Study on the impact of stoichiometric and optimal compositional ratios on physical properties of Cu₂ZnSnS₄ thin films deposited by spray pyrolysis

Cu₂ZnSnS₄ (CZTS) thin films processed by spray pyrolysis technique have attracted a great deal of attention from scientific community for solar cell applications. It is well known that Cu-poor and Zn-rich conditions are required for a good solar cell performance regardless of the method used for CZTS thin film deposition. However, few reports are found along the literature to explain such results. In this work, a comparison on the physical properties of CZTS thin films processed under stoichiometric (Cu₂ZnSnS₄) and optimal (Cu_1.6Zn_1.1Sn_0.9S₄) compositional ratios by spray pyrolysis method is presented. The impact of different substrate temperatures on structural, morphological, optical and electrical properties considering both compositional conditions is studied. For this purpose, characterization techniques such as XRD, Raman, x-ray fluorescence, EDX, AFM, transmittance and reflectance measurements, I-V, I-T and Hall measurements were performed. It is demonstrated that samples deposited under optimal compositional ratios present better structural and morphological properties as well as lower contribution of secondary phases along with band-gap energy values near to 1.5 eV as required for solar cell applications. From the electrical point of view, higher resistivity and grain boundary barrier height values along with a lower density of defects at grain boundary are reported for CZTS deposited under optimal compositional ratios. In this sense, this work helps to clarify why stoichiometric condition in CZTS samples produces lower solar cell performances.