posted on 2024-02-29, 23:30authored byXuwen Yang, Gang Ye, Karolina Tran, Yuru Liu, Jiamin Cao, Jingjin Dong, Giuseppe Portale, Jian Liu, Ping Zhang, Maria Antonietta Loi, Ryan C. Chiechi, L. Jan Anton Koster
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.