New naphthalene diimide-based n-type copolymers: The effects of conjugated side chains

[Display omitted] •Development of two new naphthalene diimide-based n-type conjugated copolymers.•Studied the effects of conjugated side chains on optoelectronic properties.•Very high absorption coefficient induced by conjugated side chains.•Weak morphological effect because of the large size of naphthalene diimide moiety. Two poly(naphthalene diimide)s (PNDIs) have been developed using p-phenylene ethynylene-based comonomers with either thiophene side chains (P(NDI-BT)) or bithiophene side chains (P(NDI-BBT)), and the effects of the length of conjugated side chains on electrochemical, optical, and morphological properties were investigated. Both polymers showed additional vibronic peaks due to the interchain π-π* transition, and a high absorption coefficient (α) of the order of 105 cm−1 was observed. P(NDI-BBT) gave higher α with narrower optical energy band gaps and lower lying the lowest unoccupied molecular orbital (LUMO) energy levels compared to those in P(NDI-BT). Interestingly, lower crystallinity was observed in P(NDI-BBT) than P(NDI-BT), which is the opposite of trends shown for reported polymers with conjugated side chains. Their charge carrier mobilities and photovoltaic properties were also investigated, and provide insight of designing NDI based n-type polymer semiconductors with conjugated side chians.