Boosts thermoelectric performance of Al/Na co-doped polycrystalline SnSe via intermediate band and multi-scale defect engineering

Thermoelectric (TE) materials have great potential in the energy recovery and environmental protection. Single crystal tin selenide (SnSe) demonstrates advantaged TE performance across a broad temperature range, but it is easy to form mechanical cracks and difficult to apply in devices. Poly-crystallization effectively enhances its mechanical properties but severely limits the hole transport reducing TE performance. Here, we provide an efficient strategy to increase hole concentration and introduce intermediate band for enhancing the electrical performance of polycrystalline SnSe in its advantaged temperature range via Al/Na co-doping. Specifically, Na dopant increases the hole concentration from 2.60 × 10¹⁷ cm⁻³ to 1.20 × 10¹⁹ cm⁻³, while Al dopant introduces intermediate band to reduce the thermal excitation temperature and promote the hole transition. As a result, the power factor of Al_0.01Na_0.01Sn_0.98Se reaches to 10.78 μW cm⁻¹ K⁻² at 823 K. In addition, we used the volatilization of carbonate to introduce dislocations and point defects in SnSe. The multi-scale defects effectively scattered phonons, making the thermal conductivity of 0.39 W m⁻¹ K⁻¹ is achieved in Al_0.03Na_0.01Sn_0.96Se. Benefit from the optimization strategies of both electrical and thermal performance, a state-of-the-art peak ZT of ∼1.73 is achieved in Al_0.01Na_0.01Sn_0.98Se. This work reveals the key roles of intermediate bands and dislocations in regulating the thermal excitation temperature and anisotropic thermal conductivity of SnSe, and it provides a new idea for improving the TE performance of SnSe-based materials.