We have studied the role of the phenyl-butyric-acid-methyl-ester side chain in the solar absorption of fullerene derivatives PC61 BM. The UV-Vis-NIR spectra are calculated with the linear response theory within time-dependent density functional theory. The initial and final orbitals of the optical transitions in solar spectrum range are analyzed in detail. The electronic states of the side chain hybridize with the states of C-60 cage, increasing the number of the initial orbitals of the solar absorption. So the side chain directly participates in the solar absorption. A distortion or length change of the side chain has obvious effects on the photoabsorption. (C) 2014 Elsevier B.V. All rights reserved.