Effects Of Thickness And Substrates On Crystal Structure And Characterization Of The FeS₂ Films

As a semiconductor material, FeS2 (pyrite) has high optical absorption coefficient ( 5 l05cm-1 for 700nm) and suitable energy band gap (about 0.95eV). These beneficial behaviors can make pyrite a promising candidate as an absorber material in thin film solar cells. Moreover, another advantage of pyrite is that it is composed of nontoxic, abundant elements.Thin films of pyrite with different thickness have been synthesized by annealing iron films in a sulfur atmosphere on Si(111) and glass substrates. Their structural, optical and electrical characteristics have been determined and the effect of the film thickness discussed. The iron films with diffident thickness on both kinds of substrates can be completely converted into pyrite by sulfurating in 80kPa sulfur atmosphere at 400 for 20h. For the pyrite films on Si(111) substrate, the band gap and optical absorption coefficient tend to decrease with increasing film thickness. The grain size of the films rises and the lattice constant reduces when the film thickness increases up to 330nm. However, in the films thicker than 330nm, the grain size decreases and the lattice constant increases with increasing film thickness. For the films on glass substrates, the gram size and lattice constant decrease with the thickness increasing from 70nm to 300nm and increase as the thickness over 300nm. With increasing in the film thickness, electrical resistivity shows a generally rising tendency and carrier concentration and optical absorption coefficient a reducing tendency. The conducting type tends to show n-type when the film is thicker.The FeS2/TiO2 films have been prepared as the photoanode in a photoelectrochemical set-up and their photoactivity also investigated. The photoelectrical conversion efficiency first decreases and then increases with the thickness of the films, and is generally at a rather low level.Pyrite films with 330nm in thickness have been prepared on the substrates of Si(100), Si(111), Al, TiO2 and glass by annealing the iron films in sulfur atmosphere. The crystal structure has been determined and orientation texture discussed for the films. The results show that the texture distribution of the film can be controlled in a certainextent by using the substrate with different crystalline type. The pyrite films prepared on the substrates Si(100), Si(111) and Al have only one preferred orientation (200) while they on the microcrystalline TiO2 substrates share two preferred orientations (200) and (220). There is an insignificant effect of amorphous glass substrates on orientation distribution in the film growth. Various substrates induce different interfacial mismatch between the film and substrate and result in the changes of lattice distortion degree, grain size and texture distribution. The film surface energy and natural orientation of grain growth mainly dominate the resultant distribution of crystal orientation and induce small lattice distortion and fine grains in the film if the substrate is amorphous or has high interfacial mismatch. Besides the film surface energy and natural orientation of grain growth the interface strain energy participates in the domination to the resultant distribution of crystal orientation and tends to induce strong lattice distortion and coarse grains if the substrate is crystalline and has low interfacial mismatch.