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Crystallinity of Nanostructured Cu2S Thin Films Deposited by Spray Pyrolysis. A Defect Chemistry Approach

Luminita Isac ,  Dana Perniu ,  Anca Duta 

Transilvania University of Brasov, Centre Product Design for Sustainable Development, Eroilor 29, Brasov 500036, Romania

Abstract

Copper sulfides (CuxS, x = 1.8-2) thin films are recognized as attractive materials for Solid State Solar Cells (SSSC). The optimum photovoltaic efficiency is obtained when chalcocite (Cu2S) is used as p-type semiconductor and/or visible-light absorbing layer in a junction with TiO2. Depending on the deposition conditions, Cu2S shows mixed electronic-ionic conduction, with predominant electronic conductivity. The p-type conduction is the result of crystalline point defects (free holes from acceptors levels of copper vacancies), while ionic conduction is due to the Cu+ ions mobility within the crystal lattice.

Chalcocite thin films were deposited onto TCO (SnO2:F) glass by chemical spray pyrolysis. As precursors, aqueous and mixtures of water:alcohol solutions of copper(II) chloride and thiourea with Cu:S molar ratio in the range 0.32-0.35 have been used. Depositions were done at 235 oC, respectively 285 oC. The X-ray diffraction analysis of as-deposited thin films showed that Cu2S may have one of the tetragonal, orthorhombic or monoclinic crystal structure. The films crystallinity can be controlled by varying the deposition parameters: precursors’ ratio and concentration, solvent and deposition temperature. According to crystallographic data (distance between lattice planes, d, lattice parameters, a, b and c, average crystallite sizes, D, average lattice strain, ε), the crystalline point defects in Cu2S crystal lattices are identified. The defect chemical equations, using the Kröger-Vink notation, are proposed, explaining the defect formation and the conduction mechanisms in Cu2S thin films.
 

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Presentation: Poster at E-MRS Fall Meeting 2007, Symposium B, by Luminita Isac
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-10 16:51
Revised:   2009-06-07 00:44