low iron glass which has better transmittance than the standard soda lime (window) glass. Some thin‐film PV modules do use regular soda lime glass to keep the cost down. In addition, most cry‐Si modules use tempered or heat‐strengthened glass to provide added strength to withstand wind and snow loads as well as hail impact. Some thin‐film modules can't use heat‐strengthened glass because the thin‐film deposition process occurs at such a high temperature that the heat strengthening would be removed from the glass. In this case, the modules are usually built with double glass (glass on front and back) to provide the strength necessary to survive in the field.
Encapsulant: The encapsulant is the material that surrounds the cells and the “glue” that holds the whole package together. The encapsulant should provide good adhesion to all of the other components within the module so that everything in the package stays stuck together for 25 or 30 years. This is usually assisted by addition of a primer into the encapsulant formulation itself. Of course, any of the encapsulant material that is used in front of active solar cells must be optically transparent and resistant to UV exposure. Since the encapsulant surrounds the solar cells, it helps to provide electrical isolation. So, materials used as encapsulants must have low‐bulk conductivity (or high‐bulk resistivity) to minimize flow of leakage currents. Some encapsulants are cross‐linked during module lamination to provide stability at the high temperatures at which they operate. Others, however, do not have to be cross‐linked since they are stable enough not to flow or creep at typical module operating temperatures. Typical examples of materials used as encapsulants are listed below:
Ethylene vinyl acetate (EVA) has been used in more modules than any other encapsulant as it is reasonably priced and readily available as a formulated film for PV with primers, cross‐link agents and UV stabilizers incorporated into the film itself.
Silicones were used in the early days of PV and worked well in the field but were abandoned due to their high costs and because liquid encapsulants were more difficult to use in manufacturing.
Polyolefins are similar to EVA and are also available as formulated sheets. They have become more popular as a replacement for EVA in recent years.
Ionomers were used in the past, especially by Mobil Solar and ASE Americas. They were typically used in glass/glass constructions but the particular formulation used had problems with delamination in the field probably due to less than ideal adhesion to the glass.
Backsheets: As the name implies, the backsheet is the outside material on the back or non‐sun side of the PV module. The functions of the backsheet include:
Protecting the rest of the module from the weather – rain, snow, hail, etc.
Screening the materials inside it from UV.
Providing protection from the high voltage within the module, so backsheets must have high resistance and high dielectric strength.
Providing protection from accidental exposure to the active components within the module. So, the backsheet must have high‐tensile strength and be scratch resistant.
The backsheet is the first line of defense in case the module catches fire so its properties will impact the fire rating that a module can obtain.
The backsheet must provide for secure bonding of junction boxes, connectors, frames and/or mounting rails.
Backsheets are usually comprised of multi‐layers of material as the different layers provide different functions. Often one of the layers (usually the center layer) is a poly(ethylene terephthalate) (PET) or polyester to provide the dielectric isolation and high resistance. The outer layer has to provide UV resistance to the layers inside and is most often composed of a fluoropolymer. The inner most layer must bond well to the encapsulant. One typical example of a multilayer backsheet is Tedlar/polyester/Tedlar.
Edge Seals: Modules that are constructed with impermeable (or extremely low permeability) front and backsheets designed to protect moisture‐sensitive PV materials, may suffer from moisture ingress from the sides. Edge Seal materials are low‐diffusivity materials that are placed around the edges of a module between the impermeable front and backsheets to prevent moisture ingress. Edge seals were borrowed from the insulated glass industry where they are used to keep moisture from penetrating between the two panes of glass. In addition to restricting moisture ingress, edge seal materials must have high electrical resistivity to provide electrical insulation as frontsheets and backsheets do. To continue to perform these functions for the lifetime of the module, edge seal materials must remain well adhered to the front and back sheets of glass. Edge seal materials are usually made of Polyisobutylene and filled with desiccants to keep moisture from penetrating throughout the useful life of the module.
Frontsheets: Frontsheets must meet all of the same requirements as backsheets, with the additional requirement of having high optical transmittance over the wavelength range that solar cells are effective, for example from about 300 nm to 1100 nm for cry‐Si. Glass is used the most as a frontsheet, but some modules are made using fluoropolymer frontsheets, particularly Tefzel or Ethylene tetrafluoroethylene (ETFE).
1.4 What Does Reliability Mean for PV Modules?
Since the book is entitled “Reliability of Photovoltaic Modules,” it is important to explain what reliability means in this context. In the simplest sense, reliability of a product means that it performs the promised functions for the expected lifetime. For a PV module, the promised function is continued production of the specified amount of electricity in a safe manner. Typically, the product warranty will specify both the amount of power retained (something like retention of 80% of initial power) and the time period over which it should retain this power level (often 25 years). So, if a particular module fails to produce this amount of power or if it becomes unsafe (maybe it has too high a leakage current or it falls out of the frame), then it is considered a failure. For a particular type of module, the term reliable would indicate that a certain percentage (maybe something like >95%) are still operating within the specified power limit throughout the length of the warranty period.
Another word often used to describe PV modules is their durability. The dictionary defines durability as “the ability to withstand wear, pressure, or damage.” PV modules must be durable since they are exposed to the stresses of the outdoor environment when deployed. For modules, durability has come to mean the continued ability to provide output power over its lifetime. Rather than focusing on failure as reliability does, durability focuses on maintaining the output power level. As a module ages, it may slowly degrade in output power. This is usually reported as an annual degradation rate although the rate is often not linear, but that is the way it is reported for simplicity. So, what does it really mean if one particular module type deployed at a specific geographic location has a reported annual degradation rate of 1% after having been deployed at the site for five years? This really means that the average module output power has dropped by 5% over the five‐year exposure time. The annual degradation rate is an important factor used in evaluating potential financial payback from the investment in the PV array. Investors want to know what annual degradation rate they can expect from the module type they are purchasing.
The final term to discuss in this section is module lifetime. This is a measure of how long the module will continue to produce power at the specified level in a safe manner. The warranty typically provides an estimated lifetime which should be the minimum time over which the product will continue to operate and meet the warrantied power. Once again, these values are typically 80% output power after 25 years. The real question is how the customer defines the lifetime. When does the owner of the PV modules decide that they have degraded too much and need to be retired and perhaps replaced? Experience indicates that the 80% level may be a reasonable benchmark to use because once modules have degraded below this power level, they often start having other problems like ground faults, breakage, etc. So, while there is certainly no consensus on defining module lifetime, this book will consider the useful life of a module over when the power has decreased to less than 80% of the original specification. For an array of modules, this would typically mean that the power of the array or the average power