Charge Transfer from Upconverting Nanocrystals to Semiconducting Electrodes: Optimizing Thermodynamic Outputs by Electronic Energy Transfer

Light-harvesting inorganic nanocrystals play an important role in emerging solar energy conversion and optoelectronic devices. We describe here a strategy for a new family of photoelectrodes with upconverting nanocrystal assemblies as the photosensitizer. The assemblies consist of oleic acid-capped cadmium selenide (CdSe) nanocrystals that coordinate directly onto a layer of surface-bound, carboxylic acid-derivatized anthracenes through displacement of the oleic acid capping ligands. Steady-state emission and transient absorption measurements show that the upconverting nano crystal assemblies, selectively excited by green light, generate singlet excitons that enable efficient charge injection into both the conduction band of TiO2 at the photoanode and the valence band of NiO at the photocathode. The singlet excitons form by sensitized triplet-triplet annihilation within the compact layer of anthracenes on the electrode surfaces. Density of state analysis reveals that the electronic coupling between the anthracene singlet excited states and the oxides provides a thermodynamic basis for light-induced charge transfer. The interplay between the excited-state populations at the surface-bound molecules and the assembled nanocrystals presents new design rules that can potentially overcome the limitations of previous dye-sensitized photoelectrochemical cells for catalytic applications.