Origin of Luminescence in Sb³⁺- and Bi³⁺-Doped Cs₂SnCl₆ Perovskites: Excited State Relaxation and Spin–Orbit Coupling

Abstract

Sb³⁺- and Bi³⁺-doped Cs₂SnCl₆ zero-dimensional perovskites are emerging as table and nontoxic phosphors for light emitting diodes. The outermost s-electrons (ns²) of the dopants are responsible for both light absorption (ns² to ns¹np¹) and emission (ns¹np¹ to ns²). At cryogenic temperatures, the Sb³⁺ dopant shows two emission peaks, but Bi³⁺ shows only one emission peak. Why? Here we address such questions, revealing the origin of luminescence in Sb³⁺- and Bi³⁺-doped Cs₂SnCl₆. We find that the emitting excited state ns¹np¹ is a triplet state ³T_7u^*. The notation “^*” implies spin–orbit coupling between the ³T_7u and ^*1_7u states. After light absorption, ³T_7u^* is occupied with one electron, which then undergoes Jahn–Teller distortion yielding a relaxed excited state (RES). For the Sb³⁺ dopant, the combination of Jahn–Teller distortion and spin–orbit coupling gives rise to two minima in RES ³T_7u^*, resulting in two emission peaks, whereas for the Bi³⁺ dopant, the spin–orbit coupling significantly dominates over the Jahn–Teller splitting yielding a single minimum in RES ³T_7u^* and, therefore, a single emission peak.