Characterization of FeS2 pyrite microcrystals synthesized in different flux media

Abstract

Pyrite FeS₂ has significant promise as a low-cost, earth-abundant photovoltaic material and has thus been the focus of solar energy researchers for years. Despite the effort, its efficiency has remained at around 3%, much lower than what is expected from the material properties. The inability to understand and control the effects of impurities in pyrite has increased difficulties in fabricating successful pyrite solar cells. Recent reports have shown evidence of phosphorus and cobalt as prospective dopants for improved optoelectronic properties and charge separation. Here, we demonstrate the optoelectric effects of alkali metal impurities in pyrite by synthesizing highly crystalline n-type pyrite microcrystals in different growth media. We find that the synthesis medium affects the impurity content of the final material. Alkali metals such as lithium, sodium, potassium, and cesium in pyrite are shown to create defect levels that cause the Fermi level to increase from −5.15 to −4.56 eV, depending on the amount of the impurity. Creation of solid solutions is suggested by the increased energy of the valence band maximum (E_VBM) from −6.17 to −5.52 eV. We show how much the concentration of these impurities can be reduced via recrystallization of FeS₂ crystals in molten salt to lower the energies of the E_VBM and Fermi levels. The effect of impurities on the photoluminescence emission of pyrite is well linked to the changes in the energy band diagram.