Indirect-to direct bandgap transition in CsRESiS 4 (RE = Sc, Y, Lanthanides) through intrinsic charge-transfer-transitions (CTT) behaviour

Abstract

The development of P-type semiconductors with high performance will drive the advancement of modern information electronics and largescale 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 analyses reveal 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 of the bandgap. The CsSmSiS4 with a charge-transfer-transitions (CTT) semiconductor, exhibits a high mobility of 610.83 cm 2 •V - 1 •s -1 and a strong absorption intensity of 0.8 × 10 5 cm -1 in the visible-light range, validating its direct bandgap nature. We also demonstrate that traversing the Ln series leads to the transition mode shifts 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.