THERMOELECTRIC TRANSPORT PROPERTIES OF LAYERED CHALCOGENIDES: CuAgSe, α-In2Se3, AND β-InSe

Abstract

Due to the urgency of our energy and environmental issues, a variety of cost-effective and pollution-free technologies have attracted considerable attention, among which thermoelectric technology has made enormous progress. Thermoelectric modules interconvert heat energy and electrical energy, work without mechanical movement, safe, low cost, and environmentally friendly green technology. The efficiency of these devices is still very low due to the limit of the thermoelectric figure of merit ZT of the material. In addition to the hundred years’ history of thermoelectric research, many efforts have been made to improve the efficiency of thermoelectric devices by improving the thermoelectric figure of merit ZT of materials. Unfortunately, the strong coupling between material parameters that determine thermoelectric efficiency, namely the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ, complicates the optimization of thermoelectric energy converters. To optimize these parameters for maximizing ZT, band engineering, complex crystal, nano-structures, nano-wire, nano-tube, superlattices approaches have been made aiming at the phonon glass electron crystal and phonon liquid electron crystal concepts. However, with persistent efforts, the number of thermoelectric materials has been dramatically increased over the last two decades. Layered chalcogenide (group VI: S, Se, Te) semiconductors have been more and more attention in a variety of energy applications, including solar cells, phase change memory, thermoelectric energy conversion. For applications in thermoelectric energy converters, layered chalcogenides have many advantages, for instance, reducing thermal conductivity with weak van der Waals bonding and heavy atomic weight, possessing a variety of structures, easily doped into p-type or n-type, and low operating costs. CuAgSe is a layered chalcogenide material that is also a robust candidate for the phonon liquid-like crystal concept with relatively high p-type and n-type ZT values (0.95 and 0.70, respectively). The stoichiometric CuAgSe has always been reported to show n-type conduction, while the p-type conduction is only present in the non-stoichiometric CuAgSe. However, it is difficult to synthesize the non-stoichiometric p-type CuAgSe without secondary phases. In 2009, single-crystalline and polycrystalline In4Se3 materials were reported with a high ZT of 1.53 at 425 C and 1.4 at 460 C. The physical properties of indium selenides were primarily determined by their intrinsic structural characteristics such as their composition, phase, crystal structure, and structural imperfections. From a structural point of view, indium selenides belong to a complex system with different stoichiometric ratios such as In4Se3, In2Se3, InSe, In6Se7, and In3Se4. However, the thermoelectric properties of In2Se3 and InSe single crystal have not been thoroughly investigated at low temperatures. In this study, the thermoelectric transport properties of the single-phase undoped, and Ni-, Co-, and Zn-doped poly-crystalline CuAgSe and the single-crystalline α-In2Se3 and β-InSe, which were grown by using the temperature gradient technique, were carried out. The carrier control in CuAgSe by growth process or doping was achieved. The maximum ZT value of 0.69 (0.56) at 623 K (673 K) in undoped n-type (p-type) CuAgSe was obtained. Zn is a promising dopant for n-type CuAgSe with a ZT value of 0.68 at 623 K. Additionally, the huge anisotropy during transport of α-In2Se3 single crystals were observed due to the anisotropic activation energy and anisotropic mobility between two directions. The β-InSe single-crystal exhibited an un-stability characteristic, which can be modified by thermal energy and potential energy.