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Carrier cooling and multiplication in semiconductor nanocrystals and superlattices (Conference Presentation)
Author(s): Laurens D. A. Siebbeles

Paper Abstract

Absorption of energetic photons in a semiconductor leads to hot electrons and holes that usually cool to the band edge by thermal relaxation. In nanomaterials this cooling can be intercepted by excitation of additional electrons across the band gap. In this way, one photon generates multiple electron-hole pairs via Carrier Multiplication (CM), which is of interest for highly efficient photovoltaics. We studied cooling and CM in: a) films of PbSe nanocrystals coupled by organic ligands, and b) 2D superlattices of directly coupled PbSe nanocrystals. The studies were performed using pump-probe spectroscopy with optical or terahertz/microwave conductivity detection. Reducing the size of ligands between nanocrystals strongly increases the charge mobility. Removal of ligands and direct coupling of nanocrystals in a square or honeycomb superlattice allowed us to further tune the mobility and to realize values as high as 150 cm^2/Vs. We found that a high mobility is essential for multiple electron-hole pairs formed via CM to escape from recombination. The coupling between the nanocrystals was found to strongly affect the competition between cooling of hot charges by phonon emission and CM. In square 2D superlattices of nanocrystals CM is much more efficient than in films with ligands between the nanocrystals. In square superlattices CM occurs in a step-like fashion with threshold near the minimum energy of twice the band gap. The factors governing the charge mobility and the efficiency of CM, as well as the impact on the efficiency of photovoltaic devices, will be discussed on the basis of theoretical modeling.

Paper Details

Date Published: 5 March 2019
Proc. SPIE 10916, Ultrafast Phenomena and Nanophotonics XXIII, 109160T (5 March 2019); doi: 10.1117/12.2506483
Show Author Affiliations
Laurens D. A. Siebbeles, Technische Univ. Delft (Netherlands)

Published in SPIE Proceedings Vol. 10916:
Ultrafast Phenomena and Nanophotonics XXIII
Markus Betz; Abdulhakem Y. Elezzabi, Editor(s)

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