Theoretical Study Of The Band Structure And Thermal Transport In Metal Halide Perovskites At Finite Temperature

The metal halide perovskite has become one of the most promising future optoelectronic materials due to its low cost,good optical band gap,small exciton binding energy and long charge carrier diffusion length.The optoelectronic properties of materials depend on microscopic characteristics such as band gap and effective carrier mass,etc.The value of band gap is closely related to its light absorption ability and temperature is one of the important factors affecting the band gap:in experiments,researchers found that the light absorption spectra of the metal halide perovskites are different because of the phase transitions as temperature changes,and even in the one phase,they are also blue-shifted with increasing temperature.It means that the band gaps in metal halide perovskites are temperature-dependent,which has a greater impact on the optoelectronic properties.Besides the phase transitions,researches showed that the change of band gap with temperature is mainly caused by two aspects:lattice thermal expansion and lattice thermal vibration.In order to well predicate the optoelectronic properties,it is important to investigate the influence of temperature on the band structure of metal halide perovskites,especially the microscopic mechanism.In addition,the optoelectronic properties are also closely related to the stability of the device.Excessive heat accumulation is not conducive to the practical application of optoelectronic devices.Therefore,the present work also investigates the heat transport properties of metal halide perovskites,focusing on the influence of temperature on the phonon spectra and lattice thermal conductivities.In this work,we use the recently-developed electron-phonon renormalization（EPR）method to study the band structures of all inorganic perovskite Cs Pb I₃ at different temperatures with different phases（orthorhombic,tetragonal and cubic phase）.In our EPR method,the lattice thermal expansion and vibration effects are both included.Besides,by using ab initio molecular dynamics simulation（AIMD）and temperature dependent effective potential method（TDEP）,the phonon spectra of Cs Pb I₃ at different temperatures are investigated,and further the lattice thermal conductivities of Cs Pb I₃ are obtained based on the Boltzmann transport equation and three-phonon scatterings.Furthermore,the AIMD simulations are used to study the band gaps of organic-inorganic hybrid perovskite MAPb(I_1-xBr_x)₃（x=0,0.33,0.67,1）at different doping concentrations of halogen elements at finite temperature.Our work shows that:Compared with only considering the lattice thermal expansion,lattice vibration leads to structural disorder at finite temperature,which makes the Pb-I-Pb bond angle decreasing greatly and the Pb-I bond length increasing slightly,especially for cubic phase.Because the main chemical character of valence band maximum（VBM）is the Pb-I antibonding in Cs Pb I₃,the bond length increasing and bond angle decreasing weaken the antibonding strength.And further VBM shifts down and band gap increasing.However,in the same phase of Cs Pb I₃,the Pb-I-Pb bond angle are almost unchanged and the Pb-I bond length increases slightly with the increase of temperature,which make the band gap increasing little.For the phonon spectra of Cs Pb I₃at different temperatures,it is found that the optical phonon frequencies move down obviously with temperature increasing,while the acoustic phonons move up weakly.Due to the small phonon velocities and life times,Cs Pb I₃ shows the low thermal conductivities in all three phases,and the values in cubic phase are smaller than those in tetragonal and orthorhombic phases.For MAPb(I_1-xBr_x)₃（x=0,0.33,0.67,1）,it is found that the band gap of MAPb(I_1-xBr_x)₃at finite temperature increases with the increase of doping concentration.The reason is that Br doping can cause VBM to move down,which is due to the lower 4p energy level of Br than 5p energy level of I.Meanwhile,it is also found that bond length of inorganic elements decreases and the bond angle increases after Br doping.In conclusion,this paper mainly studies the EPR effect on the band gap of metal halide perovskite,especially revealing the relationship between the band edge chemical bond characters and its temperature-dependent band gap.Further we deeply explore the phonon spectra and thermal transport properties of Cs Pb I₃ at finite temperatures.Our work can help us to further understand and theoretically design perovskite materials with excellent optoelectronic properties.