High Charge Carrier Mobility, Low Band Gap Donor–Acceptor Benzothiadiazole-oligothiophene Based Polymeric Semiconductors

A series of benzothiadiazole oligothiophene and oligo­(thienylene vinylene) donor–acceptor (D–A) copolymers were synthesized and characterized. These low optical band gap materials (∼1.5 eV) are capable of absorbing photons in the range of 400–800 nm and exhibit good thermal stability. Their hole mobilities, determined using an organic field-effect transistor (OFET) architecture, vary over a range of 3 orders of magnitude and strongly correlate with the molecular ordering and morphology of the respective thin films. Spin-coated films of the poly­(benzothiadiazole-sexithiophene) PBT6, which exhibits a highly crystalline lamellar π–π stacked edge-on orientation on the OFET substrate, possesses a hole mobility of ca. 0.2 cm²/V·s. Vinylene-containing analogs PBT6V2 and PBT6V2′ are amorphous and exhibit very low mobilities. The molecular weight of PBT6 has a strong influence on the electronic properties: a sample with a lower molecular weight exhibits a mobility approximately 1 order of magnitude lower than the high molecular weight homologue, and the absorption maximum is appreciably blue-shifted. The hole mobility of PBT6 is further enhanced by a factor of ca. 3 through fabrication of the OFET by drop casting. OFETs fabricated by this process exhibit mobilities of up to 0.75 cm²/V·s and I_ON/OFF ratios in the range of 10⁶–10⁷. These results demonstrate the potential of incorporating benzothiadiazole units into polythiophene derivatives to develop high-mobility semiconducting polymers.