Origin of Optical Losses in Ag/ZnO Back-Reflectors for Thin Film Si Photovoltaics

Back-reflector (BR) fabrication for thin film Si solar cells has been studied using real time spectroscopic ellipsometry (SE) with a rotating compensator instrument. The structure explored here is relevant for the substrate/ BR/n-i-p configuration and consists of opaque Ag followed by ~1500 Aring of ZnO, both prepared by sputtering. The thickness of the initial roughness layer on the Ag has been varied from 6 to 55 Aring to investigate its effects on interface formation between Ag and ZnO in the specular regime. The dielectric functions epsiv=epsiv₁+iepsiv₂ of all layers including an observed Ag/ZnO interface layer have been determined, and the latter has been fit using (i) a free electron component, (ii) a plasmon component near 2.8 eV, and (iii) a bound electron component with an absorption onset near 3.7 eV. The measured reflectance spectra of the final specular BR structures with ~1500 Aring thick ZnO are consistent with those predicted from SE models and allow one to identify the physical origins of losses. A strong minimum in reflectance centered at 2.6 eV and an associated tail extending to 1.6 eV is found to be caused by interference-enhanced plasmon absorption. Simulations for thicker ZnO (d_b=3000 Aring) demonstrate that the dominant loss from 1.2 to 1.5 eV is due to intrinsic absorption in Ag, also enhanced by interference