Developing effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.