Intra-chain and inter-chain synergistic effect gives rise to high electron mobilities for naphthalenediimide based copolymers

Abstract

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, 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 donors by means of multiscale theoretical simulations. The results indicate that these copolymers exhibit large twists between the NDI and OT units and tortuous backbone conformations, causing the intra-chain electron transfer to be dominated by the super-exchange (SE) couplings between adjacent NDI units with taking the in-between OT unit as a 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 percolation 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.