Near-Broken-Gap Alignment between FeWO₄ and Fe₂WO₆ for Ohmic Direct p-n Junction Thermoelectrics

We report a near-broken-gap alignment between p-type FeWO₄ and n-type Fe₂WO₆, a model pair for the realization of Ohmic direct junction thermoelectrics. Both undoped materials have a large Seebeck coefficient and high electrical conductivity at elevated temperatures, due to inherent electronic defects. A band-alignment diagram is proposed based on X-ray photoelectron and ultraviolet-visible light reflectance spectroscopy. Experimentally acquired nonrectifying I-V characteristics and the constructed band-alignment diagram support the proposed formation of a near-broken-gap junction. We have additionally performed computational modeling based on density functional theory (DFT) on bulk models of the individual compounds to rationalize the experimental band-alignment diagram and to provide deeper insight into the relevant band characteristics. The DFT calculations confirm an Fe-3d character of the involved band edges, which we suggest is a decisive feature for the unusual band overlap.