Low open circuit voltage (V-OC) has been recognized as one of the principle limitations in the current generation of kesterite based solar cells. In order to investigate the origin and possible solution for this problem, the influence of point defects and secondary phases on the performance of Cu2ZnSnSe4 (CZTSe) devices is explored. Compositionally graded CZTSe films have been prepared with nearly 200 solar cells, each with minute changes in cation composition. Systematic structural characterization of these absorbers with Raman spectroscopy and X-ray diffraction is used to detect secondary phases and defect clusters, and their relative concentration is correlated to device properties. The presented results show the experimental evidence of the effect of the Cu-substitutional defects, in particular [V-cu+Zn-cu], on the optoelectronic properties, especially V-OC. It is shown that V-OC can be tuned by adjusting composition, and consequently the amount of the [V-Cu+Zn-Cu] defect clusters. Furthermore, it is proven that this is intrinsic property of the CZTSe material, independent of the synthesis and post-deposition processes used for the preparation of samples. Secondary phases are also found to influence device properties, and their effects are dependent on whether they are present in the bulk or surface of the absorber. (C) 2016 Published by Elsevier B.V.