A spirobenzofluorene based phosphine oxide material (SPPO21) was used as an interlayer between the active layer and electron collecting electrode to improve the device performances of the organic solar cells (OSCs) with poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester active layer. The open-circuit voltage of the OSCs was improved by more than 50% and the power conversion efficiency was improved to 4.4% at the optimal thickness of SPPO21 interlayer (reference was 2.1%). In terms of the open-circuit voltage and efficiency, SPPO21 interlayer was even more effective than LiF. (C) 2010 Elsevier B.V. All rights reserved.