The interface of electron-selective ZnO in inverted polymer bulk-heterojunction (BHJ) solar cells was modified with a series of fullerene-based self-assembled monolayers (C60-SAM) containing different anchoring groups (catechol, carboxylic acid, and phosphonic acid), linkage location, and functionalization. The formation of the C60-SAM to the surface of ZnO was investigated by processing the SAM through either a solution immersion technique or a solution spin-coating method. It is found that the C60-SAMs with the carboxylic acid and catechol termination can be formed onto the surface of ZnO by simple solution spin-coating process, whereas all three anchoring groups can be formed by solution immersion technique. Heterojunction devices were fabricated under different processing conditions to form SAM leading to 2-fold, 75%, and 30% efficiency improvement with the carboxylic acid, catechol, and phosphonic acid C60-SAMs, respectively. The main contribution to the variation of efficiency from different SAMs is...