Ultrafast transient absorption spectroscopy of 14 and 2.1 nm CsPbBr₃ nanoparticles: Free carrier vs exciton

CsPbBr3 nanoparticles with averaged size of 14 and 2.1 nm (named as 14 and 2 nmNP, respectively) are synthesized capping with the ligand of dodecylbenzene sulfonic acid (DBSA). Using ultrafast transient absorption spectroscopy, free carriers in 14 nmNP characterize with the slow thermalization (similar to 0.76 ps) and long carrier cooling time (similar to 1 ns), which could be due to the phonon bottleneck effect. On the other hand, excitons are major photoexcitations in 2 nmNP, presenting with fast thermalization (<100 fs) and short cooling time (similar to 50 ps); this effective energy dissipation in 2 nmNPs can be ascribed to the involvement of phonons in DBSA ligands. In both 14 and 2 nmNP, we find that the photoexcitation accumulation leads to an increase in the bandgap that can be understood by the Burstein-Moss band filling effect. These results provide insights into the intrinsic photophysics of CsPbBr3 perovskite nanocrystals with different confinement effects, supplying direct implications for optoelectronic applications.