AbstractThe applications of wide‐bandgap (WBG) perovskite solar cells (PSCs) are limited by their subpar efficiency and stability due to their high density of defects, especially those at interfaces. Theoretical analyses suggest a monolayer of molecules, which is of minimum thickness and, hence, minimum resistance across the interface, possessing multifunctional groups and a permanent dipole, should effectively passivate the defects and minimize energy losses at interfaces. Herein, a self‐assembled monolayer (SAM) composed of amphiphilic molecules is designed and assembled as the interface layer to reduce the energy loss and enhance interface coupling between the perovskite and hole transport layer. It is found that the SAM also builds a back surface field through a p‐type doping effect, which promotes hole extraction and suppress the carrier recombination. Consequently, a remarkable power conversion efficiency (PCE) of 20.4% in parallel with a high open‐circuit voltage up to 1.25 V is attained. Additionally, an indoor PCE of 38.7% is realized. Both are among the best in their respective categories. Moreover, an all‐perovskite tandem solar cell is configured, presenting a decent PCE of 23.2%. This work emphasizes the significance of WBG PSCs for optoelectronic applications and indicates the eminent effects of SAMs for optimization of WBG PSCs.