Thermoelectric properties of four typical silicon allotropes

Abstract

M585-Si, mC12-Si, oC12-Si and Pbam24-Si are four typical silicon allotropes which are predicted to possess direct or quasi-direct band gap and better optical absorption coefficient than that of diamond Si (d-Si), foreshowing they are promising candidates for future thin-film photovoltaic devices. For realizing the recombination of photovoltaic and thermoelectric technologies, in this paper, we systematically investigate the thermoelectric properties by combining first-principles calculation and Boltzmann transport theory. The calculations show that the four typical silicon allotropes possess higher Seebeck coefficient (which is almost three to four times as large as that of d-Si), meanwhile anisotropic electronic conductivity and electronic thermal conductivity of these silicon allotropes are found as well. For the thermal transport properties, the four typical silicon allotropes present lower lattice thermal conductivity. At room temperature, the average thermal lattice conductivity of the four silicon allotropes is about 49.6 W m−1 K−1 (M585-Si), 54.7 W m−1 K−1 (mC12-Si), 45.1 W m−1 K−1 (oC12-Si) and 52.2 W m−1 K−1 (Pbam24-Si), respectively. Moreover, the characteristic phonon mean free path of these four silicon allotropes was also predicted. Based on the empirical electronic scattering time, the room temperature maximum thermoelectric figure of merit (ZT) of the M585-Si, mC12-Si, oC12-Si and Pbam24-Si can be as high as 0.68, 0.45, 0.34 and 0.42, respectively, which is substantially larger than that of d-Si (ZT ≈ 0.01). Better thermoelectric performance could be obtained in high temperature region (700 K), and the M585-Si along yy crystal direction is observed as large as 2.3 (1.6) with n (p)-type doping. The results presented in this work elucidate the thermoelectric performance of the four typical silicon allotropes and indicate that all of them are potential multifunctional materials for realizing the recombination of photovoltaic and thermoelectric technologies.