Ag ₂ Se Polymorphs: Structure Determination and Variations in Thermoelectric Properties

The development of high-performance, flexible thermoelectric (TE) materials is essential for advancing self-powered wearable electronics. Among n-type candidates near room temperature, Ag2Se has attracted considerable interest, but its polymorphism─especially the poorly understood metastable phase─has hindered structure–property correlations and materials optimization. In this work, we achieve phase-selective hydrothermal synthesis of Ag2Se nanocrystal polymorphs by tuning the molecular weight of polyvinylpyrrolidone and the polarity of the solvent. Using three-dimensional electron diffraction, we resolve the long-debated structure of the metastable phase as monoclinic γ-Ag2Se (space group P21/n). Differential scanning calorimetry and in situ three-dimensional electron diffraction further reveal that both orthorhombic β-Ag2Se and monoclinic γ-Ag2Se transform directly into the high-temperature cubic α-Ag2Se without interconversion. To establish structure–property relationships, we fabricate thin films of both polymorphs under identical conditions and find that β-Ag2Se exhibits electrical conductivity and power factor values over 2 orders of magnitude higher than those of γ-Ag2Se. First-principles calculations attribute the superior performance of β-Ag2Se to its more compact three-dimensional Ag–Ag framework and noncentrosymmetric structure, which result in a narrower band gap and smaller carrier effective mass. Further optimization through mechanical pressing and polymer content adjustment yields a maximum power factor of ∼466 μW·m–1·K–2 in β-Ag2Se films. Together, these results establish a clear link between synthetic parameters, polymorphic structure, and thermoelectric transport, providing a robust framework for the rational design of efficient, flexible thermoelectric materials. This work not only clarifies the structural identity of γ-Ag2Se but also advances solution-based strategies for high-performance thermoelectric systems.