A study has been conducted focusing on collecting various experiences reported in the literature regarding the electro-optical coupling between III-V materials and silicon for the development of high-efficiency tandem cells and, more generally, on the structure of such cells.

Three different technological approaches currently proposed are discussed: i) the monolithic approach, which involves the growth of metamorphic buffer layers on Si through heteroepitaxy, ii) the “wafer bonding” approach, which consists of bonding materials onto Si using appropriate surface treatments, iii) the “mechanical stacked” approach, which involves the mechanical coupling of III-V compounds with Si using suitable intermediate transparent materials, sometimes also conductive.

These different technological approaches have led to the development of two-, three-, and four-junction devices (2J, 3J, and 4J), in which silicon serves as the bottom sub-cell or an intermediate cell. In all performance simulations carried out in the literature and by RSE in this study, a critical element common to the various device designs emerges: the optical transparency of the material placed above the Si bottom sub-cell, which must be as high as possible to maximize conversion efficiency. For 2J cells, those based on InGaP for the realization of the top cell are of particular interest.

These devices, when fabricated using wafer bonding or mechanical stacked techniques, achieve the highest efficiency values, although the monolithic approach would be more cost-effective. RSE has started developing a simulation program for optimizing tandem cell structures, specifically two-terminal InGaP-on-Si (2J) cells, where the materials are mechanically coupled via a transparent and conductive intermediate layer.

For a more realistic evaluation of the simulations, RSE used the optical properties of InGaP material deposited using the MOVPE technique (metal-organic vapor phase epitaxy). The simulation results, in agreement with those reported in the literature, indicated that the energy threshold value of the material required to prevent defect formation and maximize tandem device efficiency should be lower than 1.87 eV and higher than 1.82 eV.

Furthermore, a preliminary evaluation of the monolithic approach was carried out, involving the use of ternary alloy SixGe1-x-ySny buffer layers deposited via MOVPE on Si/Ge virtual substrates. Finally, economic aspects related to the cost assessment of fabricating such tandem cells were briefly analyzed to guide future research in this field.

The Report is available on the Italian site