Damp-heat induced degradation of transparent conducting oxides for thin-film solar cells

The stability of intrinsic and Al-doped single- and bilayer ZnO for thin-film CuInGaSe2 solar cells, along with Al-doped Zn1−xMgxO alloy and Sn-doped In2O3 (ITO) and F-doped SnO2, was evaluated by direct exposure to damp heat (DH) at 85°C and 85% relative humidity. The results show that the DH-induced degradation rates followed the order of Al-doped ZnO and Zn1−xMgxO ≫ ITO ≫ F:SnO2. The degradation rates of Al:ZnO were slower for films of higher thickness, higher substrate temperature in sputterdeposition, and with dry-out intervals. As inferred from the optical micro-imaging showing the initiation and propagation of degrading patterns and regions, the degradation behavior appears similar for all TCOs, despite the obvious difference in the degradation rate. A degradation mechanism is proposed to explain the temporal process involving thermal hydrolysis.