A First Principle Study On The Charge Transfer In Titanium-based Perovskite Solar Cells

The commercialization of perovskite solar cells is mainly limited by the toxicity and poor stability of traditional organic-inorganic hybrid perovskite materials.Although lead-free all inorganic perovskite materials possess the advantages of non-toxic and environmental stability,the efficiency of lead-free all-inorganic perovskite solar cells are still low.This poor performance blames to the difficulty of interface charge extraction and the fact that the hot carrier relaxes too fast,which lead to a large number of charge transfer losses.Based on the first principle method,this thesis explores the charge transfer processes in titanium-based perovskite solar cells,expounds the main reasons for their poor performances,and presents reasonable methods to improve the efficiency.The main contents are as follows:The non-adiabatic molecular dynamics combined with the first principle are used to analyze the influence of halogen doping on the mechanism and law of hot carrier relaxation in the titanium-based perovskite material Cs2TiIyBr6-y(y=0,2 or 6).It is found that the relaxation time of the hot carriers in Cs2TiIyBr6-y ranges from 2-4 ps which indicates that the hot carriers within 10 nm from the Cs2TiIyBr6-y/ETL(Electron Transport Layer)interface can be effectively extracted before their energy loses completely.Compared with that in Cs2TiBr6 and Cs2TiI6,the non-adiabatic couplings(NAC)in Cs2TiI2Br4 is relatively weak,thus Cs2TiI2Br4 has a long hot carrier relaxation time,less hot carrier losses.Therefore,Cs2TiI2Br4 should be more suitable for being used as the light absorption layer,from the aspect of hot carrier energy loss.Further electron-phonon interaction analyses indicate that the relaxation of hot electrons mainly comes from the coupling between the electrons distributing on the Ti-X bonds and the Ti-X vibrations,and that of hot holes can be attributed to the coupling between the electrons distributing on the X atoms and the distortions of the[TiIyBr6-y]2-.Based on the first-principles calculation,La doped BaSnO3 was designed as the charge transfer layer material,and the influence of La doped BaSnO3,TiO2 and SnO2 on the charge transfer behaviors on the Cs2TiI2Br4/ETL interface was discussed.The structures of projected density of states indicates that lanthanum doping can modify the band structure of BaSnO3,the strong Coulomb interaction between La3+ ions and doped electrons can reduce the bandgap of BaSnO3,hence,the CBM value of BaSnO3 is greatly altered.The strong atomic interaction analysis showed that Cs2TiI2Br4 and Ba0.93La0.07SnO3 formed thermodynamic stable interface structure,while neither TiO2 nor SnO2 could form stable interface contact with Cs2TiI2Br4.The interface band structure on the Cs2TiI2Br4(111)/Ba0.93La0.07SnO3(001)interface is matched,the Sn-Br and Cs-O chemical bonds act as charge transport channels,and there is a large interface potential difference,which is very suitable for the extraction of interface electrons.Therefore,the lanthanum doped BaSnO3 is the most suitable ETL material for Cs2TiI2Br4 based perovskite solar cells by comparison with TiO2 and SnO2.This study provides intuitive and effective guidance for the performance improvement of titanium-based perovskite solar cells.