Thermoelectric (TE) material is a type of functional material that can realize the direct conversion between thermal and electric energy. TE materials are used in TE power generation and electric refrigeration [[1], [2], [3]]. They are mainly used in deep space detectors, space detectors, industrial waste heat recovery and utilization, solar efficient photothermal–thermoelectric composite power generation, and so on [4,5]. The efficiency of thermoelectric power generation is mainly determined by the dimensionless figure of merit ZT (ZT=σS2Tk) of materials [[6], [7], [8]]. A promising semiconductor material must have a large Seebeck coefficient S, high electrical conductivity σ, and low thermal conductivity κ at a certain temperature T [[9], [10], [11], [12]]. Seebeck coefficient and conductivity can be obtained from the following formula:S=8π2kB23eh2m*T(π3n)23σ=neμ=ne2τm*where n is the carrier concentration, m* denotes the carrier effective mass, h represents the Planck constant, kB is the Boltz constant, μ refers to carrier mobility, e stands for the unit charge, and τ is the relaxation time [[13], [14], [15]]. Therefore, reducing the carrier concentration can increase the Seebeck coefficient, but it can reduce the electrical conductivity of the material [[16], [17], [18]]. Similarly, higher carrier concentration can greatly improves the electrical conductivity of the material, but at the same time, it reduces the Seebeck coefficient of the material [[19], [20], [21]]. Therefore, the effective mass and carrier concentration should be well optimized in order to obtained high-performance thermoelectric materials [22].

Bi2Se3 is a layered chalcogenide with a hexahedron structure and group space of R-3m. The crystal structure is composed of Bi atom and Se atom in the form of Se(1)-Bi-Se(2)-Bi-Se-Se(2)-Bi-Se(1), in the layered units, five atoms stacked along the crystallographic of c-axis and repeatedly arranged; The same kind of atoms occupy the same layer. Bi atoms form a two-dimensional planar hexagonal structure in Bi layer. There are two different types of Se(1) and Se(2) in Se layer. Se(1)-Se(1)layers are connected by Van der Waals force, Bi-Se(1) is combined by covalent bond and ionic bond, Bi-Se(2) is covalent bond, and Se atoms in Se (2) layer are surrounded by octahedron composed of Bi atoms. The thickness of the penta-atomic layer is about 1 nm. This natural nanostructure greatly reduces the lattice thermal conductivity of the material and is expected to result in higher TE performance. Compared with Bi2Te3 TE material, Bi2Se3 has the same crystal and energy band structure except for a larger band gap. Compared with rare and expensive Te, the Se has a rich storage capacity in earth. Therefore, the TE material of Bi2Se3 at a medium temperature has wider application than Bi2Te3 [[23], [24], [25]].

As a potential thermoelectric material, however, the TE performance of Bi2Se3 is relatively low due to its low carrier concentration. In order to further improve the electrical transport properties, in this work, Cl and K-doped Bi2Se3 thermoelectric materials were prepared by a high-temperature melting method in KCl solvent, and its structure and thermoelectric properties were studied deeply. The results show that the carrier concentration and mobility of doped Bi2Se3 are effectively improved, the maximum PF value of 13.7 mW cm−1·K−2 is obtained in the Bi2Se3 (KCl)3.5 sample at 500 K. The TEM detection results of Bi2Se3 (KCl)3.5 show that a large number of crystal defects, such as dislocations and distortions exist in the sample, which will enhance the scattering of phonons so that the lattice thermal conductivity of the sample is significantly reduced compared with the intrinsic sample. Finally, the maximum ZTvalue of 0.79 was obtained at 550 K.