Lead halide perovskite nanocrystals (NCs) are of great promise for light-emitting devices owing to their high photoluminescence (PL) quantum yields. However, due to the instability of chloride compounds, perovskite devices have limited performances in the blue spectral range. To overcome this issue, some group focused their attention on strongly confined NC systems, such as nanoplatelets (NPls). Due to the quantum confinement of the perovskite structure, NPls exhibit different photophysical behaviors than traditional perovskite NCs. Notably, the charge carrier interactions are enhanced, leading to stable excitons and short fluorescence lifetimes. Nonetheless, because of the increased number of surface trap states, perovskite NPls display PLQY barely up to 50%.¹ Today there is an urge to fully understand the physical and structural limitations of these materials and optimize the NPl-based devices. Here, we combine transient absorption spectroscopy (TAS) and fluorescence upconversion spectroscopy (FLUPS), two ultrafast spectroscopic techniques, to probe the ultrafast exciton and multiexciton dynamics in CsPbBr₃ NPls. We first observe spectral signatures of biexciton states with a binding energy of 74 meV.² By isolating the biexciton signal, the biexciton lifetime is estimated to 10 ps due to biexciton-Auger recombination. Secondly, we highlight that the band edge exciton dynamics greatly differ from TAS to FLUPS at early timescales.³ The PL exhibits a fast decay, within a few ps, which does not follow the trend observed by TAS. We propose a simplified model by including shallow trap states lying close to the band edge. The reversible trapping of band edge excitons possibly explains the observed delayed fluorescence in perovskite NCs.

[1] B. J. Bohn, Y. Tong, M. Gramlich, M. L. Lai, M. Döblinger, K. Wang, R. L. Z. Hoye, P. Müller-Buschbaum, S. D. Stranks, A. S. Urban, L. Polavarapu, J. Feldmann, Nano Lett., 2018, 18, 5231-5238 
[2] B. R. C. Vale, E. Socie, A. Burgos-Caminal, J. Bettini, M. A. Schiavon, J.-E. Moser, J. Phys. Chem. Lett, 2019, 11(2), 387-394.
[3] E. Socie, B. R. C. Vale, A. Burgos‐Caminal, J.-E. Moser, Adv. Opt. Mat., 2021, 9(1), 2001308.