High performance low-bandgap perovskite solar cells based on a high-quality mixed Sn-Pb perovskite film prepared by vacuum-assisted thermal annealing

Tandem perovskite solar cells are an effective concept to overcome the Shockley-Queisser limit of a single-junction perovskite solar cell. For a high-performance tandem cell, besides a wide-bandgap perovskite top cell, a high-quality low-bandgap perovskite bottom cell with an optimum bandgap of ∼1.2 eV is urgently needed. Moreover, a simple process technique needs to be developed for a high-quality perovskite film with good reproducibility, in order to further simplify the whole tandem-cell fabrication. Accordingly, we develop a simple one-step process (vacuum-assisted thermal annealing) for a high-quality low-bandgap CH₃NH₃Sn_0.5Pb_0.5I_xCl_3-x film, where the absorption edge can exceed 1000 nm. After comparing CH₃NH₃Sn_0.5Pb_0.5I_xCl_3-x films annealed in a vacuum and in a nitrogen environment, we find that vacuum-assisted thermal annealing can result in CH₃NH₃Sn_0.5Pb_0.5I_xCl_3-x films with better film coverage and crystallinity. This process can also accelerate the sublimation of methylammonium chloride and reduce the trap density in the CH₃NH₃Sn_0.5Pb_0.5I_xCl_3-x film. With this process, we successfully fabricated an efficient low-bandgap perovskite solar cell with a power conversion efficiency of more than 12% and good device reproducibility as well as long-term stability.