Nanoscale coherent chemical fluctuations driving superior thermoelectric performance in vacancy-tailored ductile...

(Ag,Cu)₂(S,Se,Te) alloys, as rare examples of p-type inorganic ductile semiconductors, have emerged as promising candidates for wearable electronics because of their mechanical flexibility and favorable processability. However, the multicomponent nature of this system introduces diverse microstructures and intricate defect landscapes that complicate the optimization of thermoelectric performance. Here, we reveal the presence of compositionally inhomogeneous yet structurally coherent nanoscale chemical fluctuations within this alloy system. These fluctuations, together with abundant dislocations and twin boundaries, serve as efficient phonon scattering centers to suppress the lattice thermal conductivity to approach glass-like levels. Concurrently, tuning the cation ratios and vacancies allows for precise control over the carrier concentration and thereby leads to enhanced power factors. Last, a peak zT of 0.98 at 360 kelvins is achieved, marking a record for Ag-based ductile thermoelectrics. This study highlights the essential role of chemical fluctuations in modulating phonon and charge transports and offers valuable insight into the microstructure-defect-property interplay in ductile materials.