Sun Jianhui, An Limin, Xue Gaopeng, Li Xinghui
College of Physical Science and Technology, Heilongjiang University, Harbin 150080, People's Republic of China. Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan.
Nanotechnology. 2020 May 22;31(21):215408. doi: 10.1088/1361-6528/ab746c. Epub 2020 Feb 10.
The high efficiency of quantum dot-sensitized solar cells (QDSSCs) is a benefit of the highly efficient photoinduced-electron transfer (PET) to external electrodes. Here, we investigated how the surface defects and conduction-band (CB) offsets between core and shell materials affect the PET from CuInS quantum dots (QDs) by means of time-resolved femtosecond transient absorption and nanosecond photoluminescence spectroscopy. The transfer of 1S excited electrons from CuInS QDs to TiO films is demonstrated and we find that the surface-electron trapping can significantly reduce the efficiency of the PET. Though the electron trapping can be suppressed after ZnS surface passivation, the PET decreases significantly to a low efficiency of ∼33% from the type I CuInS/ZnS core/shell QDs because of their low electron density at the surface of the QDs. The surface-electron density is increased with the strategy of wavefunction engineering by reducing the CB offset, which allows us to achieve a quasi-type II carrier confinement in CuInS/CdS core/shell QDs. The PET efficiency appears to be as high as ∼95% from the CuInS/CdS core/shell QDs, which is ascribed to synergistic effects of the surface passivation and enhanced delocalization of the electron wavefunction from the CuInS core to the CdS shell. Finally, we demonstrate that these new mechanistic understandings of the PET processes are crucial to improving the efficiency of CuInS QDSSCs.
量子点敏化太阳能电池（QDSSCs）的高效率得益于其向外部电极的高效光致电子转移（PET）。在此，我们通过飞秒时间分辨瞬态吸收光谱和纳秒光致发光光谱，研究了核壳材料之间的表面缺陷和导带（CB）偏移如何影响来自CuInS量子点（QDs）的PET。证明了1S激发电子从CuInS量子点转移到TiO薄膜，并且我们发现表面电子俘获会显著降低PET的效率。尽管在ZnS表面钝化后电子俘获可以被抑制，但由于I型CuInS/ZnS核壳量子点表面的电子密度较低，PET效率从该量子点显著降低至约33%的低效率。通过减小CB偏移的波函数工程策略增加了表面电子密度，这使我们能够在CuInS/CdS核壳量子点中实现准II型载流子限制。来自CuInS/CdS核壳量子点的PET效率似乎高达约95%，这归因于表面钝化以及电子波函数从CuInS核到CdS壳的离域增强的协同效应。最后，我们证明了这些对PET过程的新机理理解对于提高CuInS QDSSCs的效率至关重要。
