| The efficiency of organic-inorganic hybrid perovskite solar cells has made great progress in recent years,mainly due to the excellent semiconductor properties of this type of material,such as carrier ambipolar diffusion,low temperature solution preparation,high extinction coefficient and high carrier mobility.Thereinto,ultra-long lifetime make effective exciton diffusion length L exceeding 100μm in this material,which is far superior to ordinary organic semiconductors.On the other hand,the carrier mobility anisotropy has been found in the new perovskite on experiment.However,on the microscopic scale molecules or atoms,the intrinsic mechanism of exciton ultra-long lifetime and carrier mobility anisotropy is not clear.Based on this,we mainly study the intrinsic reason of exciton ultra-long lifetime and carrier mobility anisotropy of CH3NH3PbI3 in the paper.The main research contents and results are as follows:(1)Time-resolved photoluminescence spectra is adopted to study photoluminescence spectra and lifetime of CH3NH3PbI3.The experimental results show that the photo luminescence lifetime increases with temperature,one typical feature of reverse intersystem crossing(RISC).Meanwhile,we adopt DFT method to calculate the energy difference AEst between the singlet state and the triplet state of CH3NH3PbI3,the result indicates that △Est of CH3NH3PbI3 is less than 0.1 eV,one crucial prerequisite for the happening of RISC.Based on this study,this paper puts forward one hypothesis that the ultra-long lifetime of exciton in organic-inorganic halide perovskite might be caused by the RISC process,which may provide a new theoretical basis for understanding the excellent photoelectric properties of photovoltaic materials.(2)Based on Marcus theory and DFT,carrier mobility anisotropy of HC(NH2)2PbI3,HC(NH2)2SnI3 and CH3NH3SnI3 is investigated by calculating the intramolecular vibration(namely,internal recombination energy λ)and intermolecular electronic coupling integral V.Results indicate that the holes and electrons of HC(NH2)2PbI3 show transport orientations consistency parallel to the(001),(010),(101)and(111)crystal planes while inconsistency was found parallel to(110)crystal planes(with an angle 65° between the hole and electron transport directions).The holes and electrons of HC(NH2)2SnI3 show transport orientations consistency parallel to the(001)and(101)crystal planes while inconsistency was found parallel to(110)and(111)crystal planes(with an angle ranged from 40° to 65° between the hole and electron transport directions).The holes and electrons of CH3NH3SnI3 show transport orientations consistency parallel to the(110)and(101)crystal planes while inconsistency was found parallel to(010),(001)and(111)crystal planes(with an angle ranged from 450 to 65° between the hole and electron transport directions).This shows that the internal carrier transport of the new perovskite materials has obvious anisotropy.(3)Based on the two parts above,the hole transport material(HTM)spiro-OMeTAD is studied in perovskite solar cells(PSCs)theoretically,which is closely related to the photo-excited charge separation.Based on DFT and Marcus theory,investigations on the carrier mobility anisotropy are conducted along representative crystal planes of spiro-OMeTAD by recombination energy λ and electronic coupling integral V.Results indicate that the holes and electrons show transport orientations consistency parallel to the(010),(101)and(111)crystal planes while inconsistency was found parallel to(100),(110),(011)and(001)crystal planes(with an angle ranged from 40° to 70° between the hole and electron transport directions).The anisotropy has an important influence on the effective collection of photogenerated carriers.The research provides a new theoretical understanding for the peculiar photophysical properties of organic-inorganic perovskite materials,and also points the theoretical significance of controllable and oriented fabrication of perovskite absorber and HTM in PSCs. |
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