The use of self-assembled monolayers (SAMs) of fullerene derivatives reduces the hysteresis of perovskite solar cells (PSCs). We have investigated three different fullerene derivatives observing a decrease on hysteresis for all the cases. Several processes can contribute to the hysteresis behavior on PSCs. We have determined that the reduced hysteresis observed for devices with SAMs is produced by a decrease of the capacitive hysteresis. In addition, with an appropriated functionalization, SAMs can increase photocurrent even when no electron selective contact (ESC) is present and a SAM is deposited just on top of the transparent conductive oxide. Appropriated functionalization of the fullerene derivative, as introducing -CN groups, can enhance cell performance and reduce hysteresis. This work paves the way for a future enhancement of PSCs by a tailored design of the fullerene molecules that could actuate as an ESC by themselves. ; This work was supported by the European Research Council (ERC-320441-Chirallcarbon), the CAM (FOTOCARBON Project S2013/MIT-2841), MINECO of Spain (Projects MAT2013-47192-C3-1-R, CTQ2014-52045-R, and MAT2015-70611-ERC) and by Generalitat Valenciana (Project PROMETEOII/2014/020).