Solution Processed Fe2SiS4 and Fe2GeS4 Crystalline Films and Solar Cells Using These Films

Differentformsof thin-film solar panels have been developed and commercialized in the lastdecade by identifying materials that are both efficient absorbers of solarpower and cost-effective for manufacturerand consumer.Although they operateeffectively inthin-film (1-3 microns), there are both environmental and economic concernsfor thecost and sustainability of the materials and processes employed in these approaches.An alternativesolution was seen in pursuing sustainable PV materials composed ofEarth-abundant elementssuch as FeS2 (iron sulfide) for the absorber layer.The highabsorption coefficient ofFeS2 makes the compound unique amonginorganic materials, allowingdownsizing of thethickness of the absorber layer to lower than 0.1 μm in asolar cell.The attractivenessof this thickness isvisible whencompared to 1.5–3.0 μm forcurrent thin-film technologies and over 200 μm for single-crystal Sicells. Such thin layers not only conserve material but also provideanavenue tohigh efficiency through efficient charge separation associated with a high internalelectrical field.In order to provide aligand-field splittingof sufficient magnitude for effective solar absorption, theFe2+ ion must be bound by at least six S atoms thusassuring a sufficiently large band gap. This generally requires Fe2+ in an octahedral site. Adding a thirdelement with an electronegativity that favors strongcovalent bonding with sulfur can stabilize such a site. From theseconsiderations,Fe2SiS4 andFe2GeS4 havebeen chosen to successfullydeliver the performance originally expected from FeS2.

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