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Title: 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

Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight ( M w ) of 4,8-dialkoxybenzo[1,2- b :4,5- b ]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 M w of 6.3 kg/mol. Surprisingly, by moving to the commonly employed 2-ethylhexyl branch ( P2 ), M w decreases to 3.4 kg/mol. This is despite numerous reports that this side chain increases solubility and M w . By moving the ethyl branch in one position relative to the polymer backbone (1-ethylhexyl, P3 ), M w is dramatically increased to 68.8 kg/mol. As a result of this M w increase, the shape of the absorption profile is dramatically altered, with λ 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 ~ 1 × 10 6  cm 2 /Vs for P1 and P2 to 7 × 10 4  cm 2 /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.
 [1] ;  [2] ;  [1] ;  [1] ;  [1] ;  [1] ;  [1] ;  [2] ;  [1] ;  [1]
  1. The Department of Physics and the Center for Nanotechnology and Molecular Materials, Wake Forest University, Winston-Salem, NC 27109, USA
  2. The Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA
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Journal of Nanotechnology
Additional Journal Information:
Journal Volume: 2011; Related Information: CHORUS Timestamp: 2016-11-21 19:30:47; Journal ID: ISSN 1687-9503
Hindawi Publishing Corporation
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