Interplay between annealing temperature and optimum composition and fullerene aggregation effects in bulk heterojunction photocells based on poly(3-hexylthiophene) and unmodified C60

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-C61-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 C60, which is the simplest molecule among the C60-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 C60 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 C60 molecule seems to contribute for maintaining the coherency in the present study.