The layer-dependent electronic and thermal transport properties of phosphorene offer a unique platform for designing high-performance thermoelectric devices. Here, we employ a combination of density functional theory and nonequilibrium Green's function formalism to investigate the thermoelectric performance of phosphorene-based transistors with spatially modulated layer thicknesses. By engineering the thickness of the active region from four layers to a single monolayer, we demonstrate a significant enhancement in the thermoelectric figure of merit $\left(ZT\right)$, achieving $ZT\approx 1.1$ at 300 $\mathrm{K}$ for transport along the armchair direction: a threefold increase over uniform four-layer devices. This improvement arises from the interplay between suppressed phonon thermal conductivity (reduced by $70\%$ via interfacial scattering) and maintained high electrical conductivity due to tailored band alignment and anisotropic charge transport. The armchair-oriented devices outperform zigzag configurations owing to their lower lattice thermal conductivity, despite the latter's superior power factor. Phonon transport calculations, utilizing Stillinger-Weber potentials and Green's function methods, reveal that thickness gradients induce strong backscattering, effectively decoupling electronic and thermal transport. Furthermore, the transition to monolayer regions creates a type-I heterojunction, enhancing thermopower while minimizing parasitic heat dissipation. Our results not only establish phosphorene as a versatile candidate for on-chip energy harvesting, but also provide a general framework for leveraging dimensionality and anisotropy in van der Waals heterostructures to optimize thermoelectric efficiency.

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• Thermoelectric effects
• Devices
• First-principles calculations
• Green's function methods