Microstructure and Optical Functions of Transparent Conductors and their Impact on Collection in Amorphous Silicon Solar Cells

We have developed new procedures for determining the microstructure as well as the index of refraction and extinction coefficient spectra { n ( E ), k ( E )} for textured SnO 2 thin films on glass used as the top contact layers and superstrate for amorphous silicon (a-Si:H) p-i-n solar cells. These procedures combine (i) multichannel Mueller matrix spectroscopy using a dual rotating-compensator spectroscopic ellipsometer in reflection from the surface of the SnO 2 , a measurement that is most sensitive to microstructure and n ( E ), and (ii) transmission spectroscopy through a double-thick SnO 2 sandwich contacted with index-matching fluid, a measurement that is most sensitive to k ( E ). An important optical loss in a-Si:H p-i-n solar cells is reflection from the SnO 2 /p-layer interface. In this paper, we characterize this optical loss through modeling the solar cell optical quantum efficiency and demonstrate the extent to which microscopic roughness at this interface can serve as an anti-reflection layer for enhanced collection.