title: Density of Trap States and Auger-mediated Electron Trapping in CdTe Quantum-Dot Solids.
abstract: 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 thermoelectrics. Although empirical approaches 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 trapping 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 dicoordinated 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.
authors: †TU Delft, Chemical Engineering, Optoelectronic Materials, Julianalaan 136, 2628 BL Delft, The Netherlands.
authors: Simon C
authors: SC
authors: Boehme
authors: §Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) and Donostia International Physics Center (DIPC), P.K. 1072, 20018 Donostia, Euskadi, Spain.
authors: Jon Mikel
authors: JM
authors: Azpiroz
authors: †TU Delft, Chemical Engineering, Optoelectronic Materials, Julianalaan 136, 2628 BL Delft, The Netherlands.
authors: Yaroslav V
authors: YV
authors: Aulin
authors: †TU Delft, Chemical Engineering, Optoelectronic Materials, Julianalaan 136, 2628 BL Delft, The Netherlands.
authors: Ferdinand C
authors: FC
authors: Grozema
authors: ‡Utrecht University, Debye Institute, Condensed Matter and Interfaces, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands.
authors: Daniël
authors: D
authors: Vanmaekelbergh
authors: †TU Delft, Chemical Engineering, Optoelectronic Materials, Julianalaan 136, 2628 BL Delft, The Netherlands.
authors: Laurens D A
authors: LD
authors: Siebbeles
authors: §Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) and Donostia International Physics Center (DIPC), P.K. 1072, 20018 Donostia, Euskadi, Spain.
authors: Ivan
authors: I
authors: Infante
authors: †TU Delft, Chemical Engineering, Optoelectronic Materials, Julianalaan 136, 2628 BL Delft, The Netherlands.
authors: Arjan J
authors: AJ
authors: Houtepen
keywords: Auger-mediated trapping; defect; density functional theory; electrochemistry; quantum dot; ultrafast spectroscopy