Thickness effect of Al-doped ZnO window layer on damp-heat stability of CuInGaSe2 solar cells

We investigated the damp heat (DH) stability of CuInGaSe2 (CIGS) solar cells as a function of thickness of the Al-doped ZnO (AZO) window layer from the “standard” 0.12 μm to a modest 0.50 μm over an underlying 0.10-μm intrinsic ZnO buffer layer. The CIGS cells were prepared with external electrical contact using fine Au wire to the tiny “standard” Ni/Al (0.05 μm/3 μm) metal grid contact pads. Bare cell coupons and sample sets encapsulated in a specially designed, Al-frame test structure with an opening for moisture ingress control using a TPT backsheet were exposed to DH at 85°C and 85% relative humidity, and characterized by current-voltage (I-V), quantum efficiency (QE), and (electrochemical) impedance spectroscopy (ECIS). The results show that bare cells exhibited rapid degradation within 50–100 h, accompanied by film wrinkling and delamination and corrosion of Mo and AlNi grid, regardless of AZO thickness. In contrast, the encapsulated cells did not show film wrinkling, delamination, and Mo corrosion after 168 h DH exposure; but the trend of efficiency degradation rate showed a weak correlation to the AZO thickness.