Effect of H/Ar treatment on ZnO:B transparent conducting oxide for flexible a-Si:H/μc-Si:H photovoltaic modules under damp heat stress

A flexible amorphous/microcrystalline Si:H (a-Si:H/μc-Si:H) tandem-junction photovoltaic (PV) module was produced in which a thin film of ZnO:B grown by metalorganic chemical vapor deposition (MOCVD) served as the transparent conducting oxide (TCO). The Hall mobility of ZnO:B is degraded by damp heat, simulated here using the conditions of 85 °C at 85% relative humidity; this affects the series resistance and efficiency of the PV module. In this study, ZnO:B was treated by H/Ar plasma to reduce the degradation experienced under damp heat. The degradation time of the Hall mobility of ZnO:B, defined as the time necessary for the cell to reach the efficiency loss of − 20%, was improved by ~ 54% by H/Ar treatment (ZnO:B·H/Ar). The mechanism behind this improvement was investigated by assessing the reactions of the ZnO:B and ZnO:B·H/Ar thin films to moisture. Related changes in the physical and chemical properties of ZnO:B and ZnO:B·H/Ar were analyzed by X-ray photoelectron spectroscopy, secondary-ion mass spectroscopy, and ultraviolet photoelectron spectroscopy. The analyses showed that the concentration of OH⁻ was high while those of Zn^2 + and B^3 + were low in the grain boundaries of the ZnO:B surface after exposure to humidity. After H/Ar treatment, the increase in OH⁻ concentration in ZnO:B was reduced, and the decrease in the Zn^2 + and B^3 + concentrations was much smaller. The H/Ar plasma treatment of ZnO:B affected the surface reaction forming Zn(OH)², between the OH⁻ and Zn^2 + ions at the grain boundaries under damp heat.