Unlocking Thermoelectric Performance in Cu 2 SnSe 3 Through Controlled Structural Ordering

Diamond‐like semiconductors offer a versatile platform for the design of highly efficient energy‐converting materials, owing to their tunable electronic structures and high charge carrier mobility. A thorough understanding and precise control of their structural complexity and defect chemistry are, however, essential for optimizing their transport properties. Here, we demonstrate that both the spatial arrangement of tetrahedral motifs and the polymorphic landscape of the diamond‐like semiconductor Cu 2 SnSe 3 can be precisely controlled through post‐synthesis annealing. Electron diffraction combined with precession‐assisted electron diffraction tomography reveals that structural disorder at the sub‐µm length scale, arising from the coexistence of monoclinic and cubic structures, promotes charge carrier trapping within the induced localized states. The degree of localization plays a key role in shaping the temperature dependence of the electronic properties, which evolve from amorphous‐like hopping conduction to conventional valence‐band transport as the annealing temperature increases from 573 to 947 K. Optimum hole mobility and thermoelectric performance are achieved once a single crystallographic phase is obtained by annealing above 873 K. These findings showcase the detrimental influence of local structural disorder on charge carrier transport, providing an additional degree of freedom for tailoring functional properties in crystalline semiconductors.