Are nanophotonic intermediate mirrors really effective in enhancing the efficiency of perovskite tandem solar cells?

An intermediate mirror has been proposed to enhance multijunction solar cell efficiency by selectively reflecting the light beyond higher energy bandgap of top cell, while simultaneously transmitting the rest of lower-energy light. Therefore, it reduces the higher-energy absorption spectral tail of the bottom cell (thermalization loss) and increase the absorption in the top cell. However, its effectiveness has only been theoretically validated in simplified tandem with basic components such as an antireflection coating (ARC) and top/bottom absorbers. In contrast, experimentally optimized tandem cells, such as perovskite (PVK)/silicon (Si) two-terminal configurations, include additional stacked electrodes, ultrathinned intermediate electrode, and random textures to maximize efficiency. Herein, we revisited the role of the intermediate mirror in these advanced tandem cells. Our results show that the incorporation of ideal intermediate mirror (IIM) does not improve efficiency both in textured and flat tandem cells, with its theoretical upper limit of efficiency being similar to or even lower than that of experimentally optimized cells.