Flexible Organic Thermoelectric Composites and Devices with Enhanced Performances through Fine-Tuning of Molecular Energy Levels

Abstract

Thermoelectric (TE) generators, based on thermoelectric materials, can efficiently convert thermal energy into electricity via the Seebeck effect, showing great promise for waste-heat recovery research. Recent advancements in TE composites of conductive polymer/carbon nanotubes have been significant. This study evaluates the thermoelectric properties of organic TE films and generators by combining naphthalene diimide (NDI) polymers with single-walled carbon nanotubes (SWCNTs). The results reveal that P(NDI-HTO)/SWCNT composite films containing free radicals and alkyl side chains have enhanced thermoelectric properties compared to P(NDI-HT)/SWCNT composite films without free radicals and P(NDI-TP)/SWCNT composite films containing polar side chains. Among them, maximum power factors reach 264.1 ± 21.9 μW m^–1 K^–2 for p-type and 72.2 ± 1.5 μW m^–1 K^–2 for n-type composite films, marking increases of 113% and 32%, respectively, over pristine SWCNT films. Furthermore, a flexible thermoelectric generator based on P(NDI-HTO)/SWCNT, with five pairs of p–n junctions, achieves an output voltage of 28.8 mV and an output power of 1.2 μW at a 60 K temperature differential. These improvements in thermoelectric properties are primarily due to the effective modulation of molecular energy levels, enhancing the charge transfer process between NDI polymers and SWCNTs.