Decoding and Designing Self-Trapped Excitons for Bright Prospects in Pb-Free Halide Double Perovskites

Self-trapped excitons (STEs) have evolved from being a mere photophysical feature to constituting a distinct field of study within perovskite materials. In Pb-free halide double perovskites (HDPs), STEs often dominate the optical response due to the inherently soft and anharmonic nature of the lattice. The characteristic broadband photoluminescence (PL) with a large Stokes shift observed in many HDPs is frequently attributed to exciton self-trapping without the detailed underlying mechanisms. Given the significant potential of STEs, we provide a clear understanding of their origin, formation pathways, and tunability to unlock and tailor their functional capabilities. We systematically investigate STEs in Pb-free HDPs, emphasizing our own contribution to the field, and critically review the progress, current challenges, and futuristic potentials in real-world applications.