Preparation And Optoelectronic Properties Of FeS₂ And FeS₂/TiO₂ Composite Films

Cubic structure FeS2 (pyrite) thin films is one of the most promising candidates as absorber materials for photovoltaic applications due to its suitable band gap, high absorption coefficient, which makes it only need small thickness to absorb most of the sunlight. At present, FeS2 thin film can be prepared by the one-step method or two-steps method. The resulting films prepared by the one-step method do not represent the properties of pure pyrite because of the presence of FeS2-x and some S vacancies, or deviation from ideal stoichiometry. However, FeS2 film obtained by the two-steps method, which involves the preparation of precursor films and the reaction of precursor with sulfur, always shows good crystallinity and properties.On the basis of the previous research progress, in this thesis, polycrystalline FeS2 films were prepared by thermal sulfurizing the precursor ferric oxide films obtained by sol-gel process. Effects of the sol-gel process parameters on the microstructure of precursor ferric oxide films were investigated firstly. The effects of sulfurization temperature, sulfurization times and doping-effects on the microstructure and photoelectrical properties of FeS2 films were also studied. Finally, FeS2 was successfully sensitized onto TiO2 nanotubes and porous nanocrystal films to fabricate FeS2/TiO2 composite films, the microstructure, optical and photoelectrochemical properties were preliminary explored. The main works and conclusions can be drawn as following:FeS2 thin films with an expected structure are prepared by sulfurization annealing the Fe2O3 films achieved by the sol-gel dip-coating process. The effect of sulfurization time and temperature on film properties was discussed. The results shown that the precursor Fe2O3 films could not transform into FeS2 at 300℃. However, all the precursor films transformed into FeS2 when the sulfurization temperature increased to 400～600℃. At 400℃, the FeS2 films could be obtained and no other phases appeared even though the precursor films were sulfurized only for 1 h. It should be attributed to the porous structure of the precursor films provided the short path for atomic diffusion and a large surface for the sulfurization reaction. With the increase of the photon energy, all the FeS2 films prepared under different sulfurization parameters have the similar photoabsorption coefficient curves and the absorption coefficients are approximate to 105 cm-1 in the regions of high photon energy. The band gap of the FeS2 films reduced with the increase of sulfurization temperature, but not sensitive to the sulfurization time. The dominating transport carriers of the FeS2 films are found to be hole (P-type). With the sulfurization temperature increasing, the electrical resistivity increases and carrier concentration decreases. However, with the sulfurization time increasing, the carrier concentration increases and the carrier mobility decreases while the electrical resistivity firstly tends to decrease and then increase.The introduction of Cu and Al do not change the crystal structure of the cubic FeS2 films, but have a significant effect on the microstructure and photoelectrical properties. All doped films are N-type. The FeS2 films with a low doping concentration have fine grains and smooth surface, but the carrier concentration increases more than an order of magnitude. The films with a high doping concentration may have an enrichment of the impurity ions at some place, and it could impair the photoelectric properties of the device made by FeS2 film.TiO2 nanocrystal (NC) and nanotube arrays (NT) were successfully fabricated by sol-gel process and anodic oxidation method. FeS2/TiO2 composite films with different structure were prepared by the sol-gel and electrodeposition method. The effect of preparation technology on the microstructure, optical and photoelectrochemical properties of the composite films was investigated. The results show that FeS2/TiO2 (NC) film obtained by the sol-gel method have a good microstructure, but the performance is poor. The main reasons for this result are electrode structure, interface matching, electrode contact and internal defects in the materials. Compared with the FeS2/TiO2 (NC), FeS2/TiO2 (NT) films are more advantageous to improve the photocurrent response. It should be attributing to the high porosity and the large real surface area of the TiO2 nanotube arrays.