As state-of-the-art silicon solar cells are approaching the Shockley–Queisser limit, combining a silicon bottom solar cell with a higher bandgap solar cell to form a multi-junction device is an obvious approach to enhancing the overall power conversion efficiency beyond the single junction limit. In the past years, various tandem solar cells have been demonstrated with one sun efficiency near or above 30% [1-3], indicating that it is practically achievable to fabricate such tandem devices.
There are three major ways to integrate silicon with other solar cells to achieve a multi-junction device as illustrated in Figure 1: two-terminal, mechanically-stacked four-terminal, and optical coupling four-terminal.[4, 5] In the two-terminal configurations, the sub-cells are connected in series by conductive layers that can transport the carriers from one subcell to the other. The tandem device has only two terminals and can be operated as a standard single junction solar cell which greatly simplifies its integration in a PV module and system. However, since the subcells are connected in series, the requirement of current-match limits the choices of materials in terms of bandgaps. The second configuration is mechanical stacked four-terminal tandem device, in which the subcells are also vertically stacked but each subcell has its own terminals to be operated individually to their own maximum power points. Compared with the two-terminal tandem configuration, this approach relieves the requirement of current-matching and a conductive layer between subcells. However, it is expected to have higher module and balance-of-systems costs, as well as greater power losses when scaling to module-sized areas. The third coupling configuration is enabled with an optical spectrum splitter to redirect the optimal parts of the solar spectrum to each subcell. The subcells are just operated individually without any integration which makes the choice of sub-cells more flexible. This configuration transfers the difficulty from the cell integration to the designs of the PV module. However, the spectral filters are currently still very costly.
Figure 1. Three common tandem solar cell configurations: (a) Two-terminal, (b) mechanically-stacked four-terminal, and (c) optical coupling four-terminal.
In the tandem solar cell design, the choice of bandgaps is of crucial importance when designing a high-performance device. Figure 2 shows the efficiency contours of two-junction solar cells as a function of top and bottom cell bandgap.[6-8] For a two-junction configuration, a bottom cell with a bandgap of 0.91 eV combined with a top cell with a bandgap of 1.62 eV is optimal yielding a limiting efficiency of 42.8% under one-sun illumination. Constraining the bottom cell to the bandgap of silicon (1.1 eV) only reduces the maximum efficiency by 0.4% to 42.4% when using a 1.72 eV top cell. In Figure 3, the efficiency contours of the three-junction silicon tandem cell for varying bandgap of the top and middle cells are shown. In three-junction tandem configurations with silicon bottom cells, the optimal bandgap combination of top and the middle cell is approximately 2 eV and 1.5 eV, respectively, and the resulting limiting efficiency is an impressive 49.4%.
 K.-H. Lee, K. Araki, L. Wang, N. Kojima, Y. Ohshita, and M. Yamaguchi, “Assessing material qualities and efficiency limits of III-V on silicon solar cells using external radiative efficiency,” Progress in Photovoltaics: Research and Applications, vol. 24, no. 10, pp. 1310-1318, 2016.
 J. P. Connolly, D. Mencaraglia, C. Renard, and D. Bouchier, “Designing III-V multijunction solar cells on silicon,” Progress in Photovoltaics: Research and Applications, vol. 22, no. 7, pp. 810-820, 2014.