Yan Chang, Weinberg Daniel, Jasrasaria Dipti, Kolaczkowski Matthew A, Liu Zi-Jie, Philbin John P, Balan Arunima D, Liu Yi, Schwartzberg Adam M, Rabani Eran, Alivisatos A Paul
Department of Chemistry, University of California, Berkeley, California 94720, United States.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
ACS Nano. 2021 Feb 23;15(2):2281-2291. doi: 10.1021/acsnano.0c08158. Epub 2020 Dec 18.
Understanding electronic dynamics in multiexcitonic 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 time scales 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 multiexcitonic conditions.
了解多激子量子点（QD）中的电子动力学对于设计适用于高功率场景（如太阳能聚光器和多电子电荷转移）的高效系统至关重要。量子点内的多个电荷载流子可能会发生不希望的俄歇复合事件，这种事件会在皮秒时间尺度上迅速湮灭载流子，并将吸收的光子转化为热量而非有用功。与多电子转移相比，由于空穴转移速率较慢，多空穴转移已被证明更加困难。为了探究不同尺寸的CdSe、CdS和CdSe/CdS量子点中俄歇复合与空穴转移之间的竞争关系，我们合成了一种具有优化功能的吩噻嗪衍生物，用于作为空穴接受配体与量子点结合，并用于空穴转移的光谱观察。瞬态吸收光谱用于监测在量子点中平均初始激子数范围约为1至19的激发通量下，捕获空穴和氧化配体的光致吸收特征。我们观察到通量依赖的空穴转移动力学，其持续时间比预测的俄歇复合寿命长约100 ps，并且每个量子点最多可转移3个空穴。动力学的理论模型表明，空穴受体的结合引入了与用油酸酯配体钝化的天然量子点中的陷阱态显著不同的陷阱态。这些修饰陷阱态中的空穴具有延长的寿命，这提高了空穴转移效率。这些结果突出了空穴捕获态在多激子条件下基于量子点的系统中设计空穴转移途径方面的有益作用。
