Damp-heat instability and mitigation of ZnO-based thin films for CuInGaSe2 solar cells

From our investigation of damp heat (DH)-induced degradation of the main component materials and complete CIGS devices in recent years, this paper summarizes the results on the (1) DH stability of several transparent conducting oxides deposited on glass substrates, including ZnO-based thin films, Sn-doped In₂O₃ (ITO), and InZnO, and (2) effectiveness of physical and chemical mitigations for ZnO. The electrical results showed that the DH-induced degradation rates of i-ZnO, AZO, their bilayer (BZO), and Al-doped Zn_1-xMg_xO are significantly greater than those of ITO and InZnO. Thicker AZO films are more stable than thinner ones. Structurally, upon DH exposures, the hexagonal ZnO-based thin films are transformed into highly resistive Zn(OH)₂ and/or cubic ZnO with increased transmittance and substantial morphological changes. In the physical mitigation approach, plasma-enhanced chemical vapor-deposited SiO_xN_y and sputter-deposited InZnO are employed separately as moisture barriers to protect the underlying i-ZnO, AZO, and/or BZO with good results. However, the SiO_xN_y films required working with chemical treatments to improve adhesion to the BZO surfaces. In the chemical mitigation method, simple wet-solution treatments using special formulations are found effective to protect BZO from DH attack.