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

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 pi-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-Atype 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)-4,9diyl))] (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 x 10(-2) cm(2) 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 x 10-3 mu W m(-1) K-2.