Over the course of this three-year research period, the activities conducted by RSE aim to progress from laboratory experiments on materials to the development of innovative, high-efficiency prototypes of concentrator photovoltaic (CPV) cells, thereby raising the Technology Readiness Level (TRL) from 3 to 4. Specifically, the final goal of Line of Activity 1.01 of Project RdS 1.1 is to develop 4-junction CPV cells, to be used in high solar concentration modules, by combining group III-V materials with those from group IV, as well as to develop high-efficiency InGaP cells for use in luminescent modules that can be integrated into buildings.
The research activities focus on both material growth aspects, using the MOCVD deposition system, and their characterization and post-growth processes for the final realization of CPV cells. For growth activities, innovative solutions have been developed to prevent the formation of tin segregates during SiGeSn deposition, which otherwise limit efficiency values. Initial experiments have also been conducted to integrate the SiGeSn/Ge heterostructure with III-V elements (GaAs, InGaAs, AlAs).
The development of InGaP cells has validated the predictions made in the previous three-year period, with measured power output being 100% higher than that of silicon cells currently used in luminescent modules. In the area of post-growth processes, the development of the electron beam lithography process for the fabrication of miniaturized photovoltaic cells, measuring 2.4 x 2.4 mm², has been completed. Additionally, anti-reflective oxide deposition processes using Ta2O5, SiO2, and Nb2O5, with the PVD (Physical Vapor Deposition) technique via magnetron sputtering, have been developed. These have enabled the creation of “metamaterials” with a gradually varying refractive index from 1 to 3, thus increasing light transmission within the photovoltaic cell.
Finally, two techniques for thinning the cell substrate have been selected: the Taiko process, in which the back of the wafer is removed with a grinder, and the Epitaxial Laser Lift Off (LELO) process, which separates the cell from the substrate by generating defects at their interface. The materials deposited by MOCVD and PVD have been characterized using scanning electron microscopy, profilometry, X-ray diffraction, ellipsometry, and X-ray fluorescence.