Showing papers on "Energy conversion efficiency published in 1984"

Organic electroluminescent devices having improved power conversion efficiencies

Abstract: Electroluminescent devices are disclosed comprising a hole-injecting zone and an adjacent organic luminescent zone, the device having a power conversion efficiency of at least 9×10 -5 w/w and said zones having a combined thickness no greater than about 1 micron.

High-efficiency silicon solar cells

Abstract: Silicon solar cells are described which operate at energy conversion efficiencies independently measured at 18.7 percent under standard terrestrial test conditions (AM1.5, 100 mW/cm2, 28°C). These are apparently the most efficient silicon cells fabricated to date. The high-efficiency results from a combination of high open-circuit voltage due to the careful attention paid to the passivation of the top surface of the cell; high fill factor due to the high open-circuit voltage and low parasitic resistance losses; and high short-circuit current density due to the use of shallow diffusions, a low grid coverage, and an optimized double layer antireflection coating.

Hydrogen-Evolving Solar Cells

Abstract: Sunlight is directly converted to chemical energy in hydrogen-evolving photoelectrochemical cells with semiconductor electrodes. Their Gibbs free energy efficiency of solar-to-hydrogen conversion, 13.3 percent, exceeds the solar-to-fuel conversion efficiency of green plants and approaches the solar-to-electrical conversion efficiency of the best p-n junction cells. In hydrogen-evolving photoelectrodes, electron-hole pairs photogenerated in the semiconductor are separated at electrical microcontacts between the semiconductor and group VIII metal catalyst islands. Conversion is efficient when the island diameters are small relative to the wave-lengths of sunlight exciting the semiconductor; when the island spacings are smaller than the diffusion length of electrons at the semiconductor surface; when the height of the potential energy barriers that separate the photogenerated electrons from holes at the semiconductor surface is raised by hydrogen alloying of the islands; when radiationless recombination of electron-hole pairs at the semiconductor-solution interface between the islands is suppressed by controlling the semiconductor surface chemistry; and when the semiconductor has an appropriate band gap (1.0 to 1.8 electron volts) for efficient solar conversion.

Efficiency of Splitting Water with Semiconducting Photoelectrodes

Abstract: Solar conversion efficiencies for splitting water with semiconducting photoelectrodes are calculated from basic thermodynamic principles combined with transport properties matching those of the best materials presently available. Assuming no further constraints, we derive in this way "upper limit" estimates of efficiencies achievable via semiconductor photoelectrochemical cells (PEC's), operating with no external electrical bias. Both one‐ and two‐photon configurations are considered. A one‐photon PEC is found to have an "upper limit" efficiency of ~7% (AM 1.2 solar energy to chemical potential energy stored as ). For two‐photon configurations, the "upper limit" for a p‐n PEC is ~10%, while for a tandem PEC it is ~18%. The tandem cell configuration is the least sensitive to the choice of materials parameters and transport losses and yields the highest efficiencies. Significant increases in conversion efficiencies result from assuming lower oxygen overpotentials and higher photoelectrode fill factors than have been achieved so far, with the latter being the more important, however.

High‐efficiency electroplated heterojunction solar cell

Abstract: Over 9% efficiency is demonstrated in electroplated CdS/CdTe thin film solar cells. Processing of the devices is described. A brief heat treatment step first anneals out the deep lifetime killer centers in the material then converts the originally n‐type film into solar cell grade p‐CdTe.

Point-contact silicon solar cells

Abstract: A new type of silicon photovoltaic cell designed for high-concentration applications is presented. The device is called the point-contact-cell and shows potential for achieving energy conversion efficiencies in the neighborhood of 28 percent at the design operating point of 500× geometric concentration and 60°C cell temperature. This cell has alternating n and p regions that form a polkadot array on the bottom surface. A two-layermetallization on the bottom provides contact. Initial experimental results have yielded a cell with 20-percent efficiency at a concentration of 88.

Organic electroluminescent device having inproved power conversion efficiency

Abstract: Electroluminescent devices are disclosed comprising a hole-injecting zone and an adjacent organic luminescent zone, having a combined thickness no greater than about 1 micron, which have a power conversion efficiency of at least 9 x 10-5 w/w. Preferred electron-transporting compounds for the luminescentzone provide a maximum electroluminescent quantum efficiency of at least 5 ×10-4 photons/electron when tested as described.

Phase matching limitations of high efficiency second harmonic generation

Abstract: -A discussion of second harmonic generation with imperfect phase matching in the high conversion limit is presented. Phase modulation of the fundamental pulse, dispersion, central frequency wavevector mismatch, and fundamental depletion are included. The results show that even small amounts of phase modulation or central frequency wavevector mismatch can limit harmonic generation and must be considered at high conversion efficiency. Both spatial and temporal phase matching parameters are more restrictive at high conversion efficiency because of the narrowing of the central phase matching peak. If the harmonic conversion process is overdriven, severe distortion of the harmonic and transmitted fundamental pulses can result. Cascade harmonic generation of neodymium laser radiation to 266 nm is presented as an example.

Cascaded pyroelectric energy converter

Abstract: A regenerative pyroelectric converter employing multiple staging has been constructed and tested. Operating between the temperatures of 150°C and 180°C the device converted heat into electrical energy. Measurements of the conversion efficiency and power density as functions of frequency (0.05 to 0.3 Hz) are reported. The converter's maximum power density was 33 Watts per liter of pyroelectric ceramic at 0.26 Hz.

High conversion efficiency and high radiation resistance InP homojunction solar cells

Abstract: InP homojunction solar cells have been fabricated using thermal diffusion of sulphur or selenium into p‐type InP substrates. A conversion efficiency of 16.5% (active area) was obtained for a S‐diffused cell under simulated AM1.5 illumination. The InP solar cell was found for the first time to have a higher resistance to γ‐ray radiation degradation than Si and GaAs solar cells with comparable junction depth. These results show a possibility of the InP solar cells for space applications.

High-efficiency ion-implanted silicon solar cells

Abstract: The development of solar cells with AM1 coversion efficiency of 18 percent is reported. The cells comprise an n+-p-p+structure fabricated from float zone silicon having resistivity of 0.3 Ω . cm. The n+and p+regions are formed by low energy ion implantation and thermal annealing. An important feature of cell fabrication is the growth of SiO 2 passivation for reduction of surface recombination velocity. Details of both cell fabrication and testing are reported.

A novel structure, high conversion efficiency p-SiC/graded p-SiC/i-Si/n-Si/metal substrate-type amorphous silicon solar cell

Abstract: A novel structure, high conversion efficiency amorphous silicon (a‐Si)/metal substrate‐type solar cell has been developed. The new structure, deduced from the conventional pin junction by the use of a gradual compositional grading p‐type a‐SiC:H layer between an ultrathin (∼20 A) wide optical band gap (∼2.4 eV) p‐type a‐SiC:H layer and the i layer, exhibits markedly enhanced open‐circuit voltage (Voc) and short‐circuit current density (Isc) over the conventional a‐Si pin/substrate‐type solar cell. Especially, the collection efficiency in the newly developed structure was found to be remarkably increased at short wavelengths. The experimentally observed improvement in the blue response is due to the reduction in effective interface recombination combined with the enhanced window effect. An energy conversion efficiency of 8.40% under air mass (AM) 1 (100 mW/cm2) illumination has been obtained in the first trial of a cell fabricated by the rf glow discharge decomposition of pure silane (SiH4).

Photon-transport properties of luminescent solar concentrators: analysis and optimization

Abstract: The principle of a luminescent solar concentrator is analyzed with an emphasis on the photon-transport yield. A mathematical model is developed, which takes into account the loss factors related to the photon transport in the LSC matrix. The relations obtained show that whereas the optical efficiency is still a decreasing factor with the LSC size, the concentration ratio can be optimized with regard to the geometry, the input surface, and the thickness of the LSC. The experimental analysis, carried out on two types of fluorescent PMMA, confirms the effects of these geometrical parameters on the LSC performances. A concentration ratio of 22 has been obtained experimentally with monochromatic irradiation, and a flux gain of 9.5 has also been determined in real conditions.

Physics Considerations of Solar Energy Conversion

Abstract: A comparative analysis is presented of the conversion of radiant energy to useful work by thermal and quantum processes. The operation of thermal and quantum converters and the thermodynamic conversion efficiency of each are developed in terms of the mechanism of radiation-matter interaction in thermal and quantum systems. From the analysis the maximum conversion efficiency of a single-collector thermal converter with unconcentrated solar radiation and an ambient (reservoir) temperature of 300 K is 0.540; for the same conditions the maximum conversion efficiency of a single-collector quantum system is 0.309. The analysis is extended to consider the effects on the conversion efficiency of heat reject temperature, cascaded operation, in which the reject heat of the quantum converter is used as the input to a thermal bottoming cycle, and of concentration of the solar radiation. The results obtained represent the thermodynamic limits for radiant energy conversion by thermal and quantum processes, and calculations with solar input serve as a reference against which to judge the performance and capabilities of prospective solar energy conversion systems.

Integrated‐optical frequency translator with stripe waveguide

Abstract: We demonstrate electro‐optic frequency shifting of laser light (λ=723 nm), electronically tunable through more than ± 1 MHz, in a LiNbO3: Ti single‐mode stripe waveguide, using polarization conversion at a traveling index grating, driven by two phase‐quadrature voltages of ∼27 V at the offset frequency. The optical carrier and undesired sidebands are suppressed better than 30 dB by polarizers, conversion efficiency ≥70%, device length 4.5 mm, optical bandwidth ∼1 nm; direct butt coupling to fibers is possible.

Role of surface reactions in the stabilization of n-CdS-based photoelectrochemical cells

Abstract: The potential use of II–VI semiconductor–aqueous junction cells for the conversion of optical energy to electricity has previously been limited by semiconductor photodecomposition processes combined with low energy conversion efficiencies. Decomposition processes in the prototypical n-CdS photoelectrochemical cell can be efficiently suppressed by addition of an appropriate polychalcogenide redox couple to the electrolyte1–3. However, conversion efficiencies remain low (∼5% at 488 nm)4. Moreover, although increased optical to electrical energy conversion rates can be obtained by using a redox couple such as Fe(CN)4−/3−6 (∼8% conversion efficiency at 488 nm), the cell lifetime is greatly diminished5–7 (t1/2∼1/2h). We report here that the photodecomposition of n-CdS in a Fe(CN)4−/3−6 electrolyte can be dramatically decreased and cell output parameters significantly improved by the presence of an appropriate combination of K+ and Cs+ ions. Monochromatic (488 m) conversion efficiencies in excess of 20% have been observed, with fill factors (a measure of the ideality of the cell) in the range of 65%. The enhanced stability and efficiency are associated with in situ chemical derivatization of the n-CdS surface with a layer of Kx Csy[CdIIFeII(CN)6]. (This species is an analogue of Prussian blue having a C-bound FeII/III centre and a nitrogen bound CdII centre. See, for example, ref. 8.)

Doppler shift dominated cyclotron masers

Abstract: Since in a Doppler shift dominated cyclotron maser, the Doppler term is the main contribution to the output frequency, the electron-beam momentum spread plays an important role in determing the gain and efficiency. It is demonstrated by computer simulations that the increase in beam momentum spread diminishes the gain slightly but reduces the efficiency significantly. The decrease in conversion efficiency arises mainly from the dispersion of the wave reducing the available free energy from the electron beam. Further increase in momentum spread eventually changes the saturation mechanism from phase trapping to energy depletion.

Electrochemical conversion of nitrous oxide into ammonia in the presence of iron complexes

Abstract: The electrochemical conversion of NO into NH3 has been studied using iron complexes as homogeneous catalysts. The present results show that small amounts of NH2OH and N2H4 are formed, but no detectable amounts of N2O. The conversion efficiency depends on the solution used and diminishes in the following order: FeIIphen > aquo-FeII > FeIIpy > FeIIedta > blank. The mechanism of conversion of NO into NH3 is discussed in the light of spectrophotometry, cyclic voltammetry and quantitative analyses.

Transparent Insulation System for Passive Solar Energy Utilization in Buildings

Abstract: A new concept for the passive use of solar energy, transparent insulation, is described together with the first experimental results. Transparent insulation material has the property of being transparent or translucent to solar radiation while at the same time acting as heat insulation. Elements made of this material can be attached to the walls of buildings and thus permit the utilization of solar energy for heating. Relations are given for the dependence of heat flux and conversion efficiency of radiation into useful heat on the thermal resistance of the components. Calculations using meteorological data show that with material-parameters achievable with present technology not only south but also west/east and possibly even north orientations can lead to significant contributions to heating. In order to avoid overheating in summer, control of radiation must be provided. Experiments with two different materials on two buildings showed promising results. In sunny periods in wintertime a heat flux into the house was measured for south and west orientated walls. The mean heat flux is reduced drastically depending on the material and orientation used.

12% solar conversion efficiency by interphase restructuring

Abstract: Solar energy conversion efficiency of n‐CuInSe2/p‐CuISe3 heterojunction formed in situ and contacted by I−‐I2‐Cu+‐HI electrolyte has been increased to 12% by modifying the growth conditions of the p‐CuISe3 phase. These include the use of prepolished n‐CuInSe2 (110) surfaces, higher concentrations of HI and Cu+, and an air anneal at 200 °C. X‐ray fluorescence analysis in conjunction with scanning electron micrographs of the new interphase indicates structural and compositional alterations and possibly further transformation of CuISe3 to CuIn2Se3I. Temperature dependence of the cell performance indicates solid state effects and relative independence of electrochemical kinetics.

Efficient XeF(C. -->. A) laser oscillation using electron-beam excitation

Abstract: Significantly improved XeF(C→A) laser energy density and efficiency have been obtained using electron‐beam excited Ar‐Xe gas mixtures at pressures up to 10 atm which contain both NF3 and F2. Maximum blue‐green laser pulse energy density in excess of 1.0 J/liter was obtained, corresponding to an intrinsic electrical‐optical energy conversion efficiency estimated to be in the 0.5%–1.0% range. Comprehensive, time‐resolved absolute measurements of XeF(C→A) fluorescence, laser energy, and gain were carried out for a wide variety of experimental conditions. Analysis of these data has resulted in identification of the dominant transient absorbing species in the laser medium. For the laser mixtures investigated in this work, the primary blue/green absorption processes have been identified as photoionization of the 4p, 3d, and higher lying states of Ar, and of the Xe 6p and 5d states, and photodissociation of Ar2(3∑+u) and Ar+3.

The Second Law Efficiency of Solar Energy Conversion

Abstract: The controversy over the proper expression for the theoretical maximum conversion efficiency of solar devices is resolved. The correct expression, eta(max) 1 -4/3, relates to the maximum work that can be done by a device which accepts blackbody radiation at T(s) and rejects heat to the ambient at T(o). A general bilinear dissipation equation for solar devices is derived. The effect of back-radiation is considered, and the efficiency decrease due to atmospheric scattering is determined. 9 references.

Efficient ITO/Se Heterojunction Solar Cells

Abstract: Indium-tin-oxide (ITO)/selenium heterojunction solar cells with AM 1 conversion efficiencies as high as 3.3% have been fabricated by vacuum deposition. These cells have extremely high spectral response at short wavelengths, resulting in an efficiency of 6.5% under fluorescent light. The photovoltaic performance strongly depends upon the layer thickness of selenium (0.5~10 µm) and of tellurium (0~55A) which is interposed to provide a sufficient mechanical bond between the oxide and selenium layers.

GaAs IMPATT diodes for 60 GHz

Abstract: High-performance GaAs double-drift Read IMPATT diodes have been demonstrated at 60 GHz. 1.24-W CW output power at 11.4-percent dc to RF conversion efficiency was obtained with a junction temperature rise of 225°C. The doping profiles and test circuits have not yet been optimized and we expect that still higher power and efficiency should be achievable.

GaP/phthalocyanine Langmuir-Blodgett film electroluminescent diode

Abstract: The electrical and electroluminescent properties of Au/phthalocyanine Langmuir-Blodgett film/GaP diodes are reported. The electroluminescence conversion efficiency is shown to depend on the number of Langmuir-Blodgett layers and is a maximum for a 5.6 nm-thick film. This optimum can be explained in terms of simple tunnel injection theory. A preliminary investigation reveals that the devices are relatively stable and that the maximum power conversion efficiency approaches that of an unencapsulated p-n junction diode.

A Novel Quasi-Optical Frequency Multiplier Design for Millimeter and Submillimeter Wavelengths

Abstract: This paper describes a novel design for millimeter and sub-millimeter wavelength varactor frequency triplers and quadruplers. The varactor diode is coupled to the pump source via waveguide and stripline impedance matching and filtering structures. Output power at the various harmonics of the pump frequency is fed to quasi-optical filtering and tuning elements. The low-loss quasi-optical structures enable near-optimum control of the impedances seen by the varactor diode at the idler and output frequencies, resulting in efficient high-order harmonic conversion. A minimum efficiency of 4 percent with 30-mW input power has been obtained for a tripler operating between 200 and 280 GHz, with a peak efficiency of 8 percent between 250 and 280 GHz. Another tripler, designed for the 260-350-GHz band, gave a minimum conversion efficiency of 3 percent with 30-mW input power, with a peak efficiency of 5 percent at 340 GHz.

Evidence against surface state limitations on efficiency of p-Si/CH3CN junctions

Abstract: We report here the first efficient p-type Si-based semiconductor–liquid junction system. p-Type Si photocathodes have been previously reported to yield open circuit photovoltages, Voc, of 0.38–0.40 V with several redox systems1–8. Photocurrent–voltage studies of p-Si cathodes in CH3CN solvent have demonstrated 2.5% efficiency for conversion of 632.8-nm light to electricity with the N,N′-dimethyl-4,4′-bipyridinium2+/+ redox system1, and 0.5–2.4% with other macrocyclic complexes7,8. Such modest energy conversion efficiencies for small band gap semiconductors have been attributed to surface states which pin the semiconductor Fermi level6–9 and promote recombination processes10. However, we find that p-type Si/CH3CN interfaces can yield solar conversion efficiencies in excess of 10%, and can display open circuit photovoltages within 0.08 V of the theoretical limit for an ideal junction.

The Liquid Droplet Radiator - an Ultralightweight Heat Rejection System for Efficient Energy Conversion in Space

Abstract: A heat rejection system for space is described which uses a recirculating free stream of liquid droplets in place of a solid surface to radiate waste heat. By using sufficiently small droplets ( 100 micron diameter) of low vapor pressure liquids the radiating droplet sheet can be made many times lighter than the lightest solid surface radiators (heat pipes). The liquid droplet radiator (LDR) is less vulnerable to damage by micrometeoroids than solid surface radiators, and may be transported into space far more efficiently. Analyses are presented of LDR applications in thermal and photovoltaic energy conversion which indicate that fluid handling components (droplet generator, droplet collector, heat exchanger, and pump) may comprise most of the radiator system mass. Even the unoptimized models employed yield LDR system masses less than heat pipe radiator system masses, and significant improvement is expected using design approaches that incorporate fluid handling components more efficiently. Technical problems (e.g., spacecraft contamination and electrostatic deflection of droplets) unique to this method of heat rejectioon are discussed and solutions are suggested.