Synthesis, photophysical properties and OFET application of thienothiophene and benzothiadiazole based donor-π-acceptor-π (D-π-A-π) type conjugated polymers

Abstract

Due to their distinct conductivity and potential applications in energy storage, sensors, coatings, and electronic devices like organic field-effect transistors (OFETs), organic bioelectronics, organic solar cells (OSCs), and organic light-emitting diodes (OLEDs), conjugated conducting polymers have attracted a lot of attention. Conjugated polymers are employed in organic photovoltaics (OPVs) as both donor and acceptor materials as well as channel materials in low-temperature solution produced OFETs. Numerous conjugated aromatic units, such as carbazole, benzene, thiophene, pyridine, and fluorene, have been used to create the backbone of conjugated polymers. Among these, fused thiophene-based materials have demonstrated excellent OFET characteristics due to their synthetic reproducibility to obtain replicable device performance, rigid and coplanar conformation, and closely packed conjugated back bones, enabling more effective intermolecular hopping and charge transfer. Thienothiophenes (TTs), the most basic thiophene family members, are composed of two fused thiophene units with π-extended structures. In instance, thieno[3,2-b] thiophene is the most conjugated and stable isomer of TTs. Due to their electron-rich, straight, rigid, and highly effective electron delocalized bones, extended molecular conjugation, intermolecular S-S interactions, and chemical durability properties of TTs, they have been highly desired chemical compounds for the synthesis of semiconducting polymers and are frequently used as building blocks for organic materials. Benzothiadiazole (BT) containing compounds with π-bridges exhibit intense conjugation, absorption at longer wavelengths as well and narrower band gaps. By requiring delocalization of electrons throughout the polymer chain as well as facilitating π-stacking, which enhances intramolecular charge transfer and augments charge carrier mobility, the coplanar structure of BT allows more facile for molecules to interact with one another. This thesis describes the successful Stille cross coupling of three novel polymers (P1-P3) with 4,7-bis(5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole. These novel polymers contain cyano (CN), fluoro (F), and dimethylamine (N(CH3)2) substituted thieno[3,4-b]thiophene (TT). Because of their side chains, all the polymers were discovered to be highly soluble. Through the utilization of optical, electrical, thermal, OFET application, and time-dependent DFT calculations, the effects of the functional groups (CN, F, and (N(CH3)2) on the polymers were compared and evaluated. These novel polymers' photophysical characterizations demonstrated an impressive mega Stokes shift up to 130 nm, optical and electronic band gaps of 1.70–2.00 eV, in addition to high thermal stability with a degradation temperature of around 260 oC. The charge transport characteristics of their OFET devices were studied, as well as the impact of electron donating and electron withdrawing groups on the electrical properties of the π-extended polymers. While all three polymers displayed p-type field-effect attitudes, dimethylamine substituted P3 surpassed similar p-type D-π-A-π semiconducting polymers reported in the literature with the highest average saturated hole mobility, μsat, 0.04 cm2 V -1 s -1, on/off current ratio, Ion/Ioff = 3.0×103, and the smallest subthreshold swing, SS, 250 mV dec-1. The research results of this study support the three new TT-BT polymers' exciting prospect for use in electrical and optoelectronical applications, particularly those in which the performance-critical tunability of field effect behavior is required.