To address this challenge, researchers and manufacturers are continually developing strategies and technologies to prevent or mitigate the effects of PID. This includes the development of new materials and designs for PV modules that are more resistant to PID, as well as improved testing methods and PID prevention strategies.
2. PID Mechanism
PV systems can experience PID, which leads to decreased performance of PV modules due to an electrical potential difference between their front and back electrodes 
. PID can be caused by multiple factors, including moisture ingress, elevated temperatures, and the presence of certain chemicals. The presence of an electrical potential difference between the module’s front and back electrodes is a primary mechanism responsible for PID 
. Additionally, high temperatures can exacerbate PID by elevating the electrical resistance within the PV module.
The voltage required for PID to occur can vary depending on the contributing factors and the severity of the PID. Typically, PID can be triggered by a voltage of a few hundred volts 
or higher between the front and back electrodes of the PV module. However, the effect of this voltage on the module’s performance can differ based on the extent of PID. As explained in 
, a small voltage (<500 V) may have a negligible impact on the module’s performance, while a higher voltage (e.g., 1000 V) can significantly decrease the power output of the module.
The specific voltage that is needed for PID to occur can vary depending on the specific factors that are contributing to PID and the severity of the PID. In general, PID can be caused by a voltage of a few hundred millivolts 
or more between the front and back electrodes of the PV module. However, the impact of this voltage on the performance of the PV module can vary depending on the severity of the PID. In some cases, as described in 
, a small voltage may have minimal impact on the module’s performance, while in other cases, a larger voltage may significantly reduce the module’s power output.
There are several methods that can be used to conduct a photovoltaic potential-induced degradation (PID) test on a photovoltaic (PV) module. One common method is to use a PID tester 
, which is a specialized piece of equipment that is designed specifically for testing for PID in PV modules. To conduct a PID test using a PID tester, the PV module is typically connected to the tester in a reverse bias, with the front and back electrodes of the module connected to the tester in opposite polarity. The tester then applies a voltage and current to the PV module, typically at a level that is higher than the maximum voltage and current that the module is expected to encounter under normal operating conditions.
The specific voltage and current that are required to connect with a PV module in a reverse bias for a PID test will depend on the specific tester being used and the characteristics of the PV module. In general, the tester will apply a voltage and current that are high enough to stress the module and simulate the conditions that could lead to PID, but not so high as to damage the module 
. Once the PID test is complete, the tester will typically provide data on the performance of the PV module during the test, including any changes in power output or other parameters that may be indicative of PID. This data can be used to assess the risk of PID in the PV module and identify any potential issues that may need to be addressed.
Electroluminescence (EL) imaging is a technique that is used to evaluate the performance of photovoltaic (PV) modules by measuring the light emission that occurs when an electrical current is applied to the module 
. EL imaging is typically used to identify defects or issues in PV modules that may be affecting their performance, such as cracks, damaged electrodes, or other issues 
. This information can be used to identify and address any issues that may be contributing to the durability and reliability issues of the PV modules.
One of the ways in which EL imaging can be used to detect photovoltaic PID in PV modules is by looking for changes in the light emission patterns of the module 
. PID is a phenomenon that can reduce the performance of PV modules due to the presence of an electrical potential difference between the front and back electrodes of the module. This potential difference can cause an electrical current to flow between the electrodes, which can result in changes in the light emission patterns of the module. By analyzing the EL images of a PV module, it is possible to identify any changes in the light emission patterns that may be indicative of PID. This can be useful for identifying PID in PV modules and for determining the severity of the PID 
The PID test is done to ensure that manufactured modules will perform over a long period of time under different conditions. For PID testing of solar modules, as shown in Figure 1a, the module is subjected to a temperature of 60 °C with around 85% humidity and under 1000 V load for a period of 96 h. For example, in Figure 1b,c, a polycrystalline silicon PV module was connected in a reverse bias to 1000 V, which is significantly higher than the module’s open circuit voltage of 24 V. The test was completed for a duration of 96 h. EL images of the module were taken before and after the PID test was completed. As can be seen in Figure 1b, in comparison with Figure 1c, the PID test significantly reduced the light emissions of the solar cells, indicating a severe case of PID occurring in the module. The reduction in light emissions is a clear indication that the PID has impacted the performance of the module and reduced its ability to generate electricity.
Figure 1. (a) Procedure for PID testing. A high-voltage power supply is used to power the PV module, and the leakage current of the module can also be measured. (b) Before the PID test. (c) After 96 h of the PID test.
Ionic migration is one of the mechanisms associated with PID in PV modules. In certain conditions, particularly when subjected to high voltage and high temperature, the migration of sodium ions (Na+
) through the front glass and encapsulant of conventional crystalline silicon PV modules can contribute to PID, as shown in Figure 2
. Sodium ions are naturally present in many materials used in PV module construction, such as glass and encapsulant materials. Under normal operating conditions, these ions remain relatively immobile 
. However, when the module is exposed to elevated voltages, Na+
ions can drift through the module structure due to the presence of an electric field. This drift is accelerated by the high-temperature conditions often encountered in PV systems. Leakage currents can flow from the module frame to the solar cells along several different pathways: (1) along the surface of the front glass, and through the bulk of front glass and the encapsulant; (2) through the bulk of front glass (laterally) and through the bulk of the encapsulant; (3) along the interface between the front glass and the encapsulant, and through the bulk of the encapsulant; (4) through the bulk of the encapsulant; (5) along the interface between the encapsulant and the backsheet, and through the bulk of the encapsulant; and (6) along the surface of the backsheet, and through the bulk of the backsheet and encapsulant.
The diagram illustrates the internal structure of a typical c-Si PV module, which consists of a glass-encapsulant-cell-encapsulant-backsheet arrangement. It also presents a model showcasing the potential paths for leakage currents. In this setup, the solar cells are maintained at a negative bias, while the module frame is grounded. The arrow depicted in the diagram indicates the direction of these leakage currents. Specifically, during negative voltage potential, such as through path 1, positive ions like sodium ions (Na+
) migrate towards the cells 
As Na+ ions migrate towards the negatively biased cell, they accumulate near the surface and create a localized electric field. This electric field can lead to a charge imbalance within the cell, affecting its electrical performance. The accumulation of positive charges near the surface can induce a reverse bias in the p-n junction, reducing the cell’s output voltage and ultimately its power output. The migration of sodium ions is facilitated by the presence of moisture within the module. Water molecules can provide a pathway for the movement of ions and increase the ionic conductivity of the encapsulant material. As a result, humid environments can exacerbate the ionic migration and enhance the occurrence of PID.
To mitigate PID related to ionic migration, various strategies have been developed. One approach involves the selection of low-sodium-content materials for the encapsulant and front glass to minimize the initial concentration of Na+ ions. Additionally, the use of anti-reflective coatings on the front glass can help reduce the intensity of the electric field at the surface and minimize the migration of sodium ions.
Interface charge is another mechanism that can contribute to PID in PV modules. In certain situations, the formation of a charged interface between the silicon wafer and the encapsulant or backsheet material can occur 
, leading to performance degradation. The interface charge phenomenon arises from charge separation at the interface between different materials within the PV module. This charge separation can be caused by either ionic conduction or electronic conduction processes. In some cases, impurities or contaminants present in the encapsulant or backsheet materials can facilitate the formation of charged species at the interface 
When a charged interface is formed, it creates an electric field within the module structure. This electric field can lead to various effects on the PV cell’s performance. One significant consequence is the reduction in the cell’s output voltage. The presence of the electric field can induce a reverse bias across the p-n junction, reducing the effective voltage available for the photovoltaic conversion process. As a result, the power output of the cell decreases, leading to a decline in the module’s overall performance.
3. Areas of PID Research and Development
In recent years, there has been a significant amount of research focused on understanding and mitigating PID. Some of the key findings and developments in this area are described as follows:
Improved PID testing methods: Researchers have also developed new methods for testing the susceptibility of PV modules to PID, which can help manufacturers and installers identify and mitigate potential issues before they occur. New materials and designs for PV modules: Researchers have been exploring the use of new materials and designs for PV modules that are more resistant to PID. For example, some studies have found that using materials with a higher resistance to moisture ingress, such as fluorine-doped tin oxide (FTO) or metal-plated glass, can help to reduce the risk of PID. Finally, PID prevention strategies: Researchers have identified several strategies that can be used to prevent PID, such as sealing the edges of PV modules to prevent moisture ingress, using appropriate ventilation to reduce temperatures, and avoiding the use of certain chemicals that can contribute to PID.