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 SrTiO 3 can largely reduce κ and enhance thermometric efficiency ( ZT ) without heavy elements. In this paper, we succeeded in maximizing the ZT of SrTiO 3− x H x by applying topochemical reaction directly to SrTiO 3 epitaxial films with CaH 2 , which realized wide‐range control of carrier concentration ( n e ) from 1.5 × 10 20 cm −3 to 4.1 × 10 21 cm −3 . The power factor (PF) showed a dome‐shaped behavior with respect to n e , and the maximum PF = 22.5 μW/(cmK 2 ) was obtained at the optimal n e = 3.4 × 10 20 cm −3 . Carrier transport analyses clarified that the carrier mobility was limited by impurity scattering of H‐related impurities in the SrTiO 3− x H x films, while the hydrogen substitution induced a much lower κ of 4.6 W/(mK) than other heavy‐element substituted Sr 1− x La x TiO 3 and SrTi 1− x Nb x O 3 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− x H x film with the hydrogen concentration of 1.2 × 10 21 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.