By combining III-V compounds with group IV elements in the same Metal-Organic Chemical Vapor Deposition (MOCVD) growth chamber, RSE developed the first prototypes of 2-junction (2J) III-V/ Ge/Ge cells with three terminals – something never created before – which form part of the 4J GaAs/ InGaAs/InGaP/Ge/Ge device. A comparison between the simulations of External Quantum Efficiency (EQE) curves and experimental data has allowed us to assume the presence of a ‘transistor effect’, which, due to a significant increase in the EQE of the bottom cell, could be used in the future to reduce the cell thickness. For the realization of the deep contact of the 2J, 3-terminal cell, an adequate MESA process has been developed, using new lithographic masks and appropriate chemical etchings. The cells, which measure 1×1 mm2, required the design of a new receiver and the optimization of the wire bonding parameters for soldering. The development of anti-reflection coatings (ARC) on triple junction cells continued, with a study of GaN deposition via Radio-Frequency Magnetron Sputtering, as well as the modeling and creation of 2- and 3-layer ARCs also with mixed compositions and nanostructures. The results obtained provided useful indications for the development of ARCs on future 4J cells. The characterization of ARCs took advantage of the use of X-ray reflectivity and grazing incidence X-ray diffraction techniques. The installation of a “Knife Edge Collimator” made it possible to improve the spatial selectivity of the X-ray beam incident on the sample and the angular resolution in the reflectivity measurements. In parallel with the activity on the 4J cell, a prototype of an InGaP cell for luminescent concentrator modules was also created, equipped with a new metal grid which allowed to increase the fill factor value from 80% to 85% and the conversion efficiency from 39% to 47% with reference to the spectrum emitted by the luminescent material. The prototype InGaP cell and the double junction cell were analyzed via the Electron-Beam Induced Current technique, which provided interesting results on the correlation between morphological defects and recombination centers and the position of the p/n junctions. The results obtained in this LA are a significant step forward in the development of high-efficiency, lower-cost 4J cells.