Accuracy in composition control has been one of the top issues for fabricating high-performance kesterite (Cu2ZnSn(Se,S)(4)) solar cells. A detailed understanding of the effect of Zn excess on device performance has not yet been demonstrated. Thus, specific criteria for high-performance devices, in particular discriminating between the effects of Zn-rich features at the front versus the back of the absorber, are desired. In this study, we report that co-evaporated kesterite absorbers can demonstrate high device efficiency despite the presence of large quantities of ZnSe. However, the benign presence of ZnSe is found to be conditional. While large ZnSe grains on the back of the absorbers are not harmful to device performance, the ZnSe grains produced by excess Zn near the end of the deposition degrade the cell efficiency from 8% level to 6% level (without anti-reflection coatings). The other effect related to excess Zn on the front of absorber is the facilitation of breakdown in lower reverse bias. The breakdown indicated here occurs only under the illumination of blue photons, and to our best knowledge has not been reported before. The exact mechanism of the breakdown remains open, but it is demonstrated to be related to the photoconductivity of CdS, and is thus possibly a symptom of lateral defect issues in the absorber, caused by the overdose of Zn. The same type of issue contributing to the breakdown may also be responsible for part of the parasitic loses at the working voltage, and therefore warrants further research. (C) 2013 Elsevier B.V. All rights reserved.