Plasmonic characteristics of Ag/ZnO back-reflectors for thin film Si photovoltaics

Real time spectroscopic ellipsometry (RTSE) and ex-situ normal incidence reflectance and scattering spectroscopies have been applied to analyze the plasmonic characteristics of Ag/ZnO back-reflector (BR) structures used in triple junction thin film Si:H photovoltaics. The structure explored here is relevant to the substrate/BR/(n-i-p)**3 configuration and consists of opaque Ag followed by up to ∼ 3500 Å of ZnO, both films prepared by sputtering onto Si wafers. The use of Si wafer substrates and RTSE enables controllability of the Ag final roughness layer thickness from ∼ 4 to 105 Å, i.e., from microscopic (specular) to macroscopic (low texture), in order to investigate its role in Ag/ZnO interface formation and its effect on the interface plasmonic characteristics. The analysis in ex situ measurement modes has also been extended to the optimized BR structure (with full texture). For each BR, the dielectric functions ɛ = ɛ1 + iɛ2 of all layers have been determined, including the Ag/ZnO interface layer, and the latter has been fit using a model that includes localized plasmon resonances arising from free electron oscillations within Ag interface protrusions. These resonances shift to lower energy with increasing ZnO/Ag interface thickness due to dipolar interactions, and can account for both re-radiation and dissipation in fully textured BR structures. The operative mechanism of these optimized back-reflectors for triple junction a-Si:H-based solar cells is interference-enhanced localized plasmon coupling -- and subsequent re-radiation -- in the range of 1.4–1.5 eV. In fact, this mechanism appears to be up to ∼ 80% efficient in optimized BRs. Coupling efficiency to surface plasmons is limited to ≪ 15% for 1.4–1.5 eV due to dominant protrusions in fully textured BRs.