A Universal Alkali Metal Doping Strategy Enabling High Thermoelectric Performance of IV‐VI Compounds via Phonon and Carrier Decoupling

ABSTRACT Thermoelectric performance is constrained by the coupling of phonon and carrier transport, which imposes a compromise between carrier mobility and lattice thermal conductivity. Here, we introduce a universal strategy of alkali‐metal (Na, K, Cs) doping for decoupling phonon and carrier transport in multiple IV‐VI compounds (including both n‐type PbTe and p‐type GeTe, SnTe, and TAGS), achieving highly competitive thermoelectric performance. For instance, Cs doping enables TAGS to attain a record‐high peak ZT of ∼2.1 at 723 K and an average ZT _ave of ∼1.53 from 300 to 723 K. Thermally, the larger alkali‐metal atoms induce more intense strain fields to enhance phonon scattering, while their heavy atomic mass softens phonon modes to reduce phonon group velocity. Electrically, the isovalent nature of alkali‐metal atoms minimizes ionized impurity scattering, thus preserving carrier transport. A thermoelectric module composed of optimized p‐type GeTe and n‐type PbTe achieves a conversion efficiency of ∼9.0% and a power density of 0.86 W cm ⁻² under a temperature difference of ∼370 K. This work establishes a general design principle for decoupling phonon and carrier transport in mid‐temperature thermoelectric systems.