Nano-crystalline Thin Films of Cu2ZnSnS4 Prepared with Different Compositions by Dip-coating Technique
A.E. Rakhshani, A. Bumajdad, F. Al-Sagheer, S. Sunny
Cu2ZnSnS4 (CZTS) is a promising photovoltaic material which consists from nontoxic and naturally abundant elements and has a high optical absorption coefficient and a 1.5-eV optimum band gap (Shockley-Queisser limit). Its band gap can be tailored in the range of 1.0 – 1.5 eV by partial replacement of S with Se. This makes it even more attractive for use in tandem solar cells. The conversion efficiency of the photovoltaic cells based on this absorber has already exceeded 10%. The films can be grown by several techniques among which the solution-based methods are attractive due to their simplicity and the low costs involved. In this work a dip coating method was used to prepare three films with the stoichiometry, Zn-rich, and Zn-poor compositions. The films structure, composition, surface morphology, optical transition energies, and the activation energies of the acceptor defect levels were measured using various techniques which includeX-ray diffraction, energy dispersive X-ray analysis, atomic force microscopy, photocurrent spectroscopy and admittance spectroscopy. To demonstrate the device quality of the films, CdS/CZTS heterojunction diodes were fabricated and characterized. Regardless of the composition, all three films produced excellent devices showing a high current rectification factor (103 – 105) and low reverse biased saturation current (0.01 – 20 nA). Films having an acceptor concentration of 1017 - 1018 cm-3, were composed from CZTS nano-crystallites of size below 10 nm. The optical transition energies varied among the films, but all were closely matched with those predicted theoretically. Acceptor-type defect levels with the activation energies of 5.4 meV, 18.8 meV, 70 meV and 221 meV (in Zn-rich film), 114 meV and 300 meV (in stoichiometric film), and 38 meV (in Zn-poor film) were detected and all could be assigned to the theoretically known defects.