We present a first-principles investigation of the excited-state properties of electron acceptors in organic photovoltaics including C-60, C-70, [6,6]-phenyl-C-61-butyric-acid-methyl-ester ([C-60] PCBM), and bis-[C-60] PCBM usingmany-body perturbation theory within the Hedin's G(0)W(0) approximation and an efficient Lanczos approach. Calculated vertical ionization potentials (VIP) and vertical electron affinities (VEA) of C-60 and C-70 agree very well with experimental values measured in the gas phase. The density of states of all three molecules is also compared to photoemission and inverse photoemission spectra measured on thin films, and they exhibit a close agreement-a rigid energy-gap renormalization owing to intermolecular interactions in the thin films. In addition, it is shown that the low-lying unoccupied states of [C-60] PCBM are all derived from the highest-occupied molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene C-60. The functional side group in [C-60] PCBM introduces a slight electron transfer to the fullerene cage, resulting in small decreases of both VIP and VEA. This small change of VEA provides a solid justification for the increase of open-circuit voltage when replacing fullerene C-60 with [C-60] PCBM as the electron acceptor in bulk heterojunction polymer solar cells.