The impact of atmospheric species on the degradation of CIGS solar cells

CIGS solar cells were exposed to liquid water purged with the atmospheric gases carbon dioxide (CO₂), oxygen (O₂), nitrogen (N₂) and air in order to investigate their chemical 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 solar cells in time. The 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. This resulted in an increased series resistance, which is likely related to an increase in resistivity of the ZnO:Al layer. It also led to a very rapid decrease of the short-circuit current of these samples. Therefore, the conversion efficiency was rapidly lost.