  This study shows a method of enhancing the thermoelectric properties of
GeTe-based materials by Ti and Bi co-doping on cation sites along with
self-doping with Ge via simultaneous optimization of electronic (via crystal
field engineering, and precise Fermi level optimization) and thermal (via
point-defect scattering) transport properties. The pristine GeTe possesses high
carrier concentration ($n$) due to intrinsic Ge vacancies, low Seebeck
coefficient ($\alpha$), and high thermal conductivity ($\kappa$). The Ge
vacancy optimization and crystal field engineering results 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
optimized Fermi level (carrier concentration), an enhanced power factor
($\alpha^2\sigma$) is obtained for Ti--Bi co-doped Ge$_{1.01}$Te. These
experimental results are also evidenced by theoretical calculations of band
structure, and thermoelectric parameters using density functional theory and
Boltztrap calculations, respectively. A significant reduction in the phonon
thermal conductivity ($\kappa_{ph}$) from $\sim$ 3.5 W.m$^{-1}$.K$^{-1}$ to
$\sim$ 1.06 W.m$^{-1}$.K$^{-1}$ at 300\,K for Ti--Bi co-doping in GeTe,
attributed to point-defect scattering due to mass and strain field fluctuation,
in line with the Debye-Callaway model. 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 can
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 average zT (zT$_{av}$) of 1.03 for
Ge$_{0.91}$Ti$_{0.02}$Bi$_{0.08}$Te over a temperature range of 300-773\,K.
