Optical And Electrical Properties And Photoelectrical Conversion Of FeS₂ Thin Films

The FeS₂ (pyrite) with cubic crystal structure has shown an important research value as a promising novel solar cells material due to its suitable energy band gap (E_g ≈0.95 eV), high optical absorption coefficient (α≥5×10⁵ cm^-1 for λ ≤700 nm), excellent environmental compatibility and low manufacturing cost.In the investigation, the polycrystalline FeS₂ thin films were prepared by thermal sulfurizing the precursive iron films obtained by magnetron sputtering. The effects of film thickness and crystallizing status of precursive iron films on the microstructure and photoelectrical properties of FeS₂ thin films were investigated. The FeS₂ thin films were used as sensitizers in the system of sensitized nanocrystalline solar cell. The sensitizations of TiO₂ porous films with FeS₂ thin films of different grain sizes were discussed. The main research results can be drawn as follows:In the process of sulfuration reaction, film volume expansion, sulfuration reaction perfectibility, rigid restraint level and lattice microstress can change with film thickness and affect the lattice distortion degree. There exist fine crystallites in all the sulfurized films. The thicker films show a smoother surface and more uniform microstructure than the thinner films. The carrier concentration decreases while the carrier mobility increases with increasing in film thickness until 400 nm. Furthermore, the carrier concentration increases while the carrier mobility decreases as film thickness is over 400 nm. There exists a maximum electrical conductivity at the film thickness about 130 nm.Higher substrate temperatures can result in larger crystallite scale and better crystallinity of the precursive iron films. Sulfuration reaction at 400 °C induces the change of precursive iron with various crystallizing levels into FeS₂ with fine crystallites. The geometrical completeness of film bulk increases with the increase of substrate temperature in depositing the precursive iron films. With the increase of iron grain size, the carrier concentration decreases and the mobility increases in the FeS₂ films. The electrical resistivity reaches a maximum, while the optical absorption coefficient and band gap exhibite a minimum respectively in the FeS₂ films sulfurized from the iron films of grain size about 39 nm. The precursive iron films with different grain size or crystallizing status affect the atom diffusion behavior and intergrnular cracking tendecy induced by transformation stress in the process of sulfuration reaction.The crystal defect density, transformation stress level, intergranular crack amount, atom diffusion behavior and two-layer film structure are responsible for the characteristics of the optical and electrical properties dependent on the crystallinity and continuity of the pyrite films or the crystallizing status of the precursive iron.Photoelectron in the p-conduction FeS₂ films could be more effectively injected into the conduction band of n-conduction TiO₂ film compared with the n-conduction FeS₂ films due to the build-in voltage in the junctions. FeS₂/TiO₂ composite films were prepared at different sulfuration temperatures. The optical absorption actions of composite films are improved by FeS₂ films. All the composite films show a low photoelectric current. The defects such as the transition phases from incomplete sulfuration at 300℃ increase the dark current in the films. Moreover, the modifications of TiO₂ porous films with larger FeS₂ particles in the FeS₂ films sulfurized at 400℃ decrease the effective surface for the injection of photoelectron.