Effect of secondary phases controlled by precursor composition on the efficiency of CZTS thin film solar cell

Copper Zinc Tin Sulfide (CZTS) has emerged as a promising photo absorber material for thin film solar cell applications. However, the efficiency of CZTS based solar cell is predominantly governed by the presence and absence of various secondary phases. Identification and estimation of these phases are very crucial for understanding the parameters controlling the formation of these phases. In this work, CZTS thin films are prepared on soda lime glass by magnetron co-sputtering of Cu, Zn, and Sn followed by sulfurization. Precursor composition is varied to control the secondary phase formation during the growth of CZTS layer. Study of various secondary phases formed is done using XANES spectroscopy. Other analyses, like XRD, Raman spectroscopy and optical absorption spectroscopy are used to confirm the results from XANES. It is observed that precursor composition plays a vital role in the selective formation of secondary phase during the growth of CZTS layer. An interesting observation is that bandgap of the secondary phase vis-`a-vis CZTS plays a key role in determining the device efficiency. This study highlights that different secondary phases deteriorate the device efficiency in different manner. Secondary phase with much higher band gap then absorber layer deteriorates the device efficiency less compared to the secondary phase having bandgap closer to absorber layer. This also brings out the importance of identification of secondary phase that impact the device efficiency to a greater extent and process parameter that controls its formation.