Semiconducting nanocrystals (NCs) with perovskite crystal structure have recently emerged as superstar light emitters with exceptional properties inspiring novel applications. The best known compounds are lead halide perovskites. However, concerns related to inherent toxicity and poor environmental stability of these materials have inspired a quest for alternatives. One of the most interesting of them are double perovskite (DP) metal chlorides. The most widely studied compounds are Cs₂AgBiCl₆, which is an indirect bandgap semiconductor and Cs₂AgInCl₆, which exhibits a direct gap, but the inter-band optical transition is parity-forbidden. As a consequence, these materials are poor light emitters and the photoluminescence quantum yields (PL QYs) usually are below 1%. It was discovered that the way to increase the PL QY of DP NCs was to fabricate alloyed structures with a low bismuth content. Alloying breaks the wave-function symmetry (breaking the parity constraints) and low bismuth content assures the direct bandgap. As a result, Cs₂(KNaAg)(InBi)Cl₆ NCs exhibit PL QYs reaching 70%, achieved for doping-level (i.e., below 1%) Bi contents. These results make DPs exciting materials for applications in optoelectronics, in particular in white-light LEDs and transparent photovoltaics.

Despite these achievements, the nature of the luminescent excited state is not well known. There is an agreement in the community that Bi³⁺ ions introduce a narrow absorption band in the near-UV associated with the inter-atomic transition S → P transition. However, it is not well understood what happens to electrons and holes after photoexcitation. The results of density functional theory (DFT) calculations indicate that the photoexcited hole becomes localized at a single [AgCl₆] octahedron, owing to a localized nature of the Ag d orbitals. However, regarding the fate of photoexcited electrons there are contradicting reports. 

In this talk, I will present results of temperature and magnetic field dependent photoluminescence dynamics, which reveal the fine structure of the luminescent state and show how the structure can be tuned with the NC chemical composition. I will then discuss optical transient absorption results, which show that carrier trapping occurs on a sub-ps timescale. Finally, I will present femtosecond x-ray absorption results that enable element-specific tracking of photocarrier dynamics, providing direct insight into the role of Bi ions in the carrier relaxation process. Together, our results create a comprehensive picture of the emission process of these highly emissive materials.