Indirect-to-direct bandgap transition in CsRESiS4 (RE = Sc, Y, and Lanthanides) through intrinsic charge-transfer transition (CTT) behaviour

Abstract

The development of high-performance P-type semiconductors will drive the advancement of modern information electronics and large-scale N–P complementary semiconductor technology. Herein, we demonstrate an indirect-to-direct bandgap transition of CsLaSiS₄ through the replacement of La by Sm. Band structure analysis reveals that the Ln-4f orbitals, as intermediate bands (IB), play a crucial role in the direct bandgap formation through hybridization of electronic states with S 3p and significantly enhance the quantum efficiency of CsRESiS₄ by decreasing the bandgap. CsSmSiS₄, with a charge-transfer transition (CTT) behavior, exhibits a high mobility of 610.83 cm² V⁻¹ s⁻¹ and a strong absorption intensity of 0.8 × 10⁵ cm⁻¹ in the visible-light range, validating its direct bandgap nature. We also demonstrate that traversing the Ln series leads to a shift in the transition mode from host absorption (p–d for La and Gd) to ionization transitions (f–d for Ce) and charge-transfer transitions (p–f for Pr–Eu). These findings suggest potential for achieving direct bandgaps in compounds that are constrained by their inherent indirect energy gaps, offering a strategy for tailoring energy structures to significantly improve efficiencies in optoelectronics and photovoltaics.