Low work function thin film growth for high efficiency thermionic energy converter: Coupled Kelvin probe and photoemission study of potassium oxide

Abstract

Recent researches in thermal energy harvesting have revealed the remarkable efficiency of thermionic energy converters comprising very low work function electrodes. From room temperature and above, this kind of converter could supply low power devices such as autonomous sensor networks. In this type of thermoelectric converters, current injection is mainly governed by a mechanism of thermionic emission at the hot electrode which explains the interest for low work function coating materials. In particular, alkali metal oxides have been identified as excellent candidates for coating converter electrodes. This paper is devoted to the synthesis and characterization of potassium peroxide K2O2 onto silicon surfaces. To determine optimal synthesis conditions of K2O2, we present diagrams showing the different oxides as a function of temperature and oxygen pressure from which phase stability characteristics can be determined. From the experimental standpoint, we present results on the synthesis of potassium oxide under ultra high vacuum and controlled temperature. The resulting surface is characterized in situ by means of photoemission spectroscopy (PES) and contact potential difference (CPD) measurements. A work function of 1.35 eV is measured which and the expected efficiency of the corresponding converter is discussed. It is generally assumed that the decrease of the work function in the alkali/oxygen/silicon system, is attributed to the creation of a surface dipole resulting from a charge transfer between the alkali metal and oxygen.

Synthesis and characterization of low work function alkali oxide thin films for unconventional thermionic energy converters

“…A preliminary study to determine the optimal synthesis condition has been previously performed by our group. 7 Photoemission and thermionic emission measurements are performed under a base pressure of 3 Â 10 À9 mbar. In photoemission measurement, the sample is shone with monochromatic light and the photocurrent I ph is harvested by the Kelvin probe, held at 1.5 cm from the sample.…”

“…We propose potassium and caesium oxides as silicon electrode coating to reduce the electrode's work function. It is well known that both elements present very low work functions, 6 and it has been shown [7][8][9][10][11][12][13] that they can decrease the work function of the substrate they are adsorbed on. This paper reports the results on the synthesis and characterization of the two alkali metal oxides.…”

Progress Toward High Power Output in Thermionic Energy Converters

“…To date, most TEC prototypes have achieved low work functions for their metallic electrodes using a submonolayer alkali metal (usually cesium) coating, which partially transfers electron charge from the adsorbate to the substrate and allows surface dipoles to form that lower the vacuum energy level near the surface. [76][77][78][79]…”

Thermionic enhanced heat transfer in electronic devices based on 3D Dirac materials

“…Based on thermionic emission processes, two kinds of devices, namely thermionic generators and thermionic coolers, have been proposed 3,20,21 . Thermionic generators show promising applications in harvesting thermal energy 22,23 .…”