Investigation Of Thermoelectric Properties Based On Organic-Inorganic Hybrid Perovskite Materials And Devices

Thermoelectric materials can be used for functional materials in realizing the direct conversion between heat and electric energy,and have been applied in spacecraft,semiconductor refrigeration and other fields.Recently,due to its advantages of easy preparation,high electrical conductivity,low thermal conductivity and easy application,organic/inorganic hybrid materials have become the focus of thermoelectric materials research.With the development of the new generation solar cells,a new kind of organic/inorganic hybrid perovskite material has been payed widely attention as a light absorbing layer.This organic/inorganic hybrid perovskite material exhibit the advantages of both easy preparation as organic materials and high stability as inorganic materials.The organic/inorganic hybrid perovskite in ABX3 structure has been reported for the relatively high carrier mobility,easy exciton dissociation as a result of the low exciton binding energy,ultralow thermal conductivity.Theoretically,the perovskite materials are predicted to achieve good thermoelectric properties.In this paper,we prepared thermoelectric devices based on CH3NH3 Pb I3 polycrystal film and CH3NH3PbBr3 single crystal in bulk by a mature synthesis method for organic-inorganic hybrid perovskite materials,characterized the various parameters of the materials by Kelvin probe force microscopy,UV-Vis spectra and X-ray diffraction analysis,measured the thermoelectric properties for the provskite devices including Seebeck coefficient,electrical conductivity and thermal conductivity,and improved the thermoelectricity of the perovskite by physical and chemical means such as optical excitation and doping respectively.More details about our research are listed as followed:(1)The organic-inorganic hybrid perovskite CH3NH3PbBr3 single crystals have been prepared and measured.The CH3NH3PbBr3 perovskite single crystal exhibits relatively low thermal conductivity,only 0.45 W/(m*K)at 70?,much lower than that of most inorganic thermoelectric materials.The Seebeck coefficient of CH3NH3PbBr3 single crystal material was anisotropic,and the Seebeck coefficient in horizontal direction is one order larger than that in the vertical direction.Furthermore,the CH3NH3PbI3 polycrystal films have been prepared and measured.There exists a crystalline phase transition point in CH3NH3PbI3 polycrystal film at room temperature,at which its Seebeck coefficient reached maximum.Both CH3NH3PbBr3 single crystal material and CH3NH3PbI3 polycrystal film have low electrical conductivity,which could be attributed to the large forbidden band-gap.(2)The photoexcitation were used to improve the thermoelectric property of in-film devices based on CH3NH3PbI3 polycrystal.Photoexcitation could increase electron energy in perovskite films and widened the entropy difference on both hot and cold sides of the thin-film device.Meanwhile,the photoexcitation can promote the charge carrier concentration in perovskite films,whereas the carrier mobility did not change too much.As a consequence,the photoexcitation can enhance the Seebeck coefficient and electrical conductivity of the perovskite materials simultaneously.Since the photoexcitation did not change the microstructure of the materials,it suggests little impact on the thermal conductivity of the perovskite films.Based on an simultaneously increased Seebeck coefficient and electrical conductivity,as well as unchanged thermal conductivity,the photoexcitation led to a considerable improvement in the ZT value of the perovskite materials,roughly 5 orders of higher than the ZT value in dark condition.(3)Element bismuth(Bi)was doped in CH3NH3PbI3 polycrystal film and CH3NH3PbBr3 single crystal materials,which could greatly promote the electrical properties of the provskite materials.The Seebeck coefficient of CH3NH3PbI3 polycrystal film did not change obviously after doping,whereas there was a great change taking place in the temperature-dependence of Seebeck coefficient.The Seebeck coefficient of CH3NH3PbBr3 single crystal material increased by 1-2 orders after doping,and reached its maximum when the doping concentration of Bi rose to 1%.Doping could increase the electrical conductivity of the perovskite film and single crystal perovskite,while the impurity elements could also increase the scattering strength of phonons,leading to the decreased thermal conductivity of single crystal perovskite.The AFM measurements suggest that doping would not affect the phase structure of the perovskite materials,but change the surface potential of the perovskite materials.As the Bi doping concentration rising,the surface potential of the perovskite film kept growing,suggesting that doping could cause electron enrichment on material surface.The ZT value of single crystal perovskite was 6 orders higher under the doping concentration of 5 % Bi than that of undoped single crystal perovskite.