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The terahertz absorption coefficient, index of refraction, and conductivity of nanostructured ZnO have been determined using time-resolved terahertz spectroscopy, a noncontact optical probe. ZnO properties were measured directly for thin films and were extracted from measurements of nanowire arrays and mesoporous nanoparticle films by applying Bruggeman effective medium theory to the composite samples. Annealing significantly reduces the intrinsic carrier concentration in the ZnO films and nanowires, which were grown by chemical bath deposition. The complex-valued, frequency-dependent photoconductivities for all morphologies were found to be similar at short pump-probe delay times. Fits using the Drude-Smith model show that films have the highest mobility, followed by nanowires and then nanoparticles, and that annealing the ZnO increases its mobility. Time constants for decay of photoinjected electron density in films are twice as long as those in nanowires and more than 5 times those for nanoparticles due to increased electron interaction with interfaces and grain boundaries in the smaller-grained materials. Implications for electron transport in dye-sensitized solar cells are discussed.

ZnO nanowires, grown on transparent conducting oxide substrates from aqueous solutions of methenamine and Zn(NO3)2, were integrated as the wide band gap semiconductor into dye-sensitized solar cells. ZnO nanowires and their growth mechanisms were studied using electron microscopy, x-ray diffraction and photoluminescence measurements. The solution growth method forms dense arrays of long nanowires oriented normal to the substrate surface because nanowires growing at off-normal angles are prevented from growing further when they run into neighbouring wires. Dye-sensitized solar cells with ZnO nanowires were assembled and characterized using optical and electrical measurements. Short circuit current densities of 1.3 mA cm−2, and overall power conversion efficiencies of 0.3% were achieved with 8 µm long nanowires. Photocurrent and efficiency increase with increasing nanowire length and improved light harvesting. Low surface area and a shunt that appears under light illumination limit the solar cell performance. Internal quantum efficiencies were similar for nanowires of all lengths, indicating that electron transport is not limited by the nanowire dimensions for aspect ratios less than 70.

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The creation of a sustainable energy generation, storage, and distribution infrastructure represents a global grand challenge that requires massive transnational investments in the research and development of energy technologies that will provide the amount of energy needed on a sufficient scale and timeframe with minimal impact on the environment and have limited economic and societal disruption during implementation. In this opinion paper, we focus on an important set of solar, thermal, and electrochemical energy conversion, storage, and conservation technologies specifically related to recent and prospective advances in nanoscale science and technology that offer high potential in addressing the energy challenge. We approach this task from a two-fold perspective: analyzing the fundamental physicochemical principles and engineering aspects of these energy technologies and identifying unique opportunities enabled by nanoscale design of materials, processes, and systems in order to improve performance and reduce costs. Our principal goal is to establish a roadmap for research and development activities in nanoscale science and technology that would significantly advance and accelerate the implementation of renewable energy technologies. In all cases we make specific recommendations for research needs in the near-term (2-5 years), mid-term (5-10 years) and long-term (>10 years), as well as projecting a timeline for maturation of each technological solution. We also identify a number of priority themes in basic energy science that cut across the entire spectrum Broader contextA major scientific and societal challenge of the 21st century is the conversion from a fossil-fuel-based energy economy to one that is sustainable. The energy challenge before us differs in three ways from past large scale challenges: the first is the large magnitude and relatively short time scale of the transition (a predicted doubling of energy demand by mid-century and a tripling by the end of the century); the second is the need to develop CO 2 -neutral, renewable energy sources; and the third is the cost-competitive aspect of the transition (insofar as the cost of energy to the consumer must be competitive with the fossil fuel energy supply being replaced). What is clear is that the science and engineering research communities working with industry, and policy makers (government, economists, social scientists) will have to educate the citizenry and get them to function collaboratively and globally to enhance the quality of life and to preserve the environment of our planet for future generations. Our team has prepared a technical article on the role of nanotechnology in our energy future aimed at guiding both our own community of scientists and engineers and our policy makers who interface with the public. This journal is ª The Royal Society of Chemistry 2009Energy Environ. Sci., 2009, 2, 559-588 | 559 ANALYSIS www.rsc.org/ees | Energy & Environmental Science of energy conversion, storage, and conservation technologies. We anticipate t...

In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.

This paper reports visible-light sensitization of TiO 2 nanoparticles by surface modification with Mn(II)-terpyridine complexes, as evidenced by UV-vis spectroscopy of colloidal thin films and aqueous suspensions. Photoexcitation of the [Mn II (H 2 O) 3 (catechol-terpy)] 2+ /TiO 2 (terpy ) 2,2′:6,2′′-terpyridine) complex, attached to the TiO 2 surface, leads to interfacial electron transfer within 300 fs as indicated by ultrafast optical pumpterahertz probe transient measurements and computational simulations. Photoinduced interfacial electron transfer is accompanied by Mn(II) f Mn(III) photooxidation. The half-time for regeneration of the Mn(II) complex is ca. 23 s (at 6 K), as monitored by time-resolved measurements of the Mn(II) EPR signal.