Influence of Surface Chemistry on the Binding and Electronic Coupling of CdSe Quantum Dots to Single Crystal TiO2 Surfaces

Sensitization of mesoporous nanocrystalline TiO2 solar cells with quantum confined semiconductor nanocrystals (QDs) has some advantages over organic dyes or inorganic complex sensitizers, yet the reported efficiencies of laboratory devices are not currently competitive with those of dye sensitized cells. Several methods previously utilized to bind CdSe QDs to mesoporous TiO2 films were investigated using low index faces of both anatase and ruble TiO2 polytypes as model systems. The in situ ligand exchange method, where 3-mercaptopropionic acid (MPA) covered TiO2 crystal surfaces are treated With trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO) (TOP/TOPO)-capped CdSe QDs, resulted in very irreproducible and usually low sensitized photocurrents. The ex situ ligand exchange method, whereby MPA-capped QDs are synthesized and directly adsorbed onto bare TiO2 single crystals, resulted in both reproducible sensitized photocurrents and surface coverages that are verified with atomic force microscopy (AFM). Purification of the nanocrystals and adjustment of the pH of the sensitization solution to > 10.2 was found to prevent QD agglomeration and takes advantage of the dual chemical functionality of MPA to directly link the QDs to the TiO2 surface. The spectral response of the incident photon to current efficiencies of CdSe QDs was directly compared to the commonly used sensitizer cis-di(thiocyanato)-bis(4,4;-dicarboxy-2,2'-bipyridine)ruthenium(II) (N3) on the same single crystals.