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 Cs₂AgIn_0.9Bi_0.1Cl₆ 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 ³P_n (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 Cs₂AgIn_0.9Bi_0.1Cl₆ NCs, representing a significant step forward in the precise design and synthesis of targeted lead-free double perovskite materials.