The objective of METACELLS project is to develop the epitaxy technique for the growth of metamorphic semiconductor structures that allow to realize the full efficiency potential of concentrator multijunction solar cells, paying special attention to the subsequent manufacturability and cost-effectiveness of the solar cells developed.Eventually, this growth technique will allow to fabricate a monolithic multijunction (3 - 4 junctions) solar cell exhibiting ~75% of the maximum theoretical efficiency calculated for the standard direct terrestrial spectrum (i.e. approaching 50%) at 1000 suns, using only one growth step and minimizing costly post-processing.The implementation of the optimum subcells bandgap combination, using materials with the required photovoltaic quality will be pursued. The metamorphic MOVPE growth technique will be revisited by in-depth understanding of strain and relaxation dynamics during growth of lattice-mismatched layers, using in-situ monitoring, which will be the central research topic. Development and functional multijunction cell structures, including the tunnel junctions, optimized for operation at 1000 suns, is aimed.The epitaxy routines for the growth of the step-graded buffer layers will be cost-optimized so that the process time and gases consumption are reduced while keeping the material quality unaffected. A decrease around 10% is projected to be attained, resulting in a similar reduction in fabrication cost.The accomplishment of METACELLS project objectives will contribute to widen current European excellence in high efficiency concentrator cells development and industrialization, allowing the many companies involved in this business to lead the market with the best performing product. This will enable Europe’s involvement in further research for solar cell development and will pave the way for the attainment of a photovoltaic technology suitable for mass production of electricity at competitive cost.