The light-harvesting efficiency of P3HT:PCBM solar cells can be improved by incorporating near-IR dye molecules such as silicon phthalocyanine derivatives with bulky axial groups (SiPc). In order to study the size effect of the axial groups on the dye sensitization in P3HT:PCBM solar cells, we synthesized five SiPc derivatives with different axial groups: SiPc[OSi(CnH2n+1)(3)](2) (SiPcn, n = 2, 3, 4, 6) and SiPc[OSi(iBu)(2)C18H37](2) (SiPcB18). The power conversion efficiency (PCE) increased in the order of n = 2-4, reached the maximum at around n = 4 and 6, and then decreased for SiPcB18 with the longest axial groups. As a result, the PCE was improved to 4.2%, which is larger by 10% than that of P3HT:PCBM control cells without dye molecules. We therefore conclude that the butyl or hexyl chain in the axial ligand is the most appropriate for the dye sensitization in P3HT:PCBM solar cells. (C) 2012 Elsevier Ltd. All rights reserved.