Material Design of Green-Light-Emitting Semiconductors: Perovskite-Type Sulfide SrHfS₃

A current issue facing light-emitting devices is a missing suitable material for green emission. To overcome this, we explore semiconductors possessing (i) a deep conduction band minimum (CBM) and a shallow valence band maximum (VBM), (ii) good controllability of electronic conductivity and carrier polarity, and (iii) a directly allowed band gap corresponding to green emission. We focus on early transition metal (eTM)-based perovskites. The eTM cation’s high and stable valence state makes its carrier controllability easy, and the eTM’s nonbonding d orbital and the anion’s p orbital, which constitute the deep CBM and shallow VBM, are favorable to n- and p-type doping, respectively. To obtain a direct band gap, we applied a scheme that folds the bands constituting the VBM at the zone boundary to the zone center where the CBM appears. Orthorhombic SrHfS₃ was chosen as the candidate. The electrical conductivity was tuned from 6 × 10^–7 to 7 × 10^–1 S·cm^–1 with lanthanum (La) doping and to 2 × 10^–4 S·cm^–1 with phosphorus (P) doping. Simultaneously, the major carrier polarity was controlled to n type by La doping and to p type by P doping. Both the undoped and doped SrHfS₃ exhibited intense green photoluminescence (PL) at 2.37 eV. From the PL blue shift and short lifetime, we attributed the emission to a band-to-band transition and/or exciton. These results demonstrate that SrHfS₃ is a promising green-light-emitting semiconductor.