Coumarin dyes for dye-sensitized solar cells: A long-range-corrected density functional study

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Abstract
The excited-state properties in a series of coumarin solar cell dyes are investigated with a long-range-corrected (LC) functional which asymptotically incorporates Hartree–Fock exchange. Using time-dependent density functional theory (TDDFT), we calculate excitation energies, oscillator strengths, and excited-state dipole moments in each of the dyes as a function of the range-separation parameter μ. To investigate the acceptable range of μ and to assess the quality of the LC-TDDFT formalism, an extensive comparison is made between LC-BLYP excitation energies and approximate coupled-cluster singles and doubles calculations. When using a properly optimized value of μ, we find that the LC technique provides a consistent picture of charge-transfer excitations as a function of molecular size. In contrast, we find that the widely used B3LYP hybrid functional severely overestimates excited-state dipole moments and underestimates vertical excitation energies, especially for larger dye molecules. The results of the present study emphasize the importance of long-range exchange corrections in TDDFT for investigating the excited-state properties in solar cell dyes.
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