Defects
Are Needed for Fast Photo-Induced Electron Transfer from a Nanocrystal
to a Molecule: Time-Domain <i>Ab Initio</i> Analysis
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Abstract
Quantum dot (QD)
solar cells constitute an attractive alternative
to traditional solar cells due to unique electronic and optical properties
of QDs. In order to achieve high photon-to-electron conversion efficiency,
rapid charge separation and slow charge recombination are required.
We use nonadiabatic molecular dynamics combined with time-domain density
functional theory to study electron transfer from a PbS QD to the
rhodamine B (RhB) molecule and subsequent electron return from RhB
to the QD. The time scale for the electron–hole recombination
obtained for the system without defects agrees well with the experiment,
while the simulated time scale for the charge separation is 10-fold
longer than the experimental value. By performing an atomistic simulation
with a sulfur vacancy, which is a common defect in PbS systems, we
demonstrate that the defect accelerates the charge separation. This
result is supported further by scaling arguments. Missing sulfur creates
unsaturated chemical bonds on Pb atoms, which form the PbS conduction
band. As a result, the QD lowest unoccupied molecular orbital (LUMO)
is lowered in energy, and the LUMO density extends onto the adsorbed
molecule, increasing the donor–acceptor interaction. The counterintuitive
conclusion that defects are essential rather than detrimental to functioning
of QD solar cells generates an unexpected view on the QD surface chemistry