Degradation of multijunction photovoltaic gridlines induced via thermal cycling

A well-known but heretofore uncharacterized failure mechanism in multijunction photovoltaic cells involves the development of cracks in the top cell directly adjacent to metal gridline structures. In this study, we systematically explore the potential evolution of stress, grain size, roughness, and hardness of metal gridlines during thermal cycling as it pertains to top cell cracking behavior. We discover that although top cells are found to crack after many cycles, this is not due to an accumulation of stress or damage, but rather a progression of strain hardening within the metal gridlines due to cyclic plastic deformations, quantified as an increase in hardness of as much as 57%. Furthermore, optical and topological characterization reveals morphology changes a the gridlines' top surfaces, lending some insight to commonly observed bus bar wire-bonding issues. Ultimately this suite of characterization techniques not only reveals the underlying behavior leading to gridline-induced top cell cracking failures in multijunction photovoltaics, but also suggests a route forward for the development of improved gridline materials.