Start Date: September 2008
Nick Melosh, Department of Materials Science and Engineering; Zhi-Xun Shen, Department of Physics, Stanford University
This research aims to explore a novel approach to increase the energy conversion efficiency of thermionic devices to be implemented as topping cycles for solar thermal systems. The proposed Photon Enhanced Thermionic Emission (PETE) scheme uses a wide-bandgap semiconductor absorber under highly-concentrated sunlight where electrons are first promoted to the conduction band by photon absorption and subsequently emitted into vacuum by thermal excitation. This approach has the potential to overcome traditional barriers to the realization of efficient thermionic devices, such as the identification of appropriate electrode materials.
|Figure 1:||(a) Illustration of a PETE device using a semiconductor cathode under concentrated sunlight.
(b) Energy diagram showing the three fundamental steps of the PETE process: 1 – photon excitation from the valence (VB) to the conduction band (CB); 2 – electron diffusion to the cathode surface and thermal excitation over the energy barrier ΦB with subsequent emission into the vacuum; 3 – charge collection at the anode. The output energy corresponds to the difference between the work-functions of the two electrodes: Eout ~ ΦC – ΦA
A very active research area from the 1950’s to the mid-1970’s, thermionic systems have received relatively limited attention in recent years due to numerous material and technical challenges – such as the lack of electrode materials with both low work-functions and low vapor pressures, a high operating temperature, and space-charge effects in vacuum limiting current extraction – resulting in poor performance and high fabrication cost. This research investigates a potentially breakthrough approach to enhance the conversion efficiency of thermionic systems by relying upon two fundamental processes: thermionic emission and the photoelectric effect.
As illustrated in Fig. 1, charges in a semiconducting cathode illuminated with concentrated sunlight can undergo a two-stage excitation process that firstly results in the promotion of valence-band electrons to the conduction band by photon absorption and then in thermionic emission through thermal excitation at the cathode surface. While charge emission is limited when using either of these two processes individually, their combination can create hot electron populations able to cross over the vacuum energy barrier even in materials with work-functions >2eV, thus providing substantially high operating voltage than possible with a conventional thermionic device at comparable current density. Additionally, in the PETE approach, energy losses due to the thermalization of hot electrons created by the absorption of photons with energy >ECB-EVB or to charge recombination, are fully recycled to maintain the cathode temperature. For this reason, the thermodynamic efficiency of PETE systems is higher than the Shockley-Queisser efficiency limit for single-junction photovoltaics, and it is expected that PETE devices can be demonstrated with conversion efficiencies of 15-20% at operating cathode temperatures of ~600-800oC.
This program aims at developing a system to demonstrate the potential of the PETE concept. Research encompasses theoretical and experimental investigations of the fundamental physical processes under various operating conditions, the development of a surface plasmon interface to maximize photon absorption, the study of nanostructured cathode materials to enhance the quantum yield, the engineering of the emitting cathode surface to decrease its work-function (as illustrated in Fig. 1b by the band bending at the cathode surface), and the design of an operating device.
The first step to realizing a proof-of-concept PETE device is identifying potential cathode material candidates with the following properties: low toxicity, large scale availability, bandgaps in the range of 1-2eV, good thermal stability (>600K), high photoemission efficiency, and low electron affinities. Candidates under scrutiny include silicon, silicon carbide, gallium arsenide, doped diamond, and silicon-germanium compounds. Nanostructures based on these materials in particular will be investigated for the potential of high quantum efficiency. Some of these materials have relatively low light absorption cross-sections. Therefore approaches are being considered to enhance photon absorption. These include surface texturing and the use of nanostructured cathode materials, and the implementation of photonic systems using surface plasmon resonance to absorb and concentrate light into the semiconductor absorber.
Various surface treatment approaches are being investigated to decrease the work-function of the cathode. In particular, electron emission is being measured as a function of operating temperature in the presence of specific surface adsorbates such as rare-earth metals or diamond, and as a function of surface texturing.
Finally, working PETE devices are being fabricated and their integration with existing solar thermal designs is being evaluated. Preliminary calculations show that their utilization as topping cycles for Dish/Stirling systems could potentially result in a ~45% increase in overall efficiency.
Different PETE schemes are also being evaluated in this project, such as thermal enhancement of photoelectric emission from a metallic cathode, or the use of low-bandgap semiconductor cathodes where thermally excited free electrons in the conduction band would be promoted above the vacuum energy barrier by photon absorption.