Uncovering the Role of Hole Traps in Promoting Hole Transfer from Multiexcitonic Quantum Dots to Molecular Acceptors

Abstract

Understanding electronic dynamics in multi-excitonic quantum dots (QDs) is important for designing efficient systems useful in high power scenarios, such as solar concentrators and multielectron charge transfer. The multiple charge carriers within a QD can undergo undesired Auger recombination events, which rapidly annihilate carriers on picosecond timescales and generate heat from absorbed photons instead of useful work. Compared to the transfer of multiple electrons, the transfer of multiple holes has proven to be more difficult due to slower hole transfer rates. To probe the competition between Auger recombination and hole transfer in CdSe, CdS, and CdSe/CdS QDs of varying sizes, we synthesized a phenothiazine derivative, with optimized functionalities for binding to QDs as a hole accepting ligand and for spectroscopic observation of hole transfer. Transient absorption spectroscopy was used to monitor the photoinduced absorption features from both trapped holes and oxidized ligands under excitation fluences where the averaged initial number of excitons in a QD ranged from ~1 to 19. We observed fluence-dependent hole transfer kinetics that last around 100 ps longer than the predicted Auger recombination lifetimes, and the transfer of up to 3 holes per QD. Theoretical modeling of the kinetics suggests that binding of hole acceptors introduces trapping states significantly different from those in native QDs passivated with oleate ligands. Holes in these modified trap states have prolonged lifetimes, which promotes the hole transfer efficiency. These results highlight the beneficial role of hole trapping states in devising hole transfer pathways in QD-based systems under multi-excitonic conditions.

Directed Ligand Exchange on the Surface of PbS Nanocrystals: Implications for Incoherent Photon Conversion

“…These data show that the 9-ACA exchange plateaus after the addition of only 0.1 equivalents of 9-ACA:OA, and we observe a similar rapid saturation of the chemical shift (Figure S6 and associated discussion). These behaviors are distinct from UDA and consistent with a partial exchange, as has been reported for other bulky ligands. , …”

Auger-Assisted Electron Transfer between Adjacent Quantum Wells in Two-Dimensional Layered Perovskites

“…For 2D perovskites with m = 8 and 10 ( E b > E g ), the excitation-intensity-dependent TA kinetics (Figures b and S3c) show that the early time decay component (<500 ps) becomes faster with increasing excitation intensity, whereas, in the later time window, the kinetics become consistent (Figure S4a,b). This is a typical TA feature indicative of the occurrence of Auger recombination of excitons, similar to that in semiconductor quantum dots. − By fitting the TA kinetics, we determined the Auger recombination time of τ Aug. ≈ 140.5 ps for m = 8 and 302.7 ps for m = 10 (see Figure S4c). The difference in Auger time is due to the different dielectric screening effect between m = 8 and m = 10 organic ligands.…”

Simulations of nonradiative processes in semiconductor nanocrystals

“…The decay of multiexcitonic states can be modeled by classical master equations (i.e., classical rate equations) that use decay rates of single excitons and biexcitons. These methods have proven to be surprisingly accurate for modeling the decay of a general number of excitons, N exc , in a NC 181,182 as well as Auger heating, 87 or the long-lived heating of the NC lattice that occurs due to the sequence of AR events and subsequent hot carrier cooling. 154,155 To this end, the rate that N exc excitons decays to (N exc − 1) excitons can be well-approximated by modeling AR as a bimolecular collision between exci-tons, such that the overall AR rate (K AR ) is given by…”