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 $\ensuremath{\alpha}$, and high thermal conductivity $\ensuremath{\kappa}$. The Ge vacancy optimization and crystal field engineering result in an enhanced $\ensuremath{\alpha}$ via excess Ge and Ti doping, which is further improved by band structure engineering through Bi doping. As a result of improved $\ensuremath{\alpha}$ and the optimized Fermi level (carrier concentration), an enhanced power factor ${\ensuremath{\alpha}}^{2}\ensuremath{\sigma}$ is obtained for Ti-Bi codoped ${\mathrm{Ge}}_{1.01}\mathrm{Te}$. 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 ${\ensuremath{\kappa}}_{\mathrm{ph}}$ from $\ensuremath{\sim}$3.5 to $\ensuremath{\sim}$1.06 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$ 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 ${\ensuremath{\kappa}}_{\mathrm{ph}}$ 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 ${\ensuremath{\kappa}}_{\mathrm{ph}}$. 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 ${\mathrm{Ge}}_{0.91}{\mathrm{Ti}}_{0.02}{\mathrm{Bi}}_{0.08}\mathrm{Te}$ over a temperature range of 300--773 K.