Chalcogen‐Driven Lattice Softening and Bandgap Engineering in CuAlX ₂ (X = S, Se, Te) Chalcopyrites for Photovoltaic and Thermoelectric Energy Conversion

Chalcogen substitution provides an effective strategy for simultaneously tuning the lattice dynamics, electronic structure, optical response, and transport behavior of chalcopyrite semiconductors. In this work, density functional theory (DFT) calculations are employed to establish the composition–property relationships governing CuAlX ₂ (X = S, Se, Te). All three compounds retain the tetragonal structure and are dynamically stable, as demonstrated by phonon spectra without imaginary modes. Replacing S with Se and Te progressively expands and softens the lattice, reducing the bulk modulus from 82.50 to 50.45 GPa, Young's modulus from 120.34 to 77.99 GPa, and the Debye temperature from 465.3 to 269.3 K. This lattice softening is accompanied by a chalcogen‐induced narrowing of the direct Γ‐point Perdew–Burke–Ernzerhof (PBE) bandgap from 1.68 eV in CuAlS ₂ to 1.04 and 1.01 eV in CuAlSe ₂ and CuAlTe ₂ , respectively.Because no hybrid‐functional, GW, scissor, or spin–orbit correction was applied, these semilocal‐DFT gaps are interpreted as screening values rather than quantitative photovoltaic device gaps. The optical spectra show a red‐shifting absorption edge and intrinsic absorption coefficients above 10 ⁵  cm ⁻¹ ; however, absorption strength alone does not determine solar‐cell efficiency. Boltzmann‐transport calculations within the constant‐relaxation‐time approximation (CRTA) reveal strong composition‐dependent trends in S , σ / τ , and κ _e / τ . Since lattice thermal conductivity was not explicitly calculated, a conventional total thermoelectric figure of merit is not claimed; an electronic‐only ZT _e ratio is used only as a screening indicator. Overall, the results establish composition‐dependent trends that motivate CuAlS ₂ , CuAlSe ₂ , and CuAlTe ₂ for further optoelectronic and thermoelectric screening, subject to higher‐level electronic‐structure calculations and experimental/device‐level validation.