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

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