Study of dielectric and optical constants of Benzobis(thiazole)-based copolymer thin films for designing optoelectronic devices

A new series of benzobisthiadiazole (BBT)-based donor–acceptor copolymers, namely, PBBT-FT, PBBT-T-FT, and PBBT-Tz-FT, with different π-conjugated bridges have been developed for polymer thin film transistors (TFTs). It was found that inserting different π-conjugated bridges into the backbone of the polymer allowed tailoring of opto-electrical properties, molecular organizations, and accordingly, ambipolar transport of TFTs. Copolymer PBB-T with benzo[1,2-d:4,5-d’]bis(thiazole) (BBT) as the accepting unit and benzodithiophene (BDT) as the donor unit is a promising candidate for high-performance non-fullerene polymer solar cells (PSCs). As much as we know the optical and dielectric constants of the PBB-T are not fully known. PBB-T was synthesized and subsequently thin films were properly prepared. By using the Kramers-Kronig relations and the transmission spectra of these thin films, the dielectric and optical constants were calculated and analyzed, including the absorption coefficient, dielectric constant, extinction coefficient, and refractive index of the PBB-T thin films. The results show that these dielectric and optical constants of the PBB-T thin films are quasi-independent on the thicknesses of the thin films, indicating our results are reliable. The features of the dielectric and optical constants show the PBB-T thin films are very promising candidates for high-performance non-fullerene PSCs and potential cut-off filter only permitting red and near-infrared light pass. These results are significant for designing optoelectronic devices related to the PBB-T thin films.