Tuning Spacer Interaction via Br-Substitution Position for High-Efficiency and Stable 2D Ruddlesden-Popper Perovskite...

Quasi-two-dimensional (quasi-2D) Ruddlesden-Popper (RP) perovskites with layered structures are promising photovoltaic materials owing to their improved environmental stability, but their power conversion efficiencies (PCEs) still lag behind those of state-of-the-art 3D perovskites. Here, we investigate the bromine-substitution position effect of aromatic spacers in quasi-2D RP perovskite solar cells by comparing the meta- and para-substituted spacer molecules m-BrPEAAA and p-BrPEAAA. We show that para substitution induces a larger molecular dipole, strengthens spacer-framework interaction, and promotes more ordered crystallization. As a result, the p-BrPEAAA-based quasi-2D RP perovskite film exhibits improved crystallinity, prolonged carrier lifetime, and more favorable electronic structure. The corresponding device achieves a champion PCE of 20.16%, outperforming its meta-substituted counterpart (18.91%). In addition, the para-substituted system shows improved thermal and moisture stability. This work highlights bromine substitution-position engineering as an effective strategy for regulating spacer-framework interaction and improving the efficiency and stability of quasi-2D RP perovskite solar cells.