In this work, we study the temperature-dependent Seebeck coefficient and electrical conductivity of pristine and Bi-doped SnSe to clarify the microscopic origin of carrier-type reversal in lightly doped compositions. Pristine SnSe remains $p$-type over the full temperature range, while Bi substitution drives $n$-type transport. The 2% Bi-doped sample exhibits negative Seebeck coefficients at low temperature, followed by an $n$-to-$p$ polarity reversal near 550 K, consistent with bipolar transport and carrier compensation, whereas the 6% Bi-doped sample remains robustly $n$-type up to 880 K. The experimental data are analyzed using a self-consistent two-carrier transport model with a smooth interpolation across the Pnma-to-Cmcm structural transition. To connect the extracted transport trends to electronic structure evolution, we perform first-principles calculations for Bi-doped SnSe in both phases. The calculated band structures show that polarity switching at low Bi concentration arises from the temperature-driven evolution of the band edges and the Fermi level, along with a progressive narrowing of the band gap that promotes intrinsic excitation. Electron-phonon self-energy calculations further indicate a monotonic reduction of the Pnma band gap with increasing temperature, providing a direct electronic mechanism for the onset of bipolar transport prior to completion of the structural transition.

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• Seebeck effect
• Thermal properties
• Thermoelectric transport
• Thermoelectrics
• Chalcogens
• Doped semiconductors
• Density functional calculations
• Resistivity measurements
• Thermal techniques
• X-ray photoelectron spectroscopy