A universal close-space annealing strategy towards high-quality perovskite absorbers enabling efficient all-perovskite tandem solar cells

The broad bandgap tunability of organic–inorganic metal halide perovskites enables the fabrication of multi-junction all-perovskite tandem solar cells with ultra-high power conversion efficiencies (PCEs). Controllable crystallization plays a crucial role in the formation of high-quality perovskites. Here we report a universal close-space annealing strategy that increases grain size, enhances crystallinity and prolongs carrier lifetimes in low-bandgap (low-Eg) and wide-bandgap (wide-Eg) perovskite films. By placing the intermediate-phase perovskite films with their faces towards solvent-permeable covers during the annealing process, high-quality perovskite absorber layers are obtained with a slowed solvent releasing process, enabling fabrication of efficient single-junction perovskite solar cells (PVSCs) and all-perovskite tandem solar cells. As a result, the best PCEs of 21.51% and 18.58% for single-junction low-Eg and wide-Eg PVSCs are achieved and thus ensure the fabrication of 25.15% efficiency 4-terminal and 25.05% efficiency 2-terminal all-perovskite tandem solar cells. Controlling the crystallization of perovskites is not trivial. Here Wang et al. develop a close-space annealing to improve the structural and optoelectronic quality of perovskite films with different chemical compositions, leading to over 25% efficiency in all-perovskite tandem solar cells.