SEU made new progress in the field of thermoelectric conversion

Recently, Prof. Ni Zhenhua and Prof. Lyu Junpeng from School of Physics of SEU collaborated with Researcher Wu Jing from the Institute of Materials Engineering, Singapore Science and Technology Agency. They found that the application of gate voltage into a two-dimensional Bi2O2Se-based field effect transistor can regulate the scattering of polarized optical phonons to piezoelectric scattering, thus having realized the decoupling of Seebeck coefficient and conductivity, and achieved a high thermoelectric power factor featuring a wide temperature range. Relevant results were published in Advanced Materials, an important journal in the field of materials science, with the title of "Gate-Tunable Polar Optical Phonon to Piezoelectric Scattering in Few-Layer Bi2O2Se for High-Performance Thermoelectrics". 

The thermoelectric materials based on the Seebeck effect can realize the direct conversion of thermal and electric energy, and feature very important applications in the fields of green and clean energy as well as low-temperature refrigeration. How to improve the conversion efficiency of thermoelectric materials has always been the core issue of research in this field. The strong coupling between thermoelectric parameters makes it challenging to improve the thermoelectric performance of materials. For a long time, it has been very difficult to regulate the carrier’s scattering mechanism due to the mechanism complexity. Therefore, the carrier mobility that can independently enhance the thermoelectric performance (increase the conductivity without sacrificing the Seebeck coefficient) is usually overlooked.

Different from the widely studied two-dimensional materials such as graphene, transition metal sulfide, and black phosphorus, the low phonon group velocity and strong phonon anharmonic scattering in the two-dimensional Bi2O2Se make it to have extremely low thermal conductivity (~0.92 W/mK APL 115, 193103 (2019)). At the same time, its high electron mobility and sound environmental stability provide it with great potentials in the fields of thermoelectricity and energy conversion.

 Based on the thermoelectric transport of two-dimensional Bi2O2Se field-effect transistors, the carrier's mobility of two-dimensional Bi2O2Se is dominated by the polarized optical phonon scattering at high temperature and the piezoelectric scattering at low temperature. When the piezoelectric scattering is dominant, the carrier's mobility is significantly improved. At the same time, the sharp rise in conductivity did not lead to a significant decrease in Seebeck coefficient, indicating that the regulation of the scattering mechanism can decouple the conductivity and Seebeck coefficient, which is completely different from the previous strategy of balancing the conductivity and Seebeck coefficient by regulating the carrier’s concentration. 

At the same time, the transition temperature of this scattering mechanism has a high degree of gate voltage adjustability. By applying the gate voltage of certain amount, the transition temperature from polarized optical phonon scattering to piezoelectric scattering can be significantly increased. The thermoelectric power factor and the mobility exhibit an approximately linear correlation based on the modulation of two orders of magnitude, finally achieving a high thermoelectric power factor (>400mW-1m-1K-2) in a wide range of temperature (80-200K).

This work found that the scattering mechanism of two-dimensional Bi2O2Se adjusted by the gate voltage can effectively adjust its thermoelectric performance, which has further proved that the regulation of the scattering mechanism can well decouple the thermoelectric parameters. The high-gate tunability allows fine control of the scattering mechanism, thereby revealing more in-depth physical mechanism. It is of far-reaching significance for exploring the application of low-dimensional materials in low-temperature refrigeration and self-powered IoT.

The first author of this paper is Yang Fang, a doctoral student from School of Physics of Southeast University with Prof. Ni Zhenhua, Prof. Lyu Junpeng and Researcher Wu Jing as the co-corresponding authors. This work has been funded by the National Key Research and Development Program, the National Natural Science Foundation of China, and the basic scientific research expenses by institutions of higher-learning affiliated with central departments.