Second-Order Jahn-Teller Distortions and Dynamic Lattice Polarizability as the Origin of Broadband Emission in Bi3+-Doped Cs2SnCl6

TL;DR

This study reveals that second-order Jahn-Teller distortions and dynamic lattice polarizability caused by Bi3+ doping create localized exciton traps and broad emission in Cs2SnCl6, a lead-free perovskite.

Contribution

It uncovers the electronic and structural mechanisms behind broadband emission in Bi3+-doped Cs2SnCl6 using DFT and molecular dynamics, highlighting the role of Jahn-Teller distortions and lattice heterogeneity.

Findings

BiCl5(2-) units act as exciton traps.

Lone pair induces Jahn-Teller distortions and lattice softness.

Dynamic broadening explains broadband photoluminescence.

Abstract

Lead-free vacancy-ordered perovskites (VOHPs) such as Cs2SnCl6 have emerged as promising materials for optoelectronic applications but typically suffer from wide band gaps and low photoluminescence quantum yields (PLQY). In this work, the electronic origins of broadband blue emission in Bi3+-doped Cs2SnCl6 are elucidated by combining density functional theory (DFT) calculations and ab initio molecular dynamics simulations. The study systematically explores the spatial configurations of Bi3+ dopants and Cl-vacancies, assessing their impact on structural and electronic properties. Results demonstrate that the formation of square pyramidal BiCl5(2-) units serves as efficient exciton traps, fundamentally enabling strong luminescence in an otherwise non-emitting host. The Bi3+ 6s2 lone pair in an asymmetric coordination environment drives second-order Jahn-Teller distortions, creating a highly polarizable local lattice. This softness, in contrast to the rigid SnCl6(2-) framework, produces heterogeneous distributions of bond lengths, bond angles, and band edges around dopant sites. Defect-induced states strongly localize charge carriers, while coupling to vibrational modes at 300 K dynamically broadens the band edges, accounting for the large Stokes shift and broadband photoluminescence. These results establish lone-pair-driven Jahn-Teller distortions and lattice heterogeneity as key design principles for efficient luminescence in lead-free perovskites.