Effect of Sulfurization Temperature on Properties of Cu2ZnSnS4 Thin Films and Diffusion of Ti Substrate Elements

The addition of flexible Cu2ZnSnS4 (CZTS) thin film solar cells to titanium (Ti) substrates is an attractive way to achieve the low-cost manufacturing of photovoltaics. Prior research has indicated that the appropriate diffusion of Ti elements can enhance the crystalline growth of CZTS films. However, the excessive diffusion of Ti has been shown to adversely affect the photovoltaic performance of CZTS photovoltaic devices. Therefore, it is essential to regulate the diffusion of Ti elements within CZTS thin films to optimize their photovoltaic properties. The tendency for Ti substrate elements to diffuse into CZTS films is also influenced by the activation energy associated with these Ti elements. The sulfurization temperature is posited to be a critical factor in modulating the diffusion and activation energy of Ti elements within CZTS thin films. Consequently, this research investigates the alteration of the sulfurization temperature of CZTS thin films in order to enhance the properties of these thin films and to examine the diffusion behavior of titanium elements. The results reveal that as the sulfurization temperature increases, the diffusion of Ti elements within the CZTS thin films initially increases, then decreases, and subsequently increases again. This pattern suggests that the diffusion of Ti elements is affected not only by the activation energy of the Ti elements but also by the defect hopping distance within the CZTS thin films. Notably, at a sulfurization temperature of 550 °C, the grains at the base of the CZTS thin film demonstrate an increased density, which is associated with a reduced defect hopping distance, thereby hindering the diffusion of Ti elements within the CZTS thin films. Furthermore, at this specific sulfurization temperature, the slope of the current–voltage (I–V) curve for the CZTS/Ti structure reaches its maximum, indicating optimal ohmic contact characteristics.