Carrier Mobility Modulation in Cu₂Se Composites Using Coherent Cu₄TiSe₄ Inclusions Leads to Enhanced Thermoelectric...

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-Cu₂Se and β-Cu₂Se polymorphs that the incorporation of Cu₄TiSe₄ inclusions within the Cu₂Se matrix results in a gradual large drop in the carrier mobility at temperatures below 400 K (α-Cu₂Se), whereas the carrier mobility remains unchanged at higher temperatures, where the β-Cu₂Se polymorph dominates. The sharp discrepancy in the electronic transport within the α-Cu₂Se and β-Cu₂Se matrices is associated with the formation of incoherent α-Cu₂Se/Cu₄TiSe₄ 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-Cu₂Se and Cu₄TiSe₄ gives rise to coherent phase boundaries and good band alignment, which promote carrier transport across the interfaces. Interestingly, the different cation arrangements in Cu₄TiSe₄ and β-Cu₂Se 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-Cu₂Se in the (1-x)Cu₂Se/(x)Cu₄TiSe₄ composites results in an improved ZT of 1.2 at 850 K, with an average ZT of 0.84 (500-850 K) for the composite with x = 0.01. This work highlights the importance of structural similarity between the matrix and inclusions when designing thermoelectric materials with improved energy conversion efficiency.