Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency

Quantum dot-metal oxide junctions are an integral part of nextgeneration solar cells, light emitting diodes, and nanostructured electronic arrays. Here we present a comprehensive examination of electron transfer at these junctions, using a series of CdSe quantum dot donors (sizes 2.8, 3.3, 4.0, and 4.2 nm in diameter) and metal oxide nanoparticle acceptors (SnO 2 , TiO 2 , and ZnO). Apparent electron transfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy. The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. In contrast with dye-sensitized metal oxide electron transfer studies, our systems did not exhibit unthermalized hot-electron injection due to relatively large ratios of electron cooling rate to electron transfer rate. To investigate the implications of these findings in photovoltaic cells, quantum dot-metal oxide working electrodes were constructed in an identical fashion to the films used for the electron transfer portion of the study. Interestingly, the films which exhibited the fastest electron transfer rates (SnO 2 ) were not the same as those which showed the highest photocurrent (TiO 2 ). These findings suggest that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency.Marcus theory | transient absorption spectroscopy | quantum dot sensitized solar cell | nanotechnology | energy conversion S emiconducting quantum dots (QDs) are a widely studied material with many interdisciplinary applications (1, 2). Perhaps the most appealing attribute of these materials, from both an academic and industrial perspective, is their size-dependent electronic structure-the ability to design systems and devices with tailor-made electronic properties simply by altering the size of one of the constituent materials (3). As less expensive and less complex routes are continually developed to synthesize a variety of QD materials, further implementation of QDs into nextgeneration devices and procedures is inevitable.The properties of QDs are often exploited in a system or device through their complexation with other materials of interest: functionalizing QDs with biomolecules for imaging (4); linking many QDs together with short-chain molecules to create nanostructured electronic arrays (5); creating highly emissive core-shell QD particles for sensors and optoelectronic displays (6); or sensitizing semiconducting systems with other semiconductors to create inexpensive, next-generation photovoltaic devices (7,8). In each of the aforementioned applications, QDs are utilized because of their size-dependent electronic structure.Although electronic interactions between QDs and organic molecules ...

He earned his doctoral degree (1979) in Physical Chemistry from the Bombay University and carried out postdoctoral research at Boston University (1979University ( -1981 and University of Texas at Austin (1981Austin ( -1983. He joined Notre Dame in 1983 and initiated a successful research project on utilizing semiconductor nanostructures for light energy conversion. His major research interests are in three areas: (1) to understand interfacial processes and catalytic reactions at nanostructured semiconductor interface, (2) to develop semiconductor hybrid assemblies for solar cells and solar fuels, and (3) carbon nanostructure architectures for energy conversion and storage. He has authored more than 350 peer-reviewed journal papers, review articles, and book chapters with more than 20000 citations. He has also edited three books in the area of nanoscale materials.Kevin Tvrdy (far left) received his Bachelor's degree in Chemistry from the University of Nebraska in 2005, after which he worked for one year at Streck Laboratories as an R&D technician. He is currently pursuing his doctoral degree at the University of Notre Dame in the Department of Chemistry and Biochemistry under the direction of Prashant V. Kamat. His current research centers on the application of ultrafast spectroscopic measurements to better understand and improve upon electron transfer phenomena in photovoltaic devices. In his free time, Kevin enjoys spending time with his wife Jessica outdoors. David R. Baker (far right) is a Ph.D. candidate in the Department of Chemical and Biomolecular Engineering at the University of Notre Dame, and Radiation Laboratory. He earned his Bachelor's degree in Chemical Engineering at the University of Washington in 2006, where he researched interfacial water phenomena and methylotrophic bacterial populations. His current research focuses on electrode-electrolyte interactions and developing nanoarchitectures within quantum dot solar cells. He has interned with the Jet Propulsion Laboratory and the United States Department of State. Emmy J. Radich (not pictured) is a P h.D. student in the Department of Chemical and Biomolecular Engineering at the University of Notre Dame where she works under the guidance of Prashant Kamat in the Notre Dame Radiation Laboratory. Her current focus is on carbon-based nanostructured composite materials for energy applications. Emmy earned her Bachelor's and Master's degrees in Chemical Engineering from the Dave C. Swalm School of Chemical Engineering at Mississippi State University. She also spent four years working at RespirTek, Inc., a commercial bioenvironmental laboratory. Emmy's research interests are diverse, with past projects focusing on biofuels synthesis, gas hydrates, anaerobic digestion, and novel bioremediation strategies. Emmy has also worked in various government and industrial positions ranging from groundwater assessment/ remediation regulator to a refinery process engineer to laboratory director. convert light energy into electricity. 7-10 Unlike solid state photovolt...

We demonstrate a polymer-free carbon-based photovoltaic device that relies on exciton dissociation at the SWNT/C(60) interface, as shown in the figure. Through the construction of a carbon-based photovoltaic completely free of polymeric active or transport layers, we show both the feasibility of this novel device as well as inform the mechanisms for inefficiencies in SWNTs and carbon based solar cells.

Understanding CdSe quantum dot (QD) adsorption phenomena on mesoscopic TiO 2 films is important for improving the performance of quantum dot sensitized solar cells (QDSSCs). A kinetic adsorption model has been developed to elucidate both Langmuir-like submonolayer adsorption and QD aggregation processes. Removal of surface-bound trioctylphosphine oxide as well as the use of 3-mercaptopropionic acid (MPA) as a molecular linker improved the adsorption of toluene-suspended QDs onto TiO 2 films. The adsorption constant K ad for submonolayer coverage was (6.7 ( 2.7) Â 10 3 M À1 for direct adsorption and (4.2 ( 2.0) Â 10 4 M À1 for MPA-linked assemblies. Prolonged exposure of a TiO 2 film to a CdSe QD suspension resulted in the assembly of aggregated particles regardless of the method of adsorption. A greater coverage of TiO 2 was achieved with smaller QDs due to reduced size constraints. Ultrafast transient absorption spectroscopy demonstrated faster electron injection into TiO 2 from directly adsorbed QDs (k ET = 7.2 Â 10 9 s À1 ) compared with MPA-linked QDs (k ET = 2.3 Â 10 9 s À1 ). The adsorption kinetic details presented in this study are useful for controlling CdSe QD adsorption on TiO 2 and designing efficient photoanodes for QDSSCs.