Making and Breaking of Exciton Cooling Bottlenecks in Halide Perovskite Nanocrystals

Abstract

Harnessing quantum confinement (QC) effects in semiconductors to retard hot carrier cooling (HCC) is an attractive approach for enabling efficient hot carrier extraction to overcome the Shockley−Queisser limit. However, there is a debate about whether halide perovskite nanocrystals (PNCs) can effectively exploit these effects. To address this, we utilized pump−probe and multipulse pump−push−probe spectroscopy to investigate HCC behavior in PNCs of varying sizes and cation compositions. Our results validate the presence of an intrinsic phonon bottleneck with clear manifestations of QC effects in small CsPbBr 3 PNCs exhibiting slower HCC rates compared to those of larger PNCs. However, the replacement of inorganic Cs + with organic cations suppresses this intrinsic bottleneck. Furthermore, PNCs exhibit distinct size-dependent HCC behavior in response to changes in the cold carrier densities. We attribute this to the enhanced exciton−exciton interactions in strongly confined PNCs that facilitate Auger heating. Importantly, our findings dispel the existing controversy and provide valuable insights into design principles for engineering QC effects in PNC hot carrier applications.

Spectator Exciton Effects in Nanocrystals III: Unveiling the Stimulated Emission Cross Section in Quantum Confined CsPbBr3 Nanocrystals

“…While identifying the mechanism behind this slowing will require further investigation, the most obvious candidate would be the heating of the optic phonon modes by a sequential cascade of Auger recombinations. Such a slowing of carriers following dense photoexcitation of bulk semiconductors is well-known and coined the “Hot Phonon” effect, including in bulk LHP. − An analogous effect of hot phonons in the context of semiconductor NCs has also been reported. − While a recent study reports a slow component to carrier cooling in LHP NCs after intense multiphoton absorption, the average lifetimes are far lower than observed here . Efforts are ongoing in our laboratory to clarify the impact of phonon temperatures on hot carrier cooling in QC LHP NCs by complementary methods.…”

Spectator exciton effects in nanocrystals III: Unveiling the stimulated emission cross section in quantum confined CsPbBr3 nanocrystals

“…37,38,39 While a recent study reports a slow component to carrier cooling in LHP NCs after intense multiphoton absorption, the average lifetimes are far lower than observed here. 40 Efforts are ongoing in our lab to clarify the impact of phonon temperatures on hot carrier cooling in QC LHP NCs by complementary methods. It is noteworthy that no signs of spin blockades to the relaxation of hot bi-excitons were observed in these SX experiments akin to those detected in similar experiments on CdSe NCs, 41 perhaps reflecting heavy atom induced rapid spin flipping in LHPs.…”