Charge Separation in the Hybrid CdSe Nanocrystal-Organic Interface: Role of the Ligands Studied by Ultrafast Spectroscopy and Density Functional Theory

We present a joint experimental and theoretical study of the early stage dynamics of photoexcited charges in a prototypical organic/inorganic interface. By using femtosecond pump probe experiments we compared the photophysic of a layer-by-layer hybrid structure obtained by alternating CdSe nanocrystals and poly(p-styrenesulphonic acid) and the same CdSe nanocrystals capped with hexadec-ylamine and stearic acid diluted in solutions. While in the LBL structure it is dear the appearance of a long-lived charged state, no evidence of this is instead found in the diluted solutions. Density functional calculations indicate that these states are localized dose to the nanoparticle surface, and that electrons and holes are separated across the hybrid interface, pointing out the effects of surfactant capping molecules on the optoelectronic properties of the interface. Our combined approach, allowing for unique access to the photoexcited electronic structure, opens the possibility to the fine tailoring of hybrid organic/semiconducting layers for photovoltaic applications.