Defects are needed for fast photo-induced electron transfer from a nanocrystal to a molecule: time-domain ab initio...

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.