High-entropy alloy (HEA) superconductors are known to exhibit a robust superconducting state with adaptable thermomechanical properties. In this study, we have investigated the electronic structure of HEA-type $R{\mathrm{OBiS}}_2$ (R = rare earth) layered superconductors with mixing entropy using soft x-ray absorption (XAS) and x-ray photoemission spectroscopy (XPS) measurements. The electronic structure evolution differs between equivalent and nonequivalent R mixing as a function of mixing entropy. Ce ${\mathrm{M}}_{4,5}$-edge XAS and Ce $3d$ XPS reveal clear evidence of ${\mathrm{Ce}}^{3+}$/${\mathrm{Ce}}^{4+}$ mixed valence in the HEA-type R${\mathrm{OBiS}}_2$, similar to the pristine ${\mathrm{CeOBiS}}_2$ compound, suggesting a similar charge transfer mechanism responsible for self-doping in these materials. Unlike Ce, all other R atoms exhibit a nominal R${}^{3+}$ valence state across samples with varying mixing entropy. The self-doped charge is consistent with the Luttinger volume estimated by previous angle-resolved photoemission spectroscopy (ARPES) studies. The distinct behaviors observed in equivalent and nonequivalent mixing are discussed in terms of differing potential energy landscapes of the Bi sites manifested in the Bi $4f$ XPS spectra, where intrinsic disorder is expected to have different characteristics of the superconductivity.

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• Electronic structure
• Layered crystals
• Photoemission spectroscopy
• X-ray absorption spectroscopy
• X-ray photoelectron spectroscopy