Colloidal Synthesis and Charge-Carrier Dynamics of Cs₂ AgSb_1-y Bi_y X₆ (X: Br, Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals

A series of lead-free double perovskite nanocrystals (NCs) Cs₂ AgSb_1-y Bi_y X₆ (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs₂ AgSbBr₆ NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag-Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag-Bi or Ag-Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge-carrier relaxation. The two fast trapping processes are dominated by intrinsic self-trapping (ca. 1-2 ps) arising from giant exciton-phonon coupling and surface-defect trapping (ca. 50-100 ps). Slow hot-carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot-carrier relaxation are also discussed.