Thermionic Emission Solar Energy Research Articles

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.

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.

Photon-enhanced thermionic emission solar energy converters with GaAs wire array cathode under external electric field

In this work, a theoretical emission model for GaAs wire array cathode based on photon-enhanced thermionic emission (PETE) under the action of external electric field is deduced utilizing two-dimensional continuity equations. With the electron energy distribution and elevation angle of emitted electron considered, the electron collection probability for each emission surface of GaAs wire array cathode varying with the field intensity is simulated. Combining emission current density with electron collection probability, the effective collection current density of GaAs wire array cathode is obtained. Results suggest that the external electric field can effectively enhance the collection probability of emitted electrons within GaAs wire array, which contributes to the improvement of the actual photoelectric conversion capability of GaAs wire array cathodes. For GaAs wire array cathodes, the effective collection current density can reach the maximum value with the incident angle of 20° and field intensity of 0.9 V/μm. Applying a transparent phosphorus-doped diamond film as the anode material, the simulated conversion efficiency increases from 18.85 to 44.80% as the electron affinity of GaAs wire cathode rises from 0 to 0.6 eV.

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.

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.

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.

指数掺杂GaAs阴极光子增强热电子发射型太阳能转换器转化效率理论计算

Photon-enhanced thermionic emission solar energy converter is a new highly efficient solar energy utilization technologies.An exponential doping GaAs material was presented as the cathode of photon-enhanced thermionic emission solar converter.The conversion efficiencies of the photonenhanced thermionic emission solar devices with exponential doping GaAs were theoretically analyzed by using the energy-balance and diffusion-drift-emission models.The results show that exponential doping GaAs prominently enhances photon-enhanced thermionic emission conversion,and the efficiency of photon-enhanced thermionic emission solar converters with exponential doping GaAs cathodes is a monotonically increasing function of sun concentration and a monotonically decreasing function of cathode surface recombination velocities.The efficiency of the converter can reach 30%at a flux concentration larger than 200suns and surface recombination velocities maintained at fewer than 104 cm/s.

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%.