Optimizing the Thermoelectric Performance of Ag₂S-Based Plastic Inorganic Semiconductors for Flexible Thermoelectric Generators

The pursuit of high-performance flexible thermoelectric generators (FTEGs) for wearable electronics is facing a great challenge, and thermoelectric semiconductors should show both high thermoelectric efficiency and robust mechanical flexibility. Recently, ductile inorganic thermoelectric materials have shown promising plasticity, which provides good processability and moderate thermoelectric properties in both n- and p-type solid solutions. In this work, we first improved the thermoelectric performance of n-type Ag2Te0.2S0.8 by alloying Se at the S site, resulting in a high zT value of 0.62 at room temperature for the Ag2Te0.2S0.1Se0.7 sample. The transition from the cubic phase to the monoclinic phase when alloying Se at the S site should be responsible for the improved thermoelectric performance and the well-maintained plasticity, which has been proven by the calculated lattice spacing. Then, the thermoelectric properties of p-type AgCuX (X = S, Se, Te) solid solutions were also improved by alloying S at the Se site and introducing tiny vacancies, showing a high zT value of 0.42 at room temperature for the (AgCu)0.996Se0.18S0.08Te0.74 sample. Finally, based on the warm rolling forming thick films with the optimized n- and p-type compositions, we fabricated an 18-couple FTEG with an exceptional thickness of 0.7 mm. The open circuit voltage and output power of the flexible device reached 2.32 mV and 8.81 μW at ΔT = 32.1 K, showing a high normalized power density. Crucially, due to the robust mechanical flexibility of these ductile thermoelectric materials and the flexible polyimide substrates, the variation of the internal resistance for the flexible device is within 1% after 1000 bending cycles. This work provides an effective strategy of simultaneously improving the thermoelectric performance and mechanical flexibility by tuning the crystal structures and also shows the possible applications of ductile inorganic semiconductors for wearable electronics.