Acceptor–acceptor-type conjugated polymer for use in n-type organic thin-film transistors and thermoelectric devices

Abstract

The development of n-type conjugated polymers (CPs) forming an acceptor–acceptor (A–A)-type structure are quite challenging due to the synthetic limitation of imposing a functional group on the electron deficient aromatic structure. The diketopyrrolopyrroles (DPP) structure exhibits n-type characteristics; however, the extended π-structure enables stannylation at the terminal position. The stannylated DPP is beneficial for making n-type CPs, but its use in the development of n-type CPs and their application in n-type organic thin-film transistors and organic thermoelectric devices have been not extensively studied. In the present study, we synthesized the A–A-type conjugated polymer, poly [((2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione)-5,5′-diyl)-alt-((2,7-bis(2-ethylhexyl)benzo [lmn] [3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone)-4,9-diyl))] (PDPP-NDI), by Stille coupling of distannylated DPP and dibrominated naphthalene diimide (NDI). The alternating copolymerization of DPP and NDI moieties led to a low-lying LUMO energy level of −4.4 eV, suitable for electron transport. The n-type organic thin-film transistors were fabricated with PDPP–NDI, and a substantial average electron mobility of 3.78 × 10−2 cm2 V−1 s−1 was obtained after thermal annealing. The n-type organic thermoelectric devices were developed via n-doping, and PDPP–NDI exhibited n-type thermoelectric properties, with an average power factor of 1.82 × 10−3 μW m−1 K−2.