Synthesis and characterization of donor-acceptor type copolymers based on thienylenevinylenes for organic electronics

Abstract

Over past two decades, interest in organic electronics, organic thin film transistor (OTFTs) and organic solar cells (OSCs), has increased due to their cheap, light-weight, and mechanical flexibility compared to inorganic counterparts. To improve organic electronic device performance, research on new materials for semiconducting layers has been required. Among others, conjugated polymers have good film formability compared to conjugated small molecules and oligomers. In order to improve optical and electronical properties of conjugated polymer, push-pusll structure was employed with alternating structure of electron donating units (D) and accepting units (A) in conjugated backbone. Through this concept, energy level and optical bandgap of conjugated polymers were easily tuned by changing D-A units. In this study, (E)-1,2-bis(3-alkylthiophen-2-yl)ethane, simply thienylene vinylene (TV), was used as donating units which is one of the promising molecular moiety for organic electronics due to its rigid and planar moiety. Vinyl groups between thiophenes make the conjugated backbone highly coplanar as suppressing steric hinderance caused from alkyl substituent on 3-position of thiophene. Polythienylevinylene (PTV) derivatives showed good charge carrier mobilities in OTFTs due to their tendency to form crystalline structures for efficient charge carrier transport. In some cases, despite substitution of alkyl side chains on thiophene, PTV derivatives seem to have limited solubility in common processing solvent. Thus, alkoxy-substitued benzooxadiazole (BO) and benzothiadiazole (BT) moieties were used as acceptor units not only for reducing optical bandgap but also for maintaining good solution processability. In this presentation, four D-A type conjugated polymers, BO8, BT8, TBO12 and TBT12, were designed and synthesized by Stille polymerization. These polymers showed ambipolar charge carrier transporting characterstics, and among others, TBT12 exhibited 0.77 cm2/Vs for hole mobility and 0.01 cm2/Vs for hole mobility. Photovoltaic cells based on TBO12:PC71BM showed an open-circuit voltage of 0.58 V, a short circuit current of 11.4 mA/cm2 and a power conversion efficiency of 4.7%.