Thermoelectric performance enhancement of environmentally-friendly SrTiO3 epitaxial films by hydrogen substitution

Abstract

Developing high-efficiency and environmentally-friendly thermoelectric materials has been a significant challenge. Conventional thermometric materials consist of heavy (toxic) elements to reduce thermal conductivity (κ), while we demonstrated light-element hydride anion (H⁻) substitution in SrTiO₃ can largely reduce κ and enhance thermometric efficiency (ZT) without heavy elements. In this paper, we succeeded in maximizing the ZT of SrTiO_3−xH_x by applying topochemical reaction directly to SrTiO₃ epitaxial films with CaH₂, which realized wide-range control of carrier concentration (n_e) from 1.5 × 10²⁰ cm⁻³ to 4.1 × 10²¹ cm⁻³. The power factor (PF) showed a dome-shaped behavior with respect to n_e, and the maximum PF = 22.5 μW/(cmK²) was obtained at the optimal n_e = 3.4 × 10²⁰ cm⁻³. Carrier transport analyses clarified that the carrier mobility was limited by impurity scattering of H-related impurities in the SrTiO_3−xH_x films, while the hydrogen substitution induced a much lower κ of 4.6 W/(mK) than other heavy-element substituted Sr_1−xLa_xTiO₃ and SrTi_1−xNb_xO₃ films in the wide n_e range, resulting in the higher ZT value of 0.14 in maximum at room temperature. In addition, the ZT increased to 0.17 at 373 K due to the large decrease in κ for a SrTiO_3−xH_x film with the hydrogen concentration of 1.2 × 10²¹ cm⁻³. Further study on H⁻ substitution approach and modulation of the H state in transition metal oxides would lead to development of high ZT environmentally-friendly thermoelectric materials.