Among the photovoltaic (PV) technologies developed to date, concentrating photovoltaics (CPV) show the highest conversion efficiency due to the integration of multi-junction (MJ) cells based on III-V materials in systems using both solar concentration and tracking.

CPV environmental impact is contained by the small device area, relative to the concentration factor, and can be further reduced by thinning and recycling the substrate (Ge or GaAs) on which the MJ cell structure is grown. The substrate removal and subsequent reuse for multiple epitaxial growths reduces the amount of material needed for each cell and, in addition, allows a reduction in costs. In the case of MJ cells based on III-V compounds grown on Ge substrates, the thinning of the substrate doesn’t penalize PV efficiency, because only a part of the substrate is active in PV conversion. Decreasing substrate thickness enhances the potential for employing photon recycling methods, which contributes to maintaining or even improving the conversion efficiency of thinner solar cells.

Finally, ultra-thin CPV cells can be integrated in all those PV applications where weight and/or flexibility are important factors (automotive, drones, satellites and Building Integrated PhotoVoltaic). In connection with the advantages outlined above, RSE initiated a study for the realisation of ultra-light and thin monolithic MJ cells. In this report, we described the thinning of the Ge substrate in monolithic solar cells – grown via MOCVD – by applying a low-cost chemical method. In parallel, the effect of Ge thinning on the photovoltaic performance of the device was assessed, obtaining similar short-circuit current values for a non-thinned cell and a cell in which the substrate was reduced to 40 µm.

To prevent the thinning of the substrate from causing carrier recombination at the back cell contact, RSE evaluated possible methods and technologies to passivate the semiconductor/metal interface, considering low-cost solutions. Finally, activities required for the development of the Laser Epitaxial Lift-Off (LELO) process, a technique identified for the separation of MOCVD epitaxial growths from GaAs substrates with the aim of their recycling, were evaluated.

The Report is available on the Italian site