All Direct Ink Writing of 3D Compliant Carbon Thermoelectric Generators for High-Energy Conversion Efficiency

Compliant thermoelectric generators (TEGs) can fully exploit their energy conversion efficiency by establishing conformal interfaces on arbitrarily shaped 3D heat sources. Although additive manufacturing processes allow scalable fabrication with flexibility and customizability, most printable TEGs are fabricated as planar-type devices that harvest heat only in the in-plane direction. Herein, 3D-compliant TEGs fabricated solely using direct ink writing, which enables thermal-transfer optimization for efficient through-plane heat-to-electricity conversion owing to the out-of-plane printing of viscoelastic thermoelectric (TE) inks and unique device design is proposed. The rheological properties of carbon nanotube (CNT) TE inks are engineered to ensure conformal printing along directly written vertical thermal insulators. The ink TE properties are enhanced by the fine-tuned incorporation of p- and n-type dopants, where the electrical conductivity is further facilitated by nozzle-induced CNT packing to achieve high-power factors. To minimize the parasitic thermal loss from heat sources, an ultra-thin bottom substrate is directly printed with polydimethylsiloxane, thereby realizing compliant 3D TEGs for heat harvesting in the out-of-plane direction. The TEG exhibits the highest normalized open-circuit voltage (0.28 mV K⁻¹ cm⁻²) among the additively manufactured TEGs and retains remarkable mechanical reliability against repetitive deformation, promising its potential as body heat harvesters or temperature sensors.