The impact of atmospheric species on the degradation of CIGS solar cells and molybdenum films

CIGS solar cells and non-covered molybdenum areas and scribes were exposed to liquid water purged with the atmospheric gases carbon dioxide (CO2), oxygen (O2), nitrogen (N2) and air in order to investigate their degradation behavior. The samples were analyzed by electrical, compositional and optical measurements before, during and after exposure in order to follow the degradation behavior of these samples as a function of time. The CIGS solar cells showed a rapid decrease in conversion efficiency when exposed to water purged with a combination of CO2 and N2 as well as to water purged with air, while their efficiency was slowly reduced in unpurged water and water purged with N2 or O2. Cross-section SEM showed that the exposure of samples to H2O with large concentrations of CO2 led to the dissolution of the ZnO:Al layer, likely starting from the grain boundaries. Preliminary studies showed that molybdenum films and scribes degraded in the combined presence of H2O and O2, while they were stable in the presence of H2O combined with N2 or CO2. Degradation was the most severe on positions where the molybdenum was mechanically damaged and the MoSe2 film was removed before exposure, for example in the middle of the P3 scribe. Exposure to H2O and O2 led to the disappearance of the metallic molybdenum, leaving behind an insoluble red brown material, which is likely a molybdenum oxide such as MoO2.