Photon-enhanced Thermionic Emission Research Articles

Photon-enhanced thermionic emission solar cells based on boron-doped graphene/ carbon nanospheres composite aerogel photocathode material

Photon-enhanced thermionic emission solar cells based on boron-doped graphene/ carbon nanospheres composite aerogel photocathode material

Theoretical exploration of photoemission characteristics of GaAs nanowire array cathode based on photon-enhanced thermionic emission

The theoretical photoemission models of single GaAs nanowire and nanowire array cathode based on photon-enhanced thermionic emission (PETE) mechanism are respectively developed by utilizing two-dimensional continuity equations. Based on the built models in this work, the effect of several key factors on the quantum efficiency are also discussed. Results show that GaAs nanowire cathode exhibits the superiority of photoemission performance compared with GaAs film cathode at the spectral range of 300∼600 nm. Besides, the optimal wire diameter of GaAs nanowire cathode is in the range of 340∼360 nm. In addition, the nanowire cathode with low electron affinity of 0.3 eV achieves PETE effect under low temperature of 400 K, which is conducive to improve the stability of cathode. Moreover, increasing the incident light angle or decreasing the array spacing provides an effective method of obtaining high quantum efficiency of nanowire array cathode. Furthermore, the numerical difference between quantum efficiency and effective quantum efficiency implies that the low collection efficiency of emitted electrons is the main issue that hinders the application of PETE cathodes of GaAs nanowire array. The proposed GaAs nanowire array model would provide theoretical guidance for optimum design of PETE cathodes with nanoarray structure.

The effect of space charge neutralization on the parametric design of photon enhanced thermionic solar converters

Photon-enhanced thermionic emission (PETE) is a promising method for increasing current density at relatively low cathode temperatures, but is susceptible to the space charge effect. In this study, we propose a novel PETE device model in which the space-charge barrier is relaxed by photon-excited cesium ions. The decrease in maximum efficiency with interelectrode gap is more gradual compared to previous studies. We investigate the effects of cathode materials properties (band gap and electron affinity) and operating conditions (cathode temperature and interelectrode gap) on PETE performance, and identify optimal performance conditions. Our findings show that the optimal gap range exceeds 5 μm and near field radiation loss can be minimized, providing an important opportunity for designing PETE device with larger interelectrode gaps and higher solar flux concentrations.

Inner-cascaded photovoltaic-thermionic-thermoradiative conversion for concentrated solar power

Photon enhanced thermionic emission (PETE) is a promising full-spectrum solar power technology capable of converting super-bandgap photons via photovoltaic and sub-bandgap photons through thermionic emission. However, PETE cathode with an infrared absorber, is usually accompanied with a high thermal radiation loss to the environment. This paper proposes an isothermal thermoradiative enhanced-PETE (iTR-PETE) configuration, where p-type cathode absorbs high-energy photons and n-p junction anode converts low-energy photons for thermoradiative purposes. A thermodynamic model is developed to assess the respective impacts of photovoltaic, thermionic and thermoradiative on solar iTR-PETE conversion. The positive and negative chemical potentials of the cathode and anode work together to increase thermionic power. The iTR-PETE converter achieves an excellent exergy efficiency of 55.43 % at a concentration ratio of 40 and an anode bandgap of 0.1 eV. Photovoltaic determines thermoradiative power, while thermoradiative increases waste heat to the secondary thermal cycle to improve the overall solar conversion efficiency. Along with an additional 4.24 % of power generated at anode, the thermoradiative effect reduces thermal radiation from the PETE converter by 28.54 %. The iTR-PETE combined system achieves a 92.3 % higher conversion efficiency than that of conventional PETE combined system.

Performance analysis of photon-enhanced thermionic emission systems mediated by quantum tunneling

Reducing the gap between the electrodes to the nanoscale and utilizing quantum effects are an effective way to enhance the performance of a thermionic energy device. In this work, we establish the model of a photon-enhanced thermionic emission system with a nanoscale vacuum gap, where the electron transport due to electron tunneling and the near-field radiation resulting from photon tunneling are introduced. Analytical expressions for the thermionic emission current, electron tunneling current, and heat flux due to the near-field radiation are provided. By using the energy and particle balance equations, the electron concentration and the temperature of the cathode are determined. The impacts of the voltage, electron affinity, and gap distance on the performance are further analyzed. Results show that the suggested system can achieve high efficiency at the low-temperature cathode. Up to 34.7% of solar-to-electricity efficiency is possible at a cathode temperature of 472.5 K. The proposed model provides a strategy for designing highly efficient thermionic emission devices operating at low temperatures.

Measurement of the C12A7:e- thermionic emission enhancement due to photon exposure

In this work, a test set-up device is designed, developed and manufactured to perform a direct measurement of the current emission enhancement that the photon absorption can bring to the thermionic emission in a Photon-Enhanced Thermionic Emission Device (PETED) where the C12A7:e- electride is used as the semiconductor due to its low work function value of 2.4 eV. Even though the measured thermionic emission starts at low operational temperatures, obtaining for example a current density emission of 5 mA/cm2 at 500 °C, there is barely an increase of 1% in the current emission when the device is exposed to a source of photons. This effect is mainly due to the presence of a dielectric layer at the material surface, which acts as a barrier, reduces the current enhancement effect from photon excitation, and drives to a limited efficiency of 27 μA/cm2/W.

Hybrid photon-enhanced thermionic emission and photovoltaic converter with concentrated solar power

Hybrid photon-enhanced thermionic emission and photovoltaic converter with concentrated solar power

Theoretical Analysis of InGaN Solar Energy Converters Based on Photon-Enhanced Thermionic Emission

Photon-enhanced thermionic emission (PETE) is an efficient solar energy conversion mechanism that combines photovoltaic effects and thermionic emissions. In this study, a diffusion–emission model of electrons for the InGaN cathode was deduced based on one-dimensional continuity equations. The temperature dependence of the excess electron concentration, current density, and conversion efficiency at different cathode electron affinities was simulated, and the performance of the PETE converter under isothermal and nonisothermal state was compared. The results show that the improvement in conversion efficiency under isothermal condition was limited by the increase in anode temperature and reached the maximum of ~22% at an electron affinity of 0.56–0.59 eV and the operating temperature of 710–740 K. When the anode temperature was 500 K, the conversion efficiency increased with the increase in the electron affinity and exceeded the maximum value of the isothermal state at 0.6 eV. We explored the behavior of the converter at bias voltages as well as the determination of the maximum conversion efficiency point. The open-circuit voltage in the isothermal state was lower than that in the nonisothermal state, and the output voltage at the maximum conversion efficiency was eventually greater than the flat-band voltage.

Theoretical research and simulation of GaAs nanowire arrays in reflection-type photon-enhanced thermionic emission solar converters

In this paper, GaAs was used as the cathode of the photon-enhanced thermionic emission (PETE) solar converter. Based on the one-dimensional continuity equation and boundary conditions, a reflection-type PETE theoretical model was established to simulate the effects of solar concentration, electron affinity, cathode temperature and electron recombination velocity on the current density and the conversion efficiency. For optimizing the performance of the solar converter, we used GaAs nanowire (NW) arrays as the cathode and calculated the influences of NW arrays’ period, duty ratio, and height on the absorbance and the conversion efficiency by combining the theoretical model with the three-dimensional finite-difference time-domain (3D FDTD) method. The results were used to design the structure size of NW arrays, which improved the conversion efficiency of the solar converter to 23.07%. It shows that the GaAs NW arrays are promising PETE cathode materials.

Thermionic performance of nanocrystalline diamond/silicon structures under concentrated solar radiation

Thin films of nanocrystalline diamond with thickness around 100 nm were deposited on highly doped p-type silicon substrates to evaluate the electron emission performance of these structures under illumination of concentrated sunlight in the temperature range 500–700 °C. By comparing the emitted current densities measured using a pure thermal source and a concentrated light source simulating the solar radiation spectrum (Xe lamp), an increase up to about 80 times at 600 °C was found using the concentrated light source, thus demonstrating the boost on the thermionic emission thanks to the sunlight absorption. At temperatures higher than 600 °C the action of the photon-enhanced thermionic emission (PETE) mechanism begins to vanish, starting the regime of pure thermionic emission. The opening of the quasi-Fermi levels reducing the barrier height down to 0.33 eV for electron emission is considered to explain the overall behavior of the diamond-silicon system in the PETE regime.

Photo-thermo-electric modeling of photon-enhanced thermionic emission with concentrated solar power

Photon-enhanced thermionic emission (PETE) is an advanced technology that combines both the photoelectric and the thermionic effects synergistically into a single device for direct electricity generation. However, its industrial-level application is still missing partly due to the lack of advanced models to analyze its operating characteristics and to understand the synergistic mechanism. Herein, we develop a numerical model of PETE by fully considering the optical, the electrical and the thermodynamic aspects with one-dimensional steady-state continuity equations of carriers in the semiconductor cathode. A hybrid PETE-Stirling cycle system is also proposed to yield an output power density of 162.65 kW/m 2 with 32.8% conversion efficiency. The PETE conversion efficiency keeps ∼20% with the optimal electron affinity increases from 0.5 to 1.14 eV as solar concentration ratio varies from 100 to 500. The cathode thickness should be optimized by considering both solar absorption and photon enhancement, where the thickness range of 0.78–1.52 μm is obtained for 50–500 suns. The interelectrode gap is also found to significantly affect the PETE performance by regulating both the space-charge effect and near-field radiation, where the range of 0.5–2 μm is recommended. This work can serve as a foundation to understand the working mechanism of PETE converters and provide guidelines for the performance evaluation. • Numerical models are developed for photon-enhanced thermionic emission converters. • Coupled effects of optical, electrical and thermodynamic fields are fully considered. • Characteristics of the converter for various scenarios are analyzed in detail. • A hybrid system is proposed with a 162.65 kW/m 2 power density and a 32.8% efficiency.

Theoretical analysis and experimental research of photon-enhanced thermionic emission solar energy converters with InN photocathode

In this study, an InN photocathode consisting of ZnO nanowire arrays was proposed as a cathode for a photon-enhanced thermionic emission (PETE) solar energy converter. To optimize the performance of the device, we applied InN with a phonon bottleneck effect to the PETE converter. The theoretical model for PETE conversion with this complex photocathode was developed based on one-dimensional continuous equations and boundary conditions. We simulated and analyzed the conversion efficiency as a function of operating temperature, solar concentration, surface electron affinity, and operating voltage. Furthermore, we verified the feasibility of the proposed cathode through experiments on the current-voltage characteristics of the InN photocathode at different temperatures. The results of simulations and experiments indicated that InN is a promising photocathode material and may enable high-performance PETE devices. • An InN photocathode consisting of ZnO nanowire arrays is proposed and applied to a PETE solar converter. • By using one-dimensional continuity equations, the theoretical model with this complex photocathode is deduced. • The relationships between PETE conversion efficiency of InN photocathode and various parameters are simulated and analyzed. • The feasibility of the proposed cathode is verified by experiments on the I-V characteristics at different temperatures.

Preparation of graphene aerogel and application in photon-enhanced thermionic emission.

Photon-enhanced thermionic emission (PETE) is a novel concept of solar energy conversion in recent years. Porous 3D graphene aerogels (GA) were prepared by hydrothermal reduction of graphene oxide (GO). The morphology of GO and GA was characterized by scanning electron microscopy and transmission electron microscopy respectively. The functional groups of GO and GA were characterized by Electron Microscopy and Fourier Transform infrared spectroscopy. The PETE properties of the samples were tested by a self-made device. Thermoelectron emission can be detected when the energy density of the excitation laser was higher than 35 W. The efficiency of the device was between 8.14 × 10−6% and 1.89 × 10−5%, and the output voltage was about 1 V. Compared with 3D graphene powder and 2D graphene in the control group, GA has more significant and stable thermionic emission properties. GA is a promising cathode material for a PETE solar energy converter, and the conductivity of GA should be further optimized.

Considering the effect of optical attenuation on photon-enhanced thermionic emission converter of the practical structure

Abstract Photon-enhanced thermionic emission (PETE) is a new type of solar cell. The existing papers on PETE research do not consider the structure of actual PETE devices; in practical PETE structure, the original incident light intensity is attenuated by the window layer and the buffer layer (include anode in reflection PETE devices). In this paper, according to two kinds of the common structure of PETE device, the influence of transmission and reflection of sunlight on the conversion efficiency of PETE device is analyzed. Using a light-trapping structure on the cathode of the PETE device is a valid method to reduce the reflection of the incident light. The calculation results show that the optical attenuation has a great influence on the actual photon flux received by the cathode effective layer. Under the condition of reasonable operation of the device, the efficiency of PETE can be improved by reducing the size of the material, improving the light transmittance of the buffer layer and window layer, and using the light-trapping structure.

Photoemission and photon-enhanced thermionic emission: Effect of jump in electron mass

The effect of jump in electron mass on the electron transfer through interfaces with positive and negative potential steps is considered in relation with practically important photoemission and photon-enhanced thermionic emission from semiconductors with positive and negative effective electron affinities. For a zero potential step, a direct similarity is established between the expressions for electron reflection and transmission and Fresnel equations for p-polarized light, with the ratio of masses replacing the ratio of dielectric constants. For non-zero potential steps of both signs, due to the jump in mass, there are critical electron energies that separate the energy regions with qualitatively different angular dependences of the transmission coefficient. The influence of the jump in mass on the spectra of photoemission quantum yield, which are measured experimentally under transition from negative to positive electron affinity at the p-GaAs(Cs,O)-vacuum interface, is considered. It is shown that, for a decisive proof of the jump-in-mass contribution to the electron transfer through semiconductor-vacuum interface, measuring the energy and angular distributions at the state of positive electron affinity is more advantageous, as compared to the state of negative electron affinity.

Comprehensive energy balance analysis of photon-enhanced thermionic power generation considering concentrated solar absorption distribution

The present article reports a comprehensive energy balance analysis of a photon-enhanced thermionic emission (PETE) device when it is used for concentrated solar power (CSP) generation. To this end, we consider a realistic PETE device composed of a boron-doped silicon emitter on glass and a phosphorus-doped diamond collector on tungsten separated by the interelectrode vacuum gap. Depth-dependent spectral solar absorption and its photovoltaic and photothermal energy conversion processes are rigorously calculated to predict the PETE power output and energy conversion efficiency. Our calculation predicts that when optimized, the power output of the considered PETE device can reach 1.6 W/cm2 with the energy conversion efficiency of ~18% for 100× solar concentration, which is substantially lower than those predicted in previous works under ideal conditions. In addition, the photon-enhancement ratio is lower than 10 and decreases with the increasing solar concentration due to the photothermal heating of the emitter assembly, suggesting that PETE should be more suitable for low-to-medium CSP below ~100× concentration. These observations signify the importance of a rigorous energy balance analysis based on spectral and spatial solar absorption distribution for the accurate prediction of PETE power generation.

Exploring n‐type SiC film with ultralow work function Cs coating surface for solar cell anode

In this study, the Cs adsorption behavior on n-type 3C-SiC (0001)-(2 × 1) and -(3 × 2) reconstructed surfaces are investigated via first-principles calculations. The potentiality of 3C-SiC as the anode material of solar cell based on photon-enhanced thermionic emission (PETE) is discussed. Various doping configurations are considered for the determination of an optimum n-type doping structure. Cs-covered (3 × 2) surface exhibits stronger stability compared with (2 × 1) surface. The charge transfer caused by Cs adsorption produces a [Cs+-SiC] dipole moment directing from Cs adatom to reconstructed surface, resulting in an effective declination of work function. Specifically, 0.5 ML Cs coverage surface provides an ultralow work function with 0.9 eV, demonstrating its potential application in PETE devices. Combining with a photoemission model, the maximum conversion efficiency can attain 30% utilizing SiC as an anode. Moreover, the Mulliken charge analysis provides the details of surface charge transfer and implies the inner mechanism of work function alteration. This study provides guiding significance for the synthesis of SiC material with low work function and an effective theoretical method for the design of high-performance anode materials in application of PETE devices. The adsorption mechanism of Cs on n-type doped 3C-SiC(0001)-(2 × 1) and -(3 × 2) reconstructed surfaces and its application as the anode material of solar cell are systematically investigated. For one Cs adatom adsorption, Cs-covered (3 × 2) surfaces present stronger stability compared with (2 × 1) surfaces. Cs-decorated (3 × 2) surface with 0.5 ML coverage exhibits an ultralow work function as low as 0.9 eV. Based on the photoemission model, the maximum conversion efficiency of PETE devices with SiC anode can reach 30%. Highlights Cs-decorated SiC surfaces with ultralow work function are suitable as PETE anode materials. Cs adsorption mechanism on 3C-SiC(0001)-(2 × 1) and -(3 × 2) surfaces are explored. The optimum n-type doped (2 × 1) and (3 × 2) surfaces are, respectively, determined. 0.5 ML Cs-adsorbed (3 × 2) surfaces show an ultralow work function of 0.9 eV. Cs adsorption induces a [Cs+-SiC] dipole moment on (2 × 1) and (3 × 2) surfaces.

Feasibility study on Si-doped ZnO with Cs coating surface for solar cell anode

Using first-principles calculation method, we investigate the possibility of Si-doped ZnO(0001) surface with Cs coating layer applied as the anode material of solar cell based on photon-enhanced thermionic emission (PETE) mechanism. The n-type doping and Cs adsorption process on ZnO(0001) surface are systematically discussed and analyzed. For n-type doping, five Si substitution doping models are established to study the effects of different doping sites on the formation energy, density of state and work function of n-type ZnO (0001) surfaces. Due to the lowest formation energy, the Site-2 doping model with Si replacing the Ga atom at the third bilayer is selected as the Cs adsorption substrate. Moreover, the optimum Cs adsorption site and coverage on Si-doped ZnO (0001) surface are respectively determined. Meanwhile, excessive Cs adsorption will lead to the appearance of “Cs poisoning” phenomenon. The [Cs-ZnO] dipole moment model can be employed to explain the variation mechanism of work function on Cs-covered surfaces. Based on PETE photoemission theory, the conversion efficiency of PETE devices with ZnO as anode material is predicted. These results provide guidelines for the design of PETE anode materials with high-conductivity and low work function.

High-performance solid-state photon-enhanced thermionic emission solar energy converters with graded bandgap window-layer

To realize a high-performance solid-state photon-enhanced thermionic emission (SPETE) solar energy converter, in this study, a graded bandgap window layer is therefore adopted, throughout which the bandgap gradation is generated via the variation of Al composition in the layer in the SPETE converter with a GaAs absorber. Based on one-dimension steady-state equation, an analytical model is formed in analyzing performance of the proposed device in our study. Theoretical simulation results indicate that not only are the losses of contact surface recombination being decreased via the bandgap-gradation-induced build-in electric field of the window layer, but also the photon-generated electrons are effectively collected, thereby improving the conversion efficiency. Moreover, the effect of bandgap energy of the contact surface and the width of the window layer on device performance is discussed. A trade-off of high-efficient SPETE converters is therefore realized between large contact surface bandgap and thin window layer width, to which the rationale lies in the improved process of electron collection facilitated by the enhanced build-in electric field rather than reducing the photon absorption in the window layer. Threshold values for barrier height at the emitting interface are presented to guarantee the ideal voltage-current characteristic. It is found that the threshold values of barrier increase with the increase in temperatures.

Novel concepts and nanostructured materials for thermionic-based solar and thermal energy converters

Thermal and concentrated solar solid-state converters are devices with no moving parts, corresponding to long lifetimes, limited necessity of maintenance, and scalability. Among the solid-state converters, the thermionic-based devices are attracting an increasing interest in the specific growing sector of energy conversion performed at high-temperature.During the last 10 years, hybrid thermionic-based concepts, conceived to cover operating temperatures up to 2000 °C, have been intensively developed. In this review, the thermionic-thermoelectric, photon-enhanced thermionic emission, thermionic-photovoltaic energy converters are extensively discussed. The design and development processes as well as the tailoring of the properties of nanostructured materials performed by the authors are comprehensively described and compared with the advances achieved by the international scientific community.