There are many ways reliability is viewed. A reliability engineer’s perspective, among other things, may determine how long a particular design will last in order to provide a warranty. A PV customer would like to know how long the module will last; while a PV investor is interested in the return on investment.
Thin film photovoltaic modules have been deployed in the field for over 20 years. Over time, an understanding of the importance of two key aspects has evolved. The first is the importance of the properties of the material used in different layers of the device fabrication and packaging; and the second important aspect relates to the performance of the modules with respect to these improvements and its stability in different climates. Standards like the IEC qualification tests (IEC61646) attempt to weed out the design flaws in the product pertaining to the initial failure rate, but without a firm understanding of the physics of failure, there is no guarantee that they realistically attack real life problems. A careful and systematic approach to understanding the physics of failure of the device will lead to a better understanding of random failures and also will address the problems during the wear out stage of the product which has not yet been adequately characterized.
In order to obtain the needed confidence in our products, we cannot just perform a battery of tests, we need to accurately determine what those tests are telling us. One way to determine the performance of a product under certain environmental conditions is to perform what are called “accelerated tests” where certain parameters, such as temperature, are increased to higher levels than would be experienced in the field, to obtain failures in a shorter period of time. By studying how the product fails under these accelerated conditions, one can develop suggestions as to how to improve the product, but only if there is an understanding of the failure mechanisms.
Moreover, both indoor and outdoor tests need to be performed to correlate the results to cell, mini module and module performance; understand the failure mechanism triggering a particular failure mode; model the kinetics by understanding the physics of failures ; perform visual inspection; and track performance in outdoor installations.
The PV industry has a variety of processes involved in fabrication, different types of packages, different materials used to improve efficiency, different form factors and packing density, and different mounting methods; and all of this is complicated by the fact that PV modules see various potentially harsh climatic conditions. It cannot be ruled out that a small change in any one of the processes mentioned above could severely compromise reliability. Therefore, we need to identify and, more importantly, understand any and all failure mechanisms that may be operating under a wide variety of conditions. In addition, as the technology keeps evolving, how the performance is affected by material improvements needs to be established. The desired lifetime for a PV product is 25 to 30 years, and the industry must find a way to evaluate whether a product will achieve such a lifetime through a more convenient time scale.
Currently, there is limited information available on actual performance in the field universally available, as this information is often considered proprietary. In order to relate the failure modes and corresponding mechanisms to the reliability of the product, a systematic compilation of failure data is required. The availability of correlated data is crucial to the success and competitiveness of PV technologies.
The goal of PVMC is to provide the industry with the tools to make evaluations of reliability in reasonable time frames. This will enable manufacturers to make process changes and be able to assess the viability of those changes in a short enough time to enable reasonable process development. If manufacturers make a process change that increases efficiency 20% and the product fails in two weeks, the product is worthless. In order to do this in as short a time as possible, PVMC has been soliciting manufacturers for their perceived needs and encouraging them to perform the most important experiments in order to understand the physics of the failure process.
A technical working group for reliability has been formed that asks for input from industry to ensure that the consortium is working on problems of relevance. This is achieved by conducting regular monthly meetings with the PV value chain and by conducting workshops to identify and prioritize the projects.
To address the challenges, PVMC conducts indoor and outdoor accelerated testing on cell, mini-module and module levels; performs in-depth characterizations to address failure mechanisms; and relative to both indoor and outdoor testing, develops models to understand the physics of failures and estimates the lifetime of the product. PVMC then integrates all the data, experiments and reports for members to access. PVMC has also partnered with the National Renewable Energy Laboratory to work on projects to establish necessary standards for thin films in terms of reliability.