Anomalous N-Type and P-Type Photo-Thermoelectric Effect By Controlling Charge Transfer Direction in Platinum-Oxide and Tungsten-Oxide Layered Thin Film
Authors: Omura Kaichi , Shimoyama Kohei , Irie Hiroshi 
Published: December 2020

Approximately 60% of wasted heat originating from various sources, for example, electric power plants, factories, automobiles, and so on, is unused. Also there is a huge amount of unused, ambient heat around us. Therefore, thermoelectric (TE) power generation, which is a technology for directly recovering electric energy from unused, waste and ambient heat, has been drawing attention in recent years as next-generation energy.

The combined use of thermal and photon energies has led to a new research topic based on the changes in the TE properties, i.e., Seebeck coefficient (S) and electrical conductivity (σ), under light irradiation. Two different phenomena are involved in this change. The first is the temperature gradient caused by thermal energy. The other is the contribution of photon energy of light called the photo-TE effect. In our previous research, platinum (Pt)-dispersed tungsten trioxide (WO3) thin film (Pt- WO3) after photochromic (PC) and/or gaschromic (GC) reaction (Pt-dispersed protonated WO3, Pt- dispersed HyWO3)) showed an n-type anomalous photo-TE effect where both S and s increased when irradiated with visible light in a nitrogen (N2) and formate (HCOOH) atmosphere [1]. The anomalous photo-TE effect was caused by the accumulation of electrons in Pt induced by the charge transfer from HyWO3 to Pt. In this study, we prepared Pt oxide and WO3 layered thin films (PtOx/WO3) and attempted to develop the anomalous photo-thermoelectric (photo-TE) effect for both n-type and p-type by controlling the thickness of PtOx.

WO3 thin films were prepared on SiO2 substrate plates by a spin-coating using tungsten (VI) chloride as a tungsten source, being dissolved in ethanol, followed by drying on a hot plate. Then the spin-coated films were calcined in air at 500oC to form WO3 thin films. The thickness of the WO3 films was controlled to 650 nm. Pt species was deposited on the WO3 thin films, SiO2 substrates, and Si wafers by sputtering a Pt target under an argon atmosphere at room temperature. As discussed later, the deposited Pt species was a mixture of Pt and PtOx, which is referred to as PTO hereinafter. The thickness of PTO film was controlled at 50, 70, 90, 200 and 250 nm. The composition of the PTO were measured by X-ray photoelectron spectroscopy (XPS) on the surface and the inner region from the surface subjected in situ Ar+ etching of the PTO/WO3. PTO (50, 70, 90, 200, 250 nm) /WO3 that had been subjected to the GC reaction under a N2 flow containing HCOOH for 1 h was used for measuring the photo-TE effect. Here, HCOOH acted as a proton source and contributed to the GC reaction.

Photo-TE properties (σ
photo, S
photo) were measured under visible light irradiation generated from a Xe lamp equipped with an optical filter (Y-52, > 500 nm). The same measurement was performed in the dark after the light irradiation. The measurements under these conditions were repeated several times to confirm the reproducibility of the σ
photo and S
photo data. At this time, HCHO was used instead of HCOOH to avoid the GC reaction during the measurement. In contrast to n-type conduction in PTO/Si, p-type conduction was confirmed in PTO/SiO2. The p-type conduction of PTO on SiO2 was explained by the formation of PtOx (PtO and PtO2) near the interface between PTO and SiO2. In fact, it was already reported that PtOx has the p-type characteristics [2,3]. In both cases, σ
photo increased and the absolute value of S
photo decreased, which we call the normal photo-TE effect. In contrast, in the cases of PTO (50, 90, 200, 250 nm)/WO3 (in fact, HyWO3), both the σ
photo and S
photo increased, indicating the anomalous photo-TE effect. On top of that, PTO (90, 200, 250 nm)/WO3 (HyWO3) indicated the p-type anomalous photo-TE effect, whereas PTO (50 nm)/WO3 (HyWO3) n-type anomalous photo-TE effect. We considered that the directions of photo excited electron transfer from PTO to H
y
WO3 and from H
y
WO3 to PTO are responsible for p- and n-type, respectively, of the anomalous photo-TE effect. A detailed study will be discussed at the meeting.

References

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