Failure modes of silicon heterojunction photovoltaic modules in damp heat environment: Sodium and moisture effects

Silicon heterojunction (SHJ) solar cells are expected to gain significant market share in the coming years. In the field, among identified degradation modes, moisture-induced degradation can be a significant concern for this solar cell technology and should be monitored. This work investigates the moisture-induced degradation mechanisms in SHJ cells encapsulated in different module configurations. Damp heat (DH) testing was performed under IEC 61215 standard conditions (85 °C and 85% relative humidity) for up to 2000 h. Different degradation mechanisms are identified after DH aging, due to moisture alone or in combination with sodium ions originating from photovoltaic glass leaching. Under the influence of moisture, these ions can migrate into the cell and degrade the cell passivation, resulting in massive power losses up to 57.6% of the initial value after 1500 h of DH aging. By using other types of glass, glass-glass module configurations show less than 3% of power losses after 2000 h of DH aging. The front side of the cell is much more sensitive than the rear side where the emitter of the cell is. After highlighting the impact of sodium, moisture alone was studied with a module configuration without glass. In that case, the degradation is characterized by increased series resistance without passivation losses.