Precise synthesis of copper selenide nanowires with tailored Cu vacancies through photo-induced reduction for thermoelectric applications.

Nanostructuring in α-Cu2Se while optimizing carrier concentration holds the promise of realizing further high thermoelectric performance at near room temperature. Nevertheless, controlling the amounts of Cu vacancies, which work as acceptors, in nanostructures is considerably more intricate than in bulk materials. Hence, controlling the amounts of Cu vacancies while maintaining the α-phase and nanostructure shape poses a formidable challenge. In this study, we synthesized Cu2+x Se nanowires (NWs) with various amounts of Cu vacancies at room temperature by the photoreduction method and investigated their thermoelectric properties. Cu2+x Se NWs exhibited a comparable thermoelectric power factor to that of the polycrystalline films fabricated at higher temperature. The achievement of the high power factor despite low-temperature fabrication is attributed to the precise synthesis of Cu2+x Se NWs with various amounts of Cu vacancies. We also investigated the reaction process of Cu2.00Se NWs in detail by observing the reaction intermediates. It was found that photoreduction occurred with Cu2+ ions adsorbed on Se NWs, leading to the reaction of Cu2+ ions and Se NWs without Cu deficiency. Namely, this photoreduction under the adsorbed conditions realized the control of Cu vacancies in Cu2+x Se NWs.

Thermoelectric Properties of PEDOT:PSS Containing Connected Copper Selenide Nanowires Synthesized by the Photoreduction Method.

Organic materials have attracted attention for thermoelectric materials reusing low-temperature waste heat. For the thermoelectric performance enhancement of organic materials, the introduction of inorganic nanowires is effective due to the percolation effect. In this study, we synthesized Cu2Se NWs by the photoreduction method and prepared poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) thin films containing Cu2Se NWs by spin-coating PEDOT:PSS and Cu2Se NWs alternatively. The composite films exhibited a drastic increase in electrical conductivity at more than 40 wt % Cu2Se, and the Cu2Se amount threshold was in good agreement with surface structures as observed by a scanning electron microscope. This indicates that the percolation effect of connected Cu2Se NWs brought high electrical conductivity. As a result, the composite thin films exhibited a higher power factor than the PEDOT:PSS film. This power factor enhancement by the percolation effect would be expected to contribute to the development of thermoelectric performance enhancement for organic materials.