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

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]m[Bi2Q3]n 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]m[Bi2Se3]n family effectively increases the interlayer carrier mobility (μH) and σ. Moreover, we propose an empirical index, F = Dpx,py(Bi0)/Dpx,py(Bi3+) along the kz 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 μH is maximized. Interestingly, Bi8Se7 possessing an optimal F = 1.06 is predicted to have the best μH in the [Bi2]m[Bi2Q3]n family. Our subsequent experiments confirm the as-synthesized Bi8Se7 exhibiting n-type behavior with a μH 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 Bi5.6Sb2.4Se5Te2 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 Bi5.6Sb2.4Se5Te2 may reach ∼1.2 at 425 K, suggesting a novel and promising n-type thermoelectric material near room temperature.