Optical and Electrical Transport Evaluations of n-Type Iron Pyrite Single Crystals

Iron pyrite [cubic FeS₂ (cFeS₂)] is considered as an earth-abundant and low-cost thin-film photovoltaic material. However, the conversion efficiency of cFeS₂-based solar cells remains below 3%. To elucidate this limitation, we evaluate the optical and electrical characteristics of cFeS₂ single crystals that are grown using the flux method, thus providing us an understanding of the electron transport behavior of cFeS₂ single crystals. The oxide layer on the surface of cFeS₂, which can possibly have an influence on the electrical characteristics of cFeS₂, is removed prior to characterization via optical spectroscopy and electrical transport measurement. The optical property of cFeS₂ was found to have both indirect and direct transitions. We also observed the presence of a band tail below the conduction band. The obtained electrical transport behavior indicates that cFeS₂ bulk exhibits a high defect density and a disordered phase, thus leading to the hopping conduction mechanism. Our results will pave the way for the development of photovoltaic applications with iron pyrite.