Recently, kesterite Cu2ZnSn(S,Se)4 materials have been identified as a promising indium free absorbers for the efficient low-cost thin film solar cell devices. Currently, the record efficiency of 12.6 % has been achieved on the hydrazine processed absorber layers, although the questions about the intrinsic defects and the limiting factors responsible for the large open circuit voltage deficit in these materials remain open. The incomplete picture on the point defect transitions levels in these materials arises from the fact that most of the defect characterizations were performed either by photoluminescence or by admittance spectroscopy and often involved either sulfur compounds only or a limited range of sulfur/selenium ratios or varied cation-ratios. The present work aims at filling the apparent gap in the electrical and optical defect investigations in kesterite solar cells with (i) varying anion content and (ii) fine tuning of the Sn content. The detailed and systematic characterization of the electronic properties of the active layers by means of the capacitance–voltage profiling, admittance, photoluminescence, and photovoltaic performances has been performed. It was found that: (i) with increasing sulfur content more shallow-acceptor defects tend to become slightly deeper, while the deep-acceptor defects remain at an approximately constant energy with respect to the vacuum level (ii) “Cu poor–Zn rich” conditions are necessary yet not sufficient to ensure good photovoltaic performances and (iii) fine tuning the Sn comprised in the CZTSSe layer offers a strong leverage on the density of defects present in the band gap of the photoactive materials.