# Progress in Improving Photovoltaics Longevity
## 4 Results
### PV Degradation Mechanisms
#### 4.1.2 Extrinsic Degradation (External Stress Factors)  
Extrinsic degradation [40] is a separate, broad category of degradation modes occurring on PV modules, which significantly influence their performance and long-term effectiveness during real-world operation. Bubble formation in encapsulants, which leads to delamination and hot spots, can be mitigated by using encapsulation materials with very low permeation rates for oxygen and water. Environmental Stress Factors
* **Irradiance**. The incidence of solar irradiance on PV modules can be described in terms of power, angle of incidence, and spectral distribution, which is a function of geographical features, weather conditions, climatic variations, and air pollution. Polymeric materials, with a resounding presence in PV modules, can be severely damaged by the long-term effect of ultraviolet (UV) photons (280-400 nm), as their energy is sufficient enough to provoke scissions in the element sequence (bonds between carbon and/or oxygen) of the polymer main chain. Extended exposure to UV light inevitably leads to fragility and discoloration of the polymer [77]. More specifically, despite its small power fraction in comparison with UVA (315-400 nm), UVB radiation (280-315 nm) is the most devastating for polymers in PV modules, and it can accelerate the degradation process, especially during summer at lower latitudes, when the module's exposure to the sun is prolonged. * **Temperature**. Temperature is one of the root causes of environmental stress factors related to external degradation in PV modules. Not only does it intervene in the electric behavior of the module, but it is also responsible for additional mechanical stresses, as the individual elements of the modules are characterized by different thermal expansion coefficients. Temperature is also a key factor for speeding up diffusion and chemical reactions in materials, as well as degradation mechanisms on the modules. The phenomena of metallic elements corrosion and degradation of backsheets and encapsulants are found to be in full agreement with the [PERSON] model [85]. Due to the incidence of solar irradiance on a module, a gradient in temperature between the cell or the module and the outer environment is developed. The heat flux distribution in the outer part of the cell is closely related to the geometrical and thermal properties of the neighboring materials. As previously stated, temperature variations affect the expansion and contraction of the module units in a different manner, resulting in thermo-mechanical [86] stresses, which depend on the diurnal, seasonal, and annual temperature and, consequently, appear in a periodic pattern. They can cause deformation, delamination, fatigue failures, cell cracking, and malfunctions in general in the different module components. * **Moisture**. Moisture also has a negative impact on the proper operation of PV modules, as it can cause adhesion issues among the components, corrosion of metallic materials, and delamination. In ambient conditions, moisture can be tracked in different states of matter, namely gas (humidity, water vapor), liquid (rain, water, dew, or condensed humidity), and solid (ice). Moisture in gas form can ingress through polymeric materials and concentrate in the interior of the module, causing complications and degradation of its components. In liquid forms such as rain, dew, or condensed humidity, moisture can be absorbed by the module and induce additive mechanical stresses. Among the adverse effects of liquid moisture, one can point out the erosion of surrounding materials, leakage current, and weakening of the electrical insulation of dielectric materials. Solid moisture as ice can produce mechanical stresses in the outer part of the module, delamination, or further damage to the frame and the front glass. The accumulation of snow or the formation of ice on the front side of the module provokes extra mechanical stresses, leading to possible irreparable damage, cell cracking, and even detachment of the frame [87; 88; 89].