Organic, nonfullerene semiconductors capable of self-assembly and composed of either anthraquinone (AQ) or naphthalenediimide (NDI) central fragments have been designed as electron-selective materials for n-i-p perovskite solar cells (PSCs). Both types of self-assembled monolayer (SAM) molecules contain phosphonic acid as an anchoring group, allowing covalent binding with indium tin oxide (ITO) surfaces. In particular, the NDI-based SAMs showed a more homogeneous anchoring on the ITO substrate and a stronger band bending at the ITO-SAM/perovskite interface than AQ-based SAMs. As a result, low-temperature-processed n-i-p PSCs with NDI SAMs as an electron-selective bottom contact showed a maximum power conversion efficiency (PCE) of 21.5%, representing the highest PCE among n-i-p PSCs with organic electron transporting layers (ETLs). In addition, our NDI-SAM-based devices demonstrate substantially improved long-term stability under operating temperature conditions when compared to devices using SnO2 as the ETL.