Significantly (00l)-textured Ag2Se thin films with excellent thermoelectric performance for flexible power applications

Abstract

Flexible thermoelectric power generation is a competitive candidate for powering wearable electronic devices and chip-sensors for the internet-of-things. Nevertheless, the poor thermoelectric performance of n-type flexible thin films limits their application, demonstrating strong demand for improving its properties. In the present work, flexible n-type Ag₂Se thin films with an excellent power factor of 21.6 μW cm⁻¹ K⁻² at 348 K and the high ZT value ∼0.6 at 348 ∼ 363 K are obtained via rational micro-structural manipulations. The self-assembled thin films which grew on heated organic flexible substrates have a pure Ag₂Se phase and are well-crystallized, leading to ultra-high electrical conductivity. Highly (00l)-textured thin films are obtained when the Ag/Se ratio is closed to the ideal stoichiometry, which contributes to a higher Seebeck coefficient than that of the (013)-textured ones, according to the theoretical analysis. Meanwhile, slightly Ag-rich Ag₂Se thin films have low lattice thermal conductivity, resulting in moderate total thermal conductivity. Consequently, a flexible generator module with eight pairs of p–n legs is fabricated by using present n-type Ag₂Se and self-made p-type Sb₂Te₃ films. It displays a maximum power of ∼800 nW when applying a temperature difference of 50 K, and an output voltage of 9.8 mV is achieved from the conversion of the human body heat through wearable device testing. The bending test results show an increase of only 5% in device resistance along a rod with a radius higher than 7.0 mm after 1000 cycles, indicating its good flexibility. Therefore, the novel design concept demonstrated in this work can provide an effective way to achieve high thermoelectric performance in flexible thin films and devices.