First-principles Study Of Inorganic And Nontoxic Perovskite Materials For Solar Cell Application

Since the Japanese scientist Miyasaka firstly reported on solid-state perovskite solar cells at 2006,the emergence of perovskites has opened the door to research on new materials for photovoltaics,and research on such materials has been in full swing.When organic hybrid perovskite solar cells broke through 23%of their photoelectric conversion efficiency,the research focus of researchers gradually shifted from improving the efficiency of devices to how to develop new applications such as long-range stability and environmentally friendly.It is expected to accelerate the commercial application of inorganic lead-free perovskite solar cells.In this paper,the crystal structure search of genetic algorithm and the first principle of density functional theory are used to replace the atoms at different sites and different structure in the perovskite structure ABX3.Several feasible alternatives are discussed,from the crystal structure and the stability,then to the electronic structure exploring its microscopic mechanism and performance evaluation in depth,which provides a useful theoretical basis and experimental scheme for the development and modulation of new photovoltaic materials based on inorganic lead-free perovskite semiconductors.The research content and conclusions of this thesis are as follows:(1)Using USPEX and VASP software to explore the equivalent substitution for Ca site by using the same series of alkaline earth metal in CaTiO3,it is found that the substitutional atoms do enter the Ca site,which plays the role of filling the octahedral gap and stabilizes the crystal structure,but the band gap of these semiconductor increases as the radius of the doping ions increases.When the+1 valence Na ion is substituted for the Ca site at a lower valence,it is found that the CaTiO3 can maintain the cubic phase structure when the number of substitution atoms is small,and the Fermi level enters the valence band to make the system exhibit the property of p-type TCO material;however,when the number of substitution atoms continues to increase,oxygen vacancies appear in the system,which offsets the electron deficiency in the system,the TiO6 octahedral coordination mode changes,and the Fermi level returns to the forbidden band,making the system recovering the semiconductor characteristics.Applying this method to the +3 valence Al ion,although the structure deviates from the cubic phase,the Fermi level indeed enter the conduction band,so that the system exhibits n-type TCO characteristics.From the calculation of transport properties,the obtained 2Na and 2A1 systems show good mobility,which proves that they can be used as substitutes for traditional TCO.(2)The effects of Ba+3d2?10 co-doped KNbO3 on its geometry,formation energy,magnetic moment,band structure,density of states,Bader charge and transport properties were studied by first-principles theory.The results of PBE calculation show that the lattice constant of the corresponding doping system increases with the continuous filling of the d-shell electrons in the 3d transition metal,and the structural distortion is caused by the abnormal lattice of Mn and Fe due to magnetostriction.Intensified,the formation of the system also increases.According to the electronic structure obtained by the HSE06 method,with the filling of the d-shell electrons,the impurity level brought by the 3d transition metal continuously moves from the conduction band to the low-energy valence band in the energy band structure of the system,thereby it has reached the role of modulating the wide band gap KNbO3.Four doping systems with Ti and Cr n type and Ni and Cu p type properties are suitable for photovoltaic device materials and exhibit excellent transport properties,indicating that these four systems will be widely used in future photovoltaic devices application.(3)Selecting the genomic method to replace the lead and unstable iodine in the toxic component of CsPbI3,and 12 kinds of +3 valent metal cations and +1 valent silver are selected after checking the periodic table based on the principle of non-toxic inorganic and wide sources.The ionic composition of the cation is selected from the same series of Cl-substituted I-ions to form a double perovskite structure.Based on the first principle of density functional theory,the tolerance factor and octahedral factor prediction of 12 components,global searching of crystal structure,thermodynamic and dynamic stability analysis,electronic structure analysis and transport property calculation,after layer-by-layer investigation,the two perovskites of Cs2BoAgCl6 and CsZInAgCl6 are obtained,and the high symmetry of the cubic phase is maintained.The proper optical band gap is obtained for the two perovskites with BrCl and exhibits high carrier mobility,indicating that it can be used as a candidate material for photovoltaic devices.(4)According to the first principle,the effects of anion substitution on the lattice constant,stability and electronic structure and properties of perovskite structures with different dimensions are studied.The halogen group replaces the X-position in the CsZS1nX6 structure to form a zero-dimensional point structure,and the spatial structure is no longer restricted in stretching and the band gap is large.The two-dimensional layered structure CsBiX2-Cl2 structure deviates from the cubic phase,and the octahedrons in the structure are connected in a co-edge or coplanar form,wherein the band structure of CsBiTeCl2 exhibits a suitable optical band gap and a large mobility,which is suitable for applications in photovoltaic device materials.The presence of an anionic group in the three-dimensional structure Na2FeCu(CN)6 makes its crystal structure and electronic structure less susceptible to external influences and remains stable.The study finds that the dimensionality of crystal structure does not affect the dimensionality of electronic structure.(5)According to the ATAT variable ratio search and the Gibbs free energy stable temperature region analysis,the Cl-Br ratio is used to optimize the stability of the Cs2InAgBr6 system with unstable energy.It is found that the compounds of seven Cl-Br ratios are obtained by cluster expansion.The formation is reduced relative to Cs2IngBr6 and is within the error range of 0.025 eV ambient thermal energy.The Gibbs free energy stability discrimination finds that all seven systems are stable at room temperature.The obtained CsZAglnBrsCI system exhibits an optical band gap of 1.92 eV by HSE band,and shows good light absorption and carrier transport properties.The n-i-p heterojunction(TIOZ/CsZInAgBr5CI/Cs6Ag4In4Br,SCl4)is constructed with Cs2AglnBrsCI as the light absorbing layer.The matching performance is good on lattice matching,work function and band offset,and no barrier is formed,which is favorable for carrier transport.