Organic thermoelectrics are attractive for the fabrication of flexible and cost-effective thermoelectric generators (TEGs) for waste heat recovery, in particular by exploiting large-area printing of polymer conductors. Efficient TEGs require both p- and n-type conductors: so far, the air instability of polymer n-type conductors, which typically loose orders of magnitude in electrical conductivity ({sigma}) even for short exposure time to air, has impeded processing under ambient conditions. Here we tackle this problem in a relevant class of electron transporting, naphthalene-diimide co-polymers, by substituting the imide oxygen with sulphur. n-type doping of the thionated co-polymer gives rise to a higher {sigma} with respect to the non-thionated one, and most importantly, owing to a reduced energy level of the lowest-unoccupied molecular orbital, {sigma} is substantially stable over 16 h of air exposure. This result highlights the effectiveness of chemical tuning to improve air-stability of n-type solution-processable polymer conductors and shows a path towards ambient large-area manufacturing of efficient polymer TEGs.
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