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The efficiency of poly(3-hexylthiophene) (P3HT)/6,6-phenyl C 61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells dramatically depends on the molecular weight of the donor polymer P3HT. Only for P3HTs with a number-average molecular weight ( M n ) of >10 000 have high power conversion efficiencies of >2.5% been reached. This behavior is caused by distinctly reduced charge carrier (hole) mobility in the donor phase of the devices built from lower M n P3HT samples. The reduced performance of such devices is related to a reduced intermolecular ordering (π-stacking) of the P3HT phase. These findings highlight the important role of the molecular weight in semicrystalline semiconducting polymers for the device performance and may help in the further development of novel, efficient, lower band gap donor polymers for photovoltaic applications.
Schilinsky et al. (2005) studied this question.