Abstract
The reliability of ZnO-based window layer for CuInGaSe2 (CIGS) solar cells was investigated. Samples of RF
magnetron-sputtered, single-layer intrinsic and Al-doped ZnO and their combined bilayer on glass substrates were
exposed in a weatherometer (WOM) and damp heat (DH) conditions with or without acetic acid vapor. Some
preliminary samples of single-layer Al-doped Zn1-xMgxO (ZMO) alloy, a potential replacement for Al:ZnO with a wider
bandgap, were also evaluated in the DH. The Al-doped ZnO and ZMO films showed irreversible loss in the conducting
properties, free carrier mobility, and characteristic absorption band feature after <500-h DH exposure, with the originally
clear transparent films turned into white hazy insulating films and the degradation rate follows the trend of (DH + acetic
acid) > DH > WOM. The degradation rate was also reduced by higher film thickness, higher deposition substrate
temperature, and dry-out intervals. The results of X-ray diffraction analysis indicate that the ZnO-based films underwent
structural degeneration by losing their highly (002) preferential orientation with possible transformation from hexagonal
into cubic and formation of Zn(OH)2. Periodic optical micro-imaging observations suggested a temporal process that
involves initial hydrolysis of the oxides at sporadic weak spots, swelling and popping of the hydrolyzed spots due to
volume increase, segregation of hydrolyzed regions causing discontinuity of electrical path, hydrolysis of the oxide-glass
interface, and finally, formation of insulating oxides/hydroxides with visible delamination over larger areas.
