Showing papers on "Energy conversion efficiency published in 2001"

Plastic Solar Cells

Abstract: Recent developments in conjugated-polymer-based photovoltaic elements are reviewed. The photophysics of such photoactive devices is based on the photo-induced charge transfer from donor-type semiconducting conjugated polymers to acceptor-type conjugated polymers or acceptor molecules such as Buckminsterfullerene, C60. This photo-induced charge transfer is reversible, ultrafast (within 100 fs) with a quantum efficiency approaching unity, and the charge-separated state is metastable (up to milliseconds at 80 K). Being similar to the first steps in natural photosynthesis, this photo-induced electron transfer leads to a number of potentially interesting applications, which include sensitization of the photoconductivity and photovoltaic phenomena. Examples of photovoltaic architectures are presented and their potential in terrestrial solar energy conversion discussed. Recent progress in the realization of improved photovoltaic elements with 3 % power conversion efficiency is reported.

2.5% efficient organic plastic solar cells

Abstract: We show that the power conversion efficiency of organic photovoltaic devices based on a conjugated polymer/methanofullerene blend is dramatically affected by molecular morphology. By structuring the blend to be a more intimate mixture that contains less phase segregation of methanofullerenes, and simultaneously increasing the degree of interactions between conjugated polymer chains, we have fabricated a device with a power conversion efficiency of 2.5% under AM1.5 illumination. This is a nearly threefold enhancement over previously reported values for such a device, and it approaches what is needed for the practical use of these devices for harvesting energy from sunlight.

Very-high-efficiency double-heterostructure copper phthalocyanine/C60 photovoltaic cells

Abstract: We demonstrate an external power conversion efficiency of (3.6±0.2)% under AM1.5 spectral illumination of 150 mW/cm2 (1.5 suns) with vacuum-deposited copper phthalocyanine/C60 thin-film double-heterostructure photovoltaic cells incorporating an exciton-blocking layer (EBL). We show that the anode work function influences the photocarrier collection characteristics through the built-in electric field. The cell parameters are less sensitive to the cathode work function, which is attributed to cathode-induced defect states in the EBL energy gap. The presence of these defect states also explains the surprisingly low resistance of the EBL to electron transport. We anticipate significant further improvements in power conversion efficiency by employing optimal structures in light-trapping geometries.

Efficient silicon light-emitting diodes

Abstract: Considerable effort is being expended on the development of efficient silicon light-emitting devices compatible with silicon-based integrated circuit technology. Although several approaches are being explored, all presently suffer from low emission efficiencies, with values in the 0.01-0.1% range regarded as high. Here we report a large increase in silicon light-emitting diode power conversion efficiency to values above 1% near room temperature-close to the values of representative direct bandgap emitters of a little more than a decade ago. Our devices are based on normally weak one- and two-phonon assisted sub-bandgap light-emission processes. Their design takes advantage of the reciprocity between light absorption and emission by maximizing absorption at relevant sub-bandgap wavelengths while reducing the scope for parasitic non-radiative recombination within the diode. Each feature individually is shown to improve the emission efficiency by a factor of ten, which accounts for the improvement by a factor of one hundred on the efficiency of baseline devices.

A highly efficient solar cell made from a dye-modified zno-covered tio2 nanoporous electrode

Abstract: A photoelectrochemical solar cell based on porous ZnO-covered TiO2 film has been fabricated with ruthenium bipyridyl complex as the sensitizer. The cell generated a short-circuit photocurrent of 21.3 mA cm-2 and an open-circuit voltage of 712 mV under irradiation of 81.0 mW cm-2 white light from a xenon lamp with an overall conversion efficiency of 9.8%. Compared with the pure TiO2 (anatase) film, the ZnO-covered TiO2 film possesses more outstanding ability to transport electrons with an overall power conversion efficiency increase by 27.3%. Optical elctrochemical studies show that surface modification of TiO2 with ZnO can increase the concentration of free electrons in the conduction band of TiO2. This result implies that the charge recombination is reduced in the process of electron transport through the porous network, which can decrease the photocurrent loss and hence improve both short-circuit photocurrent and open-circuit photovoltage.

Soliton self-frequency shift in a short tapered air-silica microstructure fiber.

Abstract: We report a soliton self-frequency shift of more than 20% of the optical frequency in a tapered air–silica microstructure fiber that exhibits a widely flattened large anomalous dispersion in the near infrared. Remarkably, the large frequency shift was realized in a fiber of length as short as 15 cm, 2 orders of magnitude shorter than those reported previously with similar input pulse duration and pulse energies, owing to the small mode size and the large and uniform dispersion in the tapered fiber. By varying the power of the input pulses, we generated compressed sub-100-fs soliton pulses of ∼1‐nJ pulse energy tunable from 1.3 to 1.65 μm with greater than 60% conversion efficiency.

X-ray generating mechanism using electron field emission cathode

Abstract: An x-ray generating device includes a field emission cathode formed at least partially from a nanostructure (1110) containing material having an emitted electron current density of at least 4 A/cm2. High energy conversion efficiency and compact design (1100) are achieved due to easy focusing of cold cathode emitted electron between the cathode (1110) and the gate or anode (1130) and focusing the electron beams at different anode materials (1130), pulsed x-ray radiation with varying energy can be generated from a single device.

Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions

Abstract: We report on the temperature dependence of various photovoltaic device parameters of solar cells, fabricated from interpenetrating networks of conjugated polymers with fullerenes, in the wide temperature range of their possible operating conditions ~25‐ 60 °C!. The open-circuit voltage was found to decrease linearly with increasing temperature. For the short-circuit current, we observed a monotonic increase with increasing temperature, followed by a saturation region. The rate of this increase ~coupled to a corresponding increase for the fill factor! was found to overtake the corresponding rate of decrease in voltage, resulting in an overall increase of the energy conversion efficiency. The efficiency was observed to reach a maximum value in the approximate range 47‐ 60 °C. The results are discussed with respect to possible mechanisms for photovoltage generation and charge carrier transport in the conjugated polymer-fullerene composite, and in particular, thermally activated charge carrier mobility. © 2001 American Institute of Physics. @DOI: 10.1063/1.1412270#

Enhanced Visible Light Conversion Efficiency Using Nanocrystalline WO3 Films

Abstract: Drastically improved photooxidation efficiencies,when compared to bulk WO 3 , are obtained with nanocrystalline WO 3 films (see Figure) deposited from a colloidal solution of tungstic acid. Saturation of the photocurrent, which occurs even for moderate potential differences, indicates a completely different mechanism of charge separation. The electrodes should be highly effective for photoelectrolysis of water by sunlight.

72 W Nd:Y3Al5O12 ceramic laser

Abstract: A high-power continuous-wave polycrystalline 1% Nd:Y3Al5O12 (Nd:YAG) ceramic rod laser was demonstrated. With 290 W/808 nm laser diode pumping, cw laser output of 72 W was obtained at 1064 nm. The optical-to-optical conversion efficiency is 24.8%. Thermally induced birefringence properties of Nd:YAG ceramic was also investigated.

Effects of Substrate Surface Morphology on Microcrystalline Silicon Solar Cells

Abstract: The relationship between µc-Si:H solar cell performance and surface morphology of substrate is studied systematically using textured-ZnO substrate which is formed by wet etching process in order to clarify the optimum morphology of substrate for high efficiency. An average slope, tan θ, of substrate surface textures is obtained by analysis of atomic force microscopy (AFM) images, and solar cell performance is correlated with the value os tan θ. As a result, because of the balance between the optical confinement and the grain boundary formation, a conversion efficiency shows a maximum at a tan θ of 0.08 which is 3 times smaller as compared to that used for a-Si:H. An efficiency of 9.4% (Voc=0.526 V, Jsc=25.3 mA/cm2, FF=0.710) is obtained at a deposition temperature of 140°C using the optimized morphology.

Energy management system and methods for the optimization of distributed generation

Abstract: An energy management system and method is provided for managing the generation and distribution of energy from an energy source to a building. The building has a desired building environment and a total energy profile including a thermal energy requirement and an electrical energy requirement. The energy management system comprises an energy generator arranged to convert energy from the energy source to thermal energy and electrical energy, a heat recovery unit arranged to recover byproduct heat from the energy generator, a cooling unit arranged to use a first portion of the thermal energy to drive a refrigeration unit, a heating unit arranged to use a second portion of the thermal energy to drive a heating unit, a heat storage unit arranged to store excess heat, and an energy optimizing controller.

Multiple Band Gap Semiconductor/Electrolyte Solar Energy Conversion

Abstract: Fundamental and experimental multiple band gap semiconductor/electrolyte processes to convert optical energy and utilize renewable solar energy are explored. Alternate processes discussed include an inverted (1 photon per e-) or bipolar (n ≥ 2 photons per e-) arrangement of successive band gaps. Such processes can incorporate either a Schottky or an ohmic photoelectrochemical solution interface that can drive either regenerative single or multiple different redox reactions. The multiple redox case permits solar energy storage or solar water splitting evolving H2 and provides an energy reservoir that may compensate for the intermittent nature of solar energy. Generated H2 is attractive as a clean, renewable fuel. Experimental configurations examined include GaAs/Si or AlGaAs/Si photodriven redox couples, each with a solar to electrical conversion efficiency of 19−20%. A related solar cell is configured with electrochemical storage, which provides a nearly constant energetic output in illuminated or dark co...

Molecular photovoltaics that mimic photosynthesis

Abstract: Learning from the concepts used by green plants, we have developed a photo- voltaic cell based on molecular light absorbers and mesoporous electrodes. The sensitized nanocrystalline injection solar cell employs organic dyes or transition-metal complexes for spectral sensitization of oxide semiconductors, such as TiO 2, ZnO, SnO2, and Nb2O5. Mesoporous films of these materials are contacted with redox electrolytes, amorphous organ- ic hole conductors, or conducting polymers, as well as inorganic semiconductors. Light har- vesting occurs efficiently over the whole visible and near-IR range due to the very large inter- nal surface area of the films. Judicious molecular engineering allows the photoinduced charge separation to occur quantitatively within femtoseconds. The certified overall power conversion efficiency of the new solar cell for standard air mass 1.5 solar radiation stands presently between 10 and 11. The lecture will highlight recent progress in the development of solar cells for practical use. Advancement in the understanding of the factors that govern photovoltaic performance, as well as improvement of cell components to increase further its conversion efficiency will be discussed.

Efficient laser-ultrasound generation by using heavily absorbing films as targets

Abstract: An efficient all-fiber optic source is presented; it adopts absorbing films, deposed directly over the fiber tip, as targets. It is demonstrated that the use of absorbing films made of pure graphite, or graphite powder mixed with epoxy resin, has produced a conversion efficiency increase of two orders of magnitude with respect to metallic materials. It is observed that the conversion efficiency increases monotonically as thickness is reduced down to the material optical penetration depth. Moreover, the conversion efficiency rises with the concentration of graphite powder. Principal advantages of this kind of source are the ease of production and miniaturization, the excellent electromagnetic compatibility, wide ultrasonic bandwidth and, consequently, high spatial resolution. The ultrasonic bandwidth can be controlled by varying the laser pulse duration. The possibility of generating ultrasonic signals with high frequency and flat spectral distribution makes the proposed device suitable for biological tissue spectral characterization.

Efficient cascaded difference frequency conversion in periodically poled Ti:LiNbO3 waveguides using pulsed and cw pumping

Abstract: Using an FM-mode-locked Ti:Er:LiNbO3 waveguide laser as the fundamental source, wavelength conversion by cascaded χ(2):χ(2)-difference frequency generation with a conversion efficiency of up to +3(-4.6) dB was demonstrated at a pulse repetition rate of about 2 (10) GHz. In addition, multi-channel conversion was demonstrated with a fully packaged wavelength converter using a continuous fundamental source.

Photonic band edge effects in finite structures and applications to χ ( 2 ) interactions

Abstract: Using the concept of an effective medium, we derive coupled mode equations for nonlinear quadratic interactions in photonic band gap structures of finite length. The resulting equations reveal the essential roles played by the density of modes and effective phase matching conditions necessary for the strong enhancement of the nonlinear response. Our predictions find confirmation in an experimental demonstration of significant enhancement of second harmonic generation near the photonic band edge. The measured conversion efficiency is in good agreement with the conversion efficiency predicted by the effective-medium model.

Hot electron diagnostic in a solid laser target by K-shell lines measurement from ultraintense laser–plasma interactions (3×1020 W/cm2,⩽400 J)

Abstract: Characterization of hot electron production from an ultraintense laser–solid target plasma interaction by using a buried molybdenum K-shell fluor layer technique has been reported. Laser energy was typically 400 J and its intensity was from 2×1018 up to 3×1020 W cm−2 at 20 TW to 1 PW laser power by changing pulse duration from 20 ps down to 0.5 ps. X-ray background noise level was significantly greater, i.e., gamma flash, in the shorter pulse experiments. Data analysis procedures for the experiments were developed. The conversion efficiency from the laser energy into the energy, carried by hot electrons, has been estimated to be ∼50% at 3×1020 W cm−2 laser intensity, higher than ∼18% at 1019 W cm−2 and ∼12% at 2×1018 W cm−2 intensity.

Efficient multi-keV underdense laser-produced plasma radiators.

Abstract: Novel, efficient x-ray sources have been created by supersonically heating a large volume of Xe gas. A laser-induced bleaching wave quickly ionizes the high- $Z$ gas, and the resulting plasma emits x rays. This method significantly improves the production of hard x rays because less energy is lost to kinetic energy and sub-keV x rays. The conversion efficiency of laser energy into $L$-shell radiation between 4--7 keV is measured at $\ensuremath{\sim}10%$, an order of magnitude higher than efficiencies measured from solid disk targets. This higher flux enables material testing and backlighting in new regimes and scales well to future high-powered lasers.

Computer Simulation of the Three-Dimensional Regime of Proton Acceleration in the Interaction of Laser Radiation with a Thin Spherical Target

Abstract: Results from particle-in-cell simulations of the three-dimensional regime of proton acceleration in the interaction of laser radiation with a thin spherical target are presented. It is shown that the density of accelerated protons can be several times higher than that in conventional accelerators. The focusing of fast protons created in the interaction of laser radiation with a spherical target is demonstrated. The focal spot of fast protons is localized near the center of the sphere. The conversion efficiency of laser energy into fast ion energy attains 5%. The acceleration mechanism is analyzed and the electron and proton energy spectra are obtained.

Solar cell substrate, thin-film solar cell, and multi-junction thin-film solar cell

Abstract: A solar cell substrate of the present invention has irregularities on a surface which is in contact with a photo-electric conversion layer, and light is incident on the side of the irregularities. The height of the irregularities is set so that the root mean square height is in a range of 15 nm to 600 nm, and tan θ is in a range of 0.10 to 0.30, where θ is the angle of incline of the surface of the irregularities with respect to an average line of the irregularities. According to this arrangement, the light incident on the irregularities is scattered at the interface. This increases the optical path length and thus the quantity of light absorbed in the photo-electric conversion layer. As a result, photo-electric conversion efficiency can be improved and the photo-electric conversion layer can be made thinner, thereby greatly reducing deposit time and manufacturing cost required for the photo-electric conversion layer. Further, by the foregoing ranges of the height of the irregularities, and tan θ, it is ensured that collision of crystals is not incurred. It is therefore possible to surely prevent deterioration of photo-electric conversion efficiency which is caused by defects.