Over the past decade, organic solar cells (OSCs) have made noticeable progress in photovoltaic performance thanks to the emergence of electron acceptors capable of intramolecular charge transfer, namely, nonfullerene small molecules. OSCs continue to gain momentum by employing self-assembled monolayers (SAMs) as charge transport layers, particularly those involving conjugated system in their functional groups and/or spacers. This review provides an overview of SAMs, covering molecular designs, fabrication methods, and their various functions in OSCs. Additionally, it highlights currents issues surrounding SAMs, along with efforts to address them and future perspectives.