Impact of Oligo(Ethylene Glycol) Side Chains on the Thermoelectric Properties of Naphthalenediimide-Dialkoxybithiazole Polymers

Organic thermoelectric materials have garnered significant interest as promising candidates for energy harvesting applications. In recent years, ethylene-glycol side-chain engineering in organic semiconductors has gradually become an efficient approach to boost the performance of organic thermoelectrics. Although this strategy is widely utilized, the impact of their volume and branching structure remains unknown. This contribution describes a trade-off phenomenon between the oligo(ethylene glycol) (OEG) side chains and thermoelectric properties based on the n-type doped low-bandgap conjugated polymers, achieved through the modification of the volume and structure of side chains. Three conjugated polymers comprising a naphthalenediimide-dialkoxybithiazole backbone and different linear length or branched OEG side chains exhibit good host/dopant miscibility after doping. We find that, in the linear OEG side-chain-based polymers, the increased volume of side chains slightly influences the planarity of backbones, thereby leading to similar and satisfactory thermoelectric performances. The high fraction of side chains does not consistently yield enhanced performance, as the branched OEG side-chain introduces steric hindrance. Consequently, the accordingly conjugated backbones become less planar and rigid, resulting in critical molecular packing changes and low charge carrier mobility and doping efficiency and thus low thermoelectric performance. Our work provides a unique insight into the fundamental understanding of the relationship between molecular packing and thermoelectric properties and guides the future rational design of efficient n-type organic semiconductors.