Ductile Polycrystalline AgCuSe With Preferred Orientation for High Power Density Flexible Thermoelectric Device

The recently discovered ductile inorganic thermoelectric (TE) materials provide a promising solution for developing flexible TE devices with high output power density used in wearable electronics. However, the current ductile polycrystalline TE materials are still very rare. In this work, we show that engineering a polycrystal with a strong preferred orientation along the crystal plane with a high ratio of cleavage energy to slip barrier energy can convert it from brittle into ductile. Taking AgCuSe as a case study, via controlling the rolling temperature and speed during the hot‐rolling process, the polycrystalline AgCuSe with (0 k 0) preferred orientation is successfully obtained, which exhibits excellent ductility at room temperature. Likewise, the ductile AgCuSe retains the good TE performance of brittle AgCuSe, with the room‐temperature power factor (∼21.7 µW cm −1 K −2 ) much higher than the best flexible AgCuSe‐based materials reported before. By using the ductile polycrystalline AgCuSe, an ultrathin flexible cross‐plane π‐shaped TE device is developed to show a maximum normalized power density up to 46 µW cm −2 K −2 , a record‐high value in flexible thermoelectrics. This work not only provides a novel ductile TE material for flexible thermoelectrics, but also offers an alternative route to discover more ductile inorganic materials by using the anisotropic mechanical properties.