As the development of kesterite solar cells accelerates, the bottlenecks in device performance need to be identified and ways for their circumvention defined and developed. In this work, we use 2-dimensional (2D) numerical simulations to explore possible reasons for low open-circuit voltage (V-oc) in Cu-2(Zn,Sn) Se-4 (CZTSe) solar cells. High defect density in the CZTSe absorber and at the CZTSe/CdS interface can be significant reasons for V-oc deficit, but they do not explain all of the losses observed experimentally. Local deviation from stoichiometry could create secondary phases with a lower band gap compared to the absorber. These secondary phases can be severely harmful to V-oc if located in the vicinity of the heterointerface and along the grain boundaries. (C) 2014 Elsevier B.V. All rights reserved.