Sputtering of thin pyrite films

Abstract

The high production costs of conventional solar cells make it necessary to investigate alternative production methods and materials with respect to their suitability for solar energy conversion. Semiconductors with high absorption coefficients are favorite candidates because of their potential to reduce material costs by using a thin film as active layer. One of these materials is pyrite, which is in the ideal case of composition FeS 2 and has an extraordinary high optical absorption coefficient (a>~6x 105 cm -1 for hv> 1.3 eV) [1]. A pyrite film of 0.1 ~m thickness absorbs more than 99% of the sunlight (AM 1.5) and makes the material very promising for solar cell applications. The band gap energy of synthetic crystals was measured [1,2] to be 0.92 to 0.95 eV and, at room temperature, electron mobilities in the range from 100 to 360 cm2/V s were found [3]. Problems with the material arise in connection with the open circuit voltage Uoc measured in photoelectrochemical cells [1] which are limited to 0.2 V until now. In principle it should be possible to yield half the band gap energy as open circuit voltage [4], i.e. ca. 0.45-0.5 eV for pyrite is expected. Since the efficiency of a solar cell is proportional to Uo~ this quantity must be enhanced to make the material valuable for technical applications. The small values measured for Uoc are perhaps due to the small difference of Fermi levels between pyrite and the electrolyte used (iodide-iodine). This assumption is supported by recent electro-reflectance measurements [5]. Another explanation concerns the strong non-stoichiometry in pyrite, that can lead to a sulfur deficiency in the percentage range as shown in recent works [6,7]. The sulfur defects were shown to be simple point defects for which ligand field theory predicts the development of electronic defect states in the