Charge Transfer from Single Semiconductor Nanocrystals to Single Molecules

Abstract

A candidate material for next-generation photovoltaics is a lattice of nanometer-scale semiconductor crystals (quantum dots, or QDs).¹ To efficiently convert light into current, electrons must be photoexcited out of the QD at a fast rate to avoid being trapped in defects on the QD surface.² We created a model system to quantify the rate at which excited electrons are transferred out of the QD following photoexcitation. Our model used organic molecules adsorbed to the surface of the QDs to accept excited electrons that leave the crystal. The number of surface molecules, which can be as few as one, shortens the excited-state lifetime by offering electrons more pathways out of the excited state, increasing the excited-state decay rate. We examined QD structures individually to resolve integer numbers of surface molecules on the crystals, allowing us to determine the effect a single acceptor had on the rate. By measuring the excited-state decay rate (the inverse of the characteristic time that electrons remain in an excited state), we found a discrete change in the rate corresponding to the number of sur