Narrow-bandgap thermoelectric semiconductors typically suffer from excessively high intrinsic carrier density, which severely suppresses the Seebeck coefficient and compromises the power factor. Decoupling the entangled electronic parameters to resolve this intrinsic trade-off remains a central challenge in the pursuit of high-performance materials. Here, using $n$-type Te-free BiSe as a model system, we demonstrate a “staged dual-doping” strategy to decouple the regulation of carrier density and density of states (DOS) effective mass. We show that initial Sb alloying successfully mitigates the excessive carrier density, boosting the Seebeck coefficient and carrier mobility, yet inevitably reducing the DOS effective mass. Crucially, subsequent Ge incorporation recovers this compromised effective mass by modulating the band structure while simultaneously inducing hierarchical microstructural heterogeneities ranging from point defects to dislocations that strongly scatter phonons. As a result, the optimized ${\mathrm{Bi}}_{0.69}{\mathrm{Sb}}_{0.29}{\mathrm{Ge}}_{0.01}\mathrm{Se}$ achieves a peak $\mathit{ZT}$ of 0.54 at 473 K, a room-temperature $\mathit{ZT}$ of 0.33, and an average $\mathit{ZT}$ of 0.48 over 300–523 K. This work establishes a rational, scalable pathway for optimizing Te-free BiSe and offers a transferable blueprint for managing trade-offs in narrow-bandgap thermoelectrics.

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• Density of states
• Electrical conductivity
• Electrical properties
• Thermal conductivity
• Thermal properties