPERT, have been investigated by Devoto and Halm [14]. It was concluded that the above-mentioned ARC solar cell structures cannot mitigate PID testing, and after 74 h of PID testing, there is an approximately 20% drop in their efficiency. Other experiments [15,16] have shown that p-type solar cells can degrade efficiency in the range of 5% to 25%.
The investigation of bifacial PID in bifacial mono c-Si p-PERC solar cells was developed by Carolus et al. [17]. It was evident that there is more considerable reduction in the efficiency of bifacial solar cells that have glass packing renders, which tends to be extremely sensitive to PID testing. Furthermore, the front side of bifacial solar cells tends to have more degradation than the rear junction.
One solution for mitigating the PID effect on solar cells is to change the capsulation film instead of using ethylene vinyl acetate (EVA)., Wang [18] suggests using Poly Olefin (PO) material with a low water vapour through rate. This solution can potentially reduce the power losses to around 3%. In addition, this new material was proven to improve the reliability and stability of PV solar cells, as demonstrated by López-Escalante et al. [19]. They show that PO-made solar cells are resistant, to a certain degree, to the PID effect. Other research [20] suggests using a thin silicon dioxide (SiO2) ARC layer in combination with p-type or n-type solar cells that can enhance the efficiency of the solar cell after applying the PID test. There are some other recognised methods for mitigating PID effect on solar cells, such as replacing the soda-lime glass with a quartz glass that guarantees the solar glass is free of the suspected PID ions [21]. Another appropriate method to mitigate the PID effect of solar cells is to attach narrow, thin, flexible glass strips on the glass surface along the inner edges of the solar cell frame [22]. The drop in output power after PID testing is equal to 8.8%, while it is equal to 15% prior to using this mitigation technique.
Experimental evidence of the PID effect on copper indium gallium selenide (CIGS) showed that CIGS PV cells suffer from PID testing [23]. The results showed that there is an almost 15% drop in the maximum output power after 120 h of PID testing. In addition, it was also evident that the back and front contact almost have the same drop in maximum power as well as short circuit current after completing the PID testing cycle.
Gap in Knowledge
Perhaps one of the most important factors in today’s PV manufacturing systems and production is the reliability and stability of the used materials. We have seen a Rapid increase in new ideas for how to mitigate the effect of PID on newly-developed solar cell structures. However, there is still a lack of existing ARC structures that have been proven to be effective at mitigating PID. Therefore, in this article we present the development of a suitable ARC structure created by layering SiO2 at the bottom of SiNx that effectively reduces the PID affect. Results show that there is a limited drop in the maximum power of 0.01 W, and the overall drop in efficiency is limited at 0.69% after 96 h of PID testing using an applied voltage of 1000 V and temperature setting at 85 °C.
Paper Organization
The rest of the article is organised as follows: Section 4 presents PID testing experiments performed on three different solar cell ARC structures. Section 5 presents the results of PID testing on the PV cell SiO2-free layer, SiO2 layer placed at the top of SiNx, and the SiO2 layer placed at the bottom of SiNx. Lastly, Section 6 presents the overall conclusion of the results discussed in the article, followed by the acknowledgement and the reference list.
Experiment
In this study, a PID test was conducted by constructing three polycrystalline silicon solar cells with three types of ARC structures. The test was carried out using PIDcon PID-tester according to IEC 62804 standard, at 0 h and after 96 h. The working principles of the PIDcon test setup are shown in Figure 1.
The layer consists of a solar cell, polymer foil, and glass between the two metal electrodes. A positive voltage is applied at the upper electrode, while the bottom electrode is virtually grounded/heated. The shunt resistance ( Rsh ) as a function of time is also measured. The standard test conditions were as follows: (i) voltage 1000 V, (ii) temperature 85 °C, (iii) dry conditions, no use of water and (iv) test duration 96 h.
The solar cells’ structure that has been tested in this study is shown in Figure 2. The first solar cell structure (Figure 2a) is made free of SiO2 while the second solar cell structure as in Figure 2b includes the SiO2 thin film placed on the top of the silicon nitride (SiNx). The last solar cell is where the SiO2 thin film is placed at the bottom of the SiNx. For ease of referencing the solar cells, we called them the first cell #1, the second cell #2 and the third cell #3.
To compare the PID of the three examined solar cells, we must first identify the critical electrical parameters that have to be measured and compared. Here, we complied with the IEC 62804 standard, thus comparing the maximum power ( Pmax ), efficiency, and short-circuit current density ( Jsc ). The efficiency was calculated using the following ( FF is the fill factor, and Voc corresponds to the open-circuit voltage):
4.1. SiO2 Layer Preperation and Properties
The SiO2 thin film was prepared by liquidphased deposition. The deposition system contained temperature-controlled water to maintain uniformity in the deposition temperature amongst the surface and a Teflon vessel as a liquid solution. Initially, 25 g of silica powder with 99.9% purity was mixed with 500 mL of hydrofluorosilicic acid; this mixture was stirred for almost 24 h to ensure that the hydrofluorosilicic acid became saturated. The next step was to mix 32 mL of the saturated hydrofluorosilicic acid with 25 mL boric acid for the deposition of the SiO2 film.
After the deposition process was complete, we rinsed a tin-doped indium oxide glass in water; this process is required to make a purified nitrogen gas. Finally, the thin film was annealed in the air for 10 min at 425 °C. The chemical reaction between oxygen and silicon to generate SiO2 is usually driven by a high-heat environment; however, even at room temperature, a shallow layer of native oxide, approximately 1 nm thick, can form in an air environment.
It is straightforward to deposit on various materials and grown thermally on silicon wafers, which makes it manageable for manufacturing purposes.
It can block the ion diffusion implementation of many undesired contaminants, particularly when placed on the bottom of the SiNx layer.
The interface between silicon and silicon dioxide has relatively few mechanical and electrical defects.
It has high-temperature stability of up to 1600 ˚C, making it a useful material for process and device integration [24].
The SiO2 layer causes the silicon-silicon dioxide interface to move into the wafer while the oxide grows. This would typically mean that while the oxide grows, it consumes the silicon atoms at the surface of the wafer, making it a more reliable structural layer.
4.2. Experiment Setup (Tools and Equipment)
To perform the PID test on the solar cell samples, PIDcon PID-tester has been used. The main characteristics of this tester are that no climate chamber is necessary during the PID test and no lamination of the solar cells is required [25]. The leakage current, output power and I-V curve also can be measured using this device. After the completion of PID testing, the solar cells were subjected to EL imaging. This procedure used a high-resolution Keland EL tester. This device also allows the inspection of the current density of the solar cells. Therefore, by the end of each experiment, the I-V curve, EL and current density images were analysed and compared.
The circuit diagram of the double diode model used for the analysis of the I-V curve measurements is shown in Figure 3. The junction recombination is modeled by adding a second diode (D2) in parallel with the first (D1) and setting the ideality factor typically to two.
4.3. Performance of the Examined Solar Cell Samples Before PID Testing
The EL, current density and I-V curves of the three solar cell samples before the PID test are shown in Figure 4. The critical parameters before the PID test began are summarised in Table 1: the efficiency of cell #3 is equal to 19.24% (the lowest) and 20.32% for cell #2 (the highest). In addition, the value of the current density, open-circuit voltage and the fill factor are almost identical for the three solar cell samples.
According to Figure 4, the EL image showed no cracks or significant defects in the examined solar cells before the PID test began. The current density images also show a uniform distribution of the current for all samples, meaning no defects or leakage current is present. Seeing that the negative value of current density represents a reverse current following from the solar cell, zero current density represents no flow of current at a particular area of the cell, whereas the positive value of the current density shows a direct DC current generated by the cell.
Results
5.1. SiO2-Free (Cell #1)
After the completion of the PID test over 96 h, the EL and current density images were taken, as shown in Figure 5. It is evident that the solar cell had a considerable amount of degradation. As exhibited by the current density image, after PID testing there is a significant part of the solar cell that produces even negative current density, which typically results in a decrease in the efficiency of the cell.
Figure 6 shows the actual I-V curve of the solar cell before vs. after PID testing. The summary of the comparison between all relevant parameters is presented in Table 2. As can be seen, after PID testing, all related parameters of the solar cell significantly dropped. Remarkably, the efficiency of the solar cell became 13.96%, compared with 19.80% before PID testing (loss = 5.84%). Therefore, without the SiO2 coating, the solar cell would potentially keep degrading over time.
The leakage current of this cell is generated continuously. Therefore, PV modules made of SiO2-free ARC structure would typically suffer from PID phenomena, leading to poor stability and significant decay of the output power production, and hence continuous degradation at higher rates.
5.2. SiO2 Thin Film Placed on the Top of the SiNx (Cell #2)
The I-V curve results of PID testing of the second solar cell, which has a top-layer SiO2 ARC structure, are shown in Figure 7. This result reveals that placing the SiO2 layer on the top of the SiNx does not have a significant impact on the stability of the solar cell. In fact, the experiment shows that the efficiency dropped by 7.03% (i.e., before PID it was 20.32%, and after PID it was 13.29%). Other relevant parameters are presented in Table 3.
The EL and current density images of the cell are shown in Figure 8. According to the Jsc. after completing PID testing, the solar cell dropped from 38.91 mA/cm 2 to 31.90 mA/cm 2. approximately 18%. This result illustrates the negative impact of layering SiO2 on the top of the SiNx. In addition, the results of this experiment are almost identical to cell #1 results in terms of the drop in the shunt resistance from 84 Ω to below 10 Ω and the drop in the fill factor from almost 75% to 63%. Thus, it is possible to assume that placing a layer of SiO2 on the top of the SiNx leads to lowering Jsc and Pmax and, consequently, the cell efficiency.
In summary, both solar cell samples with an SiO2 layer on the top of the SiNx and SiO2-free samples had significant leakage of the current after PID testing. Therefore, this leads us to another experiment, which will be discussed in the next subsection.
5.3. SiO2 Thin Film Placed on the Bottom of the SiNx (Cell #3)
In this subsection, the results of PID testing of the third solar cell will be discussed. This solar cell has a SiO2 layer placed on the bottom of the SiNx. Placing this layer on the bottom of the SiNx will reasonably achieve the following:
Enhance the stability of the solar cell structure, because there will be a limited leakage of the current at the top layer (SiNx), preventing mismatched conditions of the solar cell, particularly during the PID test [26].
As there will be a limited leakage of the current, the expected drop in efficiency will also be at a minimum level. In addition, a drop in the shunt resistance is expected during PID testing [27]. However, it will be a limited drop as the current density will remain at its highest.
Figure 9 shows the measured I-V curves before and after PID testing for cell #3. There is a limited drop in the maximum power of 0.01 W (approximately 2.2%); the efficiency also dropped by 0.69% (before PID 19.24% and after PID 18.55%). The shunt resistance dropped by 3.5 Ω, representing a decrease of 3.92%. All interpretive parameters before and after PID testing are presented in Table 4.
The EL and current density images of the solar cell are shown in Figure 10. As can be noticed, there is a limited loss of approximately 2.2% in the current density after completing the PID test, before PID 37.72 mA/cm 2 and after PID 36.88 mA/cm 2 ; therefore, there is a total loss of 0.84 mA/cm 2.
Table 5 summarises the percentage drop from before and after PID testing of each critical parameters analysed. Cell #3 shows lower drops for all parameters, reaffirming the effectiveness of using the SiO2 ARC layer for PID mitigation.
In contrast with the above results, one of the decisive successes of mitigating for PID testing when layering SiO2 on the bottom of the SiNx is that the dark leakage current ( J 0) is extremely low due to the effective hydrogenation process; this would characteristically reduce the trapped charge density of the solar cell structures, thereby improving the stability and reliability of the cell. In addition, PV modules consist of a series of those cells that can also produce a resolute output power with a limited degradation over time.
Conclusions
In this article, we discussed the potential of preventing PID by modifying the ARC structure in polycrystalline silicon solar cells. Three types of ARC structures were subjected to the PID test for a period of 96 h under 1000 V and 85 °C conditions, according to IEC 62804 standard. It is possible to conclude that the ARC structure containing SiO2-free or SiO2 layer on the top of the SiNx has a significant drop in the efficiency, always higher than 5%, after the PID test. A considerable value also decreases all other relevant parameters, including the shunt resistance ( Rsh ), short-circuit current density ( Jsc ) and maximum output power ( Pmax ).
We found that when the SiO2 layer is placed on the bottom of the SiNx, there is a limited leakage of the current of the solar cell after the completion of the PID test. Consequently, there was a limited drop in the maximum output power of 0.01 W, which represents approximately 2.2%, and the efficiency also dropped by 0.69%. Therefore, this ARC structure was confirmed to be an effective PID mitigation, preventing PV module degradation as well as increasing its reliability.
For future research, it would be interesting to perform PID testing on a humidity environment since it is common that PV modules are subject to these conditions.
Author Contributions
M.D.: formal analysis, methodology, software, writing—original draft, writing—review and editing. Y.H.: methodology, investigation, writing—review and editing. N.S.: formal analysis, supervision and proofreading, visualization, writing—review and editing. R.G.V.: methodology, writing—original draft, writing—review and editing. All authors have read and agreed to the submitted version of the manuscript.
References
Figure 2. Anti-reflection coating (ARC) structure of the three examined solar cells: (a) SiO2-free (cell #1), (b) SiO2 layer placed at the top of SiNx (cell #2), and (c) SiO2 layer placed at the bottom of SiNx (cell #3).
Figure 2. Anti-reflection coating (ARC) structure of the three examined solar cells: (a) SiO2-free (cell #1), (b) SiO2 layer placed at the top of SiNx (cell #2), and (c) SiO2 layer placed at the bottom of SiNx (cell #3).
Figure 3. Circuit diagram of the double diode model including the series ( Rs ) and parallel ( Rsh ) resistances.
Figure 3. Circuit diagram of the double diode model including the series ( Rs ) and parallel ( Rsh ) resistances.
Figure 4. Electroluminescence (EL), current density and the current-voltage (I-V) curve of the three examined solar cells before the potential-induced degradation (PID) test: (a) cell #1, (b) cell #2 and (c) cell #3.
Figure 4. Electroluminescence (EL), current density and the current-voltage (I-V) curve of the three examined solar cells before the potential-induced degradation (PID) test: (a) cell #1, (b) cell #2 and (c) cell #3.
Figure 6. I-V curve characteristics before and after PID testing of the first solar cell sample, SiO2-free.
Figure 6. I-V curve characteristics before and after PID testing of the first solar cell sample, SiO2-free.
Figure 7. I-V curve characteristics before and after PID testing of the second solar cell sample, with SiO2 layer placed on the top of the SiNx.
Figure 7. I-V curve characteristics before and after PID testing of the second solar cell sample, with SiO2 layer placed on the top of the SiNx.
Figure 8. EL and the current density image of the second solar cell sample, with SiO2 on the top of the SiNx.
Figure 8. EL and the current density image of the second solar cell sample, with SiO2 on the top of the SiNx.
Figure 9. I-V curve characteristics before and after PID testing of the second solar cell sample, with SiO2 layer placed on the bottom of the SiNx.
Figure 9. I-V curve characteristics before and after PID testing of the second solar cell sample, with SiO2 layer placed on the bottom of the SiNx.
Figure 10. EL and the current density image of the third solar cell sample, with SiO2 on the bottom of the SiNx.
Figure 10. EL and the current density image of the third solar cell sample, with SiO2 on the bottom of the SiNx.
| Cell #1 | 37.77 | 0.68 | 77.1 | 0.48 | 87.7 | 19.80 |
| Cell #2 | 38.91 | 0.68 | 76.8 | 0.49 | 84.4 | 20.32 |
| Cell #3 | 37.72 | 0.66 | 77.3 | 0.46 | 89.2 | 19.24 |
| Before PID | 37.77 | 0.68 | 77.1 | 0.48 | 87.7 | 19.80 |
| After PID | 33.2 | 0.66 | 63.7 | 0.37 | 9.12 | 13.96 |
| Before PID | 38.91 | 0.68 | 76.8 | 0.49 | 84.4 | 20.32 |
| After PID | 31.90 | 0.66 | 63.1 | 0.35 | 7.51 | 13.29 |
| Before PID | 37.72 | 0.66 | 77.3 | 0.46 | 89.2 | 19.24 |
| After PID | 36.88 | 0.66 | 76.2 | 0.45 | 85.7 | 18.55 |
| Jsc | 12.10% | 18.00% | 2.23% |
| Voc | 2.94% | 2.94% | 0% |
| FF | 13.40% | 13.70% | 1.1% |
| Pmax | 22.92% | 28.57% | 2.17% |
| Rsh | 89.60% | 91.10% | 3.92% |
| Efficiency | 5.84% | 7.03% | 0.69% |
© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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Dhimish, M.; Hu, Y.; Schofield, N.; G. Vieira, R. Mitigating Potential-Induced Degradation (PID) Using SiO2 ARC Layer. Energies 2020, 13, 5139. https://doi.org/10.3390/en13195139
Dhimish M, Hu Y, Schofield N, G. Vieira R. Mitigating Potential-Induced Degradation (PID) Using SiO2 ARC Layer. Energies. 2020; 13(19):5139. https://doi.org/10.3390/en13195139
Chicago/Turabian Style
Dhimish, Mahmoud, Yihua Hu, Nigel Schofield, and Romênia G. Vieira. 2020. Mitigating Potential-Induced Degradation (PID) Using SiO2 ARC Layer Energies 13, no. 19: 5139. https://doi.org/10.3390/en13195139
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Solar Panel Degradation: What Is It and Why Should You Care?
Photovoltaic (PV) technology has been heavily researched and developed for years. Most PV modules in the industry have a standard lifespan of 25 years, but some leading companies in the solar industry like Maxeon Solar have developed this technology to create solar panels lasting for 40 years or more, covered by a 40-year warranty.
To understand the lifespan limitations of PV modules, you should comprehend the concept of solar panel degradation. This is the main phenomenon affecting the lifespan of PV modules and causing them to break. In this article, we will explain everything you need to know about this and give you tips on how to reduce solar panel degradation.
What is solar panel degradation?
Solar panel degradation comprises a series of mechanisms through which a PV module degrades and reduces its efficiency year after year. Aging is the main factor affecting solar panel degradation, this can cause corrosion, and delamination, also affecting the properties of PV materials.
Other degrading mechanisms affecting PV modules include Light-Induced Degradation (LID), Potential-Induced Degradation (PID), outdoor exposure, and environmental factors. There are several tools and techniques used to determine solar panel degradation, these include visual inspection, infrared thermography, electroluminescence (EL), and performance calibration.
While PV technology has been present since the 1970s, solar panel degradation has been studied mainly in the last 25 years. Research Institutes like NREL have estimated that appropriate degradation rates of solar panels can be set at 0.5% per year with current technology.
What is the impact of solar panel degradation on your PV system?
Solar panel degradation is caused by aging and does not only affect large PV installations, but it is present on every rooftop PV installation worldwide. This is why it is of concern for homeowners with rooftop PV systems and households consuming solar energy from the grid.
Appropriate degradation rates of solar panels are estimated at 0.5% per year considering a well-maintained PV system featuring ideal conditions. However, solar panel degradation rates can reach up in some extreme cases, going as high as 1.4% or 1.54% per year.
This information highlights the importance of installing high-quality PV modules manufactured by reliable companies and performing maintenance on solar arrays. Taking every precaution will ensure minimal solar panel degradation rates and a longer lifespan for PV systems.
The higher the degradation rate, the higher energy losses the PV system will experience throughout its lifetime. Solar panel manufacturers generally establish a reference of degradation rates and each module type generally has a performance warranty graph that indicates the expected percentage output against the number of years.
A solar panel with a 0.5% degradation rate per year (Hanwha QCells 400W solar panel for instance in Figure 1) is likely to be somewhere close to 87% of its first-year output at the end of its lifetime. Meanwhile, a low-quality solar panel installed under harsh environmental conditions could have a degradation rate of 1% annually, reducing its output to just about 75% of its first-year output. Top quality manufacturers like SunPower, have been able to reduce degradation rates to as low as 0.25%, providing the maximum performance over time in the industry.
What causes solar panel degradation?
Solar panel degradation is not caused by a single isolated phenomenon, but by several degradation mechanisms that affect PV modules, but the main cause is age-related degradation. Additional causes of solar panel degradation include among others, aging, Light-Induced Degradation (LID), Potential-Induced Degradation (PID), and back-sheet failure. Let us analyze them in more detail.
Age-related degradation
Aging is the main degradation mechanism affecting PV modules throughout their years of operation. This degradation mechanism is a direct consequence of modules being exposed for years to rainfall, snowfall, extreme temperatures, hail, dust, and other external agents.
When PV modules are exposed to the aforementioned external agents, they start to decay over time and reduce their efficiency. This occurs by solar panel frames corroding, glass and back-sheet delamination, and PV materials losing their properties, all of these cause the average 0.5% yearly degradation for PV modules.
Light-Induced Degradation (LID)
Light-Induced Degradation (LID) is a phenomenon causing an acceleration in the degradation rates of solar panels, affecting modules mainly during the first year of operation. This is a consequence of sunlight accelerating the oxidation process between the boron used to dope PV materials and oxygen.
These defects occur naturally as oxygen combines with molten silicon during the Czochralski process used to grow mono-crystalline silicon (mono c-Si). The boron used to dope solar cells combines with oxygen and acts as a trap for electron-hole pairs, impacting the power generation process.
Solar panel degradation caused by LID heavily affects heavily modules manufactured with mono-crystalline silicon, especially p-type wafer ones. LID effect is also higher in PERC modules.
Potential-Induced Degradation (PID)
Potential-Induced Degradation or PID is another degradation mechanism affecting PV modules and reducing their efficiency. Unlike LID, PID does not heavily affect a particular type of PV module, but it affects mono c-Si, polycrystalline silicon (poly c-Si), and thin-film PV modules alike.
Large-scale PV installations feature a high voltage per string which causes a potential difference between the cells and the frame resulting in a leakage current, producing power losses.
Understanding of PID is still incomplete, and further study is required, but it is known that it produces high power losses in ungrounded PV systems featuring voltages over 1,500V. This is associated with large utility-scale and commercial PV systems.
Back-Sheet Failure
Back-sheet failure is another degradation cause, being the main cause of premature degradation. It is determined that 9% to 16% of PV modules suffer from backsheet failure. This is a matter of concern since the backsheet of a PV module is the first line of defense that isolates and protects inner components from external agents like moisture, wind, dust, and ultraviolet (UV) light.
The main cause for solar panel degradation due to back-sheet failure is the delamination of the backsheet or the formation of cracks in the material. When the backsheet fails, the inner components of solar panels are exposed to external agents, and the lifespan of PV modules is reduced.
Which factors increase or reduce solar panel degradation?
Just like there are different degradation rates of solar panels, there are factors that accelerate or reduce solar panel degradation. These include the materials used to manufacture PV modules, assembly process, installation process, maintenance practices, and even the weather.
Quality of Materials
Most PV modules that fall under accelerated solar panel degradation do so because of LID, PID, and back-sheet failure. These degradation mechanisms are partially caused by defects in the materials, so it can be concluded that PV modules with better higher-quality materials degrade at slower rates.
Additional materials and techniques can be used to slow corrosion and reduce solar panel degradation. It has been proven that solar panel systems can last for at least 40 years in degraded conditions, but some groundbreaking companies in the solar industry have improved the technology and are offering PV warranties for 30 years and 40 years.
Assembly of the Modules
PV modules may feature high-quality materials, but they require the strictest manufacturing processes to ensure top performance.
Improving manufacturing techniques may reduce solar panel degradation and extend the lifespan of PV modules. The U.S. Department of Energy Solar Energy Technologies Office is currently funding a research team to develop techniques that could extend the lifespan of PV modules to up to 50 years or more.
Proper Installation
When solar panels are being transported and handled during the installation, modules are subjected to mechanical stress. This stress can cause solar panel degradation due to back-sheet failure and produce partial power losses or compromise the PV module components.
To reduce solar panel degradation caused by cracking on the backsheet and increase the lifespan of PV modules, it is recommended that modules are properly handled and installed by certified professionals. This is especially important when dealing with thin-film solar panels, which are more delicate.
Regular Maintenance
Regular maintenance is a vital tactic used to reduce solar panel degradation in large and small-scale applications. Predictive and preventive maintenance can increase the operational lifetime of PV systems by reducing degradation from soiling and dust, resulting in an increased performance of the solar array.
The frequency at which maintenance should be performed may vary considering the presence of dust, snow, fallen leaves, and other climate conditions. The number of birds in the area (associated with bird drops) may also increase the required maintenance frequency.
Weather
Weather phenomena are not a variable that can be controlled, but they can be accounted for when installing PV modules and performing maintenance to avoid further solar panel degradation. Analysis previous to the installation of large-scale PV systems should consider a dedicated study on the location and historical natural disasters, ensuring the location is feasible for the installation.
An important choice that can be taken when preparing against extreme weather phenomena is selecting PV modules featuring better mechanical properties. This may include a better ingress protection (IP) index, harder frame rating, a glass with higher resistance against impact, and more. For instance, the SunPower PV modules are more vulnerable to high-speed winds compared to Q-Cells solar panels.
Final word: Choosing best PV modules to minimize degradation
Considering that solar panels have a limited lifespan, it is important to note that they can be recycled and repurposed for grid operation, EV charging stations, and other applications. The even better news is that researchers are currently working on extending the lifespan of PV modules and developing techniques to reduce further solar panel degradation.
Studies taking place are looking at increasing durability by relying on electroluminescence photography and machine learning, improving lifespan and performance for PV installation with proven results by implementing monitoring control systems. over, implementing cooling techniques using water to reduce PV module temperature has also proven effective in extending the lifespan of solar panels.
Many times solar proposals will account for first-year simulations, which may give you a misconception that this energy performance will be maintained over time when it will not. This is why choosing the solar panel with lower degradation rates is essential to keep performance over time as close as possible to the first year of installation. Most solar panel manufacturers include metrics that indicate the performance warranty for their products, choosing high-quality PV modules with degradation rates similar to the ones from SunPower or even Hanwha Q-Cells, will ensure PV systems that resist aging degradation better than conventional ones and that will provide better results in the long term.
What is PID in solar modules ? Can it be avoided ?
PID stands for potential induced degradation. It is an important issue of performance degradation in crystalline silicon solar panels. The degradation could be high as 30% or even up to 70% in some cases. The degradation occurs in solar energy systems and can be reversible or irreversible.
Potential-Induced Degradation (PID) is a common phenomenon causing PV panels to lose power generation by up to 80%. Power reduction may occur over time or can happen within days or weeks after installation.
The PID process in the PV module may grow very rapidly and in the shortest period will affect the performance of an entire PV system. Consequently, this results in damaging effects on PV system project financing, operations and economics at all installation levels: residential, commercial and utility-base. It is essential to understand and address the PID problem in its early stages, to ensure PV module performance over the entire system life – PID can be prevented and recovered on system level altogether.
Causes of Potential Induced Degradation (PID)
PID occurs because of minor, unwanted currents between the semiconductor on the one side, and the glass, anti-reflective coating (ARC). the frame, and the mounting on the other side.
The degradation in performance is associated with migration of sodium ions. from the glass plate through the encapsulation (commonly: EVA) and the Anti-Reflective Coating (ARC) to the cell.
This would be caused when due to a particular manner of string earthing, the semiconductor in a cell acquires a negative potential with respect to the encapsulation and the support structure.
The presence of these ions causes an effective shunt path across the cell and reduces the output. The effect is somewhat cumulative with time and has a greater extent when the cell is operated at a higher negative potential with respect these parts.
Temperature and humidity are both known to promote PID. However, there is not much one can do about these factors once a system is installed in a given location.
Dependence on Location in String
Recall the words “ higher negative potential ” in the explanation above. If the positive terminal of the string is taken as system ground and the mounting structure connected to the earth potential, the cell closest to the positive terminal has the least negative potential with respect to earth, and hence the least PID effect.
The cell closest to the negative terminal will experience a high negative potential relative to the grounded structure and will undergo maximum PID. Thus cells, modules and panels will experience PID according to their position in the string.
Avoiding / Mitigating PID
The following considerations are applicable:
- Location – for a new plant, within other limitations, a site with lower temperature and humidity should be selected. Note that a windy site will also keep the system cooler.
- Use PID Resistant Hardware – for a new set up, there are modules available which are resistant to PID. However, the cost will be higher due to use of more expensive encapsulating materials, anti-reflective coatings, and other materials. A compromise may have to be made for overall profitability by using panels subject to PID and adopting other mitigating techniques.
- Earthing- Use modules where there is no restriction imposed by the manufacturer on connecting the negative end of the string to system ground.
- Charge Equalizers – are built into inverters. When the inverter is inactive at night, they apply an opposite bias to the panel which cancels out the reversible type of PID effect overnight. Reversible PID is also called polarization.
Any other way to reduce PID Impact :-
Sungrow and TÜV Rheinland have jointly issued PID Zero – an anti-PID solution whitepaper for residential PV systems. Equipped with patented mirror boost topology and an intelligent control algorithm, the innovative PID Zero solution provides 24-hour anti-PID protection, enabling more effective PID suppression during the day and PID recovery at night, significantly reducing power generation losses due to module performance degradation.
What is Potential Induced Degradation?
Talking about solar panels and their lifelong warranty that ranges from 19 to 30 years, their efficiency is lost in this period. Everyone is aware of this, but sometimes your solar modules lose efficiency within a short time like say a year or two. This is stressful but somewhere both the manufacturer and the customer are responsible for this. Lack of information is another thing but getting an uncertified solar panel may be a potential cause for Potential Induced Degradation of your solar panels. Let us discuss the potential induced degradation of solar cells and panels, its causes, tests, and preventive measures.
What is Potential Induced Degradation?
The degradation occurs due to the high potential difference between the cell (semiconductor) and other parts of the module like the mount, aluminum frame, and glass. As a result of this potential difference, there is a current leakage that causes the migration of positive and negative ions.
- Negative ions: It starts leaking from the aluminum frame.
- Positive ions: Also called sodium ions, start migrating and leaking towards the surface of the cell.
This leakage reduces the photovoltaic effect by polluting the cell which further leads to power losses. Power losses due to potential induced degradation can be up to 20%. However, you may not notice it immediately, and it takes months and years for the defect to become noticeable.
What is Potential Induced Degradation of Solar Cells and Panels?
Several conjoined solar cells make solar panels. The main components of the discussion are solar cells and their aluminum frame. Potential induced degradation occurs in solar panels and cells when a solar panel is polarized with a high negative voltage. But there is a considerable voltage change in the aluminum frame and the cell.
The frame is usually grounded, thus there is 0 potential for the current. Due to the presence of impurities and a very short distance between solar cells and their frame, a current is generated between the cell and the frame. This results in leakage of the current from the entire photovoltaic module.
What is PID Effect in Solar? What causes PID Loss in Solar?
In short, it is a sign of the aging of solar cells and solar panels. It basically leads to performance deterioration of the panels. Potential Induced Degradation is the result of numerous factors that work together over a long period of time. It does not happen overnight.
Location
It includes the location of your house in terms of longitude and latitude along with climatic conditions. Also, exposure to humidity and high heat are factors related to the location where the solar panel is installed. These factors differ from place to place.
Potential induced degradation mitigation measures
Devices specially designed to lessen the effect of Potential induced degradation are not properly installed and there is negative grounding. Better devices will reduce the influence of PID otherwise it will worsen with time.
Quality of raw materials
Glass, encapsulated material, presence of impurities (sodium), etc. are considered raw materials of the solar cell. The quality of raw materials matters because they are the building block of the solar cell. For example, the presence of sodium while the glass is manufactured will cause the glass to have detrimental effects.
Quality of solar panel components
The quality of the components of solar panels, like the types of photovoltaic cells used or the material of encapsulated material, is also a factor causing PID. This includes whether they are certified as PID-resistant or not. And have these components passed the extensive reliability testing or not?
Voltage and size of system
Higher voltage systems mean a higher possibility of potential induced degradation. However, a 1000V system is less at risk than a 1500V system.
What is Potential Induced Degradation Test?
It is a type of quality assurance test for manufactured solar modules. It is to reach an estimate of the performance of those solar modules under different conditions over a long time. A PID test for solar modules subjects the module to the following conditions:
- Temperature of 140° Fahrenheit (60° Celsius)
- Humidity around 85%
- Load under 1000v
- Time period of 96 hours
- Voltage biases of 1000 V.1000 V, 1500 V, or.1500 V (as per PV module characteristics)
The time period defines the name of the test that in this case is PID (96h). During this time period, modules will be exposed to the above-mentioned conditions and their effect on power loss of solar modules will be taken into account.
What Equipment are Used in PID Test for Solar Modules?
To perform a reliable test, certain equipment is used during a Potential induced degradation test.
Electroluminescence (EL)
This is a powerful imaging tool that is used for determining the electronic and optical properties of the module. Through Electroluminescence imaging defects like PIC, micro-cracks, and cell fractures are easily noticed. The intensity of electroluminescence observed is proportional to the available number of minority charges in the modules.
Types of panels
A monocrystalline or a polycrystalline module is usually used for PID testing. The type of modules also defines the type of detection techniques to be used for testing.
How to Detect PID?
With the use of devices and techniques like Electroluminescence imaging, detecting Potential induced degradation has become easier. I-V light curves measurements of solar modules before and after imaging shows the possibility of PID. The changing slope of Voc indicates there is an increase in Rs(total resistance). The changes in I-V curves are the indicators.
What is Anti PID for Solar Panel?
To lessen the effect of Potential induced degradation, an Anti-PID box is used. It is a device attached to the inverter of solar modules. It polarizes all the photovoltaic modules that were affected in an opposite way by the negative voltage. With this box, each string keeps on changing the polarization and does not remain in a constant polarization for a long time.
Anti PID box increases the possibility for the modules to recover from the negative potential already suffered by them. It also reduces the probability of PID.
What are Anti PID Solar Panels?
Since PID occurs due to a number of reasons, material and system quality being the main of those reasons. Industries decided to manufacture Anti-PID solar panels. For this they selected:
- Solar cells with PID-free designs
- Highly resistant module encapsulation materials
Such solar panels are connected to the string and are not damaged by high string voltage. This induces more production by solar panels for longer periods. Get anti-PID solar panel here
What is PV System Configuration?
There are 5 different configurations for a Photovoltaic System that need to be finalized before installing the solar panel system.
Grid-tie that feeds all the solar-powered electricity to grid
Under this configuration, all electricity produced by your solar system is fed into the grid. You need to buy the required amount of electricity from the companies. Such a system has less installation cost, and you can even earn money by selling power to electricity companies via a feed-in tariff. This tariff scheme is available only at particular locations. Where they are available, companies buy power per kilowatt-hour for the agreed price.
Grid-tie that only feeds the surplus solar-powered electricity to grid
This configuration enables you to use the solar power generated by your panels. Plus, if there is additional power generation, it will be fed to the grid. You need to buy the required power from the company if needed. This configuration is beneficial as it reduces the reliability of big electrical companies. It is for hot and sunny climates along with people who use most of the power generated by the modules. However, a power cut from the grid means a power cut from a solar array too.
Grid-tie with battery backup
This system is also known as a grid-interactive system as it is installed with a battery bank. In case of power cuts, you get a constant power supply with stored power in these batteries. It is costlier because of additional batteries and controllers.
Stand-alone with grid power charge function
Such configuration is best applicable for home applications when the solar power is insufficient as per requirements. This system charges the battery automatically by grid alternative current (AC) power.
Stand-alone without grid power charge function
Also known as off-grid, it is provided for locations where there is no other source available. This system is the sole power provider. It has a peak power generation of less than 1 kilowatt and is usually smaller.
How to Prevent Potential Induced Degradation?
Various factors should be determined to reduce or prevent potentially induced degradation of your solar panels and modules.
Certification
A solar panel must have a certificate from in-house solar panel testing and the International Electrotechnical Commission (IEC). These systems test modules for PID resistance under IEC 62804. It is strongly recommended to be aware of the certificates provided to the system and the bill of materials used in the modules.
Solar Panel Components
a) Encapsulating materials
Potential Induced Degradation resistant encapsulate materials should be used while manufacturing and installing the modules. The encapsulant is an additional protection layer against potential induced degradation. It also ensures higher outputs from systems through different seasons.
b) Photovoltaic cells
Solar cells are primarily the cause and the most affected component by potential induced degradation. Thus, solar cells used should be certified as PID-resistant cells. It is highly recommended that additional PID testing should be done by the manufacturer, and after it is installed. Regular testing should be done by a specialized laboratory device.
Solar System Design
a) Grounding
To reduce the risk and effect of potential induced degradation, it is recommended that the grounding should be negative for solar panel systems.
b) PID-resistant devices or Anti-PID or PV offset box
The components of a solar panel system are equally important in reducing the impact of PID. Therefore, installing such devices is a plus. Like a PV offset box, it reverses the voltage on the solar panel system after sunset. In this way, it ensures solar panels are performing their best for more than 25 years.
Well, potential-induced degradation is harmful to solar panels, but you need to pay heed to the factors causing it. Also, preventive measures through mitigating devices like anti-PID solar panels must be undertaken. Precaution is better than losing.
Olivia is committed to green energy and works to help ensure our planet’s long-term habitability. She takes part in environmental conservation by recycling and avoiding single-use plastic.