Abstract
Low-grade waste heat, typically defined as thermal energy discharged at temperatures below 200 °C, represents a vast and largely unexploited resource for sustainable energy conversion. Thermoelectric generators provide a direct, solid-state route to convert this ambient heat into electrical energy, yet their widespread commercial adoption remains constrained by modest conversion efficiencies, traditionally evaluated via the dimensionless figure of merit (ZT). In this study, we demonstrate a substantial enhancement of thermoelectric performance in n-type bismuth telluride (Bi₂Te₃) thin films through the precise engineering of phonon transport via hierarchical nanostructuring. Highly textured nanostructured Bi₂Te₃ films were deposited using radio-frequency magnetron sputtering, featuring high-density coherent grain boundaries and embedded nanoscale coherent domain boundaries. Detailed temperature-dependent transport measurements revealed a remarkable suppression of lattice thermal conductivity down to 0.31 W m⁻¹ K⁻¹ at 373 K, representing a 62% reduction compared to bulk crystalline Bi₂Te₃. Theoretical modeling based on the modified Callaway model confirms that mid-to-long-wavelength acoustic phonons are scattered effectively by the engineered interfacial network without significantly compromising carrier mobility. Consequently, a peak ZT of 1.42 was achieved at 373 K, accompanied by a maximum power factor of 4.1 mW m⁻¹ K⁻². A flexible 10-leg thermoelectric generator integrated with these optimized thin films yielded an open-circuit voltage of 112 mV and a power output density of 3.8 W m⁻² under a low-grade temperature differential of 40 K, underscoring the practical viability of this approach for self-powered Internet of Things (IoT) sensors and industrial low-grade waste heat recovery systems.
