We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5-0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50-60% were also obtained.

The creation of a single electron-hole pair (i.e., exciton) per incident photon is a fundamental limitation for current optoelectronic devices including photodetectors and photovoltaic cells. The prospect of multiple exciton generation per incident photon is of great interest to fundamental science and the improvement of solar cell technology. Multiple exciton generation is known to occur in semiconductor nanostructures with increased efficiency and reduced threshold energy compared to their bulk counterparts. Here we report a significant enhancement of multiple exciton generation in PbSe quasi-one-dimensional semiconductors (nanorods) over zero-dimensional nanostructures (nanocrystals), characterized by a 2-fold increase in efficiency and reduction of the threshold energy to (2.23 ± 0.03)E(g), which approaches the theoretical limit of 2E(g). Photovoltaic cells based on PbSe nanorods are capable of improved power conversion efficiencies, in particular when operated in conjunction with solar concentrators.

We report the synthesis of oriented single crystalline titanium dioxide (TiO(2)) nanowire arrays on titanium foil. The synthesis method relies on the ability to grow single crystal sodium titanate (Na(2)Ti(2)O(5).H(2)O) nanowires on titanium foil through a novel alkali hydrothermal growth process. Following growth, the Na(2)Ti(2)O(5).H(2)O nanowires are converted to protonated bititanate (H(2)Ti(2)O(5).H(2)O) nanowires through an ion-exchange reaction without changing their morphology or crystal structure. Finally, the protonated bititanate nanowires are converted to single crystalline anatase TiO(2) nanowires through a topotactic transformation by calcination. These three sequential steps yield a carpet of 2-50 microm long single crystalline nanowires oriented in the [100] direction and primarily normal to the titanium foil. Even longer nanowires can be grown. The single crystal TiO(2) nanowire arrays on flexible titanium substrate may be used in photocatalytic and photovoltaic devices such as dye-sensitized solar cells and may enhance their performance by providing fast electron transport. The nanowires can also be used as templates for producing hierarchical nanostructures such as nanowires decorated with nanoparticles on their periphery or nanotubes with walls made of nanoparticles.

Nanocrystal quantum dots (QD) show great promise toward improving solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (Voc) is fundamentally limited due to non-radiative recombination processes in QD solar cells. Here we report the highest open-circuit voltages to date for colloidal QD based solar cells under one sun illumination. This Voc of 692 ± 7 mV for 1.4 eV PbS QDs is a result of improved passivation of the defective QD surface, demonstrating as a function of the QD bandgap (Eg). Comparing experimental Voc variation with the theoretical upper-limit obtained from one diode modeling of the cells with different Eg, these results clearly demonstrate that there is a tremendous opportunity for improvement of Voc to values greater than 1 V by using smaller QDs in QD solar cells.

Vertical arrays of crystalline zinc oxide (ZnO) nanowires grown on various substrates find applications in dyesensitized and hybrid organic/inorganic bulk-heterojunction solar cells. The ability to grow dense nanowires at high rates and the fundamental understanding of the growth process are important for these applications. Herein, we show that heterogeneous growth of ZnO nanowires on substrates seeded with ZnO nanoparticles in an aqueous solution of methenamine and zinc nitrate is mass transport limited. Mass transport limited growth leads to an inverse relationship between the nanowire dimensions (height and diameter) and the nanowire number density. This mass transport limitation also leads to nonuniform growth near the boundaries between seeded and unseeded regions. Stirring the reaction solution increases the nanowire growth rate. Experimental results were interpreted within the framework of two simple but nontrivial models of the solution phase species transport and the nanowire growth. Additionally, it was determined that the anisotropic growth is due to the intrinsic growth kinetics of the (101 j 0) and (0001) surfaces of ZnO in zinc nitrate and methenamine and not due to the growth process being mass transport limited as previously suggested.