Benzobisthiadiazole-based conjugated dono-racceptor polymers for organic thin film transistors: effects of pi-conjugated bridges on ambipolar transport

摘要

A new series of benzobisthiadiazole (BBT)-based donor-acceptor copolymers, namely, PBBT-FT, PBBT-T-FT, and PBBT-Tz-FT, with different pi-conjugated bridges have been developed for polymer thin film transistors (TFTs). It was found that inserting different pi-conjugated bridges into the backbone of the polymer allowed tailoring of opto-electrical properties, molecular organizations, and accordingly, ambipolar transport of TFTs. The UV-vis-NIR spectra of all three polymers were similar with the low band gaps of around 1.1 eV. While the lowest unoccupied molecular orbital (LUMO) energy levels were also similar (around -3.8 eV), the highest occupied molecular orbital (HOMO) energy levels varied from -5.05 to -5.42 eV because of the different pi-conjugated bridges; moreover, their TFTs exhibited different ambipolar transport. p-Type dominant TFT performances with the hole mobility (eta(h)) reaching 0.13 cm(2) V-1 s(-1) were observed for the prototype polymer PBBT-FT. However, the device based on PBBT-T-FT with thiophene bridges displayed lower but more balanced hole (mu(h)) and electron (mu(e)) mobilities of 6.5 x 10(-3) and 1.2 x 10(-3) cm(2) V-1 s(-1), respectively. The device based on PBBT-Tz-FT with the thiazole units exhibited more evenly balanced hole and electron mobilities (mu(h)/mu(e) = 0.45) along with a significantly enhanced mu(e) similar to 0.02 cm(2) V-1 s(-1). These different semiconducting features were ascribed to different molecular orientations and film morphologies revealed by wide-angle X-ray scattering (WAXS) and atomic force microscopy (AFM).