(Invited, Digital Presentation) Ultrafast Spectroscopy of Inorganic Perovskite Nanocrystals and Their Assemblies: Uncovering the Multiple Exciton Generation Rate and Band-Formation

Inorganic perovskite nanocrystals (NCs) display attractive structural and optical properties, and have perspective in a wide range of optoelectronic applications. One interesting feature is their relatively slow hot carrier cooling rate, which allows to follow carrier dynamics in great detail with ultrafast spectroscopic techniques. In this talk, we present studies of the ultrafast carrier dynamics of perovskite NCs and assembled structures. Typically, when the excitation photon energy is increased, and thus the excess energy of the created carriers, the cooling time of the carriers towards the bandedge increases along. However, for CsPbI3 NCs, when the photon energy exceeds a certain threshold energy the cooling time starts to decrease, indicating that a competing relaxation pathway opens up which funnels carriers towards the bandedge. This threshold energy indicates the onset of a carrier multiplication process, in which absorption of a single photon leads to the creation of multiple electron-hole pairs. This process can also be inferred by following the carrier concentration, which shows a delayed build-up following the excitation pulse. The carrier dynamics in this system can be modeled within a framework of two competing cooling mechanisms. With the experimentally determined carrier cooling rate and threshold energy it is possible to extract the time-constant associated with the carrier multiplication process. Experiments on highly ordered CsPbBr3 NCs, so-called supercrystals, revealed how the photophysics are altered due to the inter-NC coupling as compared to the individual NCs. Next to a red-shift of the absorption and photoluminescence peak, the supercrystals show signatures of band-like states. The observation of a reduced Stokes-shift, decreased biexciton binding energy and increased carrier cooling rates, support the formation of delocalized states resulting from the coupling between individual NCs. These results open perspectives for assembled NCs for application in optoelectronic devices, with design opportunities exceeding the level of nanocrystal and bulk material.