Enhanced Cross-Plane Thermoelectric Transport of Rotationally Disordered SnSe₂ via Se-Vapor Annealing

We report cross-plane thermoelectric measurements of SnSe and SnSe₂ films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (-38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe₂ as the Se source in a sealed tube), a transition from SnSe to SnSe₂ is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from -38.6 to -631 μV/K) after Se annealing due to both the SnSe-to-SnSe₂ transition and the mitigation of unintentional doping by Se vacancies. We also observe a corresponding 6-fold drop in the electrical conductivity (from 3 to 0.5 S/m) after Se annealing, which is consistent with both a drop in the carrier concentration and an increase in band gap. The power factor S²σ increased by 44× (from 4.5 nW/m·K² to 0.2 μW/m·K²) after Se annealing. We believe that these results demonstrate a robust method for mitigating unintentional doping in a promising class of materials for thermoelectric applications.