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

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 Na_xMoO₃ ‘molybdenum bronze’ following this redox reaction: xNa⁺ + MoO₃ + xe⁻ ↔ Na_xMoO₃ It is proposed that the formed oxide layer contains Na_xMoO₃ 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.