Self-assemble monolayers (SAMs) have become state-of-the-art hole-selective contacts for high-efficiency perovskite-based solar cells due to their easy processing, passivation capability, and low parasitic absorption. Nevertheless, for the deposition of SAMs with a monolayer thickness and a high packing density on metal oxide substrates, critical challenges persist. To overcome these, the study focuses on the impact of annealing temperature – an intrinsic yet so far unexplored process parameter – during the formation of SAMs. By performing in situ angle-resolved X-ray photoelectron spectroscopy combined with advanced data analysis routines, it is revealed that increasing the annealing temperature reduces the formed SAM layer thickness from a multilayer stack of ≈5 nm at 100 °C (conventional temperature employed in literature) to a monolayer at 150 °C. Furthermore, denser adsorption of the SAM to the metal oxide surface is promoted at high temperatures, which enhances the interfacial SAM/perovskite passivation quality. With this strategy, a 1.3%abs power conversion efficiency (PCE) increment is obtained in fully-textured perovskite/silicon tandem solar cells, with improved reproducibility, and a champion device approaching 30% PCE. This study advances the understanding of SAMs formation and presents a promising strategy for further progress in high-efficiency perovskite-based solar cells.