Strategies to Improve the Stability of Perovskite-based Tandem Solar Cells

Organic-inorganic metal halide perovskite-based tandem solar cells have attracted significant research attention in recent years. The power conversion efficiency of perovskite-based tandem can efficiently meet the requirements of practical applications; however, their instability limits their commercialization. The most commonly used wide-bandgap perovskites suitable for top sub-cells, which are based on I/Br alloying at X site, often suffer from severe phase segregation. When exposed to light illumination, a smaller bandgap phase appears and acts as a carrier trap, leading to a reduction in the quasi-Fermi level splitting and large Voc deficit. The narrow-bandgap perovskites suitable for bottom sub-cells, which are based on Sn/Pb alloying at B sites, always face atmospheric instability. When exposed to air, Sn2+ is rapidly oxidized to Se, which can shorten the carrier diffusion length and result in a drop in efficiency. Herein, we summarize the recent advances in perovskite-based tandem solar cells from the viewpoint of stability. We analyzed the stability data of highly efficient perovskite-based tandems reported so far, such as perovskite/silicon, perovskite/perovskite, and perovskite/copper indium gallium selenide (CIGS) tandems. We found that the key to improve the perovskite-based tandems is to improve the stability of the perovskite sub-cells. Then, we systematically analyzed the phase and atmospheric instability of wide- and narrow-bandgap perovskite, respectively, providing some reasonable strategies to tackle the instability. Compositional engineering, crystallinity optimization, and employing other perovskites with wide bandgaps are effective means to avoid phase instability of the I/Br alloying perovskite. Introducing the reducing additives, improving the film morphology, and forming a 2D/3D structure can help in improving the atmospheric stability of Sn-Pb narrow bandgap perovskites. Furthermore, we review the intrinsic instability of perovskite and corresponding improvement methods, which are inevitable in future tandem solar cells. By reducing the methylamine (MA) content in perovskite component and suppressing ion migration, the long-term operational stability is greatly enhanced. Finally, we briefly summarize the instability issues related to the interconnecting layer. In addition to the optimization of perovskite-based tandem devices, encapsulation also plays a crucial role in improving stability against environmental stressors. Studies based on improving the stability of perovskite-based tandems are still in the early stage. However, with a deeper understanding of the stability of perovskite sub-cells and the interconnecting layer, the commercialization of perovskite-based tandems, especially perovskite/silicon tandem devices, is promising to be achieved in the near future.