Damp Heat Induced Degradation of Silicon Heterojunction Solar Cells With Cu-Plated Contacts
Damp heat exposure is one of the most stringent environments for testing the durability of solar cells in packaged modules. Damp heat stresses and induces a variety of degradation modes in solar cells and modules: for example, moisture-induced corrosion of electrodes and interconnections, deterioration of polymeric materials, and/or thermally activated diffusion processes. To screen for these and other potential degradation modes, we subject one-cell modules containing silicon heterojunction (SHJ) solar cells with Cu-plated contacts to extended damp heat tests at 85 °C/85% relative humidity. SHJ cells were laminated with two common encapsulants: ethylene vinyl acetate (EVA) and polyolefin elastomer (POE), and two constructions: glass-backsheet and glass-glass. We observe degradation in all components of solar cell maximum power (PMP): current, voltage, and fill factor, and find evidence of increased carrier recombination and nonideal diode behavior with increasing stress. For glass-backsheet constructions, EVA samples generally degrade more than POE by a factor of approximately 1.5x PMP, and the different encapsulants produce different degradation patterns. Similar trends are observed in glass-glass modules, but to a lesser degree. In a different experiment, we observe a decrease in effective minority carrier lifetime of nonmetallized SHJ precursors measured after damp heat. This implies that some degradation unrelated to the contacts is to be expected and confirms the observation of increasing recombination.