Promoting Ruddlesden–Popper Perovskite Formation by Tailoring Spacer Intramolecular Interaction for Efficient and Stable Solar Cells

Abstract Low‐dimensional Ruddlesden–Popper phase (LDRP) perovskites are widely studied in the field of photovoltaics due to their tunable energy‐band properties, enhanced photostability, and improved environmental stability compared to the 3D perovskites. However, the insulating spacers with weak intramolecular interaction used in LDRP materials limit the out‐of‐plane charge transport, leading to poor device performance of LDRP perovskite solar cells (PSCs). Here, a functional ligand, 3‐guanidinopropanoic acid (GPA), which is capable of forming strong intramolecular hydrogen bonds through the carboxylic acid group, is employed as an organic spacer for LDRP PSCs. Owing to the strong interaction between GPA molecules, high‐quality LDRP (GPA) ₂ (MA) _n‐1 Pb _n I _3n+1 film with promoted formation of n = 5 phase, improved crystallinity, preferential vertical growth orientations, reduced trap‐state density, and prolonged carrier lifetime is achieved using GPAI as the dimensionality regulator compared to butylamine hydroiodide (BAI). As a result, GPA‐based LDRP PSC exhibits a champion power conversion efficiency of 18.16% that is much superior to the BA‐based LDRP PSC (15.43%). Importantly, the optimized GPA‐based LDRP PSCs without encapsulation show enhanced illumination, thermal, storage, and humidity stability compared to BA‐based ones. This work provides new insights into producing high n value LDRP films and their efficient and stable PSCs.