Enhancing Field-Effect Mobility of Conjugated Polymers Through Rational Design of Branched Side Chains (original) (raw)

2014, Advanced Functional Materials

Optimization of the solution processability and intermolecular interactions in thin fi lms of these materials is important in the design of new polymeric semiconductors. [ 4,5 ] Here we describe the infl uence of the structure of side chains on the solubility and interchain interactions that govern charge carrier transport in a new low bandgap polymer. We recently introduced poly(benzothiadiazolealt-sexithiophene), pBT6 , a polymeric semiconductor composed of alternating benzothiadiazole (acceptor) and sexithiophene (donor) units with n-dodecyl side chains to impart solubility. It displays a bandgap of 1.5 eV and a hole mobility of up to 0.75 cm 2 V −1 s −1. [ 6 ] To further enhance the semiconducting performance of this class of materials, we incorporated diketopyrrolopyrrole units as a second acceptor into the structure of pBT6. The new polymer consists of electrondonating terthiophene units (T) between two different electron acceptors, benzothiadiazole (B) and diketopyrrolopyrrole (D), in an alternating arrangement to afford the donor-acceptor(1)-donor-acceptor(2) (D-AD -A′) copolymer, pTBTD , as shown in Scheme 1. Diketopyrrolopyrrole and its isomer [ 7 ] have been reported as attractive acceptors in the development of semiconductors for use in high performance polymer-based organic fi eld effect transistors (OFETs) and organic photovoltaics, exhibiting mobility and power conversion effi ciency beyond 1 cm 2 V −1 s −1 [ 8-12 ] and 7%, respectively. [ 13-17 ] The D-AD -A' nature of the new copolymers is expected to: i) lower the HOMO energy level of the polymer; ii) enhance intramolecular charge transfer, and iii) enhance π-π intermolecular interactions due to the coplanar nature of diketopyrrolopyrrole. [ 8,13,18 ] Incorporation of branched side-chains improves polymer solubility compared with analogs with linear side chains. [ 1,2,19,20 ] However, branched side chains in which the branched point is close to the polymer backbone (e.g., 2-octyldodecyl, 2-decyltetradecyl) interferes with close packing of the polymers and weakens π-π intermolecular interactions between conjugated backbones, compared to analogs with linear side chains. [ 21-23 ] Therefore, a design strategy that