1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C-61 (PCBM) grafted to multi-walled carbon nanotubes (MWNTs) was for the first time synthesized and utilized for enhancing the electron transport in bulk-heterojunction solar cells. Minimal interaction between poly (3-hexylthiophene) (P3HT) and PCBM-grafted MWNT (PCBM-g-MWNT) reduced the chance for MWNT to function as a hole transporter. The comparison of charge carrier mobilities revealed that the electron moves ca. twice faster in a device containing 0.3 wt % PCBM-g-MWNT than in devices with 0.1 wt % MWNT or no MWNT. PCBM-g-MWNT, blended with P3HT and PCBM for polymer solar cells, enhanced the cell efficiency (eta) by 39% at 0.3 wt %. (C) 2011 The Electrochemical Society. [DOI:10.1149/1.3530197] All rights reserved.