Abstract
Amorphization using impurity doping is a promising approach to improve the thermoelectric properties of tin-doped indium oxide (ITO) thin films. However, an abnormal phenomenon has been observed where an excessive concentration of doped atoms increases the lattice thermal conductivity (κl). To elucidate this paradox, we propose two hypotheses: (1) metal hydroxide formation due to the low bond enthalpy energy of O and metal atoms and (2) localized vibration due to excessive impurity doping. To verify these hypotheses, we doped ZnO and CeO2, which have low and high bond enthalpies with oxygen, respectively, into the ITO thin film. Regardless of the bond enthalpy energy, the κl values of the two thin films increased due to excessive doping. Fourier transform infrared spectroscopy was conducted to determine the metal hydroxide formation. There was no significant difference in wave absorbance originating from the OH stretching vibration. Therefore, the increase in κl due to the excessive doping was due to the formation of localized regions in the thin film. These results could be valuable for various applications using other transparent conductive oxides and guide the control of the properties of thin films.
Highlights
As regulations on carbon emissions are strengthened to prevent global warming, numerous studies of renewable energy are being conducted in various fields to achieve higher energy conversion efficiencies [1,2,3,4,5]
As a transparent TE material, tin-doped indium oxide (In2 O3 :SnO2, ITO) thin film is promising owing to its high electrical conductivity, chemical stability, low toxicity, and low price compared to TE alloys [12,13,14,15]
Numerous studies have been carried out to improve the physical properties of ITO, including chemical treatment, impurity doping, and heat treatment, to improve the efficiency compared to opaque TE materials [16,17,18,19,20]
Summary
As regulations on carbon emissions are strengthened to prevent global warming, numerous studies of renewable energy are being conducted in various fields to achieve higher energy conversion efficiencies [1,2,3,4,5]. To employ thermoelectric (TE) materials in various devices and fields, extensive studies have been carried out to improve the energy conversion efficiency of thin-film thermoelectric materials [6,7,8,9,10,11]. As a transparent TE material, tin-doped indium oxide (In2 O3 :SnO2 , ITO) thin film is promising owing to its high electrical conductivity, chemical stability, low toxicity, and low price compared to TE alloys [12,13,14,15]. Numerous studies have been carried out to improve the physical properties of ITO, including chemical treatment, impurity doping, and heat treatment, to improve the efficiency compared to opaque TE materials [16,17,18,19,20]. Heat treatment to improve the thin film crystallinity is not promising for ITO-based
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