Spray pyrolysis deposited CuSbS₂ absorber layers for thin-film solar cells

CuSbS₂ thin films were fabricated by spray pyrolysis from metal chloride aqueous solutions, followed by a post-deposition sulfurization step. The structural, chemical, optical and electrical properties of CuSbS₂ and the effect of various sulfurization temperatures on CuSbS₂ thin film have been systematically studied. We used a two-step sulfurization method. Step 1 at lower temperature was to encourage complete saturation of the as-deposited film with sulfur vapor. And step 2 at higher temperature was to promote the formation and crystallization of CuSbS₂. The sulfurization temperature of step 2 is very important for the formation of device-grade CuSbS₂ films. With the increase in sulfurization temperature, impurities such as Sb₂S₃ decreased and the crystallinity of CuSbS₂ improved. Until 400 °C, impurities disappeared and phase-pure well-crystallinity CuSbS₂ thin films were obtained. When the sulfurization temperature is higher than 400 °C, CuSbS₂ gradually changes to Cu₃SbS₄. The CuSbS₂ films sulfurized at 400 °C with optimum band gap of 1.53 eV are p type, and absorption coefficient is larger than 10⁵ cm⁻¹ in the visible light wavelength range. The temperature dependence of electrical conductivity of CuSbS₂ has been studied for the first time. At measurement temperatures higher than 140 K the electrical conductivity of the CuSbS₂ film is dominated by band conduction and nearest neighbor hopping (NNH). However, at temperatures below 140 K the conduction is predominantly affected by variable range hopping (VRH). Finally, thin-film solar cells based on the sprayed CuSbS₂ absorber layers with a maximum photoelectric conversion efficiency of 0.34% have been fabricated.