Photon-enhanced Thermionic Emission Devices Research Articles

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.

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.

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.

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.

Exploring the n-GaN films with Cs coating surface for low work function solar cell anode

In this study, the first principle calculation is utilized to investigate the possibility of GaN to be applied as the anode of solar cells based on photon-enhanced thermionic emission (PETE). The results suggest that Si-doped GaN(0001) surface can obtain satisfactory ionization in N-rich environment. Substituted atoms are inclined to replace Ga atoms in the outmost layer. The highest carrier concentration is obtained when substitutional atoms are in the first bilayer. Surface coating technology utilizing Cs adatoms can effectively modify the surface work function. The optimal adsorption site on GaN(0 0 0 1) surface is N-top site. The work function attains the minimum at 0.75 ML coverage as multiple Cs atoms adsorbed. Simultaneously, the conversion efficiency reaches a peak value of 37% at 0.75 ML Cs coverage according to the theoretical prediction. Besides, the conversion efficiency largely depends on the temperature when the work function of the anode is low enough (at least lower than 1 eV). Our results demonstrate the possibility of GaN as the anode of PETE devices and provides a method for discovering more candidate materials for both the cathode and anode.

Theoretical analysis and modeling of photoemission ability and photoelectric conversion characteristics of GaAs nanowire cathodes based on photon-enhanced thermionic emission

An axially exponential-doping GaAs nanowire cathode is proposed as the cathode material of photon-enhanced thermionic emission (PETE) device. Two-type theoretical models are respectively developed based on the numerical solution of multi-dimensional continuous equations with the boundary conditions. The quantum efficiency and the conversion efficiency of exponential-doping GaAs nanowire cathode based on PETE mechanism as a function of wire length, wire width, operating temperature, emissive surface recombination velocity and surface electron affinity are simulated and analyzed, respectively. Results show that the PETE devices with exponential-doping cathode can exhibit more superiority than those with uniform-doping structure by comparing the conversion efficiency, which mainly attributes to the built-in electric field induced by the axially exponential-doping structure. In addition, the optimal wire height and wire width for exponential-doping GaAs nanowire cathode are about 6 μm and 300 nm, respectively. Moreover, the emissive surface recombination velocity has an obvious inhibition on the quantum efficiency and conversion efficiency of GaAs nanowire PETE devices. The simulations would provide valuable theoretical guidance to achieve high-performance nano-based PETE devices.

High-performance GaAs nanowire cathode for photon-enhanced thermionic emission solar converters

GaAs nanowire cathodes with exponential doping and graded Al composition structures are proposed for photon-enhanced thermionic emission (PETE) devices. The conversion efficiency models with these two nanowire cathodes are deduced on the basis of one-dimensional continuity equations. The conversion efficiency as a function of wire length, wire width, Al composition distribution, cathode temperature, emissive surface and back interface recombination velocity are also simulated, respectively. Results show that exponential doping and graded Al composition cathode structures can obviously improve the conversion efficiency of devices through introducing a built-in electric field along the growth direction of nanowire. Besides, the optimum wire length and wire width range are 300–340 nm and 5.9–6.4 μm, respectively. Moreover, wider Al composition range is beneficial to achieve higher conversion efficiency. These simulations provide an interesting attempt to explore the working mechanism of GaAs nano-based PETE devices and are expected to be verified by the experimental results in the future.

Effects of near-field photon tunneling on the performance of photon–enhanced thermionic emission energy conversion

Photon-enhanced thermionic emission (PETE) has been recently proposed as a new concept to achieve full-spectrum solar energy harvesting. Here, we develop a model to consider both space charge effects and near-field photon tunneling losses in evaluating PETE's performance. An optimal gap distance is found to exist, and the underlying mechanism is attributed to soaring thermal radiative losses with decreasing inter-electrode distance beyond the blackbody limit. Further efficiency enhancement approaches are proposed based on electron affinity optimization and mismatched thermal radiation of electrodes. This work helps deepen the understanding of energy transport and conversion processes of PETE devices and guide their optimum designs.

High-performance Photon-enhanced thermionic emission solar energy converters with [formula omitted] cathode under multilevel built-in electric field

A transmission mode AlxGa1−xAs∕GaAs photocathode consisting of a composition-graded window layer and an exponential-doping emission layer is proposed to improve the performance of photon-enhanced thermionic emission (PETE) solar energy converters. The theoretical model with this complex photocathode is deduced based on one-dimensional continuity equations to analyze the characteristics of the device. It is found the multilevel built-in electric fields can effectively enhance the conversion efficiency and response of the entire spectrum in contrast to other photocathode structures. Because of less heterojunction interface recombination losses and greater transport capacity of photoelectrons toward emissive surface. Moreover, the effect of Al proportion variation in the window layer and the thickness of the window layer and emission layer on performance is discussed. The model would provide theoretical guidance for better performance of PETE device.

Effects of nanoscale vacuum gap on photon-enhanced thermionic emission devices

A new model of the photon-enhanced thermionic emission (PETE) device with a nanoscale vacuum gap is established by introducing the quantum tunneling effect and the image force correction. Analytic expressions for both the thermionic emission and tunneling currents are derived. The electron concentration and the temperature of the cathode are determined by the particle conservation and energy balance equations. The effects of the operating voltage on the maximum potential barrier, cathode temperature, electron concentration and equilibrium electron concentration of the conduction band, and efficiency of the PETE device are discussed in detail for different given values of the vacuum gap length. The influence of the band gap of the cathode and flux concentration on the efficiency is further analyzed. The maximum efficiency of the PETE and the corresponding optimum values of the band gap and the operating voltage are determined. The results obtained here show that the efficiency of the PETE device can be significantly improved by employing a nanoscale vacuum gap.

Limit of efficiency for photon-enhanced thermionic emission vs. photovoltaic and thermal conversion

Conversion of sunlight by photon-enhanced thermionic emission (PETE) combines a photonic process similar to photovoltaic cells, and a thermal process similar to conventional thermionic converters. As a result, the upper limit on the conversion efficiency of PETE devices is not the same as the Shockley–Queisser (SQ) limit that corresponds to the bandgap of the absorbing material, nor to the Carnot efficiency corresponding to its temperature. Here we analyze the upper limit on efficiency of ideal PETE devices in several possible configurations, in comparison to ideal photovoltaic cells and ideal solar thermal converters. Isothermal PETE converters are shown to be restricted to less than the SQ limit, but non-isothermal devices can exceed this limit. The limit of efficiency increases with the flux concentration reaching for example 52% at concentration of 1000 suns. Spectral splitting leads to a modest increase in conversion efficiency to 56% at 1000 suns. Addition of a secondary thermal cycle increases the efficiency limit for all PETE configurations, up to 69.8% and 70.4% for the cases of isothermal PETE and a dual bandgap PETE system at 1000 suns.

Diffusion emission model for solid-state photon-enhanced thermionic emission solar energy converters

Solid-state photon-enhanced thermionic emission (PETE) solar energy converters are new devices that can directly convert solar energy into electrical power at elevated temperatures. This study proposes a new solid-state PETE device structure with a p-type doped semiconductor as the absorber. A model based on a 1D steady-state continuity equation is presented to simulate the diffusion and emission of photogenerated carriers and to calculate the efficiencies. This model can evaluate the effect of device structure and material parameters on performance and efficiency. Calculation results show that the new device is more efficient compared with the original device. A longer electron diffusion length is favourable for enhancing efficiency. Moreover, the optimal absorber thickness is predicted. Our analyses also show that the front interface strongly affects conversion efficiency, which emphasises the need to reduce interface recombination losses. The results of this study may serve as bases for the optimum design of practical solid-state PETE devices.

Si, GaAs, and InP as cathode materials for photon-enhanced thermionic emission solar cells

Photon-enhanced thermionic emission (PETE) solar cells are photovoltaic devices designed for high temperature operation. The use of Si, GaAs, and InP as the cathode (i.e. the absorber and the electron-emitter electrode) materials in PETE solar cells is investigated with numerical device models. The models describe the cathode one dimensionally and are valid also at high injection levels. The temperature dependence of the photon absorption coefficients and temperature and doping dependencies of electron mobilities are modelled. Simulated device characteristics are presented and the factors determining the efficiency of the PETE devices are discussed. Our results show that Si, GaAs, and InP are all promising materials for PETE solar cells, if the surface recombination, effective electron affinities, and thermal stability of these materials can be optimized. Owing to their strong absorption, GaAs and InP show higher efficiencies (20–25%) than Si (10–15%) in spite of their intense radiative recombination. Especially, InP is a promising candidate for PETE cathodes as it shows higher efficiency than GaAs due to its stronger photon absorption properties.

Photon-enhanced thermionic emission from p-GaAs with nonequilibrium Cs overlayers

Photon-enhanced thermionic emission (PETE), which is promising for increasing the efficiency of solar energy conversion, is studied during cesium deposition on the As- and Ga-rich p-GaAs(001) surfaces and subsequent relaxation in the nonequilibrium Cs overlayer by means of photoemission quantum yield spectroscopy adapted for systems with time-variable parameters. Along with direct photoemission of “hot” electrons excited by light above the vacuum level, the spectra contain PETE contribution of “thermalized” electrons, which are excited below the vacuum level and emit in vacuum due to thermalization up in energy by phonon absorption. Comparing the measured and calculated spectra, the effective electron affinity and escape probabilities of hot and thermalized electrons are obtained as functions of submonolayer Cs coverage. The minima in the affinity and pronounced peaks in the escape probabilities are observed for Cs deposition on both the As- and Ga-rich surfaces. Possible reasons for the low mean values of the electron escape probabilities and for the observed enhancement of the probabilities at certain Cs coverages are discussed, along with the implications for the PETE device realization.

Heterostructured cathode with graded bandgap window-layer for photon-enhanced thermionic emission solar energy converters

A heterostructured cathode having a graded bandgap window-layer is proposed in this study to improve the performance of photon-enhanced thermionic emission (PETE) solar energy converters. A model based on a one-dimensional steady-state equation is developed to analyze the characteristics of the proposed device. This model is used to calculate the conversion efficiency of a PETE device with an AlxGa1−xAs/GaAs cathode. The built-in electric field induced by the bandgap gradation in the window-layer is shown to improve the efficiency because of the reduction of contact surface recombination losses and efficient collection of photogenerated electrons. This field is unaffected by temperature variation, and the improvement of efficiency is sustained at elevated temperatures. The effect of window-layer parameters and cathode thickness on efficiency is discussed. We also show that the efficiency maximizes at lower temperatures for cathodes with lower electron affinities. Moreover an optimal value exists for anode work function at a given anode temperature, thus providing guidance for the optimum design of barrier conditions for both cathode and anode.

Material optimum choices and parametric design strategies of a photon-enhanced solar cell hybrid system

A novel design concept of the solar hybrid system consisting of a solar concentrator, a photon enhanced thermionic emission device (PETED), and a thermoelectric generator (TEG) is proposed. The main irreversible losses in the system are taken into account. The energy balance equations of the cathode and anode of the PETED are directly used to expound that the cathode and anode temperatures of the PETED may be changed through the different choices of the energy bandgap and electron affinity of the cathode material, the reduced electric current of the TEG, and the structure parameter of the hybrid device. Analytic expressions for the power output and efficiency of the hybrid system are derived. The maximum efficiency of the hybrid system is calculated and it is proved that the hybrid system can effectively enhance the conversion efficiency of solar energy. The optimal values of some important parameters, such as the energy bandgap and electron affinity of the PETED, the reduced electric current of the TEG, the structure parameter of the hybrid device, and the solar concentration of the hybrid system, are determined. The results obtained here will be helpful for the reasonable choice of cathode materials and the optimum design of practical hybrid devices.

Diffusion-emission theory of photon enhanced thermionic emission solar energy harvesters

Numerical and semi-analytical models are presented for photon-enhanced-thermionic-emission (PETE) devices. The models take diffusion of electrons, inhomogeneous photogeneration, and bulk and surface recombination into account. The efficiencies of PETE devices with silicon cathodes are calculated. Our model predicts significantly different electron affinity and temperature dependence for the device than the earlier model based on a rate-equation description of the cathode. We show that surface recombination can reduce the efficiency below 10% at the cathode temperature of 800 K and the concentration of 1000 suns, but operating the device at high injection levels can increase the efficiency to 15%.

On the nondirect optical transition characteristics in the photon-enhanced field emission spectra

A heterostructured cathode having a graded bandgap window-layer is proposed in this study to improve the performance of photon-enhanced thermionic emission (PETE) solar energy converters. A model based on a one-dimensional steady-state equation is developed to analyze the characteristics of the proposed device. This model is used to calculate the conversion efficiency of a PETE device with an AlxGa1−xAs/GaAs cathode. The built-in electric field induced by the bandgap gradation in the window-layer is shown to improve the efficiency because of the reduction of contact surface recombination losses and efficient collection of photogenerated electrons. This field is unaffected by temperature variation, and the improvement of efficiency is sustained at elevated temperatures. The effect of window-layer parameters and cathode thickness on efficiency is discussed. We also show that the efficiency maximizes at lower temperatures for cathodes with lower electron affinities. Moreover an optimal value exists for anode work function at a given anode temperature, thus providing guidance for the optimum design of barrier conditions for both cathode and anode.