Cu2ZnSnSe4 materials and solar cells using 2SSS processing

Because of the anticipated high demand for Indium ongoing growth of CIGS technology may be limited. Kesterite materials which replace Indium with a Zn/Sn couple are thought to be a solution to this issue. However, efficiencies are still below the 10% level, and these materials are proving to be complex. Even determination of the bandgap is not settled because of the occurrence of secondary phases. We use a film growth process, 2SSS, which we believe helps control the formation of secondary phases. Under the right growth conditions we find 1/1 Zn/Sn ratios and XRD signatures for Cu₂ZnSnSe₄ with no evidence of secondary phases. The optical absorption profile of our films is also a good match to the CIS profile even for films annealed at 500°C, and we see no evidence of phase separation. The effect of intentional variation of the Zn/Sn ratio on material and device properties is also presented and suggests that a given composition might not produce a unique bandgap.