Effects Of Process Parameters On The Photoelectrical Characteristic FeS₂ Thin Films Prepared By Sulfurizing The Electrodeposited Precursive Films

The iron pyrite (FeS₂) with cubic crystal structure has attracted considerable attention as a potential candidate for the absorber materials for photovoltaic applications or thin-film solar cells due to its high absorption coefficient (α≥5×10⁵cm^-1 as λ≤700nm) and suitable energy band gap (Eg≈0.95eV). Moreover, the constituent components of FeS₂ are abundant, cheap and non-toxic.In the investigation, the precursive Fe₃O₄ films were prepared by constant current electrodepositing and oxidation treating. The polycrystalline FeS₂ films were obtained by annealing the Fe₃O₄ in sulfurizing atmosphere. The effects of the sulfidation parameters, such as temperature, time and sulfur vapor pressure, on the microstructure and photoelectrical characteristics of FeS₂ thin films were investigated. Some research results were obtained as follows:The polyporous Fe₃O₄ films can be obtained by electrodepositing in the aqueous solution of Na₂S₂O₃ and FeSO₄ and annealing at 200 ℃.The reaction transformed from Fe₃O₄ to FeS₂ occurs under the condition of sulfurizing at 400 ℃ only for 2h. The FeS₂ thin films prepared by sulfurizing the precursive Fe₃O₄ films for a shorter period show a polyporous morphology similar to the aspect of precursive films. With prolonging the sulfidation time, the conversion reaction from Fe₃O₄ to FeS₂ tends to be further complete to results in the FeS₂ grain size propagates, lattice parameter decrease and polyporous morphology disappears. As a result, the optical absorption coefficient, electrical resistivity and charge carrier concentration increase. The electrical properties show an insignificant change when the sulfidation time is longer than 10h.There is an insufficient sulfurizing reaction of the precursive Fe₃O₄ to result in a mixed film structure of FeS₂ and Fe₃O₄ at lower sulfur vapor pressures. However, there is an inexpectant sulfurizing reaction in the substrate films beneath the Fe₃O₄ films at higher sulfur vapor pressures. The precursive Fe₃O₄ films are completely transformed into the polycrystalline FeS₂ films by sulfurization annealing at the sulfur vapor pressures higher than 20kPa. The morphology of the FeS₂ films can change from porous and loose structure intosmooth and compact structure with increasing the sulfur vapor pressure. The change of the sulfur vapor pressure can alter the microstrain level of phase transformation and the concentration of crystal point defects consequently to result in the possible variations in film geometrical integrality and defect energy state distributed in forbidden band. Therefore, the optical absorption properties of FeS2 films can change with the sulfur vapor pressure and there is the minimum value of absorption coefficient as sulfurization annealing at 40kPa.Obvious reaction transformed from Fe3C>4 to FeS2 can occur during sulfidation annealing at 300 °C. A higher temperature than 400°C makes a nearly complete transformation. However, higher sulfidation temperature has a disadvantage effect on film quality. For example, some unexpected reaction products can easily be formed in the substrate film at 500°C and the FeS2 grains heavily coarsened at 600 °C. With increasing the sulfidation temperature, the grain size of the FeS2 thin films increases and the presursive morphology disappears to display the granular-crystalline aspect because the crystallizing characterization is improved in higher sulfidation temperatures. The approximately normal lattice parameter and the maximal absorption coefficient are obtained in the FeS2 film from the sulfidation annealing at 500 °C. Moreover, the electrical resistivity decreases while the carrier concentration increases with increasing the sulfidation temperature.