Book Description

Use of Earth-abundant materials in solar absorber films is critical for expanding the reach of photovoltaic (PV) technologies. The use of Earth-abundant and inexpensive Fe in PV was proposed more than 25 years ago in the form of FeS2 pyrite - fool's gold. Unfortunately, the material has been plagued by performance problems that to this day are both persistent and not well understood. Researchers from the National Renewable Energy Laboratory (NREL) and Oregon State University, working collaboratively in the Center for Inverse Design, an Energy Frontier Research Center, have uncovered several new insights into the problems of FeS2. They have used these advances to propose and implement design rules that can be used to identify new Fe-containing materials that can circumvent the limitations of FeS2 pyrite. The team has identified that it is the unavoidable metallic secondary phases and surface defects coexisting near the FeS2 thin-film surfaces and grain boundaries that limit its open-circuit voltage, rather than the S vacancies in the bulk, which has long been commonly assumed. The materials Fe2SiS4 and Fe2GeS4 hold considerable promise as PV absorbers. The ternary Si compound is especially attractive, as it contains three of the more abundant low-cost elements available today. The band gap (E{sub g} = 1.5 eV) from both theory and experiment is higher than those of c-Si and FeS2, offering better absorption of the solar spectrum and potentially higher solar cell efficiencies. More importantly, these materials do not have metallic secondary phase problems as seen in FeS2. High calculated formation energies of donor-type defects are consistent with p-type carriers in thin films and are prospects for high open-circuit voltages in cells.