Fe–Al–Si-based thermoelectric (FAST) materials are non-toxic and low-cost materials that can be used for autonomous power supplies to drive internet-of-things wireless sensor devices. The conduction type can be controlled by changing the Al/Si ratio, which is suitable for fabricating reliable thermoelectric power-generation modules consisting of materials with similar thermal expansion coefficients. In this work, we evaluated the electronic structures of p- and n-type FAST materials with relatively large absolute values of the Seebeck coefficient by photoemission spectroscopy to obtain deeper insight into controlling the p-n characteristics of FAST materials. The core-level spectra suggested that the FAST materials have a covalent bonding nature. The chemical-potential shift should be the dominant factor of the core-level shift, which is consistent with the expected behavior of carrier doping of thermoelectric semiconductors, that is, rigid-band-like behavior. The size of the core-level shift of ~0.15 eV is close to the band gap of ~0.18 eV obtained from transport measurements. The observed electronic structure can qualitatively explain the experimental results.

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