Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Charge trapping is an ubiquitous process in colloidal Quantum-Dot Solids and a major limitation to the efficiency of quantum dot based devices such as solar cells, LEDs, and thermo-electrics. While much experimental work led to a reduction of trapping and thereby efficiency enhancements, the exact chemical nature of the trapping mechanism remains largely unidentified. In this study, we determine the density of trap states in CdTe Quantum-Dot solids both experimentally, using a combination of electrochemical control of the Fermi level with ultrafast transient absorption and time-resolved photoluminescence spectroscopy, and theoretically, via Density Functional Theory calculations. We find a high density of very efficient electron traps centered ~0.42 eV above the valence band. Electrochemical filling of these traps increases the electron lifetime and the photoluminescence quantum yield by more than an order of magnitude. The capture rate constant for holes is an order of magnitude lower that for electrons. These observations can be explained by Auger mediated electron trapping. From Density Functional Theory calculations we infer that the traps are formed by di-coordinated Te atoms at the quantum dot surface. The combination of our unique experimental determination of the density of trap states with the theoretical modeling of the quantum dot surface allows us to identify the trapping mechanism and chemical reaction at play during charge trapping in these quantum dots.
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