Influence of Mo/MoSe2 microstructure on the damp heat stability of the Cu(In,Ga)Se2 back contact molybdenum

• SLG/high pressure Mo/low pressure Mo/MoSe2 stacks were exposed to damp heat.
• Molybdenum deposited at low pressure retained the best reflectivity and conductivity.
• Damp heat exposure leads to a NaxMoO3/Mo multilayer structure.
• The NaxMoO3 probably consists of Na+ intercalated in a (reduced) MoO3 matrix.
• Intercalation can explain the migration of Na+ through molybdenum grain boundaries.

The degradation behavior of Mo/MoSe2 layers have been investigated using damp heat exposure. The two studied molybdenum based films with different densities and microstructures were obtained by lifting off Cu(In,Ga)Se2 layers from a bilayer molybdenum stack on soda lime glass. Hereby, a glass/Mo/MoSe2 was obtained, which resembles the back contact as present in Cu(In,Ga)Se2 solar cells. The samples were degraded for 150 h under standard damp heat conditions and analyzed before, during and after degradation. It was observed that the degradation resulted in the formation of needles and molybdenum oxide layers near the glass/Mo and the Mo/Cu(In,Ga)Se2 interfaces. X-ray Photoelectron Spectroscopy measurements have shown that the sodium was also present at the surface of the degraded material and it is proposed that the degraded material consists mostly of MoO3 with intercalated Na+. This element has likely migrated from the soda lime glass. This intercalation process could have led to the formation of NaxMoO3 ‘molybdenum bronze’ following this redox reaction:xNa+ + MoO3 + xe− ↔ NaxMoO3It is proposed that the formed oxide layer contains NaxMoO3 with different Na+ contents and different grades of conductivity. This intercalation process can also explain the high mobility of Na+ via the grain boundaries in molybdenum. It was also observed that the molybdenum film with a top layer deposited at a high pressure is more susceptible for damp heat degradation.