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

Sb³⁺- and Bi³⁺-doped Cs₂SnCl₆ zero-dimensional perovskites are emerging as stable 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_1u^*. The notation "*" implies spin-orbit coupling between the ³T_1u and ¹T_1u states. After light absorption, ³T_1u^* 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_1u^*, 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_1u^* and, therefore, a single emission peak.