A First-principles Study On The Modulation Of Optoelectronic Properties Of Two-dimensional Perovskite Heterojunctions

In recent years,two-dimensional materials and their heterojunctions have made outstanding contributions to breaking through the difficulties of nanoelectronics and optoelectronics,so they have always been research hotspots in the fields of materials chemistry and condensed matter physics.Perovskite-based van der Waals heterostructures exhibit high carrier mobility and perfect photoresponse properties,which can be used to design transistors and photovoltaic devices.However,a single van der Waals heterojunction has relatively limited functionality due to its specific energy band type.Among them,the heterostructure with the type-I energy band arrangement is suitable for light emitting diodes due to its high emission efficiency,and is generally widely used in the field of light emitting.Heterostructures with type-II band arrangements can be applied to photovoltaic fields such as solar cells.Heterostructures with type-III band arrangements are mainly used as tunneling field effect transistors.We can achieve more controllable device function by tuning the type of band alignment of the heterojunction.In this thesis,three-layer perovskite Cs₂PbI₂Cl₂and single-layer Pd₂Se₃are used to build a heterojunction model.By gradually replacing Cl atoms in Cs₂PbI₂Cl₂with I atoms,the effect of halogen substitution on the optoelectronic properties of 2D perovskite heterojunctions was investigated.Based on density functional theory,the optoelectronic properties of the Cs₂PbI₂Cl₂/Pd₂Se₃heterojunction model and its doped model were investigated,including band structure,density of states,effective mass,carrier mobility,interfacial charge transfer and light absorption spectrum and other related calculations were performed,and the following research results were obtained:The calculation shows that the Cs₂PbI₂Cl₂/Pd₂Se₃heterojunction is a type-I energy band type with a band gap of 0.99 e V,and the band gap of the Cs₂PbI₂Cl₂/Pd₂Se₃heterojunction can be effectively controlled after doping.With the increase of the doping ratio,the transition from type-I to type-II of the energy band alignment occurs.Due to the change of the energy band arrangement type,the effective mass of holes in Cs₂PbI_2（1+x）Cl_2（1-x）/Pd₂Se₃is greatly reduced,and the carrier migration rate is improved,which is more suitable for photovoltaic devices.Differential charge density map can see that the charge is mainly depleted in the Cs₂PbI_2（1+x）Cl_2（1-x）interface and accumulated in the Pd₂Se₃interface,proving that the charge is transferred from the Cs₂PbI_2（1+x）Cl_2（1-x）in the heterostructure transferred to Pd₂Se₃.The optical absorption spectrum shows that the total absorption curve of the doped Cs₂PbI_2（1+x）Cl_2（1-x）heterostructure also increases gradually.Comparing the performance of heterojunctions before and after doping,the heterojunctions with type-I energy band structure are suitable for light-emitting devices,while the heterojunctions with type-II energy band structure are suitable for photovoltaic devices.By adjusting the doping ratio,more controllable device functions can be designed.