# Progress in Improving Photovoltaics Longevity
## 4 Results
### PV Degradation Mechanisms
#### 4.1.2 Extrinsic Degradation (External Stress Factors)
Poor cell interconnection, deterioration of frames, and increased Rs [49] are only some of the remarkable consequences of corrosion related to encapsulant degradation. * **Backsheet discoloration and cracking**. Due to the variety and possible combinations of the composition materials, the degree to which degradation mechanisms affect the backsheet of a PV module depends on its manufacturing characteristics. Backsheet degradation mainly includes discoloration and cracking. _Discoloration_. In operating conditions, the production of chromophores, which are by-products released in chemical reactions, adds a yellow tint to the backsheet [50; 51]. The appearance of the yellow color in the backsheet foreshadows reduced power output on a long-term scale, as it entails amendments in the polymer structure. In order to combat the undesirable impact of UV irradiance on the module, it is a common practice to insert colorants in the polymers. The gradual deterioration of the outer polymer layer of the backsheet can lead to a local gathering of these colorants, which adds a chalky tint to the module. Backsheet discoloration is more likely to cause delamination and cracking. _Cracking_. Backsheet cracking [52] can be observed in either the outer or the inner layer and is identified as one of the most frequent problems in a PV module, which is directly linked to safety issues. Due to weakened electrical insulation, current leakage and ground faults can arise. Cracking can, furthermore, expedite the entry of moisture, water, chemical substances, and oxygen into the internal part of the module, putting an obvious risk to all its components. Weather conditions also exert a major influence on cracking initiation, as they undermine the stability of the polymer chains and abet fragility. Once created, mechanical stresses lead to rapid crack sprawl, with a rate related to environmental conditions and material processing [53]. * **Potential-induced degradation (PID)**. The term \"potential induced degradation\" has been introduced to describe all the degradation phenomena that can occur on a PV module due to high levels of voltage. The series connection of modules in solar power plants leads to the measured voltage on the order of kilovolts in operation service. More specifically, these high electrical potential levels are developed between solar cells and frames, with the latter being grounded in accordance with safety protocols and regulations. The aforementioned potential difference leads to an inevitable leakage current between the encapsulant of the module and the grounded frame. Regardless of the manufacturing techniques and architecture, all PV modules are prone to PID, while the development of this degradation mechanism depends on the selection of construction materials. Although PID can be observed by climate conditions and environmental agents, such as temperature and relative humidity, potential bias remains its main cause. The magnitude of the effects is strongly associated with the polarity of the applied voltage. For instance, in c-Si-based modules, two different PID mechanisms have been detected [54]: surface polarization (PID-p) and shunting (PID-s). PID-p is a high voltage creepage-related phenomenon that arises from surface polarization as an extension effect of net charge hoarding in the dielectric stack that separates the cell and the frame of the module. This phenomenon occurs more frequently in the cases of a positive biased n-type wafer. The gathered charge changes the surface field distribution of the solar cell, enhancing the surface recombination processes, which significantly diminish both the short circuit current and open circuit voltage. The complete understanding of the PID-p phenomenon, along with a coherent interpretation of the responsible physical mechanisms, remains a challenge for scientists. On the other hand, in PID-s [99], local shunts over the emitter are created.