Self-Trapped Excitons or Bi 3+ Ions for Broad Emission in a Lead-Free Double Perovskite? Hearing What Pressure Says

The broad emission origin of lead-free double perovskites with ns 2 -metal ion doping remains a long-standing controversy. Herein, pressure is introduced as a robust tool to determine the mechanism of broad emission from Cs 2 AgIn 0.9 Bi 0.1 Cl 6 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 Bi 3+ emission from 3 P n ( n = 0, 1, or 2) to 1 S 0 as an alternative mechanism. The broad emission is composed of two types of self-trapped states due to the different structures of BiCl 6 –AgCl 6 and InCl 6 –AgCl 6 . The abnormal emission enhancement within the range of 5.01–10.01 GPa results from the local distortion of BiCl 6 octahedra that increases the exciton–phonon coupling strength. Our study elucidates the long-term dispute about the origin of broad emission in Cs 2 AgIn 0.9 Bi 0.1 Cl 6 NCs, representing a significant step forward in the precise design and synthesis of targeted lead-free double perovskite materials.