In situ monitoring of material properties during thin-film deposition provides researchers with a valuable tool for maximizing solar cell performance, while also enabling efficient exploration of deposition parameter space. This article describes how our research team has utilized in situ real-time spectroscopic ellipsometry (RTSE) to maximize our productivity in two related projects. We are using hot wire chemical vapor deposition (HWCVD) for low-temperature (90-235 degrees C) deposition of very thin films of amorphous hydrogenated silicon for amorphous Si/crystal-Si heterojunction (SHJ) solar cells. We are also using HWCVD for low-temperature (200-440 degrees C) deposition of epitaxial films of silicon on crystal-Si substrates. We utilize RTSE as both an in situ diagnostic and a postgrowth analysis tool for SHJ solar cells and epi-Si films grown by HWCVD. Using input from RTSE analysis we have achieved a photovoltaic energy conversion efficiency of 17.1% on an Al-backed p-type float-zone c-Si wafer. Epi-Si films have been grown as thick as 500 nm utilizing parameter optimization based on RTSE analysis. (c) 2006 American Vacuum Society.