Role of a [6,6]-phenyl C61 butyric acid methyl ester homologue buffer layer for bulk-heterojunction solar cells

•N-type homologue buffer layer (HBL) was invested in organic solar cell.•The HBL significantly reduced series resistance of the device.•The HBL tremendously improved rectification ratio of the device.•The HBL enlarged donor/acceptor interface area through interdiffusion.•Solar cell with a controlled HBL showed near 40% higher efficiency than reference device. 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 (30nm) had an enhanced power conversion efficiency of nearly 40% compared to the reference device.