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E5.02 - Adhesion and Reliability of Complex Multijunction Photovoltaic Structures 
April 8, 2015   10:45am - 11:00am

Concentrating photovoltaics (CPV) incorporating high efficiency multijunction photovoltaic cells are a leading candidate for large-scale ground-based solar energy installations. As in other solar technologies, reliability over extended operating lifetimes is an important metric for success. Despite the successful application of multijunction cells in space based systems, the application of multijunction cells with their complex layered structures in terrestrial applications requires an improved understanding of thermomechanical reliability and testing metrologies as a basis for improved lifetime predictions. Of particular concern is the adhesion of the many internal interfaces including those involving backside metal contacts, substrates, active layers, antireflective coatings, and frontside metal gridlines. The effects of stressing parameters that include mechanical stress, temperature, and UV exposure in the presence of humidity from terrestrial environments are also of significant interest. We discuss modified thin-film adhesion testing metrologies together with the first quantitative measurement of adhesion of selected interfaces within state-of-the-art multijunction cells. In particular, we address the adhesion of several 2- and 3-layer antireflective coating systems on multijunction cells along with frontside gridlines and backside metal contacts. Special modifications to previously established techniques were necessitated by the fragility of germanium and gallium arsenide substrates ubiquitously used in these devices. By varying interface chemistry and morphology through processing, we initially demonstrate the marked effects on adhesion and help to develop an understanding of how high adhesion can be achieved, as adhesion values ranging from 2 J/m2 to 12 J/m2 were measured. Furthermore, long-term temperature cycling and high-humidity exposures demonstrate environmental degradation modes and begin to build the basis for physics-based degradation models.

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