LAUSR

Quasi‐2D semiconductors have garnered immense research interest for next‐generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi‐2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi‐2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron–phonon coupling strength mediated by phonons with purely out‐of‐plane vibrational vectors. This is demonstrated in ZrBeSi‐type quasi‐2D systems, where the representative sample Ba1.01AgSb shows a high room‐temperature hole mobility of 344 cm2 V−1 S−1, a record value among quasi‐2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p‐type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi‐type compounds. This work uncovers the relation between electron–phonon coupling and crystal symmetry in quasi‐2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.