Enhancing thermoelectric efficiency of Bi₂Se₃ alloys via significant reduction of lattice thermal conductivity by complementary defect structure formed by heavy Fe addition

Mid-temperature thermoelectric applications require materials with stable performance without bipolar conduction degradation. Bi₂Se₃ is attractive due to its wider bandgap (∼0.25 eV) compared to Bi₂Te₃ (∼0.15 eV), but simultaneous optimization of electrical and thermal transport remains challenging. Here we demonstrate that heavy Fe addition in Bi₂Se₃ up to 6 at% enables simultaneous optimization of thermoelectric transport properties through complementary mechanisms: enhanced electrical conductivity via increased carrier concentration and reduced lattice thermal conductivity via in-situ FeSe₂ formation. Fe incorporation creates multiple beneficial effects: enhanced electrical conductivity through increased carrier concentration via multiple Fe valence states (Fe^{2 +}/Fe³⁺) and formation of FeSe₂ precipitates that scatter heat-carrying phonons while preserving electron transport. The optimized Fe-doped Bi₂Se₃ achieves a 57 % improvement in thermoelectric performance, reaching a figure of merit zT of 0.30 at 520 K. This enhancement results from simultaneous increases in power factor to 0.77 mW/mK² and reductions in thermal conductivity by 35 % (from 1.15 to 0.74 W/mK) through complementary frequency-dependent scattering mechanisms involving Fe point defects and FeSe₂ inclusions. Our approach demonstrates that controlled impurity engineering can effectively decouple electronic and thermal transport, providing a general strategy for developing high-performance thermoelectric materials suitable.