Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight(Mw)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 withMwof 6.3 kg/mol. Surprisingly, by moving to the commonly employed 2-ethylhexyl branch (P2),Mwdecreases to 3.4 kg/mol. This is despite numerous reports that this side chain increases solubility andMw. By moving the ethyl branch in one position relative to the polymer backbone (1-ethylhexyl,P3),Mwis dramatically increased to 68.8 kg/mol. As a result of thisMwincrease, the shape of the absorption profile is dramatically altered, withλmax= 637 nm compared with 598 nm forP1and 579 nm forP2. The hole mobility as determined by thin film transistor (TFT) measurements is improved from~1×10−6 cm2/Vs forP1andP2to7×10−4 cm2/Vs forP3, while solar cell power conversion efficiency in increased to2.91%forP3relative to0.31%and0.19%forP1andP2, respectively.
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
