Photovoltaic (PV) energy production is the fastest growing renewable energy source with approximately 100 GW installed in 2018, which brings the cumulative total to over 500 GW. This is the result of an ever-decreasing levelized cost of electricity (LCOE) for PV, mainly attributed to: i) gaining efficiencies, ii) lowering PV module production costs, and iii) improving lifetime.
Especially lifetime of PV systems, and thus reliability, has been gaining importance recently. Different types of degradation are studied to assess the power generation during the lifetime of a module. Potential-induced degradation (PID) of PV modules is an important degradation mechanism caused by a high system voltage that drives ion drift towards the solar cell. These ions affect the proper functioning of the silicon solar cells and have been shown to induce rapid and significant performance losses of up to 50% at module level within one year in the field.
WHAT IS POTENTIAL-INDUCED DEGRADATION?
Today, PV system voltages of 1000 V and more tend to become common practice. The evolution towards higher system voltages is encouraged by the increasing scale of PV parks since it leads to efficient cabling of the system: for larger power values, when raising the voltage and keeping the current low, less copper is needed and losses are reduced. However, the high system voltage induces an electric field between the grounded PV module frame and the solar cell matrix, leading to alkaline metal ion (mostly Na+) drift into the solar cell, as shown in figure 2. This phenomenon causes severe power losses and is called potential-induced degradation (PID). The Na+ origin is still unclear; some state the soda lime glass as the source, others initiated PID in PV modules in the absence of a front glass and hypothesized that the Na+ is present on the solar cell after the manufacturing process.
Figure 2: Alkaline metal ion drift towards the solar cell under influence of a strong electric field.
The occurrence of this failure mode depends on both the magnitude and the polarity of the electric field. When looking at PV modules in the system, those located at the positive pole of the string are not affected by PID since the Na+ drifts away from the solar cell. The modules located at the negative pole of the string are affected by PID, presented as dark-coloured solar cells in figure 3. PID starts evolving around the perimeter of the PV modules and gradually spreads further away from the PV modules frame as PID progresses. Further quantitative measurements also revealed that the voltage has a superlinear effect on PID progression.
Figure 3: PID occurrence within one PV string. Only solar cells negatively biased towards the PV module frame are suffering from PID.
Mitigating PID solutions
Mitigating PID solutions for new products are demonstrated at cell, module and system level and are finding their way towards industrial applications. At cell level, the anti-reflection coating (ARC) was found to play an important role in PID susceptibility. By increasing the conductivity of this layer during the production process, i.e. by increasing the Si/N ratio in the SiNX layer, the solar cell is less susceptible to PID. This can be explained by a twofold effect: (i) a reduction of the electric field responsible for Na+ drift through the SiNX ARC and (ii) a neutralization of the advancing Na+ ions.
At module level, alternative materials such as PID-free encapsulation materials or aluminosilicate glass can be used in the manufacturing stage, so that they render PV modules PID resistant (or so-called “PID-free”). Such materials are mainly contributing to PID mitigation due to their high electrical resistivity, and thus cancel out the charge drift towards the solar cell.
At system level, the grounding configuration of the PV system can be adapted in such a way that the electrical field causes the alkaline metal ions to migrate away from the solar cell. However, the use of high-efficiency transformerless inverters does not always allow this approach. In addition, the use of module-level inverters or power optimizers significantly decreases the risk of PID due to the limited voltage build-up between the solar cell matrix and the grounded PV module frame.
Preventive measures are very interesting to incorporate in new solar cells, PV modules, and systems. However, they do not bring a solution for PV plants which are already installed in the field and suffering PID. Therefore, PID recovery techniques are investigated and translated into industrial applications. In the field, this is achieved by placing the solar cell matrix at a positive bias to the PV module frame during night time. This renders an out-diffusion process of the alkaline metal ions, cancelling out its detrimental effect at cell level which emerged during day time. This approach is commercialized and can be installed easily in operating PV systems. However, the reversibility of PID on full-size modules depends on the degradation level due to PID. This is shown in figure 4, in which the reversibility of 49 different PV modules is presented. A high recoverability is found for degradation levels under 40% performance loss. When the PV modules performance loss exceeds 85%, they exhibit an irreversible behaviour. From this point of view, it is important to detect and recover PID before the point of no return.
Figure 4: PID reversibility: almost all performance loss can be regained by reversing the high voltage polarity when the performance loss does not exceed 85%.
In big PV plants, PID can be detected by comparing the electrical parameters of different PV strings with each other under operating conditions. For example, a big spread in the operating voltage (VMPP) between different strings has been shown to be a great PID indicator, this is shown in figure 5. Both graphs show the VMPP at string level of a PV park during the day. At the top graph, a PID affected park is presented and a big spread in VMPP can be observed. The bottom graph shows a PV park which is not affected by PID, exhibiting a negligible spread in VMPP.
Figure 5: A big spread in VMPP during the day between different strings of one PV park is a good indicator of PID.
However, comparing the electrical properties of different strings is only possible in big PV parks. Residential installations with only one or two strings do not allow this approach. For PID detection in such residential sites, electroluminescence (EL) imaging is a good PID detection technique. By applying a current through the solar cells (at night), radiative recombinations of carriers cause light emission in the near infrared spectrum, which can be visualized by a simple CCD camera. PV cells affected by PID exhibit a reduced EL signal and are thus observable as dark areas within the PV module. A typical EL image of a PID affected string is shown in figure 6. Note that only the solar cells which are negatively biased to the PV module frame are suffering PID.
Figure 6: EL signal of a PID affected string. Only PV modules located at the negative pole are affected.
Nevertheless, the latter two techniques are useful to detect PID, but they cannot be deployed to quantify the emerged performance loss. In order to achieve this, current-voltage (IV) tracing should be performed at standard test conditions (STC). This is conducted by sweeping the voltage and measuring the PV module’s output current under 1000 W/m2 illumination. However, these measurements are time-consuming as every PV module under test is measured separately and must be disconnected from the string. The normalized IV curves of a full-size PV module with standard crystalline silicon n+/p solar cells before (initial) and after (stressed) PID is presented in figure 7.
Figure 7: Normalized IV curve of a full-size PV module before and after PID.
A thorough understanding of the physics and the underlying signatures of PID mechanisms, both theoretically and experimentally, is of key importance to develop adapted characterization methods and mitigation solutions. Over the last 15 years, PID research mainly focussed on crystalline silicon n+/p solar cells, the dominating technology in today's PV market. Elaborate research has led to prevention and recovery solutions, which are finding their way into industrialization. However, the combination of increasing system voltages, up to 1500 V and more, and the emergence of new PV technologies, such as bifacial PERC and PERT solar cells, render new (and possibly irreversible) PID mechanisms.
Bifacial solar cells are manufactured in such a way they can generate electricity by capturing the incident light at both the front and rear side of the solar cell. For this purpose, they lack a metal contact on the entire rear side of the solar cell. Hence, their rear side looks similar to their front side. Because of this, additional degradation mechanisms due to PID are discovered, as shown in figure 8. Such new degradation mechanisms are occurring at the front and rear side simultaneously, rendering a complex behaviour. However, microstructural studies to proof the root cause and possible mitigation solutions are yet to be addressed for these new bifacial technologies.
Next to the bifacial crystalline silicon market, PID also has to be looked into for thin film PV technologies such as perovskite solar cells where little is known on the PID sensitivity and mechanisms in these materials. This will become even more important when these technologies will be combined in tandem cells in PV parks of the future.
Figure 8: Bifacial PID of bifacial p-PERC solar cells when using a glass/glass module configuration.
- PID of PV modules is caused by a high voltage difference (in the order of 1000 V and more) between the grounded PV module’s frame and the solar cell matrix;
- drifting alkaline metal ions, predominantly Na+, into the solar cell under influence of a strong electric field are shown to be the root cause of this failure mode;
- PID can be mitigated at cell, module and system level;
- PV modules exhibiting a performance loss of up to 85% are fully recoverable, which underlines the importance of early PID detection;
- the combination of increasing system voltages and the emergence of new PV technologies renders new PID mechanisms and further research is needed.
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