Controlling The Electronic Structures Of Halide Perovskite Via Heterovalent Doping And Halogen Ratio Modification Via First-principles Studies

Halide perovskite,with a form of ABX₃(A=Cs⁺,CH₃NH₃⁺…;B=Pb²⁺,Sn²⁺…;X=Cl^-,Br^-,I^-),is an excellent optoelectronic material and has been proposed to have great potential in solar cells,displays,photodetectors and other optoelectronic devices.In experiment,solar cells using CH₃NH₃PbX₃ nanocrystals as a light absorbing layer have demonstrated to have a conversion efficiency of more than 22%.Depending on cationic A site,halide perovskite can be classified into inorganic perovskite and organic-inorganic hybrid perovskite.In order to enhance the performance of optoelectronic devices,halide perovskites are usually engineered with respect to structural or compositional modifications.In this thesis,we have studied in detail the effects of IIIA and VA element doping on the band gap,light absorption and stability of all-inorganic perovskite CsPbI₃ by first-principles calculations.In addition,effects of different halogen ratio of organic-inorganic hybrid perovskite FAPbBr₃（FA=NH₂CHNH₂⁺）on its electronic structures.The followings are the main content:We systematically studied the effects of inorganic perovskite CsPbI₃ doping with In,Tl,Sb and Bi.Each element was doped at three different concentrations of 1.6%,3.7%and 12.5%.Through first-principles calculations,we found that CsPbI₃ exhibits p-type semiconductor properties when doped with In and Tl elements,and n-type semiconductor properties are demonstrated when doped with Sb and Bi.When the same element is doped at different concentrations,the band gap decreases as the doping concentration increases.The range of band gap change is between 1.2 and 0.4 eV.Moreover,we found that the new electronic states appeared at the minimum of conduction band or the maximum of valence band are mainly contributed by the s,p orbitals of the doped element.Moreover,when doped with In element,we observed an enhanced optical absorption at long wavelength,which may benefit the performance of optoelectronic devices.We have also calculated the stability of the doped system by estimating the formation energies.We found that when In,Sb,and Bi are doped,the formation energy decreases as the doping concentration increases,while doping of Tl element has little change and is relatively difficult.We can understand this phenomenon by calculating the tolerance factor of the doped element.We have also investigated the effects of halogen ratio on the electronic structures of organic-inorganic hybrid perovskite FAPbBr₃ by engineering the ratio of Cl/Br and I/Br by incorporating Cl and I into the compound.While the band gap decreases from FAPbCl₃ to FAPbBr₃,and to FAPbI₃ in the pristine form,we found that when Cl is doped in FAPbBr₃,the band gap increases.The more Cl is incorporated,the larger the band gap becomes.When I is doped into FAPbBr₃,the band gap becomes smaller.The larger the doping concentration is,the smaller the band gap becomes.Therefore,by changing the halogen ratio of FAPbBr₃,we can effectively engineer the band gap of FAPbBr₃ to a large range（2.43 eV to 1.58 eV）,which may provide a useful way for future synthesis of optoelectronic materials and devices.