# Progress in Improving Photovoltaics Longevity
## 4 Results
### PV Degradation Mechanisms
#### 4.1.2 Extrinsic Degradation (External Stress Factors)
The PV module is characterized by a moisture saturation limit, which can be exceeded with a decrease in temperature. This excess causes the creation of droplets at the interfaces, metallic elements, and cell surfaces, which, in turn, lead to reduced module performance and losses. From a manufacturing point of view, modules contain low water levels, but on a long-term scale and in operating conditions, internal moisture levels will increase. In order to extend the lifespan of PV modules, the attention must be focused on the time needed to achieve the equilibrium moisture concentration. This temporal limit is calculated up to a week and to several years for a glass/backsheet module (breathable construction) and a glass/glass module (unbreathable construction), respectively [90; 91]. This specific difference is directly associated with the ease or difficulty in moisture movement from the module to the environment and vice versa. * **Soiling**. Soiling [92; 93] is a very broad category of PV module degradation that affects its performance and effectiveness. It may arise from the accumulation of dust and bird droppings on the surfaces, or it can be attributed to air pollution. The level of soiling is a complicated function of the properties and the location of the surface, the height from the ground in which the module has been installed, and the way all components of the modules have been implemented. The climate conditions are also a key parameter to the degree of soiling, as they can obstruct the path of light into the cell. For example, in desert climates [94], levels of dust soiling are extremely high. A uniform distribution of biological soiling or dust does not weaken the reliability of PV modules on a long-term scale, but it can significantly impact the performance of the module on the field. However, bird droppings, with an irregular shape and a random distribution on the front surface, can interrupt the light transmission of the module, leading to reliability and performance failures [95]. More specifically, this type of soiling can induce the formation of hot spots, localized areas where temperatures are particularly high (on the order of hundreds of degrees). Due to their catastrophic impact on the module packaging and the solar cell, sources of soiling must be treated with particular attention, including frequent cleaning procedures. * **Air Pollutants**. Air pollutants [96, 97, 98] of natural or artificial/industrial origin in a high concentration can significantly degrade the performance of the various components of a PV module, such as backsheets, junction boxes, connectors, and wiring. Different kinds of pollutants are dominant in various regions: ammonia in rural areas and salt mist in coastal areas. This type of extrinsic degradation also raises legitimate concerns about the safety of the PV modules, as problems with decreased electrical insulation may arise. Summarizing results are presented in Table 2.  
Increased temperature can, in most cases, cause solar cell bonding issues, which explains bubble formation near the interfaces between different solar cells. It should be mentioned that the appearance of bubbles increases the reflectance of solar irradiance, which further degrades the PV module's performance. * **Corrosion and decreased adherence to contacts**. Metal corrosion and poor adherence of contacts are two of the most frequent degradation modes a solar module can face. Corrosion can be caused by the permeation of oxygen and moisture through the laminated edges of a PV module [47]. The entry of moisture can furthermore lead to current leakage, which diminishes power, weakens metal contacts of the junction box, and increases series resistance (Rs). In order for corrosion to take place in a PV panel, a metallic element, an electrolyte, and an oxidation factor should coexist [48]. Although being a long process, the impact of aging on the PV components, especially the encapsulant and the backsheet, can gradually lead to corrosion.