Grain Boundary-Confined Lead Halides Mitigate Post-Treatment Surface Lattice Distortion in Wide-Bandgap Perovskites for Efficient Tandem Solar Cells

Wide-bandgap (WBG) perovskite solar cells (PSCs) have attracted increasing attention due to their essential role in high-efficiency tandem architectures. Post-treatment strategies are commonly employed to improve WBG perovskite films by modifying surface states and enhancing device performance. However, such treatments risk simultaneously inducing surface lattice distortion, thereby undermining device efficiency. To address this issue, we strategically introduce lead thiocyanate into perovskite precursor solutions to induce the formation of excess lead halides at grain boundaries. These halide species act as protective buffers that react preferentially with post-treatment salts, effectively hindering the formation of post-treatment-induced surface lattice distortion. Consequently, the resulting single-junction PSCs with a 1.77 eV bandgap achieve an impressive power conversion efficiency (PCE) of 19.60%. Furthermore, by integrating these high-performance WBG subcells with a narrow-bandgap perovskite counterpart, we realize two-terminal all-perovskite tandem devices that deliver a remarkable PCE of 28.61%, highlighting their strong potential for next-generation multijunction photovoltaic systems.