The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Alireza Nojeh - One of the best experts on this subject based on the ideXlab platform.
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semiconductor Thermionics for next generation solar cells photon enhanced or pure thermionic
Nature Communications, 2021Co-Authors: Ehsanur Rahman, Alireza NojehAbstract:Semiconductors have been used in solar energy conversion for decades based on the photovoltaic effect. An important challenge of photovoltaics is the undesired heat generated within the device. An alternative approach is Thermionics, which uses the thermal excitation of electrons from an emitter to a collector across a vacuum gap. If the emitter is a p-type semiconductor, the photogeneration-induced quasi-Fermi level splitting can reduce the effective barrier for electron emission—a mechanism used by a photon enhanced thermionic emission device. Here, we evaluate the prospects of this alternative solar conversion technology considering different semiconductor materials and thermionic device configurations. We also reveal that whether such a device operates in the photon enhanced or purely thermionic mode, depends on the complex interplay among materials properties, device physics and solar concentration level. A semiconductor thermionic device, which utilises thermally excited electrons, is considered as an alternative in solar conversion technology, yet its working mechanism is not clear. Here, the authors reveal that whether such a device operates in the photon enhanced or purely thermionic mode, greatly depends on the material properties and device physics.
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light induced nanostructured thermionic energy converters the effect of cathode anode gap on the output current
International Vacuum Nanoelectronics Conference, 2017Co-Authors: Kais Dridi, Mike Chang, Kevin Voon, Alireza NojehAbstract:Nanostructured thermionic energy converters may offer substantial advantages over traditional devices by reducing system complexity and requirements. However, they still face similar fundamental challenges, such as anode collection efficiency and the space charge effect, which have limited their performance to date. Here, we investigate a carbon nanotube-based thermionic converter, demonstrating that its output current is enhanced substantially by making the cathode-anode gap smaller, presumably due to the improved collection efficiency and reduction of space charge.
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Nanostructured Thermionics for Conversion of Light to Electricity: Simultaneous Extraction of Device Parameters
IEEE Transactions on Nanotechnology, 2015Co-Authors: Amir H. Khoshaman, Mehran Vahdani Moghaddam, Andrew T. Koch, Mike Chang, Alireza NojehAbstract:Thermionic conversion involves the direct conversion of heat, including light-induced heat, from a heat source, e.g., solar energy, to electricity. Although the concept is almost a hundred years old, the progress of thermionic convertors has been limited by issues such as the space-charge effect and availability of materials with desirable mechanical and electrical properties, while maintaining a low work function. Nanotechnology could help address some of the main challenges that thermionic convertors face. However, existing models, which were developed for macroscopic convertors, are not capable of describing all aspects of nanostructured devices. We present a method to evaluate the output characteristics of thermionic convertors with a higher precision than the existing models and the ability to simulate a broader range of parameters, including temperatures, active surface areas, interelectrode distances, and work functions. These features are crucial for the characterization of emergent devices due to the unknowns involved in their internal parameters; the model's high numerical precision and flexibility allows one to solve the reverse problem and to evaluate the internal parameters of the device from a set of simple experimental data. As an experimental case, a carbon nanotube forest was used as the emitter and locally heated to thermionic emission temperatures using a 50-mW-focused laser beam. The current-voltage characteristics were measured and used to solve the reverse problem to obtain the internal parameters of the device, which were shown to be consistent with the values obtained using other methods.
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polarization dependent light induced thermionic electron emission from carbon nanotube arrays using a wide range of wavelengths
Applied Physics Letters, 2012Co-Authors: Mehran Vahdani Moghaddam, Parham Yaghoobi, Alireza NojehAbstract:Light-induced thermionic electron emission from arrays of carbon nanotubes is observed using low-power, continuous-wave lasers with a broad set of wavelengths ranging from violet to infrared. The thermionic emission current is highest when the electric field of the laser is parallel to the axis of the nanotubes and lowest when it is perpendicular. The polarization dependence is stronger for the longer-wavelength beam.
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solar electron source and thermionic solar cell
AIP Advances, 2012Co-Authors: Parham Yaghoobi, Mehran Vahdani Moghaddam, Alireza NojehAbstract:Common solar technologies are either photovoltaic/thermophotovoltaic, or use indirect methods of electricity generation such as boiling water for a steam turbine. Thermionic energy conversion based on the emission of electrons from a hot cathode into vacuum and their collection by an anode is also a promising route. However, thermionic solar conversion is extremely challenging as the sunlight intensity is too low for heating a conventional cathode to thermionic emission temperatures in a practical manner. Therefore, compared to other technologies, little has been done in this area, and the devices have been mainly limited to large experimental apparatus investigated for space power applications. Based on a recently observed “Heat Trap” effect in carbon nanotube arrays, allowing their efficient heating with low-power light, we report the first compact thermionic solar cell. Even using a simple off-the-shelf focusing lens, the device delivered over 1 V across a load. The device also shows intrinsic storage capacity.
Yonghang Zhang - One of the best experts on this subject based on the ideXlab platform.
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ultralow interface recombination velocity 1 cm s at cdte mgxcd _ 1 hbox xte heterointerface
IEEE Journal of Photovoltaics, 2017Co-Authors: Xinhao Zhao, Shi Liu, Calli M Campbell, Maxwell B Lassise, Zhao Yuan, Yonghang ZhangAbstract:The interface recombination velocity (IRV) at the interfaces in CdTe/Mg x Cd ${}_{1\hbox{-}}$ x Te double heterostructures (DHs) is studied using time-resolved photoluminescence. It is found that both thermionic emission and tunneling effects can cause photogenerated carrier loss over or through the Mg x Cd ${}_{1\hbox{-}}$ x Te barriers, either due to the low barrier potential or the thin barrier thickness. Thus, carrier lifetime measurements reveal only an effective IRV. The thermionic emission induced interface recombination can be distinguished by conducting temperature-dependent carrier lifetime measurements, and the tunneling induced IRV can be quantified by comparing samples with different barrier thicknesses. When both thermionic emission and tunneling effects are suppressed or even eliminated, the actual IRV (due to the recombination at the DH interface trap states) is measured to be ∼1 cm/s, with a very long carrier lifetime of 3.6 μ s achieved in the DHs.
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ultralow interface recombination velocity 1 cm s in cdte mg x cd 1 x te double heterostructures
Photovoltaic Specialists Conference, 2016Co-Authors: Xinhao Zhao, Shi Liu, Calli M Campbell, Yuan Zhao, Maxwell B Lassise, Yonghang ZhangAbstract:CdTe/Mg x Cd 1−x Te double heterostructures (DHs) grown on InSb (001) substrates using molecular beam epitaxy have demonstrated very long carrier lifetime and low interface recombination velocity (IRV) due to the effective carrier confinement and surface passivation provided by Mg x Cd 1−x Te. However, both thermionic emission and tunneling effects can cause carrier loss over or through the Mg x Cd 1−x Te barriers when the barrier potential is low or when the barrier is thin. Thus carrier lifetime measurement can only give an effective IRV, which consists of the actual IRV that is purely due to recombination through interface trap states, and carrier loss due to thermionic emission and tunneling. By conducting temperature dependent carrier lifetime measurements, the thermionic emission induced interface recombination can be distinguished. Also by comparing samples with different barrier layer thicknesses, the contribution to effective IRV from tunneling effect can be quantified. When both thermionic emission and tunneling effects are eliminated, the actual IRV is measured to be ∼1 cm/s and a very long carrier lifetime of 3.6 μs is observed.
A. Datas - One of the best experts on this subject based on the ideXlab platform.
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Thermionic-enhanced near-field thermophotovoltaics
Nano Energy, 2019Co-Authors: A. Datas, Rodolphe VaillonAbstract:Abstract Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and Thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ∼ 30% and ∼ 70 W/cm2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (∼10% and ∼10 W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.
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thermionic enhanced near field thermophotovoltaics for medium grade heat sources
Applied Physics Letters, 2019Co-Authors: A. Datas, Rodolphe VaillonAbstract:Conversion of medium-grade heat (temperature from 500 to 1000 K) into electricity is important in applications such as waste heat recovery or power generation in solar thermal and co-generation systems. At such temperatures, current solid-state devices lack either high conversion efficiency (thermoelectrics) or high-power density capacity (thermophotovoltaics and Thermionics). Near-field thermophotovoltaics (nTPV) theoretically enables high-power density and conversion efficiency by exploiting the enhancement of thermal radiation between a hot emitter and a photovoltaic cell separated by nanometric vacuum gaps. However, significant improvements are possible only at very small gap distances (<100 nm) and when ohmic losses in the photovoltaic cell are negligible. Both requirements are very challenging for current device designs. In this work, we present a thermionic-enhanced near-field thermophotovoltaic (nTiPV) converter consisting of a thermionic emitter (graphite) and a narrow bandgap photovoltaic cell (InAs) coated with low-workfunction nanodiamond films. Thermionic emission through the vacuum gap electrically interconnects the emitter with the front side of the photovoltaic cell and generates an additional thermionic voltage. This avoids the use of metal grids at the front of the cell and virtually eliminates the ohmic losses, which are unavoidable in realistic nTPV devices. We show that nTiPV operating at 1000 K and with a realizable vacuum gap distance of 100 nm enables a 10.7-fold enhancement of electrical power (6.73 W/cm2) and a 2.8-fold enhancement of conversion efficiency (18%) in comparison with a realistic nTPV device having a series resistance of 10 mΩ·cm2.
Rodolphe Vaillon - One of the best experts on this subject based on the ideXlab platform.
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Thermionic-enhanced near-field thermophotovoltaics
Nano Energy, 2019Co-Authors: A. Datas, Rodolphe VaillonAbstract:Abstract Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and Thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ∼ 30% and ∼ 70 W/cm2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (∼10% and ∼10 W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.
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thermionic enhanced near field thermophotovoltaics for medium grade heat sources
Applied Physics Letters, 2019Co-Authors: A. Datas, Rodolphe VaillonAbstract:Conversion of medium-grade heat (temperature from 500 to 1000 K) into electricity is important in applications such as waste heat recovery or power generation in solar thermal and co-generation systems. At such temperatures, current solid-state devices lack either high conversion efficiency (thermoelectrics) or high-power density capacity (thermophotovoltaics and Thermionics). Near-field thermophotovoltaics (nTPV) theoretically enables high-power density and conversion efficiency by exploiting the enhancement of thermal radiation between a hot emitter and a photovoltaic cell separated by nanometric vacuum gaps. However, significant improvements are possible only at very small gap distances (<100 nm) and when ohmic losses in the photovoltaic cell are negligible. Both requirements are very challenging for current device designs. In this work, we present a thermionic-enhanced near-field thermophotovoltaic (nTiPV) converter consisting of a thermionic emitter (graphite) and a narrow bandgap photovoltaic cell (InAs) coated with low-workfunction nanodiamond films. Thermionic emission through the vacuum gap electrically interconnects the emitter with the front side of the photovoltaic cell and generates an additional thermionic voltage. This avoids the use of metal grids at the front of the cell and virtually eliminates the ohmic losses, which are unavoidable in realistic nTPV devices. We show that nTiPV operating at 1000 K and with a realizable vacuum gap distance of 100 nm enables a 10.7-fold enhancement of electrical power (6.73 W/cm2) and a 2.8-fold enhancement of conversion efficiency (18%) in comparison with a realistic nTPV device having a series resistance of 10 mΩ·cm2.
Christopher M. Day - One of the best experts on this subject based on the ideXlab platform.
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enhanced electron emission from functionalized carbon nanotubes with a barium strontium oxide coating produced by magnetron sputtering
Carbon, 2007Co-Authors: Feng Jin, Christopher M. Day, Yan Liu, Scott LittleAbstract:Abstract Carbon nanotubes (CNTs) were functionalized with a surface coating using magnetron sputter deposition. The CNT samples used were prepared by plasma enhanced chemical vapor deposition and were vertically aligned to the surface of the tungsten substrate. A thin layer of barium strontium oxide approximately 100 nm in thickness was deposited on their surface using magnetron sputtering. The oxide coating was uniform, covering the whole surface of the CNTs and significantly lowered the work function while preserving the geometry. The resulting oxide coated CNTs had a work function of 1.9 eV and a field enhancement factor of 467, which led to a significant improvement in both field and thermionic emission. Compared to uncoated CNTs, the field emission was increased by a factor of two, while the thermionic emission increased by more than four orders of magnitude. At 4.4 V/μm, a field emission current of 23.6 μA was obtained from an emitting surface of 0.012 cm2. Similarly, at 1.1 V/μm and 1221 K, a thermionic emission current of 14.6 mA was obtained.
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Thermionic emission from carbon nanotubes with a thin layer of low work function barium strontium oxide surface coating
Applied Physics Letters, 2006Co-Authors: Feng Jin, Christopher M. DayAbstract:We have created a thermionic cathode structure that consists of a thin tungsten ribbon, carbon nanotubes(CNTs) on the ribbon surface, and a thin layer of low work function barium strontium oxide coating on the CNTs. This oxide coated CNTcathode was designed to combine the benefits from the high field enhancement factor from CNTs and the low work function from the emissive oxide coating. The field emission and thermionic emission properties of the cathode have been characterized. A field enhancement factor of 266 and a work function of 2.1 eV were obtained. At 1437 K , a thermionic emissioncurrent density of 15 mA ∕ cm 2 in an electric field of 0.9 V ∕ μ m was obtained, which is 50 times greater than the emission current density from the uncoated CNTcathode at the same temperature.