Fe-mediated phase evolution and defect regulation in flexible Cu2Se films for enhanced thermoelectric transport

The monoclinic copper selenide (α-Cu2Se) is a promising thermoelectric (TE) material due to its ordered Cu sublattice and intrinsically low thermal conductivity for near-room-temperature applications. However, excessive Cu vacancies in α-Cu2Se lead to carrier overconcentration and limit TE performance, whereas the disordered Cu sublattice of β-Cu2Se provides a potential route for carrier transport regulation. In this work, flexible Fe-incorporated Cu2Se films were fabricated by combining low-temperature hydrothermal synthesis with screen printing and subsequent thermal annealing. Moderate Fe incorporation enables the controllable α/β phase evolution and defect landscape modulation, promoting structural reconstruction and optimizing Cu-related defect chemistry. Structural analysis is consistent with partial Fe occupation at Cu sites in β-Cu2Se, together with the formation of a crystalline Fe2O3 secondary phase. Such regulation exploits the complementary characteristics of ordered α-Cu2Se and defect-mediated β-Cu2Se, leading to a balanced carrier concentration and mobility relationship for enhanced thermoelectric transport. As a result, the optimized Fe-doped Cu2Se film exhibits a maximum power factor of 657 μW m−1 K−2 at room temperature. Furthermore, a flexible six-leg thermoelectric generator fabricated from the optimized film delivers an open-circuit voltage of ∼12 mV at ΔT = 40 K and a maximum power of 9.46 nW at ΔT = 29.1 K. This work provides insights into Fe-regulated α/β phase evolution and Cu-related defect engineering in flexible Cu2Se films, offering a promising strategy for developing near-room-temperature flexible thermoelectric generators.