Synthesis and characterization of polycrystalline SnSe/SnSe₂ heterophase thermoelectric thin films via chemical spray pyrolysis

In this study, we report the successful growth of polycrystalline SnSe/SnSe₂ heterophase thin films using single precursor compounds consisting of SnCl₂.2H₂O and 1,1-dimethyl l-2-selenourea via chemical spray pyrolysis. The films were deposited on soda-lime glass substrates within a narrow temperature range of 270–300 °C followed by annealing in vacuum at temperature in the range of 250–450 °C for 1 h. The structural and morphological properties of the films were investigated using X-ray diffraction (XRD), Raman mapping, high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The presence of both SnSe₂ (hexagonal) and SnSe (orthorhombic) phases was confirmed and their ratios were determined using the Reference Intensity Ratio (RIR) method from XRD data. This narrow deposition temperature induces intrinsic atomic defects (V_Sn, V_Se, Sn_Se) with lower formation energies during nucleation, enabling the formation and growth of SnSe/SnSe₂ heterophase. These defects act as electron donors, and in addition the energy band bending at the SnSe/SnSe₂ interface favors the formation of electron-rich regions showing n-type conductivity. The excess electrons contribute to a higher electron concentration leading to a higher Seebeck coefficient resulting in an improved power factor. Interestingly, the as-deposited films with a higher ratio of SnSe₂ phase exhibited an increased power factor compared to their annealed counterparts, indicating the importance of the preferential growth of SnSe₂ in enhancing carrier concentration and carrier mobility at the interfaces of the SnSe₂/SnSe heterophase.