Synthesis, Characterization, and Field-Effect Transistors Properties of Novel Copolymers Incorporating Nonplanar Biindeno[2,1-b]thiophenylidene Building Blocks

A nonplanar but conjugated heteroacene, biindeno[2,1-b]thiophenylidene (BTP), is employed to design and synthesize solution-processable polymer semiconductors for organic field-effect transistors (OFETs) applications first. By copolymerizing with isoindigo (IDG), diketopyrrolopyrrole (DPP), and naphthalenediimide (NDI) derivatives, three novel BTP-based copolymers (PBTP-IDG, PBTP-DPP, and PBTP-NDI) have been synthesized and characterized successfully. The results indicate that three BTP-based polymers exhibit broad absorption spectra and good solubility in most common solvents. Because of the dominantly electron-deficient contributions to the whole polymer backbones, the energy levels of the lowest unoccupied molecular orbitals are decreased to ca. -4.0 eV for all these polymers, thus exhibiting good electron affinities. Moreover, the deep-lying energy levels of the highest occupied molecular orbitals (HOMO) have been demonstrated for three BTP-based polymers, with the HOMO values ranging from -5.48 to -5.80 eV. Investigation of the OFETs performance indicates that three BTP-based polymers exhibit well hole transport properties in ambient air and excellent ambipolar performance in a N-2 glovebox. Compared with PBTP-IDG and PBTP-NDI, the uniform morphological structure, interconnected polycrystalline grain, and close pi-pi stacking distance endow PBTP-DPP with higher hole mobility of 1.43 cm(2) V-1 s(-1). Particularly, the well-balanced hole and electron mobilities of 0.68 and 0.13 cm(2) V-1 s(-1) have been demonstrated for the PBTP-DPP-based OFETs in a N-2 atmosphere, respectively. The results suggest that the nonplanar BTP unit and its derivatives are promising p-conjugated building blocks for the design and synthesis of solution-processable polymer semiconductors with high charge-transporting performance.