5 - Degradation and Failure Mechanisms of PV Module Interconnects

Abstract

The interconnect of a photovoltaic (PV) module enables the photoelectric current of the PV cell's p–n junctions to be harvested and brought out to the load and power system. In typical crystalline silicon PV modules, the interconnect is made using lead-/tin-coated copper ribbons, which connect the front-side busbar of 1cell to the back side contact on the adjacent cell in that substring of the module. The front-side cell metallization consists of a busbar, finer grid lines (or fingers), and the back-side aluminum contact in an aluminum back surface field cell design. The front-side cell metallization requires that there are tabs of copper ribbon that have a sigmoid shape and connect the front to the back of the cells. The reliability of interconnection technologies is critical to crystalline silicon solar modules to achieve 25years of designed lifetime. The two main PV interconnect challenges have been the reliability of the solder bonding to the cell metallization and the resistance of the ribbons and tabs to cyclic fatigue over lifetime. This chapter reviews the major reliability issue of PV module interconnects, including the PV cells screen printed silver busbar and grid line corrosion, solder joint degradation, and interconnect ribbon failures. New interconnect strategies using electrically conductive adhesives to replace the ribbon, such as the cell carrier sheets used with emitter wrap-through cells or the newer shingled cells and half-cell or smaller sliver cells are also being developed, and their reliability is a focus of current research.