Charge Transport and Thermoelectric Properties of Layered BiCuOCh (Ch = Se, Te) compounds

Abstract

The demand for alternative energy of fossil fuel is becoming a major social issue. Thermoelectric materials have received a lot of attention recently as one of the key technologies to solve energy problems because they can directly and reversibly convert waste heat into electrical power. The efficiency of a thermoelectric material is determined by the dimensionless figure of merit ZT = S2σT/κ, where S, σ, T, and κ are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. All those parameters are interrelated with carrier concentration, band structure, and the dominant scattering mechanisms, for example, in terms of carrier concentration, the increase in carrier concentration leads to the increase in electrical conductivity, the decrease in Seebeck coefficient, and the increase in electronic thermal conductivity. Therefore, optimization of the carrier concentration is needed to improve the efficiency of a thermoelectric device. BiCuOCh (Ch =Se and Te) oxychalcogenide has been reported as a promising oxide thermoelectric material since 2010 due to intrinsically low thermal conductivity originated from its weak inter-layer bonding, low Youngs modulus, and anharmonicity and high Seebeck coefficient originated from the layered structure with alternately stacked (Bi2O2)2+ insulating and (Cu2Ch2)2- conducting layers along the c-axis. However, in case of BiCuOSe, the main limitation of undoped BiCuOSe for thermoelectric applications originates from its poor electrical conductivity. Therefore, enhancement of electrical conductivity has to be achieved in order to improve thermoelectric performances of BiCuOSe thermoelectric materials. First, we report the density of state effective mass and related charge transport properties in K-doped BiCuOSe. As compared with undoped BiCuOSe, simultaneous increase in both the carrier concentration and the Hall mobility was achieved in the K-doped BiCuOSe. The origin of the enhancement was discussed in terms of the two-band structure in the valence band of the BiCuOSe. The decrease in thermal conductivity with increasing K doping amounts in Bi1-xKxCuOSe was observed due to the phonon scattering by K atoms. With optimized hole concentration and lower thermal conductivity than other compounds, the highest ZT of 0.41 was obtained at 640K in Bi0.948K0.052CuOSe compound. Second, the effects of Bi deficiency on thermoelectric properties in Bi1-xCuOSe (x = 0-0.1) were investigated. The electrical conductivity of the compound increased with the increase in the Bi deficiency due to the increase in hole concentration by introducing Bi deficiencies. The drastic reduction in Seebeck coefficient was observed and it was discussed in terms of electronic structures calculated by density functional theory (DFT) of Bi1-xCuOSe compounds. Thermal conductivity increased due to the increase in hole concertation and reduced anharmonicity with increasing amounts of Bi-deficiency in the Bi1-xCuOSe compounds. The highest figure of merit (0.4 at 810K) was obtained for Bi0.975CuOSe through the optimized hole concentration, and this value was ~ 8% higher than that in the stoichiometric compound. Third, we investigated point defect-assisted doping mechanism and related thermoelectric transport properties in Pb-doped BiCuOTe compounds. The substitution of trivalent Bi3+ with divalent Pb2+ led to the generation of more than one hole per single Pb atom. The origin of the extra charge carrier was discussed in terms of the formation energy of p-type native point defects, and it could be evidenced by the density functional theory calculations. Related charge transport properties indicated that control of the native point defect is critical to achieve high thermoelectric performance in BiCuOTe material system. Finally, the effects of Pb doping on thermoelectric properties in Bi1-xPbxCuOSe0.8Te0.2 (x = 0-0.06) were investigated. The Pb doping in BiCuOSe0.8Te0.2 led to the increase in electrical and thermal conductivity and the decrease in Seebeck coefficient due to the increase in hole concentration. The generation of unexpected free hole was also observed in these compounds. It was confirmed that the change in temperature-dependent power factor of Bi1-xPbxCuOSe0.8Te0.2 compounds when comparing with temperature-dependent power factor of Bi1-xPbxCuOSe and Bi1-xPbxCuOTe compounds. The highest figure of merit of 1.06 at 910K was obtained for Bi0.94Pb0.06CuOSe0.8Te0.2 with enhance power factor.