The degradation of photovoltaic (PV) systems is one of the key factors to be addressed in order to reduce the cost of electricity produced by increasing the operational lifetime of PV systems. In order to reduce it, it is essential to know the phenomena of degradation and failure. This article has been prepared to present an overview of the state-of-the-art knowledge on the reliability of photovoltaic modules.
Introduction
The economic and social impact of photovoltaics (PV) is enormous and will continue to grow rapidly.
The quality and commercial attractiveness of a photovoltaic system is mainly determined by its field performance, cost and lifetime, to which the photovoltaic module contributes significantly. During the operational life of a module, it will be exposed to simultaneous environmental stresses such as sunlight, heat and cold, humidity and mechanical loads. These factors often lead to a gradual decrease in performance and, in some cases, to sudden failures and power losses. It is desirable to limit these effects.
Crucial is ensuring long-term reliability. Our engineers strive to minimise degradation as much as possible and quantitatively predict phenomena that cannot be eliminated. This will allow a more accurate estimation of the expected lifetime of a module and its electrical performance. Such estimates are necessary for large-scale investments, as investors, banks and insurance companies want to minimise risks and uncertainties. To optimise reliability and predictability and to increase module lifetime, it is crucial that degradation and failure mechanisms are known and can be easily recognised and contained.
Renewable energy technology such as photovoltaics will play a key role in the energy transition to a net-zero energy system, however, it is crucial that different photovoltaic technologies continue to evolve to enable further cost reductions, but also to reduce environmental impact and facilitate reuse and recycling. It is clear that new device architectures and materials must take into account scale effects, the ability to have a long lifetime and a low environmental impact. This paper summarises the reliability challenges that must be met in order to achieve a longer lifetime and further cost reduction.
During actual operation, PV modules are exposed to various external and internal stress factors that affect their long-term performance and reliability. While external stress factors are related to environmental conditions, internal stress factors are caused by the PV modules’ bill of materials and processing effects.
Environmental stress factors
1. Irradiation
The main characteristics of solar radiation incident on PV modules are its power, spectral distribution and angle of incidence.
The spectral distribution and intensity level of incident irradiation depend on a number of factors, such as variations in stratospheric ozone with latitude and season, time of day, season, azimuth (compass angle), tilt angle to the horizontal, cloud cover, surface reflection, altitude and air pollution. UVB (280-315 nm) is the most damaging part of UV light, particularly for polymeric materials in photovoltaic modules, although its power fraction (∼1.5%) is small compared to that of UVA (315-400 nm; ∼98.5%) according to the reference spectrum. This fraction fluctuates for incident UV light: it is highest at lower latitudes, during summer and in the mid-afternoon hours, when the sun’s rays are strongest, and faster degradation can be expected in these outdoor conditions.
2. Temperature
Temperature is a key environmental stress factor, as it can directly affect the electrical performance of a photovoltaic module, accelerate permeation and reaction rates in materials, and induce mechanical stress due to differences in the thermal expansion coefficients of module parts. The temperature inside the cell or module may differ from the ambient temperature, in particular due to incident radiation. The heat flow out of the cells depends on the geometry and thermal conductivity of the surrounding materials, the wind speed and the installation configuration of the photovoltaic module.
Temperature has an accelerating effect on many mechanisms of module degradation, particularly those related to chemical reactions and diffusion. The temperature dependence of these effects is often modelled using the Arrhenius equation with a mechanism-dependent activation energy. The degradation of encapsulating and backsheet films and the corrosion of metallising elements follow the Arrhenius behaviour.
Due to the mismatch of thermal expansion coefficients, module materials expand and contract at varying rates as the temperature changes. This mismatch can induce thermomechanical stresses within the module structure. The mechanical stability of active electrical elements, such as cells, solder joints and interconnecting strips, is particularly affected by such stresses. Deformations, delaminations at the module interfaces and even cracks in the cells can occur. Diurnal and seasonal temperature variations produce cyclic thermomechanical stresses that can lead to fatigue-induced failures in the various module components.
3. Humidity
Moisture is another important stress factor for photovoltaic modules, as the ingress of moisture can deteriorate the adhesive bonds at the interfaces between module components, causing delamination, causing loss of passivation and degrading anti-reflective coatings; it also leads to corrosion of metallising elements. Under external conditions, moisture occurs in different forms, such as water vapour (or humidity), condensation (or dew), rain, snow and ice.
Since water vapour is in gaseous form, it can permeate through polymeric packaging materials, accumulate within the module structure and induce degradation of module components. Liquid water, especially condensed moisture, dew or rain, can also be absorbed or desorbed. The ingress of water in large quantities can induce mechanical stresses due to the hydrodynamic expansion and contraction of the volume. It can also erode low molecular weight species and additives from polymeric materials. It can also dissolve ions, deteriorate the electrical insulation of dielectric materials and cause leakage currents. In solid form as ice, it can undergo volume changes during freeze-thaw cycles and produce mechanical stresses on the outer side of the photovoltaic module that lead to delamination of the front glass or damage to the frame.
When the module is saturated with moisture, a drop in temperature can cause the moisture level to exceed the saturation limit, leading to condensation in the form of water droplets, particularly on the interfaces, cell surfaces and metallisation elements. While weakened interfaces can delaminate and create additional pathways for moisture to enter, short circuiting and corrosion of metallisation can cause a significant loss of performance due to increased resistance. Modules hardly contain any water after manufacture, but internal moisture concentrations increase over time in the field. The time required to reach the equilibrium moisture concentration level is one of the key parameters for the lifetime of photovoltaic modules. It has been estimated from a few days to a week in a breathable structure (glass/backsheet module), but up to a few years in a non-breathable structure (glass/glass module).
4. Mechanical load
Photovoltaic modules can be subjected to various mechanical stresses due to manufacturing processes, transport, handling during installation, wind, hail, snow and thermo-mechanical loads. Since solar cells and metallisation elements are thin and fragile, they are susceptible to such stress conditions and must be protected against cracking or breaking. Embrittled backsheets are also subject to mechanical loads. The front glass and the frame can also be damaged by mechanical loads, which can lead to the breakage of the photovoltaic module. For polymeric packaging materials, mechanical stresses can create or extend cracks, particularly when the mechanical strength of a polymer is already weakened by other environmental stress factors, such as UV radiation or moisture.
Depending on the orientation, modules often experience mechanical stress on the front side during operation. Snow load is a static stress factor and long-term accumulation on the front side can exert a significant force on the module, causing cracks in the cell. If the module is tilted, snow accumulation and ice formation on the edge of the module can induce bending forces and damage the rigidity of the frame. Detachment of the frame from the module can even occur. In this case, the modules lose their environmental and electrical protection. Wind, on the other hand, is a dynamic stress factor and can apply forces to the front and rear depending on its direction. The mechanical loads due to wind can be significant, as its direction and speed can change suddenly during gusts. Prolonged exposure to cyclic wind gusts can cause micro-cracks to form in the cells and induce fatigue failures of the metallising elements. The modules in the mounting structure must therefore have a certain degree of torsion to withstand wind-induced vibrations.
5. Dirt
Dirt can result from dust accumulation, air pollution, microbial algae growth or bird droppings on the module surface. Uniform accumulation of dust or biological dirt does not affect the long-term reliability of PV modules, unlike other failure mechanisms such as corrosion, delamination and cell cracking, but it can affect power performance and this must be considered when measuring degradation outdoors. The degree of soiling may depend on the module surface properties, installation location and module mounting configuration, such as tilt angle and height above ground. Dew formation or drying cycles can cause dust particles to cement together, making them difficult to remove by natural cleaning. Dust dirt can be significant especially in desert climates. On the other hand, in tropical climates with high humidity and frequent dew formation, biological dirt can also hinder light transmission in the solar cell.
Bird droppings can be considered a form of biological dirt, but their effect is different from that of dust and biological dirt. Due to their larger size and non-uniform formation on the front surface, they can block the transmission of light locally and thus significantly affect the performance and reliability of the module. Thus, they can act as partial shading of the module and lead to cell mismatch phenomena which, if not cleaned, could lead to hotspot formation. Hotspots are localised areas of high temperature, sometimes exceeding several hundred degrees. They are potentially one of the most serious types of module degradation because they can be dangerous and cause significant damage to the solar cell and module packaging. Hotspots form in areas where large currents pass through a small resistive area and can be caused by shading, dirt and damaged cells or connections (metallisation, interconnections).
6. Chemicals (natural and industrial pollutants)
Certain naturally occurring or industrially produced chemical species can cause corrosion of PV modules. The most common are salt spray in offshore areas (particularly harmful in tropical climates), ammonia in rural agricultural areas, and sulphuric and nitric acid in industrial areas. These stress factors can degrade various PV module components, such as backsheets, adhesive edge sealants, junction boxes, wiring harnesses and connectors. In addition to performance degradation, they can cause safety problems due to compromised module insulation.
In addition to the environmental stress factors described above, internal factors such as module design, bill of materials and processing effects can also cause or influence module degradation.