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

Abstract

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.

Progress Toward High Power Output in Thermionic Energy Converters

“…[ 132,133 ] and was reviewed recently by Kribus and Segev [ 134 ] ; early related work was performed by Smestad. [ 135 ] While numerous experiments have explored emitter materials for PETE, [ 136–139 ] and several modeling studies have proposed optimal PETE configurations, [ 140–147 ] none have demonstrated complete energy conversion systems that can operate at elevated temperatures. [ 134 ] For instance, the heterostructure emitter quantum efficiency measurements conducted by Schwede et al.…”

Near-field enhanced thermionic energy conversion for renewable energy recycling

“…However, thermionic converters have not been widely implemented due to their general requirement of high cathode temperatures (i.e., >1500 K) and a low conversion efficiency below ∼15%, even at such high temperatures. As a novel technology to circumvent these challenges, photon-enhanced thermionic emission has been recently proposed and received keen attention [25][26][27][28][29][30]. In photon-enhanced thermionic emission, a p-type semiconductor cathode is illuminated by highintensity light to make use of photon energy absorption for thermionic emission.…”

Solar energy conversion with photon-enhanced thermionic emission

“…However, this should be modified with an emission probability, indicating that electrons having sufficient energy to reach the vacuum may still fail to do so. Measurement of the emission probability for a GaAs with CsO coating show that at the best coating coverage ratio (about 0.4 of a monolayer), emission probability is 12% for thermalized electrons, and 24% for hot electrons [43]. The authors suggest that this is due to fundamental effects that are inherent to any emitting surface, such as electron scattering at the semiconductor-vacuum interface, the influence of an interface potential barrier, or a low probability of a conversion of a Bloch electron in the crystal into a free electron in vacuum.…”