Beyond Thiophene: Unraveling the Role of Selenophene‐Based Spacer in Dion‐Jacobson Perovskites for Efficient Solar Cells

2D Dion-Jacobson (DJ) perovskites, which eliminate van der Waals gaps between organic spacer layers, are emerged as promising photovoltaic materials due to their excellent environmental and structural stability. Here, a selenophene-based dicationic spacer, namely selenophene dimethylammonium (SeDMA) is developed, for stable and efficient DJ perovskite solar cells (PSCs). In situ grazing incidence wide-angle X-ray scattering (GIWAXS) and in situ photoluminescence (PL) measurements show that the SeDMA-based DJ perovskite film exhibits a faster nucleation rate compared to the thiophene dimethylammonium (ThDMA)-based film. This accelerated nucleation, driven by the robust Se···Se and Se···Pb interactions, promotes vertically oriented crystal growth, enhancing film quality with larger grain size, improved charge carrier mobility, and reduced trap density. Consequently, the optimized DJ PSCs based on SeDMA using methylammonium (MA) as the A-site cation (nominal n = 5) achieve a power conversion efficiency (PCE) of 17.85%. This efficiency is further enhanced to 19.43% through a binary formamidinium/methylammonium (FA/MA) A-site cations strategy. The SeDMA-based DJ PSCs exhibit significantly improved thermal, moisture, and light stability compared to their thiophene-based counterparts. These findings underscore the potential of selenophene-based spacers in enhancing the efficiency and stability of DJ PSCs.