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Various transparent conducting oxide (TCO) films are used as window layers to permit light transmission, photocurrent generation, and for electrical current collection in thin-film photovoltaic technologies. Bilayer composites (BZO) of intrinsic ZnO (i-ZnO) and Al-doped ZnO (AZO) have been used on NREL's high-efficiency CuInGaSe2 (CIGS) solar cells. Previously, we demonstrated that, when tested in damp heat (DH) condition at 85°C and 85% relative humidity (RH), the stability trend of some TCOs was in decreasing order of SnO2:F > In2O3:SnO2 (ITO) > ZnO-based films of AZO, BZO and Al-doped Zn1-xMgxO (ZMO). We also observed that the degradation rate of AZO, BZO, and ZMO was influenced by additional factors such as film thickness, deposition conditions, and exposure history. This work continued our efforts in searching for a high-performance and high-stability window layer TCO, as well as in finding mitigation methods to protect the ZnO layer, either i-ZnO or BZO, for use on the CIGS solar cells. The current study, which involved the third experimental set of TCOs deposited on glass, further examined in DH test conditions the thickness effect on single-layer AZO films, the glass substrate effect on BZO, the stability and protective effect of amorphous In2O3:ZnO (InZnO or IZO) as a conducting window layer for the underlying i-ZnO, and the stability and protective power of a protective transparent metal oxide (PTMO) coating for all three types of ZnO (AZO, BZO, and i-ZnO). The samples were periodically characterized with optical, electrical, and structural measurements during the course of DH exposure. The results show that the DH stability of AZO increased as the film thickness increased, BZO on Corning® Eagle 2000™ glass degraded somehow faster than on Corning® 7059™, and both the IZO and PTMO showed generally high DH stability and good protective power for the ZnO layers underneath. However, the results of decreased (002) peak intensity of ZnO from X-ray diffraction analysis indicated that both IZO and PTMO still allowed certain levels of moisture penetration.
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