Unlocking Room‐Temperature Thermoelectric and Flexibility in Freestanding Single‐Crystalline SnSe Membrane through Metastable‐Phase Engineering

Abstract Despite SnSe's high thermoelectric performance, its application in flexible devices is limited by a negligible room‐temperature power factor ( PF ) and poor flexibility from inherent rigidity. Here, first‐principles calculations of Gibbs free energy and phonon spectra for SnSe phases revealed that metastable F m3m‐SnSe exhibits relative stability. Freestanding single‐crystalline metastable F m3m‐SnSe membranes are then fabricated. These membranes exhibit a remarkable room‐temperature PF of 2.39 µW cm −1 K −2 , nearly two orders of magnitude higher than that of stable P nma‐phase SnSe films (0.03 µW cm −1 K −2 ). Notably, the freestanding F m3m‐SnSe membranes demonstrate exceptional mechanical robustness: they spontaneously curl into tubular architectures and retain over 80% of their electrical conductivity after 200 bending cycles (bending radius: 12 mm). Microstructural analyses confirm polycrystalline P nma‐SnSe films suffer from grain boundary cracking under bending due to stress concentration, but single‐crystal F m3m‐SnSe membranes—without grain boundaries to concentrate stress—exhibit better flexibility and resistance to cracking. This work establishes freestanding single‐crystalline metastable F m3m‐SnSe as a highly efficient and flexible thermoelectric material and provides a fundamental strategy for designing room‐temperature SnSe‐based self‐powered flexible electronics.