Realizing n-type GeSe thermoelectrics: boosting solubility of donor dopants by enhancing crystal symmetry

The successful deployment of thermoelectric materials necessitates the concurrent development of high-performance p-type and n-type pairs situated within an identical matrix. Nevertheless, limiting by the low dopant solubility, the conventional doping often cannot transfer the Fermi level to the opposite carrier type. Here, the solubility limit of donor dopants was enhanced to achieve n-type GeSe by inducing additional cationic vacancies through raising crystal symmetry. Converting the intrinsic p-type nature of GeSe to n-type poses significant challenges, primarily due to the exceedingly low dopant solubility within its native orthorhombic structure. To overcome this, the In2Te3 alloying was initially employed to transition GeSe from orthorhombic to rhombohedral structure, simultaneously generating a large number of Ge vacancies. Following this, the introduction of Pb acts to mitigate the excessive Ge vacancies, steering the material toward a weak p-type character. Crucially, the elevated Ge vacancy concentration serves to extend the solubility limit of Bi donor dopant, which not only promotes the formation of cubic phase, but also enables the p–n type transition. As a result, a peak zT of 0.18 at 773 K was attained for the n-type cubic Ge0.55Bi0.2Pb0.25Se(In2Te3)0.1, marking an 18-fold enhancement in comparison with its n-type orthorhombic counterpart. This work attests to the efficacy of introducing vacancies through enhancing crystal symmetry as an effective means to expand dopant solubility, thereby offering valuable insights into the achievement of compatible p- and n-type chalcogenides within the same matrix.