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This study shows a method of enhancing the thermoelectric properties ofGeTe-based materials by Ti and Bi co-doping on cation sites along withself-doping with Ge via simultaneous optimization of electronic (via crystalfield engineering, and precise Fermi level optimization) and thermal (viapoint-defect scattering) transport properties. The pristine GeTe possesses highcarrier concentration ($n$) due to intrinsic Ge vacancies, low Seebeckcoefficient ($\alpha$), and high thermal conductivity ($\kappa$). The Gevacancy optimization and crystal field engineering results in an enhanced$\alpha$ via excess Ge and Ti doping, which is further improved by bandstructure engineering through Bi doping. As a result of improved $\alpha$ andoptimized Fermi level (carrier concentration), an enhanced power factor($\alpha^2\sigma$) is obtained for Ti--Bi co-doped Ge$_{1.01}$Te. Theseexperimental results are also evidenced by theoretical calculations of bandstructure, and thermoelectric parameters using density functional theory andBoltztrap calculations, respectively. A significant reduction in the phononthermal 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 showa decreasing group velocity in Ti--Bi co-doped GeTe, supporting the obtainedreduced $\kappa_{ph}$. The strategies used in the present study cansignificantly increase the effective mass, optimize the carrier concentration,and decrease phonon thermal conductivity while achieving an impressive maximumzT value of 1.75 at 773\,K and average zT (zT$_{av}$) of 1.03 forGe$_{0.91}$Ti$_{0.02}$Bi$_{0.08}$Te over a temperature range of 300-773\,K.

Effect of crystal field engineering and Fermi level optimization on  thermoelectric properties of Ge$_{1.01}$Te: Experimental investigation and  theoretical insight