Bi₈Se₇: Delocalized Interlayer π-Bond Interactions Enhancing Carrier Mobility and Thermoelectric Performance near...

Environmental heat-to-electric energy conversion provides a promising solution to power sensors used for wearable and portable devices. Yet the near-room-temperature thermoelectric materials are extremely rare. The natural heterostructure [Bi₂][Bi₂Q₃] family provides an important platform to search and develop the cheaper and less toxic of such materials. However, the bottleneck problem in this family is how to enhance the interlayer electrical conductivity (σ). Herein, we uncover for the first time that the delocalized π-bond interaction between the stacking layers in the [Bi₂][Bi₂Se₃] family effectively increases the interlayer carrier mobility (μ_H) and σ. Moreover, we propose an empirical index, F = Dp,p(Bi⁰)/Dp,p(Bi³⁺) along the k direction in the Brillouin zone to evaluate the strength of the interlayer delocalized π-bond. F is optimized at a value of 1, under which μ_H is maximized. Interestingly, Bi₈Se₇ possessing an optimal F = 1.06 is predicted to have the best μ_H in the [Bi₂][Bi₂Q₃] family. Our subsequent experiments confirm the as-synthesized Bi₈Se₇ exhibiting n-type behavior with a μ_H value (33.08 cm²/(V s) at 300 K) that is higher than that of BiSe (26.19 cm²/(V s) at 300 K) and an enhanced σ value. Furthermore, the Te/Sb codoping, via varying the top of the valence band, significantly increases the Seebeck coefficient and eventually enhances the ZT value to ∼0.7 in Bi_5.6Sb_2.4Se₅Te₂ at 425 K along the hot-pressing direction, which is comparable to the optimized value of BiSe. According to the single parabolic band model prediction, the ZT of Bi_5.6Sb_2.4Se₅Te₂ may reach ∼1.2 at 425 K, suggesting a novel and promising n-type thermoelectric material near room temperature.