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combined manuscript file v190319.pdf (3.74 MB)
Epitaxial Dimers and Auger-Assisted De-Trapping in PbS Quantum Dot Solids
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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.