Studies On Preparation And Optical Properties Of FeS₂Thin Films Obtained Via Chemical Bath Deposition

Cubic structure of FeS₂（pyrite） thin films is one of the most candidates as theabsorber materials for photovoltaic applications or thin-film solar cells due to itssuitable band gap （Eg=0.95eV）, high absorption coeffient and infinite elementalabundance. However, it suffers from high preparation cost and low conversionefficiency, which prevents it from commercial use.In order to explore low-cost preparation methods, in this thesis, FeS₂films wereprepared by thermal sulfurizing the precursor iron monosulfide films obtained bychemical bath deposition. The influence of sulfurization temperature, sulfurizationtime and cooling rate on the phase composition of the obtained films was discussed inthe aspects of thermodynamics and kinetics. The results showed that alteringsulfurization temperature and time were unable to eliminate the impurity phasesexisting in FeS₂films and rapid cooling was proved to be an effective way to obtainhigh purity pyrite films. Samples obtained through rapid cooling at a rate of3.5°Cmin-1and2.5°C min^-1from500°C to200°C presented the only crystalline pyritephase films. The further optical absorption test indicated that the pyrite filmssulfurized at500°C for3h and then cooling with a rate of2.5°C min^-1had a higherabsorption coefficient (α≈2.0×10⁵cm^-1) than the films with2h sulfurization at500°C with the same cooling rate. And the band gaps of these two samples sulfurizedfor3h and2h were calculated to be0.82eV and0.88eV, respectively.Based on the investigation described above, in order to adjust the band gap ofFeS₂to be in the range of1.3¹.7eV and obtain high conversion efficiency, Mn-dopedFeS₂thin films were prepared by introducing Mn element in the process of chemicalbath depositon and the following sulfurization treatment. The effects of Mn contentson the grain morphology, structural and optical properties of the synthesized filmswere studied. Results showed that the Mn-doped FeS₂films showed （200） preferredorientation after increasing doping content. Films with special light-trapping networkwere obtained by increasing doping content, as results the （200） preferred orientation.It was confirmed that the raising of absorption coefficients of the films from2.0×10⁵cm^-1to3.7×10⁵cm^-1was mainly attributed to the special microstructure, andthe band gap of thin films are also modified to1.31eV because of the quantum effectand the decreasing impurities phases caused by Mn-doping. The change of the band gap was closely related to the electronic structure densityof FeS₂. The study of electronic structure could provide theoretical guidance to theproperties of the FeS₂materials doped with some elements. The band structure andthe state density of the films were calculated based on the first-principles of DensityFunctional Theory. The reckoning showed that the band structure of the films and thelocal density of the Fe atom and the S atom both changed because of doping Mn.