A lot of efforts have been devoted to synthesize novel fullerene derivatives to improve the performance of polymer-based bulk heterojunction photocells. Although the archetypical soluble fullerene derivative [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) is frequently treated as a reference material, PCBM, which carries both aromatic and aliphatic moieties, is not very simple molecule. In this context, unmodified C-60, which is the simplest molecule among the C-60-based fullerenes, seems to be the most suitable for the reference material. Here, a set of systematic experiments on the device performances of the bulk heterojunction composites consisting of P3HT and unmodified C-60 over various compositions and annealing temperatures has been carried out. It has been found that the apparent optimum composition strongly depends on the annealing temperature chosen. Additionally, a monotonic relationship between the shift in the open-circuit voltage and the multiplication factor of the short-circuit photocurrent has been also found, directly indicating that the fullerene aggregation results in increased electron affinity to enhance the charge separation in the composites. The simple structure of the C-60 molecule seems to contribute for maintaining the coherency in the present study. (C) 2013 Elsevier B.V. All rights reserved.