Materials Science

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

Abstract

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.

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