Contrasting strategies of optimizing carrier concentration in bulk InSe for enhanced thermoelectric performance

Indium selenide (InSe), as a wide-bandgap semiconductor, has received extensive attention in the flexible electronics field in recent years due to its exceptional plasticity and promising thermoelectric performance. However, the low carrier concentration severely limits its thermoelectric performance improvement. In this work, we conducted contrasting strategies that can be employed to increase the carrier concentration of InSe, including bandgap narrowing and heterovalent doping. Specifically, the carrier concentration initially increases as a result of the reduced bandgap upon Te alloying and then slightly decreases due to the weak electronegativity of Te. Whereas Br doping realizes high carrier concentration by pushing the Fermi level into the conduction bands and activating the multiple bands. On the other hand, both Te and Br obviously suppress the thermal conductivity due to the point defect scattering. By contrast, Br doping realizes a higher thermoelectric performance with a maximum ZT of ~ 0.13 at 773 K benefiting from the better optimization of carrier concentration. This work elucidates the strategies for enhancing carrier concentration at anion sites and demonstrates the high efficiency of halogen doping in InSe. Moreover, the carrier concentration of InSe is promising to be further optimized, and future work should focus on employing approaches such as cation doping or second-phase compositing. Graphical abstract: (Figure presented.)