We investigated the influence of thermal annealing on the structural and optical properties of two variations of organic nanocomposite materials; 1) poly [3-hexylthiophene] and [6,6]-phenyl C(61) butyric acid methyl ester (P3HT:PC(61)BM), and 2) poly [3-hexylthiophene] and [6,6]-phenyl C(71) butyric acid methyl ester) (P3HT:PC(71)BM). The evolution of Surface roughness and morphology was investigated using atomic force microscopy while device electrical properties were investigated by measuring current-voltage (I-V) characteristics. Upon thermal treatment, results show that P3HT:PC(71)BM nanocomposites produce a more homogeneous mixture of finer grain size than P3HT:PC(61)BM. Furthermore, stronger optical absorption in the visible region is observed in P3HT:PC(71)BM compared to P3HT:PC(61)BM. Since optical absorption is closely related to film crystallinity, it is inferred that P3HT:PC(71)BM composites undergo more extensive crystallization upon annealing. Photoluminescence spectra of both P3HT:PC(61)BM and P3HT:PC(71)BM nanocomposites (dissolved in chlorobenzene) show that each has excellent quenching effects. I-V characteristic curves show that P3HT:PC(71)BM registers higher current density under AM 1.5 illumination than does P3HT:PC(61)BM. For the devices described in this paper having active areas of approximately 12 mm(2), efficiency is approximately 33% better for C(71)-based solar cells than that observed for devices made using C(61) fullerene. (C) 2009 Elsevier B.V. All rights reserved.