First-Principles Study Of Novel Sulfides And Halides Based Optoelectronic Semiconductors

The improvement of modern society is closely related to the fast development of electronic information technology which is firmly dependent on the development of semiconductors.Semiconductors are critical parts of solar cells,field-effect transistors and luminescent devices.Properties of conventional semiconductors have been understood well and the application of these materials are deeply mature.Currently,sulfides and halides based optoelectronic semiconductors such as Cu₂ZnSnS₄ and CH₃NH₃PbI₃ have drawn intensive attentions.We now have a clear understanding of their properties.However,theoretical studies of properties of some novel sulfides and halides based optoelectronic semiconductors such as low-dimensional Bi₂S₃,WSe₂ and Cs₄PbBr₆ are limited.Therefore,in this thesis,we did a research of four novel sulfides and halides based optoelectronic semiconductors.The thesis is composed of seven chapters.The first chapter gives an introduction to the development of semiconductors and application of sulfides and halides based optoelectronic semiconductors in solar cells,field-effect transistors and luminescent devices.The second chapter introduces the simulation methods used in the study,including density functional theory and defect calculation methods.The third chapter and fourth chapter discuss the defect properties of Bi₂S₃ and WSe₂ which are two novel sulfides based optoelectronic semiconductors with low-dimensional structures.Bi₂S₃ has a unique quasi one-dimensional structure which strongly affects the defect properties.Additonally,the defect properites of Bi₂S₃ are quite different from those of conventional covalent binary semiconductors.The dominant intrinsic defects are donor defect,explaining the observed n-type conductivity of Bi₂S₃.However,intrinsic defects of Bi₂S₃ have deep levels which cloud limit its performance as the light-absorber or sensitizer material.Futhermore,we study a series of extrinsic dopant elements in Bi₂S₃ as well.Due to the doping limit rule,it is difficult to achieve supreme p-type doping efficiency in Bi₂S₃.In our studies,only Pb doping could make the sample show weak p-type conductivity while other dopant elements make the sample show n-type conductivity.Among them,Cu,Br and Cl are good candidates of n-type doping with high electron carrier concentration.And we predict that Cu,Br and Cl doped Bi₂S₃ may be ideal n-type electron acceptor or counter electrode material in solar cells.Defect properites are important characteristics of optoelectronic semiconductors which act as key parts in solar cell and field-effect transistors.WSe₂ has applications in field-effect transistors which should show good ambipolar conductivity.However,it was reported that n-type doping is less efficient than the p-type doping in WSe₂.Thus,we study the defect properties of a series of dopants in WSe₂ and screen suitable n-type and p-type dopants.We find that formation energies of intrinsic defects are high,thus making doping with high concentration possible.The results of extrinsic doping show that Nb and Ta doped sample has good p-type conductivity while Re,Ru and Os doped sample show less efficient n-type conductivity compared to the former.The fifth chapter turns to double-perovskite halides which attract intensive attentions.We predict the thermodynamic stability of Cs₂AgBiBr₆ and Cs₂AgBiCl₆using different exchange-correlation functionals and find that the stability prediction can be dependent on the approximations to the exchange-correlation functionals.It is pointed out that optB88-vdW functional is the most suitbale for describling the themodynamic stability.The sixth chapter concentrates on unraveling the luminescence mechanisms in(C₄N₂H₁₄X)₄SnX₆?X=Br,I?and Cs₄PbBr₆.We discuss the electronic properties,self-trapping exciton and defect-assisted exciton properties and give a comprehensive and reasonable understanding of photophysical properties in(C₄N₂H₁₄X)₄SnX₆?X=Br,I?and Cs₄PbBr₆.The seventh chapter summaries all the research and clarifies the application of our studies.