In-doping induced resonant level and thermoelectric performance enhancement in n-type GeBi2Te4 single crystals with intrinsically low lattice thermal conductivity

•Growth of large-sized Ge1–xInxBi2Te4 single crystals by Bridgman method.•Ultralow κL of GeBi2Te4 driven by cation disorder and low phonon velocity.•Resonant level introduced by In-doping greatly improves electrical performance.•Achieving zTave of 0.36 in the range of 323–573 K for Ge0.97In0.03Bi2Te4 sample.