Lanthanide‐Like Contraction Enables the Fabrication of High‐Purity Selenium Films for Efficient Indoor Photovoltaics

Abstract The lanthanide contraction involves a reduction in atomic radius among f‐block elements below the expected level. A similar contraction is observed in group‐16 elements. The atomic radius of Se (117 pm) is slightly larger than that of S (104 pm) arising from the presence of d electrons, compared to the significant increase in atomic radius from O (73 pm) to S. This lanthanide‐like contraction contributes to Se's robust oxidative resistance. Here we report a selective oxidation strategy utilizing Se's strong antioxidative property to remove coexisting narrow‐band gap Te impurities from Se feedstocks. This strategy selectively oxidizes volatile Te impurities into involatile TeO 2 that remains in the evaporation source, while only volatile Se deposits onto the substrate during the thermal‐evaporation deposition process. This enables the fabrication of high‐purity Se films possessing a wide band gap of 1.88 eV, ideally suited to the optimal band gap for indoor photovoltaics (IPVs). The resulting Se photovoltaics exhibit an efficiency of 20.1 % under 1000‐lux indoor illumination, outperforming market‐dominant amorphous silicon and all types of lead‐free perovskite IPVs. Unencapsulated Se devices show no efficiency degradation after 20,000 hours of storage in ambient atmosphere.