Enhancement of the Thermoelectric Performance of n-Type Naphthalene Diimide-Based Conjugated Polymer by Engineering of Side Alkyl Chains

Developing n-type doped semiconducting polymers with high thermoelectric (TE) performance still remains challenging. In this paper, we show a new strategy to enhance the TE performance of n-doped naphthalene diimide (NDI)-based conjugated donor–acceptor (D-A) polymer by introducing one linear alkyl chain for each NDI unit. Film of the PNDI2T-1, in which each NDI unit is attached with one linear and one branching alkyl chains, exhibits higher electrical conductivity and a greater power factor (PF) than those of PNDI2T-2 with the same conjugated backbone and two branching side chains, after doping with either (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) or decahydro-3a,6a,9a-triazaphenalene (TAM) under the same conditions. The optimal PF values of the doped films of PNDI2T-1 with N-DMBI and TAM can reach 1.6 and 3.5 μW m–1K–2, respectively, which are among the highest reported TE performances for NDI-based semiconducting polymers after n-doping. The higher TE performance of the doped films of PNDI2T-1 is attributed to the enhancement of charge mobility and improvement of doping degree after side-chain modification. These results provide a new molecular design rationale for the future development of high-performance n-type thermoelectric semiconducting polymers.