Effect of crystal field engineering and Fermi level optimization on thermoelectric properties of Ge1.01Te: Experimental investigation and theoretical insight

This study shows a method of enhancing the thermoelectric properties of GeTe-based materials through Ti and Bi codoping on cation sites along with self-doping of Ge via simultaneous optimization of electronic (via crystal field engineering and precise Fermi level optimization) and thermal (via point-defect scattering) transport properties. Pristine GeTe has high carrier concentration n due to intrinsic Ge vacancies, a low Seebeck coefficient alpha, and high thermal conductivity kappa. The Ge vacancy optimization and crystal field engineering result in an enhanced alpha via excess Ge and Ti doping, which is further improved by band structure engineering through Bi doping. As a result of improved alpha and the optimized Fermi level (carrier concentration), an enhanced power factor alpha(2)sigma is obtained for Ti-Bi codoped Ge1.01Te. These experimental results are also evidenced by theoretical calculations of band structure and thermoelectric parameters using density functional theory and BOLTZTRAP calculations. A significant reduction in the phonon thermal conductivity kappa(ph) from similar to 3.5 to similar to 1.06 W m(-1) K-1 at 300 K for Ti-Bi codoping in GeTe is attributed to point-defect scattering due to mass and strain field fluctuations. This decrease in kappa(ph) is in line with the Debye-Callaway model. Also, the phonon dispersion calculations show a decreasing group velocity in Ti-Bi co-doped GeTe, supporting the obtained reduced kappa(ph). The strategies used in the present study significantly increase the effective mass, optimize the carrier concentration, and decrease phonon thermal conductivity while achieving an impressive maximum zT value of 1.75 at 773 K and an average zT of 1.03 for Ge0.91Ti0.02Bi0.08Te over a temperature range of 300-773 K.