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Epitaxial Dimers and Auger-Assisted De-Trapping in PbS Quantum Dot Solids

preprint
submitted on 19.03.2019 and posted on 20.03.2019 by Rachel H. Gilmore, Yun Liu, Wenbi Shcherbakov-Wu, Nabeel Dahod, Elizabeth Lee, Mark Weidman, Huashan Li, Joel Jean, Vladimir Bulovic, Adam Willard, Jeffrey C. Grossman, William Tisdale
Electronic trap states limit the overall power conversion efficiency of quantum dot (QD) solar cells by inhibiting charge carrier transport and reducing the open-circuit voltage. Here, we explore the dynamic interaction of charge carriers between band edge states and sub-band trap states using broadband transient absorption spectroscopy. In monodisperse arrays of 4-5 nm diameter PbS QDs, we observe an optically active trap state ~100-200 meV below the band edge that occurs at a frequency of 1 in ~2500 QDs. Uncoupled QD solids with oleic acid ligands show trap-to-ground-state recombination that resembles Auger recombination. In electronically coupled QD solids, we observe entropically-driven uphill thermalization of trapped charge carriers from the trap state to the band edge via two distinct mechanisms: Auger-assisted charge transfer (~35 ps) and thermally activated hopping (~500 ps). Photophysical characterization combined with atomistic simulations and high-resolution transmission electron microscopy suggest that these states arise from epitaxially fused pairs of QDs – rather than electron or hole traps at the QD surface – offering new strategies for improving the efficiency of QD solar cells.

Funding

Imaging Interfacial Electric Fields on Ultrafast Timescales

Basic Energy Sciences

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U.S. Department of Energy, Office of Science, Basic Energy Sciences, DE-SC0019345

CAREER: Ligand-Mediated Photothermal Energy Dissipation in Semiconductor Nanocrystals

Directorate for Mathematical & Physical Sciences

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U.S. Department of Energy, Office of Science, Brookhaven National Lab, DE-SC0012704

DOE National Energy Research Scientific Computing Center DE-AC02-05CH11231

NSF Extreme Science and Engineering Discovery Environment (XSEDE) ACI-1053575

History

Email Address of Submitting Author

tisdale@mit.edu

Institution

Massachusetts Institute of Technology

Country

USA

ORCID For Submitting Author

0000-0002-6615-5342

Declaration of Conflict of Interest

The authors declare no conflicts of interest.

Version Notes

version 1.0

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