Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO(2) single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO(2) facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.

The splitting of water photoelectrochemically into hydrogen and oxygen represents a promising technology for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled--yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalysing water oxidation also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, especially for nanostructured photoanodes benefiting from small charge-carrier transport distances. Here we use super-resolution imaging, operated in a charge-carrier-selective manner and with a spatiotemporal resolution of approximately 30 nanometres and 15 milliseconds, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resolution (about 390 nanometres), the photocurrent associated with water oxidation, and find that the most active sites for water oxidation are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod--even though more improvement in photocurrent efficiency correlates with less reduction in onset potential (and vice versa) at the sub-particle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential reduction the optimal sites are the sites with more positive onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.

Sensitization of mesoporous nanocrystalline TiO(2) 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 TiO(2) films were investigated using low index faces of both anatase and rutile TiO(2) polytypes as model systems. The in situ ligand exchange method, where 3-mercaptopropionic acid (MPA) covered TiO(2) 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 TiO(2) 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 TiO(2) 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.