Zinc tin nitride (ZnSnN2) is a promising semiconductor candidate for solar cell applications and optoelectronic devices. However, the practical use of this material has been limited by several issues as, among others, oxygen contamination, a very high concentration of free carriers and the difficulty to reach the theoretically predicted band gap. Here, we deposit thin films of ZnSnN2 by reactive DC magnetron cosputtering at room temperature with an RF bias power (Pb) in the range 0-50 W. Using first principle calculations, we explore the structural and opto-electronic properties that are favorably compared to experimental results. We demonstrate that the optical band gap energy can be decreased from 1.7 eV to 1.34 eV, close to the predicted value of 1.37 eV. The free electron concentration is decreased down to 1017 cm-3 which results in the reduction of the absorption by free electrons in the IR range. In addition, in a given range of applied bias powers, we observe a densification of the films and significant decrease of their oxygen contamination from 6.7 down to 2.0 at. % . The study underlines that a value of 20 W power bias leads to the optimal structural, optical and electrical properties. Our results provide an interesting method to obtain a potential candidate for photovoltaic application, in an environmental friendly way, for a low-cost industrialization.