Formation mechanism of secondary phases in Cu2ZnSnS4 growth under different copper content

Understanding the formation mechanism of the secondary phases in Cu2ZnSnS4 (CZTS) absorber layer is necessary for the development of CZTS thin films solar cells. With this aim, the XRD, Raman and SEM were used to investigate the phases coexistence in CZTS thin films grown under different copper content. The surface of the Cu-rich CZTS thin films is rough, and the secondary phase CuxS emerges as front surface phases. Besides, another secondary phase ZnS segregates towards both front and back regions of off-stoichiometric CZTS thin films (Cu rich or Cu poor), which was evidenced by the 2nd and 3rd order of ZnS using Raman analysis excitated at 325 nm. The pure phase CZTS exists only as its composition was stoichiometric. So the preparation of pure phase CZTS is sensitive to composition, suggesting that the precise composition control plays an important role in CZTS growth.