First Principles Study Of The Microscopic Configurations,Electronic Structures,and Optical Properties Of Defective TiO₂,Double Doped TiO₂,and Organometal Halide Perovskites Materials

Along with the traditional energy depletion and deterioration of ecological environment,mankind will face double challenges of the energy shortage and environmental pollution.Solar energy is a kind of new energy with more environmental protection and more abundance than the tranditional energy sources.How to make full use of solar energy is an important issue to solve the energy and environment problems.Thus,people have been dedicated to studying and improving various kinds of photocatalytic materials.Titanium dioxide(TiO2)has been becoming one of the best prospectively photocatalytic materials with wide band gap because of its high catalytic activity,high structural stability,low cost,and nontoxicity.It is widely used in the fields of new sources of energy and environmental pollution treatments.Due to the bigger band gap,TiO2 could only absorb the ultraviolet part-5%of the solar energy.In addition,the photo-generated electron-hole pairs of TiO2 are easy to recombine,which reduces the quantum efficiency of photocatalysis of TiO2.All of the above mentioned issues will limit the large-scale use of TiO2.In order to expand the application scope of photocatalysis of TiO2,it is an effective method to improve the visible light photocatalytic activity of TiO2 by means of carrier doping.Solar photovoltaic battery is also an extremely important scheme for exploring new energy.How to reduce the cost of photovoltaic power generation and improve the photoelectric conversion efficiency of solar cells is the core issue of the current photovoltaic research field.Organometal halide perovskite material CH3NH3PbI3 was first used in solar cells in 2009.Fortunately,compared to the original photoelectric conversion efficiency 3.8%,the current value has reached up to 20.2%.Many experimental and theoretical researches on the structure,mechanism,and modification of organometal halide perovskite material CH3NH3PbI3 are gradually emerging.With the sustainable enhancement of computing capability of modern computers,material simulation plays a more and more important role in physics researches.At present,the principles calculation based on density functional theory is effective.It can not only allow understanding of the micro-structure and electronic properties,but also predict unknown properties of some materials.Recent studies have shown that different ions co-doped TiO2 materials can significantly improve the visible light photocatalytic activity.However,our understanding of the details of the electronic structure and optoelectronic properties is still lacking.In addition,researches on CH3NH3PbI3 and related derivative structure have mainly focused on substitution of the halogen elements and adjustment of different elements substitutions.Theoretical study about Pb cationic replacement is still lacking.In this thesis,using the first principles method based on density functional theory,we investigate the influences of a variety of intrinsic point defects,C-Nb co-doping,and Mo-C co-doping respectively on the electronic structure and photocatalytic activity of TiO2.Becides,we also calculate the electronic structure and optical properties of organometal halide perovskite materials CH3NH3XI3(X=Pb,Bi,Mn,Fe).The whole thesis is organized as follows:Chapter one introduces the basic characteristics,photocatalytic properties,and major application potentials of TiO2.Various approaches to modify the structure and properties of TiO2are briefly introduced.Subsequently,we highlight recent research progress of organic metal halide perovskite material CH3NH3PbI3.In chapter two,attention is paid to the fundamental framework of density functional theory and first principles calculations.The simulation software package VASP used in our thesis is briefly introduced.In chapter three,we present a systematical theoretical investigation on the geometry,formation energy,electronic,and optical consequences of four types of defects in TiO2,including oxygen vacancy(Vo),Ti vacancy(VTi),Ti interstitial(Tii),and Vo-Tii dimer.The results show that the defects embedded in TiO2 lead to local lattice distortion in different manner.The existence of oxygen vacancy leads to a deep donor defect level in the forbidden band,located at 1.2 eV below the conduction band minimum(CBM).While the Ti interstitial forms two local states,a deep donor level and a shallow donor level at?1.25 eV and 0.28 eV respectively below the CBM in the forbidden band.For the Vo-Tii defect,by comparing the total energy terms of different defect configurations,the formation energy,and electronic structure,we predict that oxygen vacancies are more likely to combine with the titanium interstitial to form Vo-Tii dimer defects when Vo and Tii defects co-exist in TiO2.We calculate the optical properties of TiO2 lattice with these four kinds of defects.The calculated results show that the infrared absorption peak of TiO2 lattice with Vo-Ti;defect is consistent with observed infrared absorption peak.The results that we have predicted can explain experimental observations.In fourth chapter,we are concerned with(C,Nb)-and(Mo,C)-doped TiO2.For(C,Nb)-doped TiO2,we investigate the electronic structures and optical properties of the C,C-Nb and C-2Nb doped anatase TiO2 in details.For the C-doped TiO2,three subbands in the band gap are generated due to the C-2p states,which suppress the effective band gap for electron transferring from the valence band maximum(VBM)to the CBM,while the gap states are partially occupied and probably serve as the recombination centers for electrons and holes.The C-2Nb co-doping is investigated for charge compensation consideration in order to eliminate these recombination centers,and increase the efficiency of separating electron-hole pairs.For the C-2Nb co-doped TiO2,there are two possible co-doping configurations.We comprehensively study and compare the electronic structure and optical characteristics of the two different configurations.The three subbands in the band gap of the C mono-doped TiO2 become fully occupied because the two Nb atoms contribute sufficient electrons for compensation,and the effective band gap is also narrowed.Furthermore,the optical properties of the co-doped TiO2 and pure TiO2 are calculated.It is found that the optical absorption of the C-2Nb co-doped TiO2 extends its coverage over the visible light region.For(Mo,C)-doped TiO2,we study in detail the electronic and optical properties of Mo and Mo-C doped anatase TiO2.For the Mo mono-doped TiO2,the band gap reduces little,and the largest perturbation occurs at the CBM of TiO2.Therefore,the Mo-C co-doping is investigated for the charge compensation consideration.We discuss six doped configurations and find that the total energy of the system is increased with increasing distance of C and Mo.It is found that co-doped configurations with C nearest to Mo possess the lowest total energy.Then,we focus on discussing three possible Mo-C adjacent co-doped configurations.The subbands mainly induced by C-2p states in the band gap become fully occupied because the Mo atom contributes sufficient electrons to C anion for compensation.At the same time,the effective band gap is narrowed about 0.9 eV and the perturbation at the CBM occurred in Mo mono-doped TiO2 disappears,which means the band edges of doped system still straddle the redox potentials of water.Furthermore,the optical properties of the compensated Mo-C adjacent co-doped TiO2 and pure TiO2 are calculated.The optical absorption edges of the Mo-C co-doped TiO2 also shift towards the visible light region.In chapter five,we study the electronic structure and optical properties o CH3NH3XI3(X=Pb,Bi,Mn,Fe).For CH3NH3XI3(X=Pb,Bi),both the orthorhombic tetragonal structures are considered.CH3NH3PbI3 is a direct band gap semiconductor.strong antibonding coupling state of Pb-s orbital and I-p orbital compose of the top of valence band distribution.For CH3NH3BiI3,the band gap is narrow with respect to CH3NH3PbI3.The strong antibonding coupling states of Bi-s orbital and I-p orbital compose of the top of the valence band distribution of CH3NH3BiI3,and the band edge structure is similar to that of CH3NH3PbI3.The optical data show that CH3NH3BiI3 has stronger visible light absorption than CH3NH3PbI3 and infrared absorption is predicted.These similar electronic structures and better spectral absorption indicate that CH3NH3BiI3 system is likely to be a good candidate for solar battery.For CH3NH3XI3(X=Mn,Fe),the results show that the G-type antiferromagnetic order is the magnetic ground state for both materials.The band gap in the FM state is smaller than that in the G-type antiferromagnetic state.The sixth charpter is devoted to the conclusion and perspectives to the future work...