# Progress in Improving Photovoltaics Longevity
## 4 Results
### PV Degradation Mechanisms
#### 4.1.2 Extrinsic Degradation (External Stress Factors)
This phenomenon is more likely to be found in a negative p-type wafer, mainly because the external bias pushes the sodium ions to move from the cover glass to the cell and turn into metallic sodium in the emitter. PID in a PV module is inextricably linked to increased recombination and shunt formation, which negatively affect the proper functioning of solar cells. In addition, it can cause considerable power losses due to the decrease in efficiency and performance. PID exhibits reversibility, provided it is detected in time. Once the phenomenon is identified, the module is unshackled from the potential and is left to naturally cool to room temperature in order to restrict power loss [57]. Alternatively, the recovery procedure is focused on a reverse bias application and was estimated at 100 h, as found by [PERSON] et al. [58]. The supervision of the PID effects during stress tests in the laboratory is often carried out by electroluminescence (EL) imaging. * **Light-induced degradation (LID). ** Light-induced degradation (LID) refers to the degradation of a PV module due to exposure to solar light. LID occurs in either thin-film [100] or mono-crystalline Si solar cells and directly or indirectly impacts all module components. The magnitude of the effect is relatively small, measuring up to 5% of total module power. The injurious outcomes caused by LID could be summarized as the appearance of metastabilities in the semiconductor layers, which are induced by high-temperature processing and diffusion phenomena among the different layers [59]. The two main recognized causes of LID in c-Si PV modules are the Czochralski process utilized for cell construction [60; 101] and iron-boron pair dissociation [62]. In the first case, the crystal lattice is enriched with oxygen atoms, while the doping process with boron atoms leads to boron-oxygen complex formation on the wafer. A prerequisite for the formation of these defects is light exposure, with subsidiary action of recombination carrier-triggered mechanisms. This effect results in a decreased minority-carrier lifespan in the layers of the cells. In the second case, the interstitial iron impurities contained in c-Si solar cells create pairs with boron atoms. Light exposure causes pair dissociation and interstitial iron as defects are found in the lattice, acting as recombination centers. The performance of the module can, in most cases, partially recover during summer [102], while in winter, a further decrease is observed. The LID degradation effect can be identified by constant monitoring of the module and continuous comparison of its I-V characteristic curves, which provide information about the current and the voltage of the module. The LID stabilization procedure, although it requires a long period of time, has proven promising for the efficiency of the module. * **By-pass diode failure**. The installation of bypass diodes [40; 64], often integrated with the junction box, is of utmost importance in the attempt to moderate the losses when the module operates in the reversed-bias mode, allowing the current to circulate by bypassing the bad cells. In cases of significant differences in short circuit current among some cells, those with the lowest values of current can experience reverse bias and, consequently, overheating. The phenomenon can be observed either in the case of a shaded cell, where the cell is illuminated by less light than it is designed for, or in a damaged cell/internal electric circuit. Bypass diodes not only serve an instrumental role in the degradation of the performance of the module, but they are also prone to safety issues, as they exhibit a variety of failure modes, such as thermal leakage,electrostatic discharge, and arching [103]. In extreme weather conditions, like thunderbolts and lightning, the increased electrostatic discharge leads to a high current flowing through the diode in a short time frame. As a result, thermal runaway takes place, and the diode in the cell gradually overheats until it is permanently damaged.