Pulse Electrodeposition Synthesis and Characterization of Sns Thin Films Obtained from Dimethyl Sulfoxide Solutions

Abstract

Thin film materials for solar photovoltaic energy conversion have been considered a good option to provide a low cost power supply for a progressive increasing demand across the world. The most intensively studied and commercialized thin film solar cells are those based on (CIGS), Cu(In,Ga)Se2 and CdTe. Concerns regarding the cadmium because its toxicity and the limited abundance of indium and gallium could limit the future of these technologies. For these reasons alternative materials to develop new photovoltaic solar cells are currently investigated around the world. From the point of view of earth abundance, cost and toxicity SnS appears as a promising candidate. It has a direct optical band gap value (1.5 eV) which is within the optimum range for photovoltaic solar energy conversion. It exhibits p-type conductivity and high optical absorption coefficient meaning that only some microns of thickness could absorb most of the incident light. Theoretical calculations predict a conversion efficiency of 24% for materials with the same band gap1, however the reported efficiency for solar cells based on SnS is less than 2%2. Such low efficiencies were attributed to factors like poor quality of the films, grain boundaries, lower thickness employed, insufficient diffusion lengths in the layers and large band discontinuities. Hence, in order to obtain better efficiencies in SnS based solar cells the quality of the layers need to be substantially improved. Electrodeposition is one of the economical methods for large surface coating and has been successfully used to prepare semiconductor films. The method allows the control of the bandgap width and the doping level through the control of variables such as solution composition, applied potential, pH and working temperature. Furthermore, monitoring the circulating charge it is also possible to control the thickness of the deposited layers. Metal chalcogenides have been generally electrodeposited by the cathodic co-reduction of the metal and chalcogenide ions. However, compositional heterogeneity is usually a problem with this approach. An attractive option involves the use of aprotic deposition baths such as dimethylsulfoxide which presents a high boiling point and also allows the solubility of the chalcogene precursor in nonionized, molecularly dissolved form. In this solvent it is possible to form binary compounds starting from the electrochemical reduction of the elementary chalcogen precursor and the consecutive formation of a chalcogenide anion that further precipitates heterogeneously onto the surface of a convenient substrate for the formation of II-VI semiconductors. In current work SnS thin films were prepared on molybdenum substrates by pulsed electrodeposition at differents synthesis times. The structure, morphology and composition were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and Raman spectroscopy. The optical and electrical properties were studied by UV–vis absorption and capacitance-potential curves, respectively. The electrodeposited SnS films showed p-type semi-conductive characteristics and exhibited cathodic photocurrent under visible light illumination. Highly compact, dense, homogenous and almost stoichiometric films were obtained by properly selecting the electrodeposition parameters. These results show that the as grown films show composition and thickness compatible with the requirements of SnS based solar cell devices Acknowledgements: ToEuropean Commission, NanoCIS project FP7-PEOPLE-2010-IRSES grant 269279 and to CONICYT (Chile), FONDECYT (Fondo de Desarrollo Cientifico y Tecnológico, Chile) through Project N° 3160217.