This project targets building a proposal for a H2020 FETOPEN- Research and Innovation Actions (FETOPEN1) call of the work programme 2016-2017. It consist in two main actions consisting, on the one hand, to complete and validate the consortium, based on pre-existing informal contacts, and, on the other hand, to write the proposal. The core network is based on two French partners, CNRS (including two laboratories, IEMN and IJL) and CEA-INES (LITEN/DTS) as well as a German partner, Fraunhofer Institute for Solar Energy Systems. The targeted FETOPEN project will aim demonstrating the feasibility of a high efficiency solar cell which figure of merit in €/W should be ultimately in line with the crystalline silicon one. We will use an inorganic/inorganic tandem cell concept using earth abundant and nontoxic raw materials. Fabrication processes will be matched and up scalable to mass production. Today, such a tandem cell concept appears among the best solutions exhibiting cells with 30% and more efficiency. The approach is based on the use of a crystalline silicon bottom cell, in homo- and hetero-junction structure, since these technologies are already well established and display optimised efficiency values as well as stability and cost performance. The technological breakthrough is more particularly located in the development of a thin film technology for top cell using earth abundant materials. The Zn(Sn,Ge,Si)N2 material line is proposed since its bandgap energy is in perfect matching with silicon one for a tandem cell structure. It is a most challenging and exploratory material line, only a few American groups started to work on it. The second challenge will be to combine such a nitride cell with an usual silicon cell in a monolithic (2-terminal) tandem cell. Research will be devoted to the development of theoretical and experimental background on the different materials required for the top nitride cell, the design and fabrication of such a cell, the design of an optimum structure of tandem cell, including the mandatory tunnel junction, the fabrication of demonstrators and their characterization. The envisaged consortium will be composed of European partners with matched expertise allowing this multidisciplinary (material growth, modelling, cell design, cell fabrication and characterization among the main topics) problematic to be answered. Such a success will pave the way to develop a new high efficiency solar cell concept meeting cost and lifetime demands