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Harvesting energy directly from sunlight using photovoltaic (PV) technology or concentrating solar power (solar thermal energy conversion) is increasingly being recognized as an essential component of future global energy production. The decreased availability of fossil fuel sources and the realization of the detrimental long-term effects of emissions of CO 2 and other greenhouse gases into the atmosphere are driving research and deployment for new environmentally friendly energy sources, especially renewable energy resources. An additional driving force is the increasing worldwide sensitivity toward energy security and price stability. Capturing even a small fraction of the 162,000 terawatts (TW) that reaches the earth would significantly impact the overall energy balance. PV systems, in addition, are portable and well-suited to distributed applications. The largescale manufacturing of photovoltaics is increasingly economically viable. The rapid expansion of manufacturing capability in PV components and the deployment of concentrating solar power (CSP) systems offers the potential for supplying a significant fraction (10% without need for storage) of our energy demand with minimal environmental impact. (See accompanying sidebar by Mehos for additional detail.) In addition, it is clear that these technologies represent one of the next major high-technology economic drivers eventually succeeding microelectronics, telecommunications, and display industries. Truly achieving this goal will require materials-science-driven cost reductions, not just incremental cost reductions through economies of scale.In 2004, the average total worldwide power consumption was 15 TW (1.5 × 10 13 W), with 86.5% from the burning of fossil fuels, according to U.S. Department of Energy statistics. In 2003, 39.6 quads (1 quad = 1 quadrillion BTU = 1.055 × 10 9 GJ, 29.9 quad = 1 TW-year) of energy, largely from fossil fuels, was consumed to produce electricity just in the United States. After conversion losses, 13.1 quads of net electrical energy was output by power plants for general consumption.1 This amount of electricity could be produced by a 100 km × 100 km area of high solar insolation, such as in the desert southwestern United States, covered with solar modules with a power conversion efficiency of 15%. In order to meet the U.S. Department of Energy cost goal of $0.33/W or $0.05-0.06/kWh for utility-scale production, these modules would need to be manufactured at a cost of $50/ m 2 or less. Goals for solar thermal power are comparable. Although the costs of modules are falling substantially, reaching these objectives with today's technology will require significant improvements in cell performance, as well as in the additional components making up the balance of solar systems. In addition, a variety of new technologies including thin films, thin silicon, organic photovoltaics, multijunction concentrator approaches, and next-generation nanostructured devices have the potential to significantly reduce the cost per watt.With the recog...

We have studied the stability of the luminescence from porous Si in the presence of a variety of ambient gases (e.g., N2, H2, forming gas, and O2). Although the optical properties are fairly stable under most conditions, illumination in the presence of O2 causes a substantial decrease in luminescence efficiency. Infrared measurements show that the surfaces of degraded samples are oxidized. The luminescence lifetime of the degraded material is found to be substantially reduced, and the density of Si dangling bonds increases by more than two orders of magnitude, which suggests that oxidation of the surface introduces nonradiative recombination channels. These observations indicate that the electronic properties at the surface of the porous Si play a key role in obtaining efficient luminescence from this material.

We present detailed investigations of the fabrication and characterization of photovoltaic devices consisting of poly(3-hexylthiophene) (P3HT) intercalated into a mesoporous structure of ZnO nanofibers. ZnO nanofibers were grown via a low-temperature hydrothermal route from a solution of zinc nitrate precursor. P3HT was spin-coated on top of the structure, and intercalation into the voids between the nanofibers was induced with annealing. A silver electrode was used as the top contact. Spin-coating, storage, and testing of the device were performed in air. We discuss the effects of atmosphere and ZnO nanofiber morphology on device performance. Optimized nanofiber devices exhibited a 4-fold increase in the short circuit current (2.17 mA/cm2) as compared to that of a planar ZnO−P3HT bilayer device (0.52 mA/cm2) as a result of the increased donor−acceptor interfacial area. The efficiency of the nanofiber based device under 1 sun-simulated solar illumination was 0.53% and was found to increase at higher incident light intensities, reaching a value of 0.61% at 2.5 suns. Additionally, we found that for these devices fabrication in and exposure to air is required to obtain good diode characteristics. We also show that the morphology of the ZnO nanostructures in the nanocomposite directly impacts device performance. Treatment of the ZnO surface using surfactants increased the open circuit voltage at the expense of the short circuit current; however, there was little effect on overall device efficiency. Because of the inverted geometry of this device that allowed for the use of a silver top contact, the device was not susceptible to oxidative degradation when stored in the dark.