Carrier Mobility Modulation in Cu 2 Se Composites Using Coherent Cu 4 TiSe 4 Inclusions Leads to Enhanced Thermoelectric Performance

Carrier transport engineering in bulk semiconductors using inclusion phases often results in the deterioration of carrier mobility (μ) owing to enhanced carrier scattering at phase boundaries. Here, we show by leveraging the temperature-induced structural transition between the α-Cu Se and β-Cu Se polymorphs that the incorporation of Cu TiSe inclusions within the Cu Se matrix results in a gradual large drop in the carrier mobility at temperatures below 400 K (α-Cu Se), whereas the carrier mobility remains unchanged at higher temperatures, where the β-Cu Se polymorph dominates. The sharp discrepancy in the electronic transport within the α-Cu Se and β-Cu Se matrices is associated with the formation of incoherent α-Cu Se/Cu TiSe interfaces, owing to the difference in their atomic structures and lattice parameters, which results in enhanced carrier scattering. In contrast, the similarity of the Se sublattices between β-Cu Se and Cu TiSe gives rise to coherent phase boundaries and good band alignment, which promote carrier transport across the interfaces. Interestingly, the different cation arrangements in Cu TiSe and β-Cu Se contribute to enhanced phonon scattering at the interfaces, which leads to a reduction in the lattice thermal conductivity. The large reduction in the total thermal conductivity while preserving the high power factor of β-Cu Se in the (1- )Cu Se/( )Cu TiSe composites results in an improved of 1.2 at 850 K, with an average of 0.84 (500-850 K) for the composite with = 0.01. This work highlights the importance of structural similarity between the matrix and inclusions when designing thermoelectric materials with improved energy conversion efficiency.