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 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.