β-Alkyl substituted Dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene Semiconducting Materials and Their Application to Solution-Processed Organic Transistors

A novel highly pi-extended heteroacene with four symmetrically fused thiophene-ring units and solubilizing substituents at the terminal a-positions on the central ring, dithieno[2,3-d;2',3'-d]benzo[1,2-b;4,5-b]dithiophene (DTBDT) was synthesized via intramolecular electrophilic coupling reaction. The a-positions availability in the DTBDT motif enables the preparation of solution-processable DTBDT-based polymers such as PDTBDT, PDTBDT-BT, PDTBDT-DTBT, and PDTBDT-DTDPP. Even with its highly extended acene-like pi-framework, all polymers show fairly good environmental stability of their highest occupied molecular orbitals (HOMOs) from -5.21 to -5.59 eV. In the course of our study to assess a profile of semiconductor properties, field-effect transistor performance of the four DTBDT-containing copolymers via solution-process is characterized, and PDTBDT-DTDPP exhibits the best electrical performance with a hole mobility of 1.70 X 10(-2) cm(2) V-1 s(-1). PDTBDT-DTDPP has a relatively smaller charge injection barrier for a hole from the gold electrodes and maintains good coplanarity of the polymer backbone, indicating the enhanced pi-pi stacking characteristic and charge carrier transport. The experimental results demonstrate that our molecular design strategy for air-stable, high-performance organic semiconductors is highly promising.