Department of Chemistry, Emory University , Atlanta, Georgia 30322, United States.
Acc Chem Res. 2013 Jun 18;46(6):1270-9. doi: 10.1021/ar300202d. Epub 2012 Nov 13.
Recent reports of multiexciton generation (MEG), a process by which one absorbed photon generates multiple excitons, in lead chalcogenide nanocrystals (NCs) have intensified research interest in using this phenomenon to improve the efficiency of solar energy conversion. Practical implementation of MEG processes in solar cells and solar-to-fuel conversion devices requires the development of materials with higher MEG efficiencies and lower excitation thresholds than are currently available, as well as schemes for efficient multiexciton extraction before the ultrafast exciton-exciton annihilation occurs. This Account focuses on the extraction of multiexcitons by interfacial electron transfer in model NC-molecular acceptor complexes. We provide an overview of multiexciton annihilation and multiexciton dissociation (MED) processes in NC-acceptor complexes of (i) CdSe quantum dots (QDs), (ii) CdSe/CdS quasi-type II core/shell QDs, (iii) CdSe quantum confined nanorods (QRs), and (iv) PbS QDs. We show that ultrafast electron transfer to adsorbed molecular acceptors can efficiently dissociate multiexcitons generated by absorption of multiple photons in (i), (ii), and (iii). Compared to core-only CdSe QDs, the electron hole distributions in CdSe/CdS quasi-type II QDs and CdSe QRs significantly improve their MED efficiencies by simultaneously retarding Auger recombination and facilitating interfacial electron transfer. Finally, in PbS-methylene blue (MB(+)) complexes, we show that the presence of electron acceptors does not affect the MEG efficiency and electron transfer to MB(+) efficiently dissociates the multiple excitons generated in PbS QDs. Our findings demonstrate that ultrafast interfacial charge transfer can be an efficient approach for extracting multiexcitons, and wavefunction engineering in quantum confined NCs can further improve MED efficiency.
最近有报道称，在 lead chalcogenide nanocrystals（NCs）中存在多激子生成（MEG）现象，即在一个被吸收的光子生成多个激子的过程。这一现象引发了人们对利用该现象来提高太阳能转换效率的研究兴趣。为了在太阳能电池和太阳能到燃料转换器件中实际应用 MEG 过程，需要开发具有比现有材料更高的 MEG 效率和更低的激发阈值的材料，以及在超快激子-激子湮灭发生之前有效提取多激子的方案。本报告重点介绍了在模型 NC-分子受体复合物中通过界面电子转移提取多激子的方法。我们提供了关于 NC-受体复合物中多激子湮灭和多激子解离（MED）过程的概述，这些复合物包括（i）CdSe 量子点（QD）、（ii）CdSe/CdS 准 II 型核/壳 QD、（iii）CdSe 量子限制纳米棒（QR）和（iv）PbS QD。我们表明，超快电子转移到吸附的分子受体可以有效地将（i）、（ii）和（iii）中吸收多个光子产生的多激子解离。与仅含有核的 CdSe QD 相比，CdSe/CdS 准 II 型 QD 和 CdSe QR 中的电子空穴分布通过同时延迟俄歇复合和促进界面电子转移，显著提高了它们的 MED 效率。最后，在 PbS-亚甲蓝（MB(+)）复合物中，我们表明电子受体的存在不会影响 MEG 效率，并且电子向 MB(+)的转移可以有效地将 PbS QD 中生成的多激子解离。我们的发现表明，超快界面电荷转移可以是提取多激子的有效方法，并且在量子限制 NC 中进行波函数工程可以进一步提高 MED 效率。
