Li Yulu, Lai Runchen, Luo Xiao, Liu Xue, Ding Tao, Lu Xin, Wu Kaifeng
State Key Laboratory of Molecular Reaction Dynamics , Dynamics Research Center for Energy and Environmental Materials , Dalian Institute of Chemical Physics , Chinese Academy of Sciences , Dalian , 116023 , China . Email:
Departmental of Chemistry , College of Chemistry and Chemical Engineering , Xiamen University , Xiamen , Fujian 361005 , China.
Chem Sci. 2019 May 6;10(23):5983-5989. doi: 10.1039/c9sc01339c. eCollection 2019 Jun 21.
In traditional solar cells, photogenerated energetic carriers (so-called hot carriers) rapidly relax to band edges emission of phonons, prohibiting the extraction of their excess energy above the band gap. Quantum confined semiconductor nanocrystals, or quantum dots (QDs), were predicted to have long-lived hot carriers enabled by a phonon bottleneck, , the large inter-level spacings in QDs should result in inefficient phonon emissions. Here we study the effect of quantum confinement on hot carrier/exciton lifetime in lead halide perovskite nanocrystals. We synthesized a series of strongly confined CsPbBr nanocrystals with edge lengths down to 2.6 nm, the smallest reported to date, and studied their hot exciton relaxation using ultrafast spectroscopy. We observed sub-ps hot exciton lifetimes in all the samples with edge lengths within 2.6-6.2 nm and thus the absence of a phonon bottleneck. Their well-resolved excitonic peaks allowed us to quantify hot carrier/exciton energy loss rates which increased with decreasing NC sizes. This behavior can be well reproduced by a nonadiabatic transition mechanism between excitonic states induced by coupling to surface ligands.
在传统太阳能电池中，光生高能载流子（即所谓的热载流子）通过发射声子迅速弛豫到能带边缘，从而无法提取其高于带隙的多余能量。量子限域半导体纳米晶体，即量子点（QDs），被预测具有由声子瓶颈导致的长寿命热载流子，也就是说，量子点中的大能级间距应会导致低效的声子发射。在此，我们研究了量子限域对卤化铅钙钛矿纳米晶体中热载流子/激子寿命的影响。我们合成了一系列边长低至2.6 nm（这是迄今为止报道的最小尺寸）的强限域CsPbBr纳米晶体，并使用超快光谱研究了它们的热激子弛豫。我们在所有边长在2.6 - 6.2 nm范围内的样品中观察到了亚皮秒级的热激子寿命，因此不存在声子瓶颈。它们分辨率良好的激子峰使我们能够量化热载流子/激子能量损失率随纳米晶体尺寸减小而增加的情况。这种行为可以通过与表面配体耦合诱导的激子态之间的非绝热跃迁机制很好地再现。
