The influence of atmospheric species on the degradation of the Mo/MoSe2 back contact in CIGS solar cells

The degradation mechanisms of the molybdenum (Mo) back contact for Cu(In,Ga)Se2 solar cells in the presence of atmospheric species have been investigated. Mo/MoSe2 films on glass were immersed in water purged with the gases oxygen (O2), nitrogen (N2), carbon dioxide (CO2), air, in non-purged water, and exposed to air. It was found that the degradation rate is the highest in the combined presence of liquid water and oxygen. Degradation in water was less in the absence of oxygen, while air exposure displayed the least degradation. The main degradation product was MoO2, which was found at places where metallic molybdenum was not fully covered by MoSe2. MoSe2 is thus a good protective measure against molybdenum oxidation and can play an important role to protect CIGS devices against environmental degradation.