First-principles thermoelectric transport in pristine and substitutionally doped monolayer 1T-TeSe₂

Assessing carrier control in two-dimensional chalcogenides requires both electronic and phonon transport. Here we study pristine and substitutionally doped monolayer 1T-TeSe₂ using first-principles electronic structure, phonon Boltzmann transport, semiclassical electronic transport, and defect-scattering analysis. PBE, HSE06, and mBJ calculations including spin-orbit coupling give indirect band gaps of 0.41, 0.83, and 1.38 eV, respectively. The rotationally corrected harmonic phonons support dynamical stability within numerical resolution. The effective-thickness-corrected in-plane lattice thermal conductivity is 5.85 W m^-1 K^-1 at 300 K and is dominated by low-frequency acoustic modes of the heavy Te-Se framework. Two-dimensional-corrected charged-defect calculations place the As, P, and Si (0/-) acceptor levels 0.180, 0.155, and 0.230 eV above the PBE + SOC valence-band maximum. Cl and Br have no transition inside the PBE + SOC gap and remain in the q = +1 charge state throughout it, consistent with resonant donor behaviour. At 800 K and 10 fs, the in-plane xx sampled peak ZT is 1.296 on a 3 × 3 × 1 DFT k mesh and 1.469 on a 4 × 4 × 1 mesh. The remaining mesh dependence limits the precision of these absolute values. Calculations with alternative harmonic IFCs and mode-inclusion thresholds show that the absolute ZT is sensitive to the treatment of low-frequency phonons, the sampled peak spanning 1.30-1.90 across these thermal models and the two DFT meshes. Rigidly increasing the gap to 0.83 or 1.38 eV raises both mesh values by about 16%; this gap-only test retains the PBE band velocities. State-resolved electron-phonon lifetimes provide a microscopic scattering scale, whereas the CRTA result depends on a transport relaxation time and remains sensitive to the assumed τ. The Klemens model estimates at most 0.25% mass-disorder reduction of lattice thermal conductivity for As and Br, compared with 11-15% for P, Si, and Cl. These calculations determine defect charge-state behaviour and quantify transport-model sensitivity. Assessing a substitution-induced performance gain requires dopant-dependent carrier populations and electron and phonon scattering rates.