Transformation between the Dark and Bright Self-Trapped Excitons in Lead-Free Double-Perovskite Cs2NaBiCl6 under Pressure.

Understanding the relationships between the structure and the properties in lead-free double perovskites is significant for their applications in the optoelectronic field. Here the nonluminous Cs2NaBiCl6 crystal exhibits an unexpected broadband dual-color emission as the external pressure is increased to 6.77 GPa. The emission intensity is remarkably enhanced with further compression to 8.50 GPa. By analyzing the results of in situ high-pressure experiments and the density functional theory, we conclude that the dual-color emission is attributed to singlet self-trapped excitons (STEs) and triplet STEs, respectively. This phenomenon originates from the tilting and twisting of [BiCl6]3- caused by the transition of cubic Cs2NaBiCl6 to the tetragonal phase. Notably, the transformation between the dark and bright STEs in the Cs2NaBiCl6 crystal is demonstrated by ultrafast transient absorption experiments under different pressures. This work not only offers deep insight into the structure-property relationship in lead-free double perovskites but also opens the door for the design of new lead-free double perovskites.