Fabrication of ternary Cu–Sn–S sulfides by a modified successive ionic layer adsorption and reaction (SILAR) method

Abstract

Ternary Cu–Sn–S chalcogenides, Cu₂SnS₃, Cu₅Sn₂S₇ and Cu₃SnS₄, have been successfully synthesized by annealing three precursor film samples deposited via a modified successive ionic layer adsorption and reaction (SILAR) method. The mechanism of ion-exchange and improvement of the rinsing procedure were introduced into the SILAR process for the purpose of achieving the codeposition of different metal sulfides and increasing the growth rate of thin films. The crystal structure, composition, surface morphology, optical and electrical properties of three ternary sulfide samples have been characterized. The temperature dependence of the Seebeck coefficient, electrical conductivity, thermal conductivity and ZT values of the Cu₃SnS₄ thin film sample have also been measured between 293 K and 573 K. Owing to the intrinsic advantages of the SILAR method and the improvement of the SILAR process, ternary Cu–Sn–S thin films can be deposited on glass substrates at a speed of 400 nm per hour and the surface morphologies of the thin films are comparable with those of thin films prepared by vacuum based methods, which can satisfy the requirements for high quality, high efficiency and low-cost production of thin films. With the appropriate band gap energies (1.0 eV, 1.45 eV and 1.47 eV for Cu₂SnS₃, Cu₅Sn₂S₇ and Cu₃SnS₄ respectively) and considerable absorption coefficients (α > 10⁴ cm⁻¹), most importantly with earth-abundant elements, Cu–Sn–S thin films can be used as alternative absorber layer materials in thin film solar cells. Additionally for the Cu₅Sn₂S₇ and Cu₃SnS₄ film samples, some novel properties (such as strong optical absorption in the NIR band, excellent conductivity and suitable carrier concentration) make them attractive for potential research interest as thermoelectric materials.