Revisiting Cobalt Dopability in GeTe System to Design Modulation-Doped Thermoelectrics

Dopability plays a pivotal role in determining the limit of carrier concentration and the chemical potential of semiconductor thermoelectric materials, which are directly related to the figure of merit. Here, the doping behavior and mechanism of cobalt (Co) in GeTe-based thermoelectric materials are first investigated. According to theoretical calculations and tentative experiments, the extremely hard Co dopability in GeTe system is ascribed to the formation of an insoluble intermetallic phase in eutectics, even though the point defect formation energy and charge transition level indicate a 3at.% doping limit. A two-step method is developed to synthesize (CoGe₂)_xGe_0.85Sb_0.10Te composited thermoelectric materials and use synchrotron technologies to investigate both average and local structures. CoGe₂ nanoprecipitates are observed endotaxially and uniformly embedded in Ge_0.85Sb_0.10Te matrix, which acts as an electron reservoir to optimize carrier concentration without deteriorating carrier mobility, conceiving an ideal modulation doping scheme. Moreover, the phonon mismatch at the semi-coherent CoGe₂/Ge_0.85Sb_0.10Te interfaces gives rise to the Kapitza resistance to impede phonon propagation. The synergistic manipulation of electronic and thermal transport leads to a desirable figure of merit of 2.2 at 775 K and a conversion efficiency of 8.2% under a temperature difference of 420 K, representing a promising performance in this field and providing a benchmark workflow to design composited thermoelectrics.