Enhancement of thermoelectric power in layered Bi₂Sr₂Co_2−x Ir _x O _y single crystals

We report on the structural, electrical, and thermal transport properties of the single crystals of Bi₂Sr₂Co_2−x Ir _x O _y (0 ≤ x ≤ 0.2). Large-sized (centimeter-level) and good-quality single crystals were grown by a modified flux method. The substitution of Ir ions for Co ones makes the in-plane resistivity ρ _ab increase monotonically, whereas, the in-plane thermopower S _ab initially increases and then decreases as x is larger than 0.1. Therefore, among all samples, the in-plane power factor P _ab (=S _ab ² /ρ _ab) of Bi₂Sr₂Co_1.9Ir_0.1O _y would reach a maximum value of 116.4 μWm⁻¹K⁻² at 300 K and shows a significant improvement of about 20 % compared to that of the parent sample. Our results indicate that an appropriate doping of Ir ions at Co-sites may provide an effective way to enhance the thermoelectric performance of Bi₂Sr₂Co₂O _y system.