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

Abstract

Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight $\left(M_w\right)$ 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 ${\lambda }_{\text{max}}$ = 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 $~1\times {10}^{-6}$  cm ² /Vs for P1 and P2 to $7\times {10}^{-4}$  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.

Authors:

Coffin, Robert C. ^[1]; MacNeill, Christopher M. ^[2]; Peterson, Eric D. ^[1]; Ward, Jeremy W. ^[1]; Owen, Jack W. ^[1]; McLellan, Claire A. ^[1]; Smith, Gregory M. ^[1]; Noftle, Ronald E. ^[2]; Jurchescu, Oana D. ^[1]; Carroll, David L. ^[1]

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+ Show Author Affiliations

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

Publication Date:

Sat Jan 01 00:00:00 EST 2011

Sponsoring Org.:

USDOE

OSTI Identifier:

1197856

Grant/Contract Number:  

FG02-07ER46428

Resource Type:

Published Article

Journal Name:

Journal of Nanotechnology

Additional Journal Information:

Journal Name: Journal of Nanotechnology Journal Volume: 2011; Journal ID: ISSN 1687-9503

Publisher:

Hindawi Publishing Corporation

Country of Publication:

United Kingdom

Language:

English