Single Crystal Preparation And Thermal And Mechanical Properties Of Organic-inorganic Hybrid Perovskite

The application of organic-inorganic hybrid perovskite in optoelectronic devices has broad prospects.Improving device lifetime and stability remains the stumbling-block of the commercialization of hybrid perovskite based devices?HPDs?.Since advanced encapsulation technology has been keeping solar cells from moisture,oxygen and ultraviolet light,the intrinsic stability of the material and structure itself become extremely important.Then the heat transfer during the operation of the device,as well as the matching between the layers of the layered device,and the ability of the material to resist external damage and stress during the service,are issues should be consider to improve device life during device design and service.The thermal properties of hybrid perovskite materials including thermal conductivity and thermal expansion coefficient,and mechanical properties including hardness and Young's modulus are such inherent properties.Currently,the thermal conductivity and thermal expansion coefficient of MAPbX₃?MA=CH₃NH₃,X=Cl,Br,I?are controversial.The reports on hardness and modulus are not comprehensive and systematic,which is not conducive to further solving the influence of thermal and mechanical properties on device stability.Single crystals are undoubtedly the best form of the material itself because of its absence of grain boundaries and few defects.For the first time,using the single crystal,we investigate the thermal properties and mechanical properties of MAPbX₃ at room temperature and above systematically.The test methods used were all commercial test methods,and the data were reliable.Combined with the first principle calculation,the internal mechanism is explained and discussed.The experimental results show that the three hybrid perovskites have extremely low thermal conductivity only 0.3-0.5W·m^-1·K^-1,and the thermal expansion coefficient is between 29-58×10^-6 K^-1.The hardness are very small at around 0.5-0.7 GPa and Young's modulus are between 10 and 30 GPa.Particularly,for MAPbI₃,thermal conductivity is observed being only 0.3 W·m^-1·K^-1 and linear thermal expansion coefficient along[100]direction is as high as 57.8×10^-6 K^-1?tetragonal?and much higher at the structural phase transition point larger than 200×10^-6 K^-1.The hardness is about 0.3 GPa and the Young's modulus is only about 13 GPa.Through experimental and theoretical calculations,we attribute the ultralow thermal conductivity,ultrahigh thermal expansion,low hardness and modulus to the low phonon velocity caused by the weak chemical bonding of Pb-X associated with the soft perovskite materials.The presence of organic ions can further reduce the thermal conductivity,thermal expansion,hardness and modulus of perovskite materials.,but not the determinant factor.We speculate that materials containing Pb/Sn-X chemical bonds,even non-perovskite structures,may also have lower thermal conductivity.Although the higher thermal expansion coefficient brings the matching problem in the layered devices,the softer materials represented by the smaller hardness and modulus indicate the controllability of the materials in the device application.And its ultra-low thermal conductivity may create a new era of thermoelectric materials.