Exciton and Biexciton Transition Reveals Auger and Energy Transfer Dynamics in Mn-Doped CsPbCl₃ Nanocrystals

Femtosecond transient absorption (TA) spectroscopy is a powerful technique that has been used to disentangle ultrafast exciton dynamics in halide perovskite nanocrystals (NCs). The bleaching caused by the state-filling effect of the excitons is extensively and almost exclusively used to extract the exciton dynamics in perovskite NCs, whereas the induced absorption signals remain largely overlooked. We show that optical pumping of the higher-lying excitons results in exciton bleaching and red- and blue-shifted induced absorption resonances, caused by the exciton-to-biexciton transitions, in both undoped and Mn-doped CsPbCl₃ NCs. While both exciton bleaching and biexciton induced absorption signals deliver the exciton-to-Mn energy transfer (ET) time scale, our analysis suggests that the biexciton induced absorption resonance may provide a more accurate ET time scale and shows a strong Mn-content dependence. Importantly, the bleaching dynamics provide information on the biexciton Auger recombination (and carrier trapping) rates, which become faster with increasing Mn-content from ∼37 to ∼20 ps (and ∼13 to ∼6 ps), a direct consequence of the enhancement of the ET efficiency. The meticulous analysis presented herein thus delivers the importance of the induced absorption signals in TA spectra that often remain ignored and can be used to gain deeper fundamental knowledge.