Composition tuning of a mixture of thienothiophene-based polymer (PTB7) and PC70BM using a novel additive, tetrabromothiophene (Br-ADD)

A novel additive, tetrabromothiophene (Br-ADD), is shown to improve the properties of bulk heterojunction consisting of poly(3-hexylthiophene: [6,6]-phenyl-C-60 (P3HT:PC60BM) and a thienothiophene-based polymer: C-70-butyric acid methyl ester (PTB7:PC70BM). The hydrophobic Br-ADD selectively interacts with the fused heteroaromatic rings on the polymers to increase the polymer surface energy, which improves the bulk heterojunction and photovoltaic performances of the resulting polymer solar cells (23.5% for P3HT and 12.3% for PTB7). The selective interactions between Br-ADD and the polymer and the properties of the bulk heterojunctions are characterized using UV-vis spectroscopy, photoluminescence (PL), atomic force microscopy (AFM), secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and J-V measurements. (C) 2014 Elsevier B.V. All rights reserved.