Self-Trapped Excitons or Bi3+ Ions for Broad Emission in a Lead-Free Double Perovskite? Hearing What Pressure Says

The broad emission origin of lead-free double perovskites with ns2-metal ion doping remains a long-standing controversy. Herein, pressure is introduced as a robust tool to determine the mechanism of broad emission from Cs2AgIn0.9Bi0.1Cl6 nanocrystals (NCs). The negative correlation between the crystal field strength and broad emission wavelength under compression corroborates that the broad emission is indeed attributed to the radiative recombination of self-trapped excitons, ruling out Bi3+ emission from 3Pn (n = 0, 1, or 2) to 1S0 as an alternative mechanism. The broad emission is composed of two types of self-trapped states due to the different structures of BiCl6-AgCl6 and InCl6-AgCl6. The abnormal emission enhancement within the range of 5.01-10.01 GPa results from the local distortion of BiCl6 octahedra that increases the exciton-phonon coupling strength. Our study elucidates the long-term dispute about the origin of broad emission in Cs2AgIn0.9Bi0.1Cl6 NCs, representing a significant step forward in the precise design and synthesis of targeted lead-free double perovskite materials.