The effect of replacing [6,6]-phenyl-C-61 butyric acid methyl ester (PCBM) by its multiadduct analogs (bis-PCBM and tris-PCBM) in bulk heterojunction organic solar cells with poly(3-hexylthiophene-2,5-diyl) (P3HT) is studied in terms of blend film microstructure, photophysics, electron transport properties, and device performance. Although the power conversion efficiency of the blend with bis-PCBM is similar to the blend with PCBM, the performance of the devices with tris-PCBM is considerably lower as a result of small photocurrent. Despite the lower electron affinity of the fullerene multiadducts, mu s-ms transient absorption measurements show that the charge generation efficiency is similar for all three fullerenes. The annealed blend films with multiadducts show a lower degree of fullerene aggregation and lower P3HT crystallinity than the annealed blend films with PCBM. We conclude that the reduction in performance is due largely to poorer electron transport in the blend films from higher adducts, due to the poorer fullerene network formation as well as the slower electron transport within the fullerene phase, confirmed here by field effect transistor measurements. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 45-51, 2011