The Preparation Of MAPbI₃ And The Study Of Its Structure And Properties

Since MAPbI₃ was used as a light-absorbing layer in solar cells by Akihiro Kojima in 2009 for the first time,the perovskite solar cells have attracted the attention of the researchers for the simple and low-cost preparation process.The power conversion efficiency of the perovskite solar cells has increased from 3.8%to 25.5%in just a few years,while silicon-based solar cells have taken about 30 years to achieve the same efficiency.So MAPbI₃ has become one of the most promising material in the photovoltaic field.MAPbI₃ has a large electron-hole diffusion distance,even as high as175?m in a single crystal,a low electron-hole recombination rate,a high carrier lifetime,and a large dielectric constant at low frequency.However,there are some basic physical properties still need to be clear.There are some controversies regarding the dielectric properties,electrostriction,and piezoelectricity.In order to resolve these disputes,this thesis conducted the follow research:(1)Systematically study the dielectric properties of MAPbI₃ single crystals synthesized by inverse temperature crystallization.The study found that MAPbI₃ single crystals exhibit a giant dielectric constant at low frequency at room temperature,with a dielectric constant up to 10⁶ at 0.01 Hz at 303 K.This phenomena stem from the interface polarization and dielectric relaxation.The relaxation time and conductivity as a function of temperature are divided into two regions according to the fitting results of ac conductivity.The activation energy in the low temperature region is 0.27 e V which is related to the Debye relaxation.The activation energy in the high temperature region is 0.80 e V which is related to the cole-cole relaxation.The change in the relaxation type is originated from the more defects at high temperature and the long-range migration of ions.This model can also explain the abnormal increase in dielectric constant under illumination.The dielectric constant as a function of temperature shows that the MA⁺exhibit an anti-parallel orientation in the orthorhombic phase.And the anti-parallel orientation will gradually break down as the temperature rises at tetragonal phase.The effect of the Cs⁺doping on the structure and the properties of MAPbI₃ was studied.With the increase of Cs⁺doping concentration,the band gap of MAPbI₃increases.The phase transition temperature of the orthorhombic phase to the tetragonal phase increase with the increase of the amount of doping indicating that the Cs⁺doping can improve the stability of this material.According to the Kirkwood–Fr(?)hlich–Onsager theory,it is found that Cs⁺doping can enhance the anti-parallel arrangement of MA⁺.(2)The effect of ac and dc poling on the electrostriction and piezoelectricity of MAPbI₃ single crystal is studied.The electrostrictive coefficient of the non-poling sample is only-0.49 nm²V^-2and after ac poling the electrostrictive coefficient reaches-49.3 nm²V^-2which is nearly 100 times higher than the non-poling samples.The electrostrictive coefficient of the dc poling is-14 nm²V^-2.The increase in the electrostrictive coefficient is attributed to the electric field induced defects.Similarly,after ac poling and dc poling,the piezoelectric coefficient of the material is also improved which may be due to the distortion of the crystal structure caused by the electric field.(3)By adding a slight excess of MAI,pure MAPbI₃ films and powder were prepared in the ambient air,then the powder was pressed into a pellet.The study on the hysteresis loops of the MAPbI₃ films and pellets revealed that neither of them show a sudden change in current caused by the domain switch and show the characteristics of the typical linear resistance.The phenomena may be due to the weak ferroelectricity of the material and the current is dominated by the leakage current.Piezoelectric force microscopy found that there exists the domain structure in films indicating that the material has weak ferroelectricity.High-resolution XRD was used to study the phase transition of MAPbI₃ from the tetragonal phase to the cubic phase.The study showed that there exists a sudden change in lattice parameter at 59?.however,the twist of the Pb I6 octahedron still exists at70?indicating that there is coexistence of tetragonal phase and cubic phase.The lattice parameter has no obvious hysteresis during the heating and cooling process indicating that it is not the first order phase transition.According to the change of rotation angle of Pb I6 with the temperature,exponent?indicates that the phase transition is between the first order and the second order.The phase transition maybe take place near the tricritical point.By studying the XRD of single crystal using the(?)scan model,it is found that the single crystal has a pre-transitional effect near 59?.(4)The MAPbI₃ and MAPbI₃/TiO₂ powders were prepared by the anti-solvent method.The SEM topography and EDS energy spectrum show that MAPbI₃ and TiO₂were uniformly distributed.The results show that MAPbI₃ have weak photocatalytic oxidation of benzylic alcohols.However,MAPbI₃ will be decomposed because of the oxidation of I^-at the same time.The catalytic oxidation performance of MAPbI₃/TiO₂is lower than that of TiO₂ because I^-is preferentially oxidized by TiO₂.