Research On Process Parameters And Optical And Electrical Properties Of Pyrite (FeS₂) Thin Films

The polycrystalline FeS₂ (pyrite) with a cubic system has shown an important research value to develop into a promising candidate for the manufacture of solar energy cells due to its suitable energy band gap, high optical absorption coefficient, abundant constituent resource in nature, excellent environmental safety and low preparation cost.On the basis of the previous research progress, in this thesis, the polycrystalline FeS₂ thin films were prepared by thermal sulfiirizing the precursory sputtered Fe films and electrodeposited Fe₃O₄ films. The effects of sulfurizing process parameters, crystal texture distributions and crystal planar defects on the microstructure and photoelectrical properties were investigated. The measured system of dye sensitization for FeS₂ solar cells was design and used to determine the photoelectrical conversion characteristics. Some significant results have been obtained with regard to the law and mechanism in the conversion from different precursory films to resultant FeS₂ films, the control and induction of preferred orientation in crystal growth, the preparation process of FeS₂/In₂S₃ composite films, the effect of the change of specific surface energy in the FeS₂ films, and the measurement technology on photoelectncal conversion characteristics from the optical anode of the films composite with FeS₂ and porous TiO₂. The main conclusions can be drawn as following:The FeS₂ films with an expected structure are obtained by annealing the Fe films from magnetron sputtering and the Fe₃O₄ films from electrolytic depositing at 400 ℃ sulfurizing temperature and 80 kPa sulfurizing pressure. The Fe₃O₄ films have a higher sulfurizing reaction rate to transform into the FeS₂ films than the Fe films so that the suitable sulfurizing time is 10h for the former while 20h for the latter.The FeS₂ films evolved from the precursory Fe films show smooth surface, coarse grains, high optical absorption coefficient and approximately normal band gap. The FeS₂ films evolved from the precursory Fe₃O₄ films have the porous morphology, small grain scale, high carrier density and high electrical resistivity. The FeS₂ films associated with In₂S₃ are formed by sulfurizing the precursory Fe₃O₄ films electrodeposited on the substrate of ITO films in the electrolyte with low pH values.There are some film defects, such as crystal lattice distortions, transition phases, vacancies, interstitial atoms, grain boundaries and geometric discontinuities in the FeS₂films obtained from the sulfuration of different precursory films. The internal stress from the specific volume change of phase transformation and the crystal point defects from the atomic diffusion during sulfuration result in the change of lattice parameters of the FeS2 films. The transition phases from incomplete sulfuration decrease optical absorption coefficient. The transition phases, vacancies and interstitial atoms can introduce the additional defect states in forbidden band and therefore decrease the band gap. Moreover, the band gap widens with the grain scale decreasing.The change of film thickness can result in the changes of specific surface area, orientation distribution ratio, internal stress distribution, transition phase volume fraction, crystal defect concentration and planar defect density. The crystallographic orientation (200) displays the highest ratio for the FeS2 films with the thickness range from 120 to 550 nm although the ratios of other crystallographic orientations can change to a certain degree with the change of the film thickness. With the film thickness increasing, the grain size increases until there exists a maximum grain size in the films of 380 nm thickness while the optical absorption coefficient, band gap and carrier concentration decrease and electrical resistivity increases. In general, the lattice parameter of FeS^I^Ss composite films is lower than the normal value and decreases with sulfurizing time prolonging.The crystallographic orientations of the FeS2 films can be controlled to a certain degree by changing the substrate materials with different crystal structure. The preferred orientation (200) exists in the FeS2 films from the sulfuration at 400 °C for the precursive Fe films sputtered on the substrates single crystal Si(lOO) slices, single crystal Si(lll) slices and aluminum sheets with (111) crystallographic texture. There are both preferred orientations (200) and (220) in the FeS2 films sputtered on the substrate of the TIO2 films with (101) crystallographic texture and small crystallites. There is an insignificant effect of amorphous glass substrates on the preferred crystallographic orientations.The substrates with amorphous structure or high lattice misfit to pyrite structure can hardly offer the effective interfaces to induce the crystallizing course or change the ratio of crystallographic orientations. Therefore, the distribution of FeS2 crystallographic orientations is mainly controlled by film surface energy and preferred orientations of grain propagation. The crystallographic orientations depend on the interface strain energy besides the film surface energy and preferred orientations of grain propagation if the FeS2 films from the precursory Fe films sputtered on the substrates with crystalline structure or low lattice misfit to pyrite structure ? The FeS2 films crystallized on the substrates of amorphousglass plates have a smaller lattice distortion and lower microstrain but a higher optical absorption coefficient than those crystallized on the substrates of single crystal Si(lOO) slices.The optical anode prepared by the composite films of crystallized FeS2 and porous T1O2 can be utilized to measure the photoelectrical conversion characteristics. The open-circuit voltage of the measurement system decreases with the film thickness increasing. The FeS2 films with a 540 nm thickness show maximum photoelectrical conversion efficiency.