Copper-based chalcopyrite semiconductors have attracted sustained interest owing to their promising thermoelectric (TE) performance, yet the microscopic origins of their TE behavior remain incompletely understood. Here, we systematically investigate the TE properties of $\mathrm{Cu}M{X}_2$ ($M=\mathrm{Al}$, Ga, and In; $X=\mathrm{S}$, Se, and Te) using first-principles calculations. Electronic transport is computed from explicit electron–phonon coupling based on the Perdew-Burke-Ernzerhof (PBE) functional-calculated band structures including spin–orbit coupling (SOC), with band gaps and dielectric constants corrected using the screened hybrid functional Heyd-Scuseria-Ernzerhof (HSE06). Lattice thermal conductivities (${\kappa }_{\mathrm{L}}$) are obtained by solving the phonon Boltzmann transport equation with three- and four-phonon scattering rates, using temperature-renormalized second-order force constants from 300 to 800 K. For $p$-type doping, at a fixed temperature and hole concentration, as $X$ varies from S to Te, the hole mobility ($\mu$) increases markedly due to progressively weaker polar optical phonon (POP) scattering, reflecting the reduced ionic contribution to the dielectric response in compounds with heavier chalcogens. Combined with smaller transport effective masses, $\mathrm{Cu}M{\mathrm{Te}}_2$ compounds therefore exhibit high electrical conductivities ($\sigma$) and large power factors (PFs). Across the $\mathrm{Cu}M{X}_2$ family, the anomalously lower ${\kappa }_{\mathrm{L}}$ of $\mathrm{Cu}M{\mathrm{Se}}_2$ relative to $\mathrm{Cu}M{\mathrm{Te}}_2$ arises primarily from enhanced three-phonon scattering at low-frequency regions. For a given $M$, $\mathrm{Cu}M{\mathrm{S}}_2$ displays the steepest temperature-induced decrease in ${\kappa }_{\mathrm{L}}$ and attains a smaller ${\kappa }_{\mathrm{L}}$ than $\mathrm{Cu}M{\mathrm{Se}}_2$ and $\mathrm{Cu}M{\mathrm{Te}}_2$ at 800 K. Given the low band degeneracy and comparatively modest hole mobilities of $\mathrm{Cu}M{X}_2$ compounds, the most effective routes to further improve their TE performance are to enhance $\sigma$ and reduce ${\kappa }_{\mathrm{L}}$ through doping.

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• Electron-phonon coupling
• Lattice thermal conductivity
• Thermoelectric effects
• Semiconductor compounds
• Thermoelectrics
• Density functional calculations