Preparation And Performance Optimization Of C-axis-oriented BiCuSeO-based Thermoelectric Thin Films

The rural remote areas are economically underdeveloped areas in our country.The infrastructure of the power grid is generally relatively backward and large power grids cannot be fully covered everywhere.The traditional energy sources cannot meet the needs of local economic and social development.The development and utilization of renewable energy can not only meet the electricity demand of people and solve the energy problems in remote and poor areas that lack energy,but also can improve the effective utilization rate of resources and the quality of rural residents'life.Solar radiation energy can be used as the heat source of solar thermal power generation module.Thermoelectric materials can directly convert solar thermal energy into electricity by Seebeck effect.The study of thermal power generation technology is of great practical significance to solve the problem of power supply shortage in remote areas.However,the conversion efficiency of thermal power generation is mainly limited by the performance of thermoelectric materials.Therefore,it is of great significance to develop thermoelectric materials with good performances.BiCuSeO is a p-type semiconductor material with good thermoelectric properties,which is very promising in the thermoelectrics fields.BiCuSeO-based thin film fabrication is challenging due to the difficulty of controlling a stoichiometry transfer of such complex structures and the presence of volatile elements of Bi and Se.Over the past several years,extensive works have been done in enhancing the thermoelectric performance of BiCuSeO bulks.However,so far there have been little reports on the TE properties of BiCuSeO-based c-axis orientedepitaxial thin films.For miniaturizing the TE devices and optimizing the thermoelectric performances,nanoscale thin film may be advantageous because it is compatible with microelectromechanical system.Moreover,thin film TE devices can achieve very high cooling power densities and very fast cooling.In the present work,c-axis oriented epitaxial BiCuSeO thin films were fabricated by pulsed laser deposition?PLD?.The thermoelectric properties of the films were studied in detail and various strategies?such as introducing amorphous defects,element doping,nanocomposite thin films and ultra thin films?were adopted to optimize the thermoelectric performances of BiCuSeO-based materials,the main results are listed as below:1)BiCuSeO thin films were prepared on single crystal SrTiO₃?001?substrates by pulsed laser deposition.Detailed microstructural analysis revealed that the epitaxial relationships between the substrates and the films were[010]SrTiO₃//[010]BiCuSeO and[001]SrTiO₃//[-100]BiCuSeO.It could be found from the results of electrical properties test that the film exhibited a metallic-like conducting behavior over range of 20³50 K,with the room temperature resistivity of only 12.5 m?cm and Seebeck coefficient of 202?V K^-1.The calculated room temperature power factor?PF?of the films was about 0.32mW m^-1 K^-2,which was 2-6 times higher than that of the corresponding polycrystalline bulk materials.2)c-axis-textured BiCuSeO thin films with amount of amorphous grain boundaries were grown directly on amorphous glass substrates by pulsed laser deposition.We investigated the effect of the amorphous defects on the thermoelectric transport properties of thin films of BiCuSeO.As the temperature decrease from room temperature,the resistivity of the film increases monotonously,while the Seebeck coefficient exhibits an opposite behavior over the entire temperature range from 10 to 300 K.Here,a three dimensional variable-range-hopping conduction process was suggested to govern the electrical transport properties of the films at the lower temperature below room temperature.The power factor of the thin film was a little decreased(the room temperature power factor is 0.173 mW m^-1 K^-2)due to the introducing amount of amorphous defects.While the existence of amount of amorphous grain boundaries and interfaces lead to a significant decrease of the thermal conductivity.Thus,the thermoelectric performance of the BiCuSeO thin films is expected to be greatly enhanced as compared to that of bulks with the same nominal composition.3)The effect of M²⁺elements doping on the structure,valence state of elements and thermoelectric properties of the c-axis oriented BiCuSeO thin films were investigated in the present work.It can be found that the substitution of Ba²⁺,Pb²⁺and Ca²⁺for Bi³⁺can be effectively decreased the resistivity,and further optimized the power factor of the films.A highest power factor?PF?1.2 mW m^-1 K^-2 at about 673 K has been achieved in the 6%Pb-doped and 7.5%Ba-doped thin film samples,which is about 2.8 times higher than the undoped BiCuSeO thin films.4)Noble metal Ag and Au were chosen as the nano dispersed phase to construct the Bi_0.94Pb_0.06CuSeO/metal nanocomposite thin films.The carrier energy filtering effect resulted from Bi_0.94Pb_0.06CuSeO/metal interfaces can enhanced the Seebeck coefficient of the composites,and led to improved power factor;while the strong phonon scatterings from metal nano particles and Bi_0.94Pb_0.06CuSeO/metal interfaces in the composites further suppressed the thermal conductivity.Finally,synergistic optimization of electrical and thermal transport properties was realized in Bi_0.94Pb_0.06CuSeO/metal nanocomposite thin films.5)Ultrathin c-axis-textured Bi_0.94Pb_0.06CuSeO film with a 18 nm thickness were prepared on single crystal SrTiO₃?001?substrates.The calculated room temperature power factor?PF?of the films was about 0.85 mW m^-1 K^-2,which was comparable with the corresponding polycrystalline bulk materials.The thermal conductivity was suppressed due to the strong phonon scatterings of the ultrathin film,which make a large improvement on the ZT values of the ultrathin film.