Showing papers on "Energy conversion efficiency published in 2002"
TL;DR: It is demonstrated that semiconductor nanorods can be used to fabricate readily processed and efficient hybrid solar cells together with polymers and Tuning the band gap by altering the nanorod radius enabled us to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum.
Abstract: We demonstrate that semiconductor nanorods can be used to fabricate readily processed and efficient hybrid solar cells together with polymers. By controlling nanorod length, we can change the distance on which electrons are transported directly through the thin film device. Tuning the band gap by altering the nanorod radius enabled us to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum. A photovoltaic device consisting of 7-nanometer by 60-nanometer CdSe nanorods and the conjugated polymer poly-3(hexylthiophene) was assembled from solution with an external quantum efficiency of over 54% and a monochromatic power conversion efficiency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers. Under Air Mass (A.M.) 1.5 Global solar conditions, we obtained a power conversion efficiency of 1.7%.
4,898 citations
TL;DR: A chemically modified n-type TiO2 is synthesized by controlled combustion of Ti metal in a natural gas flame and performs water splitting with a total conversion efficiency of 11% and a maximum photoconversion efficiency of 8.35% when illuminated at 40 milliwatts per square centimeter.
Abstract: Although n-type titanium dioxide (TiO2) is a promising substrate for photogeneration of hydrogen from water, most attempts at doping this material so that it absorbs light in the visible region of the solar spectrum have met with limited success. We synthesized a chemically modified n-type TiO2 by controlled combustion of Ti metal in a natural gas flame. This material, in which carbon substitutes for some of the lattice oxygen atoms, absorbs light at wavelengths below 535 nanometers and has a lower band-gap energy than rutile (2.32 versus 3.00 electron volts). At an applied potential of 0.3 volt, chemically modified n-type TiO2 performs water splitting with a total conversion efficiency of 11% and a maximum photoconversion efficiency of 8.35% when illuminated at 40 milliwatts per square centimeter. The latter value compares favorably with a maximum photoconversion efficiency of 1% for n-type TiO2 biased at 0.6 volt.
3,911 citations
TL;DR: In this article, three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2, and (3) QDs dispersed in a blend of electron- and hole-conducting polymers.
Abstract: Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.
2,405 citations
TL;DR: In this paper, the authors reported on stimulated Raman scattering in an approximately 1-meter-long hollow-core photonic crystal fiber filled with hydrogen gas under pressure, which was guided and confined in the 15-micrometer-diameter hollow core by a two-dimensional photonic bandgap.
Abstract: We report on stimulated Raman scattering in an approximately 1-meter-long hollow-core photonic crystal fiber filled with hydrogen gas under pressure. Light was guided and confined in the 15-micrometer-diameter hollow core by a two-dimensional photonic bandgap. Using a pulsed laser source (pulse duration, 6 nanoseconds; wavelength, 532 nanometers), the threshold for Stokes (longer wavelength) generation was observed at pulse energies as low as 800 ± 200 nanojoules, followed by a coherent anti-Stokes (shorter wavelength) generation threshold at 3.4 ± 0.7 microjoules. The pump-to-Stokes conversion efficiency was 30 ± 3% at a pulse energy of only 4.5 microjoules. These energies are almost two orders of magnitude lower than any other reported energy, moving gas-based nonlinear optics to previously inaccessible parameter regimes of high intensity and long interaction length.
961 citations
TL;DR: In this paper, a method to realize multiple electron-hole pair generation per incident photon is proposed, and the theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell's band gap using detailed balance calculations.
Abstract: One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. These losses can largely be reduced in a solar cell if more than one electron–hole pair can be generated per incident photon. A method to realize multiple electron–hole pair generation per incident photon is proposed in this article. Incident photons with energies larger than twice the band gap of the solar cell are absorbed by a luminescence converter, which transforms them into two or more lower energy photons. The theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell’s band gap using detailed balance calculations. It is shown that a maximum conversion efficiency of 39.63% can be achieved for a 6000 K blackbody spectrum and for a luminescence converter with one intermediate level. This is a substantial improvement over the limiting efficiency of 30.9%, whi...
938 citations
TL;DR: With buried waveguides formed in periodically poled lithium niobate by annealed and reverse proton exchange, what is believed to be the highest normalized conversion efficiency for SHG in the 1550-nm communications band reported to date is demonstrated.
Abstract: Efficient three-wave mixing devices have numerous applications, including wavelength conversion, dispersion compensation, and all-optical switching. Second-harmonic generation (SHG) is a useful diagnostic for near-degenerate operation of these devices. With buried waveguides formed in periodically poled lithium niobate by annealed and reverse proton exchange, we demonstrate what is believed to be the highest normalized conversion efficiency (150%/W cm(2)) for SHG in the 1550-nm communications band reported to date.
312 citations
TL;DR: In this article, a method of generating white light by combining a blue organic light-emitting diode with a down-conversion phosphor system is presented, which leads to an overall power efficiency increase, an effect attributed to the high quantum efficiency of phosphor materials and the presence of light scattering in the phosphor layers.
Abstract: A method of generating white light by combining a blue organic light-emitting diode with a down-conversion phosphor system is presented. It is demonstrated that the use of the down-conversion phosphor system actually leads to an overall power efficiency increase, an effect attributed to the high quantum efficiency of phosphor materials and the presence of light scattering in the phosphor layers. It is also shown that this approach permits the generation of illumination quality white light over the full range of color temperatures required for lighting applications. For the model device demonstrated in this work, an overall electrical to optical power conversion efficiency of 1.3% was achieved at a brightness of 1080 cd/m 2 .
267 citations
TL;DR: In this paper, the synthesis and characterization of a core−shell nanoporous electrode consisting of an inner SnO2 matrix and a thin shell of TiO2 has been described.
Abstract: This paper describes the synthesis and characterization of a core−shell nanoporous electrode consisting of an inner SnO2 matrix and a thin shell of TiO2. The coating is characterized as a very thin rutile TiO2 layer whose conduction band level is located between the levels of bare SnO2 and TiO2. The TiO2 shell acts as an energy barrier at the electrode−electrolyte interface, thus slowing the interaction between the electrons in the electrode and the electrolyte ions. When applied in a dye-sensitized solar cell, the coated electrode is significantly superior to a bare SnO2 electrode. The increase of all cell parameters improves the conversion efficiency by a factor of 2.2. The combination of improved electron collection efficiency with respect to bare SnO2 and a more positive conduction band with respect to bare TiO2 should make dyes having a relatively positive excited-state potential usable in dye-sensitized systems.
258 citations
TL;DR: In this article, the authors discuss recent research and progress using nanostructures for solid-state energy converxion, focusing on fundamental issues that distinguish energy transport and conversion between nanoscale and macroscale.
Abstract: Solid-state energy conversion technologies such as thermoelectric and thermionic refrigeration and power generation require materials with low thermal conductivity but good electrical conductivity and Seebeck coefficient, which are difficult to realize in bulk semi-conductors. Nanostructures such as superlattices, quantum wires, and quantum dots provide alternative approaches to improve the solid-state energy conversion efficiency through size and interface effects on the electron and phonon transport. In this review, we discuss recent research and progress using nanostructures for solid-state energy converxion. The emphasis is placed on fundamental issues that distinguish energy transport and conversion between nanoscale and macroscale, as well as heat transfer issues related to device development and property characterization.
246 citations
TL;DR: In this paper, the power conversion efficiency of an organic photovoltaic device based on a bulk heterojunction of a conjugated polymer and a methanofullerene was investigated.
216 citations
TL;DR: Quasi-phase-matched GaAs structures, characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP(2) (ZGP) optical parametric oscillator as a pump source, and an internal SHG conversion efficiency of 33% was measured in QPM GaAs.
Abstract: Quasi-phase-matched (QPM) GaAs structures, 0.5 mm thick, 10 mm long, and with 61-mm grating periods, were grown by a combination of molecular-beam epitaxy and hydride vapor phase epitaxy. These were characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP2 (ZGP) optical parametric oscillator as a pump source. The SHG efficiencies of QPM GaAs and QPM LiNbO3 were directly compared, and a ratio of nonlinear coefficients d14GaAsd33LiNbO3 5.01 6 0.3 was found at 4.1-mm fundamental wavelength. For input pulse energies as low as 50 mJ and 60-ns pulse duration, an internal SHG conversion efficiency of 33% was measured in QPM GaAs. © 2002 Optical Society of America OCIS codes: 190.2620, 190.4360, 190.4160, 190.4400, 190.4720. Gallium arsenide (GaAs) has excellent potential as a mid-IR nonlinear optical material because of its large nonlinear susceptibility, broad transparency range 0.9 17 mm, low optical absorption, high thermal conductivity, high laser-damage threshold, and well-developed material technology. To realize its potential for nonlinear optical frequency conversion, including second-harmonic generation (SHG), difference-frequency generation, and optical parametric oscillators (OPOs) requires an efficient scheme for quasi-phase matching. Recently an all-epitaxial process for fabrication of orientation-patterned (OP) structures in GaAs was developed, 1–3 and it was shown that quasi-phase-matched (QPM) layers can be grown to a thickness of 0.5 mm and a length of as much as 2 cm with good quality. 4 Here we characterize the nonlinear optical properties of these crystals. There is a signif icant scatter of reported values for the second-order nonlinear coeff icient d14 d36 of GaAs, even when its possible dispersion is taken into account by Miller’s rule. For example, for SHG
TL;DR: It is demonstrated that the liquid-crystal cell exhibits an efficiency in energy conversion near 100%, making the device appealing for laser trapping methods used in single-molecule biomechanics and for optical guiding of cold atoms.
Abstract: We present a novel technique for producing a doughnut laser beam by use of a liquid-crystal cell. It is demonstrated that the liquid-crystal cell exhibits an efficiency in energy conversion near 100%. One of the main advantages of this method is its capability of dynamic switching between a Gaussian mode and a doughnut mode of different topological charges. The liquid-crystal cell is also dynamically tunable over the visible and near-infrared wavelength range. These advantages make the device appealing for laser trapping methods used in single-molecule biomechanics and for optical guiding of cold atoms.
Patent•
28 May 2002TL;DR: In this article, a material having small thermal conductivity is used as a member of a protective case for protecting the power converter to be fixed to the solar cell module main body containing the solar cells.
Abstract: To improve the conversion efficiency of a power converter in a solar cell module in which a solar cell module main body containing a solar cell is integrated with a power converter for converting the output power from the solar cell module main body, a material having small thermal conductivity is used as a member ( 205 ) of a protective case for protecting the power converter to be fixed to the solar cell module main body containing the solar cell, thereby preventing easy conduction of heat from the solar cell module main body heated to high temperatures to a power conversion circuit ( 201 ). A material having large thermal conductivity is used as a member ( 206 ) of a protective case for fixing the power conversion circuit ( 201 ), thereby allowing easy radiation of heat generated in the power conversion circuit ( 201 ).
TL;DR: In this paper, a reduction in linewidth enhancement factor due to the discreteness of the electron states in quantum dots is attributed to the reduction of the linearly-enhanced electron states.
Abstract: Conversion efficiency to longer wavelengths in four-wave-mixing-based wavelength conversion in optical semiconductor amplifiers is generally much lower than that in the opposite direction. This study demonstrates experimentally that this feature is drastically improved, and the asymmetry between conversion directions is eliminated by using quantum dots in the active layer. We attribute this to a reduction in linewidth enhancement factor due to the discreteness of the electron states in quantum dots.
TL;DR: The data show that with the PCT a KBBF crystal can produce deep-UV coherent light with measurable power output and conversion efficiency as high as 13% from 400 to 200 nm without any surface-loss correction.
Abstract: By use of KBe2BO3F2 (KBBF) crystal with a size of 10 mm×10 mm×1.2 mm and a special prism-coupling technique (PCT), fourth-harmonic generation of Ti:sapphire laser systems from 200 to 179.4 nm has been achieved. Moreover, with a Ti:sapphire laser with a 50-fs pulse duration and a 1-kHz repetition rate, conversion efficiency as high as 13% from 400 to 200 nm without any surface-loss correction has also been obtained. The data show that with the PCT a KBBF crystal can produce deep-UV coherent light with measurable power output.
TL;DR: In this article, the principle of detailed balance is used to calculate the limiting efficiency of solar cells with the inclusion of the impurity photovoltaic effect, in the idealized case when all transitions are assumed to be radiative.
Abstract: In the past, minimal improvements have been predicted for efficiency enhancement of solar cells using the impurity photovoltaic (IPV) effect, where optical excitation through midgap defect levels allows the use of long wavelength photons to increase the conversion efficiency of sunlight to electricity In the present work, the principle of detailed balance is used to calculate the limiting efficiency of solar cells with the inclusion of the impurity photovoltaic effect, in the idealized case when all transitions are assumed to be radiative Based on these calculations, the limiting efficiency of the IPV device with a large number of different defect species is determined to be 772% The terrestrial performance of the IPV device is also investigated by comparing its spectral sensitivity with that of tandem solar cell designs
Abstract: A non-sintering method for preparing nanoporous semiconductor layers for dye-sensitized photocells was developed in which TiO2 particles were electrophoretically deposited on an electrode and post-treated by chemical vapor deposition of Ti alkoxide and microwave irradiation. The method provides photocells with energy conversion efficiency up to 4.1% under irradiation of AM1.5 light (100 mW/cm2).
TL;DR: In this paper, the second principle of thermodynamics stating that entropy cannot be destroyed limits the efficiency of solar energy conversion to 0.93 for reversible operation, and it is recognized that processing the incident solar energy current is not possible without the generation of entropy.
Abstract: Solar energy is heat of 5800 K . Any system converting solar energy in mechanical, chemical or electrical energy is a heat engine. The second principle of thermodynamics stating that entropy cannot be destroyed limits the efficiency of solar energy conversion to 0.93 for reversible operation. In addition, it is recognized that processing the incident solar energy current is not possible without the generation of entropy. This sets an upper limit of 0.86 for maximally concentrated solar radiation. Several processes which allow to achieve this limit are discussed and how they can possibly be realised in photovoltaic solar energy conversion.
TL;DR: In this article, a unit cell of multi-junction structures was investigated for a thinner microcrystalline silicon-germanium (μc-SiGe) solar cells, and a conversion efficiency of 5.6% was achieved by using a μcSiGe film with 20% Ge concentration.
TL;DR: Stimulated Raman scattering of 35-ps pulses in BaWO(4) crystal can be considered an efficient Raman-active material for utilization in picosecond solid-state laser systems.
Abstract: Stimulated Raman scattering of 35-ps pulses in BaWO4 crystal was studied. The second harmonic of a Nd:YAG double-mode-locked laser system was used as a pump radiation source. The first Stokes conversion efficiency reached 38% in a single-pass setup and 85% in a double-pass setup. The second Stokes was generated with 20% (single-pass) and 50% (double-pass) conversion efficiency. Measurement of the temporal profiles of pump and first Stokes radiation with picosecond resolution was performed. The obtained conversion efficiency data were compared with the a plane-wave numerical model. BaWO4 crystal can be considered an efficient Raman-active material for utilization in picosecond solid-state laser systems.
TL;DR: In this article, a loop configuration formed by a polarization beam splitter was used to demonstrate that the existing WDM sources in C-band can be wavelength converted to the S-band with low polarization sensitivity and low crosstalk.
Abstract: By using a loop configuration formed by a polarization beam splitter, we experimentally demonstrate that the existing wavelength-division multiplexing (WDM) sources in C-band can be wavelength converted to the S-band with low polarization sensitivity and low crosstalk. Using a fiber parametric amplifier as a band converter, we achieve experimentally 4.7-dB conversion efficiency over 30-nm conversion bandwidth in 315 m of fiber. Compared to the conventional straight fiber wavelength conversion scheme, a more than 2-dB improvement in polarization sensitivity is measured. In addition to the polarization insensitivity, channel crosstalk is measured to be <-27 dB in 315 m of high nonlinearity fiber. In a detailed experimental study, the pattern of crosstalk in longer fiber lengths and the coupling between the polarization sensitivity and crosstalk are measured. For example, with a 430-m fiber length, we measure the degradation in polarization sensitivity to be /spl sim/4 dB for 12-dB increased signal power. The experimental results are also confirmed by theoretical calculations. Moreover, in a 32 channels systems simulation, the signal-to-noise ratio (SNR) of the converted signals after 800-km propagation is calculated to be only 0.8-dB degraded compared to using laser diodes with the same initial SNR values. Furthermore, we calculate the effect of pump noise and show that the relative intensity noise of the pump is transferred to the converted signals with an additional 8-dB/Hz degradation.
TL;DR: In this article, the state-of-the-art organic bulk hetero-junction photovoltaic cells are based on an interpenetrating donor-acceptor network in the bulk to form efficient nanostructured p-n junctions.
TL;DR: In this paper, the authors extended the theory to four bands and calculated a limiting efficiency of 71.7% for the three and four bands, respectively, and showed that superlattices and quantum dots offer flexibility in artificially designing the energy and width of the bands.
Abstract: Multi-band solar cells provide a possible approach to obtain photovoltaic efficiencies that are greater than that of a single junction solar cell. In the three-band case, an intermediate band harnesses photons of energy less than that between the two main bands, allowing these photons to contribute to the power output of the device. Previous work has shown that introducing a third band offers an efficiency of 63.2%. This paper extends the theory to four bands and calculates a limiting efficiency of 71.7%. Finite bandwidths of all bands can be used to ensure photon absorption selectivity, assumed in deducing the previous limits, but at the cost of reduced limiting efficiency. The maximum efficiency using this feature for the three and four bands is 58.9% and 59.0%, respectively. Superlattices and quantum dots offer flexibility in artificially designing the energy and widths of the bands.
TL;DR: In this paper, a thermoelectric incorporated cogeneration system is proposed, and its key elements and advantages discussed, and a novel idea, which provides an alternative solution to overcome the low efficiency drawback, is to operate the thermodynamic converter in a combined heat and power generation mode (cogeneration).
TL;DR: In this paper, the stimulated Raman scattering process in a BaWO/sub 4/ crystal was employed to frequency downshift the first, second, and third harmonics of a Nd:YAG actively-passively mode-locked laser system.
Abstract: The stimulated Raman scattering process in a BaWO/sub 4/ crystal was employed to frequency downshift the first, second, and third harmonics of a Nd:YAG actively-passively mode-locked laser system. Single-pass, double-pass, and external cavity configurations were investigated for this purpose. In each experimental arrangement, the Stokes radiation properties were characterized regarding energy, beam profile, spectrum, and temporal development. The peak pump-to-first-Stokes conversion efficiency was measured to be 55% in the near infrared and 85% in the visible spectral region. The BaWO/sub 4/ picosecond Raman gain at a pump wavelength of 355 nm was measured to be 38 cm/GW, and a 15% conversion efficiency was achieved. A study of the Raman-cavity output beam profile development as a function of pumping energy was conducted. This work shows that the BaWO/sub 4/ crystal can be used in picosecond solid-state laser systems as an efficient frequency converter with a wide range of pump radiation wavelengths.
TL;DR: In this article, a direct synthesis of methanol from methane and water vapor mixtures has been successfully realized by non-equilibrium plasma chemical reactions under atmospheric pressure using a newly developed ultra-short pulsed barrier discharge in an extremely thin glass tube reactor.
TL;DR: In this paper, a photovoltaic cell made from a mixture of regioregular poly(3-alkylthiophenes) and soluble fullerene derivatives is compared.
Abstract: Photovoltaic cells made, at ambient conditions, from blends of regioregular poly(3-alkylthiophenes) and soluble fullerene derivatives are compared. It is shown that the cell performance is affected by the length of the donor side-chain, as it occurs in devices based on pure polythiophenes. The highest value of power conversion efficiency (0.84%) were obtained by using poly(3-hexylthiophene) and a fullerene concentration of 33-40% by weight. We anticipate a significant further improvement in power conversion efficiency upon a mild thermal treatment of the cells.
TL;DR: The double-layer photobioreactor using two types of photosynthetic bacteria, Rhodobacter sphaeroides RV and its reduced-pigment mutant, MTP4, was developed for efficient hydrogen production as mentioned in this paper.
TL;DR: In this article, the absorption of a nanoporous TiO2 thin film was enhanced throughout the visible wavelengths with surface modification of Sr2+, especially in the range of shorter wavelengths.
Abstract: The absorption of a nanoporous TiO2 thin film was enhanced throughout the visible wavelengths with surface modification of Sr2+, especially in the range of shorter wavelengths. Charge separation efficiency from laser experiment was measured. Data show that the incident photon to current efficiency of a solar cell based on the dye Ru[LL‘(NCS)2] (L = 2,2‘-bipyridine-4,4‘-dicarboxylic acid, L‘ = bis(tetrabutylammonium) 2,2‘-bipyridine-4,4‘-dicarboxylate) sensitized Sr2+-modified TiO2 electrode increased from 9.2% to about 11.8% under the irradiation of a laser pulse energy of 23 μJ at 532 nm. The photoelectrical conversion efficiency is increased from 7.3% to 9.3% under the illumination of a white light of 93.1 mW/cm2.
TL;DR: In this article, the authors proposed a solar triple cycle with a high-temperature MHD generator and two additional cycles in series, which achieved a higher peak conversion efficiency than the solar combined cycle scheme.