Cu2ZnSnSe4 (CZTSe) is a promising thin film photovoltaic absorber material. However, up to now its main problem is the low open circuit voltage (Voc) compared to more mature thin film photovoltaic technologies. Most kesterite synthesis routes employ a two-step approach consisting of precursor deposition followed by a crystallization step. In this study, a Ge-assisted crystallization process for nanocrystalline CZTSe precursors is presented. This process shows a strong beneficial influence on crystal growth and further solar cell device performance, especially the open circuit voltage. Detailed morphological and structural analyses as well as in depth solar cell device characterization are presented for different Ge doping. For low Ge doping an increase in charge carrier density is observed, resulting in devices with 8.5% efficiencies without antireflective coating and Voc of over 470 mV, which corresponds to Voc deficits (Eg/q – Voc) below 580 mV, a value comparable to current record devices. High Ge amounts, despite enhancing grain growth, deteriorate cell performance. Admittance spectroscopy measurements identified the appearance of a deep defect for high Ge doping, and this may be the cause for this deterioration. These results indicate that an accurate control of group IV (Ge, Sn) elemental composition during crystallization may be mandatory for high device performance. In view of that we propose a defect model based in the interaction of group IV elements with Cu, giving important insights into the understanding of fundamental limitations of current kesterite device performances.