Effect of ruthenium alloy on the band gap value of FeS2-pyrite

Abstract

In the aim of increasing the band gap value of FeS2-pyrite thin films having good crystallinity, high absorption coefficient (∼105cm−1) and a band gap of about 0.95eV, which were synthesized by a simple and low cost method consisting of sulphuration, under vacuum (≅10−4Pa), of amorphous iron oxide thin films pre-deposited by spray pyrolysis of FeCl3·6H2O (0.03M)-based aqueous solution onto glass substrates heated at 350°C, we draw attention in this work to the fabrication of these films after alloying with Ru. We followed two methods: the first one consists of spraying aqueous RuCl3·3H2O solution, during shorter time, on heated pre-deposited oxide layer at the same spray conditions with molar ratio as RuCl3·3H2O:FeCl3·6H2O=x:1−x (x=0.3966, 0.1586, 0.0396, 0.0317, 0.0156 and 0.00). The second consists of spraying on heated substrate, an aqueous solution prepared by dissolving ferric chloride (FeCl3·6H2O) and Ruthenium(III) chloride hydrate (RuCl3·3H2O) with molar ratio as RuCl3·3H2O:FeCl3·6H2O=x:1−x (x=0.3171, 0.1586, 0.234, 0.0119, 0.0051, 0.0025 and 0.00). Afterward, the as obtained films are sulphured at the optimum conditions (pressure≅10−4Pa, duration=6h, temperature=450°C). Dark layers having granular structure, were obtained. The effect of alloying on atomic structure, as well as optical properties of Ru-alloyed FeS2-pyrite films were examined by XRD, optical and MEB characterisations. Our results show that the band gap value of Fe1−xRuxS2 layers increases versus the alloy percentage. An optimum band gap value was obtained according to the first method of about 1.48eV for x=0.0156; which is considered a very interesting result for the photovoltaic applications of our films. An increase of the band gap value versus the Ru concentration with the second method was observed, as well.