Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology , Van der Maasweg 9, 2629 HZ Delft, The Netherlands.
Joule Physics Laboratory, School of Computing, Science and Engineering, University of Salford , Manchester M5 4WT, United Kingdom.
ACS Nano. 2017 Jun 27;11(6):6286-6294. doi: 10.1021/acsnano.7b02506. Epub 2017 Jun 6.
Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and holes and build up a broadband cooling spectrum for both charge carriers. Cooling of electrons is slower than of holes. At energies near the band gap we find cooling times between successive electronic energy levels in the order of 0.5 ps. We argue that here the large spacing between successive electronic energy levels requires cooling to occur by energy transfer to vibrational modes of ligand molecules or phonon modes associated with the QD surface. At high excess energy the energy loss rate of electrons is 1-5 eV/ps and exceeds 8 eV/ps for holes. Here charge carrier cooling can be understood in terms of emission of LO phonons with a higher density-of-states in the valence band than the conduction band. The complete mapping of the broadband cooling spectrum for both charge carriers in PbSe QDs is a big step toward understanding and controlling the cooling of hot charge carriers in colloidal QDs.
了解半导体量子点（QD）中热载流子的冷却对于提高 QD 在光电器件中的性能至关重要。我们研究了 PbSe QD 中电子和空穴在载流子倍增发生的高能下的冷却动力学。我们描述了热电子和空穴的不同冷却步骤，并为两种载流子构建了宽带冷却光谱。电子的冷却速度比空穴慢。在接近带隙的能量处，我们发现相邻电子能级之间的冷却时间约为 0.5 ps。我们认为，这里相邻电子能级之间的大间距要求通过能量转移到配体分子的振动模式或与 QD 表面相关的声子模式来发生冷却。在高过剩能量下，电子的能量损失率为 1-5 eV/ps，而空穴的能量损失率超过 8 eV/ps。在这里，可以根据价带中比导带具有更高态密度的 LO 声子发射来理解载流子冷却。在 PbSe QD 中对两种载流子的宽带冷却光谱进行全面映射是朝着理解和控制胶体 QD 中热载流子冷却迈出的重要一步。
