Theoretical Simulation Of Electronic Structure And Optical-electronic Property Of FeS₂, And Its Surface Passivation

Using ab initio norm-conserving psendo-potential computational simulation method based on density functional theory, electronic structure and optical-electronic property of FeS2 have been artificially modulated in order to increase theoretical conversion efficiencies of solar cell. From the viewpoint of electronic level, reasons for the limitation of the open circuit photovoltage have been systematically studied and defective electronic energy band model has been built to lay the foundation for the increase of the photovoltage. Based on this theoretical study, technique of increasing the photovoltage has been carried out.The modulated electronic structure of FeS2 under external pressure is characterized by the widening of its band gap with the increasement of external pressure and the reduction of lattice constant. The widening of the band gap is caused by an increase in Fe-S covalence under pressure due to the reduction of the Fe-S bond length.The optical properties of FeS2 under pressure perturbation are characterized by the widening of band gap with increase of external pressure. External pressure not only leads to the increase of the optical absorption edge, but also lead to the energy scope of the optimum spectroscopy moving towards higher energy so that the optimum energy scope matches with the optimum solar spectroscopy (1.5~3.1eV). In this energy scope, there exits higher optical absorption coefficient αλ>1.0 x 105cm-1). Therefore, the modulated optical-electronic material FeS2 under external pressure not only can increase the theoretical inversion efficiency of FeS2 solar cell, but also can be used as thin film material (<0.1m), which provide theoretical and experimental foundation to develop cheap FeS2 thin film solar cells of high efficiency.Electronic structure and optical constant of the solid solution RuxFei-xS2 have been theoretically calculated. Both the optical absorption edge and the optimum spectroscopy scope move towards higher energy with the increase of the Ru-concentration X. At the same time, in this optimum spectroscopy scope, still keeps the same high absorption coefficient as FeS2 single crystal.Therefore, not only the adsorption edge but also the optimum spectrum scope of solid solution RuxFei.xSi can be controlled by means of controlling the Ru-concentration. Thus, artificially modulating the theoretical conversion efficiency of FeSa silar cell has been realized and provides theoretical foundation and experimental way to increase theoretical conversion efficiency of FeS2 solar cell.The theoretical sulfur vacancy formation energy in FeS2 (pyrite) has been calculated. Based on the theoretically calculated results, FeSa (pyrite) can be regarded as a stoichiometric compound, and the sulfur vacancy is the dominating intrinsic point defect in FeS2 (pyrite). On the basis of ab initio theoretical calculation and ligand-field theory considerations, the band-structure model of the ideal composition FeS2 is modified due to the presence of S vacancies, and the modified band-structure model with defect states for FeS2 with sulfur vacancy has been built. The most important electronic effect is the formation of defect states in the forbidden zone.Calculation of the electronic structure of (100) surface indicates that a new type of unoccupied surface states exists in the forbidden zone. The calculated band gaps of the planar (100) surfaces approximately amount to 0.446eV less than that of bulk. On the basis of ligand-field-theory consideration, surface states on the planar FeSa (100) surface have been found to be due to symmetry reduction of the iron atom coordination sphere.Theoretical calculation of the electronic structure of stepped (100) surface indicates that surface states increase in the forbidden zone and band gap of stepped (100) surface amounts to 0.15eV. Calculation of the electronic structure of sulfur-deficient planar (100) surface indicates that some new occupied surface states exits in the forbidden zone and new unoccupied surface states lie above the surface state of pl...