This report summarizes the progress of the Basic Sciences Branch of the National Renewable Energy Laboratory (NREL) from October 1, 1990, through September 30, 1991. Seven technical sections of the report cover these main areas of NREL's in-house research: Semiconductor Crystal Growth, Amorphous Silicon Research, Polycrystalline Thin Films, III-V High-Efficiency Photovoltaic Cells, Solid-State Theory, Solid-State Spectroscopy, and Superconductivity. Each section explains the purpose and major accomplishments of the work in the context of the US Department of Energy's National Photovoltaic Research Program plans.