The role of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as an n-type homologue buffer layer (HBL) was investigated in a bulk-heterojunction solar cell. The n-type HBL was inserted between the photoactive layer and the cathode. We found that the n-type HBL formed an ideal contact between the photoactive layer and the cathode, enhancing the rectifying ability of the device. In addition, the enlarged donor/acceptor interface between the bulk-heterojunction layer and the n-type HBL generated an additional photocurrent through an interdiffusion phenomenon. A device with the controlled n-type HBL (30 nm) had an enhanced power conversion efficiency of nearly 40% compared to the reference device. (c) 2013 Elsevier B.V. All rights reserved.