A research team led by HAN Keli from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences synthesized new lead-free double perovskite nanocrystals (NCs). The detailed charge carrier dynamics of these NCs were also clarified. Their findings were published in Angew. Chem. Int. Ed..

The structure of double perovskite has a general chemical formula of A₂MM'X₆ (where A is a monovalent cation, X is the halogen anion, M and M' are +1 and +3 cations).

Although, the double perovskite family consists of several thousand combinations from a theoretical perspective, only a few bulk materials have been synthesized experimentally. Among these, only two families of double perovskite colloidal NCs namely, Cs₂AgBiX₆ (X = Cl, Br, I) and Cs₂AgInCl₆ (Mn²⁺ doped Cs₂AgInCl₆) have been prepared. On the other hand, the charge carrier dynamics of double perovskite NCs are still not clear.

Crystal structure and charge carrier dynamics of Cs2AgSb1-yBiyBr6(0 ≤y ≤ 1)) (Image by YANG Bin) 

The scientists synthesized a series of lead-free double perovskite NCs: Cs₂AgSb_1-yBi_yX₆ (X: Br, Cl; 0≤y≤1) NCs. In particular, Cs₂AgSbBr₆ 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 indicated the presence of two prominent fast trapping processes in the charge carrier relaxation. The two fast trapping processes were dominated by intrinsic self-trapping (1~2 ps) due to giant exciton-phonon coupling and surface defects trapping (50~100 ps), respectively.

In this work, the slow hot-carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot-carrier relaxation are also discussed.

This study was supported by the key research project of National Natural Science Foundation.