Designer Entropy Enables High Thermoelectric Power and Efficiency

ABSTRACT Thermoelectric (TE) materials promise sustainable energy conversion from waste heat, but their practical utility is often governed by a fundamental trade‐off between conversion efficiency ( η ) and output power ( P ). High‐entropy engineering has emerged as a powerful strategy to boost efficiency by maximizing phonon scattering, yet this compositional complexity can inadvertently degrade electronic properties and thus suppress output power. Here, we introduce “Designer Entropy”—a chemically guided paradigm that moves beyond maximizing disorder to achieve targeted transport properties by consciously selecting alloying elements. We demonstrate this principle by systematically contrasting two distinct pathways in a SnTe(GeSe) 0.25 matrix: doping with Sb (near‐matched atomic radius) versus Bi (large radius/mass mismatch). While both pathways yield nearly identical high conversion efficiencies of ~5.7% at a temperature difference of 500 K, their power generation capabilities diverge significantly. The Sb‐doped device, designed to preserve electronic transport, delivers a maximum output power of 23.1 mW, an ~20% enhancement over its Bi‐doped counterpart. This non‐equivalent optimization stems from a mechanistic dichotomy: Sb doping preserves high carrier mobility and suppresses bipolar effects, leading to a superior power factor, whereas Bi doping, despite inducing ultralow lattice thermal conductivity by creating strong local distortions, suffers from severe carrier scattering and a premature bipolar onset. Our work establishes a rational design framework that decouples the optimization of efficiency and power, providing a blueprint for developing next‐generation TE materials that shift the focus from compositional complexity to compositional intelligence for high‐power applications.