Intra-chain and inter-chain synergistic effect gives rise to high electron mobilities for naphthalenediimide based copolymers
Naphthalenediimide (NDI) based copolymers have been widely used as n-type materials in organic field-effect transistors and organic solar cells due to their good electron-accepting abilities and high electron mobilities. However, the electron transport mechanisms for these copolymers remain elusive. In particular, whether intra-chain or inter-chain electron transfer controls the transport performance is debated. Here, we have investigated the intra-chain conformations, inter-chain stacking structures, and electron transport properties in a series of NDI based copolymers using oligothiophenes (OT) as donor by multiscale theoretical simulations. The results point out that these copolymers exhibit large twists between the NDI and OT units and tortuous backbone conformations, making that the intra-chain electron transfer is dominated by the super-exchange (SE) couplings between adjacent NDI units with taking the in-between OT unit as bridge. Also, the twisted intra-chain geometries prevent long-range ordered inter-chain packing. Consequently, neither intra-chain nor inter-chain electron transfer can individually form a continuous transport network. However, NDI units have plenty of inter-chain short contacts and the combination of intra-chain and inter-chain couplings results in a robust percolating network for efficient electron transport. In addition, the electron mobility can be improved by balancing the intra-chain SE couplings and inter-chain direct couplings with an appropriate donor length.