The operating hypothesis for using nanoparticle precursors to generate thin-film chalcogenide photovoltaics absorbers in Gen 2 PV is that the final crystalline film resulted from thermal treatment would mirror the original nanoprecursors composition. In addition, melting point depression as a result of the nano-confinement would enable relatively low processing temperatures. With nanoparticles melting occurring at several hundred degrees lower than the melting point in bulk, the thermal treatment of NP precursors enables fabrication of uniform, crystalline thin-films on inexpensive substrates that only sustain moderate annealing temperatures (such as soda lime glass, SLG). A narrow particle size distribution contributes to film uniformity and enables film thickness control. Therefore, tremendous efforts have been reported toward rigorous control of nanoparticles size and composition. Our efforts, focused on two iron chalcogenides: Fe2SiS4 and Fe2GeS4, demonstrate that processing often compromises the compositional control and final film purity, given that the reactivity associated with nano-confinement also leads to elemental loss during annealing. We report our findings related to thermal processing (annealing) and discuss current strategies to mitigate the elemental loss during annealing.
