Thermoelectric properties of orthorhombic silicon allotrope Si (oP32) from first-principles calculations* *Project supported by the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT13093), the National Natural Science Foundation of China (Grant Nos. 11304262 and 11404275), the Scientific Research Fund of Hunan Provincial Education Department, China (Grant Nos. 17B252, 17K086, and 16K084), the Natural Science

Abstract

The diamond-like cubic silicon (d-Si) is widely used in modern electronics and solar cell industries. However, it is not an optimal candidate for thermoelectric application due to its high lattice thermal conductivity. Si (oP32) is a recently predicted orthorhombic silicon allotrope, whose total energy is close to that of d-Si. Using first-principles calculations and Boltzmann transport theory, we systematically investigate the thermoelectric properties of Si (oP32). The lower phonon thermal conductivity and higher power factor are obtained in Si (oP32) than those in diamond silicon. The low phonon thermal conductivity (33.77 W/mK at 300 K) is mainly due to the reduction of the phonon group velocity and enhancement of phonon–phonon scattering (including scattering phase space and strength). Meanwhile, the results also show that the thermoelectric performance along the zz lattice direction is better than that along the xx and yy lattice directions, and the figure of merit (700 K) along the zz lattice direction could approach to 2.45 and 1.75 for p-type and n-type Si (oP32), respectively. The values are much higher than those of d-Si (about 0.06)) and Si24 (0.6), indicating that the Si (oP32) is a promising candidate for thermoelectric applications. Our theoretical studies shed light on the thermoelectric properties of Si (oP32) and could stimulate further experimental studies.