Comparison of Al/ZnO and Ag/ZnO interfaces of back-reflectors for thin film Si∶H photovoltaics

Real time spectroscopic ellipsometry (RTSE) and ex-situ normal incidence reflectance and scattering spectroscopies have been applied to analyze the optical characteristics of Al/ZnO back-reflector (BR) structures used in thin film Si:H photovoltaics. The results of this study have been compared with those of previous detailed studies of Ag/ZnO structures. The structures explored here are relevant to the substrate/BR/n-i-p solar cell configuration and consist of an opaque Al film having a controllable thickness of surface roughness, followed by up to ~3000 ¿ of ZnO. All Al and ZnO films have been prepared by rf magnetron sputtering, the ZnO under a standardized set of conditions at room temperature. The thickness of the final roughness thickness on Al has been varied by adjusting its deposition conditions in order to investigate the effects of Al film roughness on interface formation and Al/ZnO interface optical properties. From a comparison of the complex dielectric functions ¿ = ¿₁ + i¿₂ of the Al/ZnO and Ag/ZnO interface layers, we find that the localized plasmon feature is observed at higher energies for the Al/ZnO than for the Ag/ZnO interface, as would be expected based on the higher bulk plasma energy of Al. As a result, the primary loss mechanism for Al/ZnO is dissipation, not through localized plasmon modes as in Ag, but rather through intraband and interband absorption intrinsic to the Al. General approaches for reducing losses for both Al/ZnO and Ag/ZnO structures are discussed.