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Journal of Nanotechnology
Volume 2011, Article ID 572329, 10 pages
Research Article

Variation of the Side Chain Branch Position Leads to Vastly Improved Molecular Weight and OPV Performance in 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene/2,1,3-benzothiadiazole Copolymers

1The Department of Physics and the Center for Nanotechnology and Molecular Materials, Wake Forest University, Winston-Salem, NC 27109, USA
2The Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA

Received 2 February 2011; Accepted 13 April 2011

Academic Editor: Román López-Sandoval

Copyright Β© 2011 Robert C. Coffin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight (𝑀𝑀) of 4,8-dialkoxybenzo[1,2-b:4,5-bβ€²]dithiophene (BDT)/2,1,3-benzothiadiazole (BT) copolymers. When dodecyl side chains (P1) are employed at the 4- and 8-positions of the BDT unit, we obtain a chloroform-soluble copolymer fraction with 𝑀𝑀 of 6.3 kg/mol. Surprisingly, by moving to the commonly employed 2-ethylhexyl branch (P2), 𝑀𝑀 decreases to 3.4 kg/mol. This is despite numerous reports that this side chain increases solubility and 𝑀𝑀. By moving the ethyl branch in one position relative to the polymer backbone (1-ethylhexyl, P3), 𝑀𝑀 is dramatically increased to 68.8 kg/mol. As a result of this 𝑀𝑀 increase, the shape of the absorption profile is dramatically altered, with πœ†max = 637 nm compared with 598 nm for P1 and 579 nm for P2. The hole mobility as determined by thin film transistor (TFT) measurements is improved from ∼1Γ—10βˆ’6 cm2/Vs for P1 and P2 to 7Γ—10βˆ’4 cm2/Vs for P3, while solar cell power conversion efficiency in increased to 2.91% for P3 relative to 0.31% and 0.19% for P1 and P2, respectively.