Perovskite solar cells (PSCs) have received considerable attention for their increasing photovoltaic performance achieved through fine optimization of stacking layers and experimentation with device architecture. The incorporation of interlayers is shown to positively impact the fabrication process by improving photovoltaic parameters. In recent years, carbazole‐based self‐assembled monolayers (SAMs) are investigated as a potential hole‐transport layer (HTL), due to their efficient passivating nature at the hole‐selective interface and faster charge extraction. In this study, a novel interlayer 2‐decyl[1]benzothieno [3,2‐b][1]benzothiophene (C10‐BTBT) is introduced, over the HTL SAM (2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl) ethyl) phosphonic acid, also known as MeO–2PACz. This new interlayer over SAMs significantly improves charge transfer at the interface, resulting in a high fill factor of 85.89% and a boost in power conversion efficiency from 18.04% to 20.50%. In this research, the potential of interlayer–SAM combinations is highlighted in advancing PSC technology.