Substrate Engineering of SWCNT <i>p</i> – <i>n</i> Junctions for Dual‐Mode Power Generation and Heat‐Flux Sensing

Autonomous thermoelectric power generation under uniform heating, without an external cold reservoir, remains an open challenge for self-powered Internet-of-Things (IoT) sensors. This work shows that, for a fixed single-walled carbon nanotube (SWCNT) p-n junction film, circuit, and adhesive-bonded device structure, the substrate alone produces a substrate-dependent transition between continuous power generation and high-sensitivity heat-flux sensing. Across three flexible substrates, cycloolefin polymer (COP), polyimide (PI), and polyethylene naphthalate (PEN), substrate infrared absorptivity and per-unit-area thermal inertia jointly govern the sign, magnitude, and temporal evolution of the output voltage. COP (absorbance 0.34; transmittance 45.8% at 8.8 µm) heats the film preferentially, sustaining a stable in-plane gradient and delivering +0.38 mV at steady state. PI and PEN, with near-complete absorption, transiently invert the gradient, producing excursions of -0.58 and -0.96 mV; the larger PEN response reflects 2.4-fold greater thermal inertia, yielding -45 µV/K per junction pair. A transient thermal model and thermographic imaging reproduce these dynamics and indicate that the substrate optical contrast, rather than the interfacial factors common to all devices, principally governs the switching. For the present device structure and testing conditions, these results establish a substrate-engineering framework for dual thermoelectric functions in a single SWCNT architecture.