Coumarin Dyes for Dye-Sensitized Solar Cells - A Long-Range-Corrected Density Functional Study

TL;DR

This study uses long-range-corrected TDDFT to analyze excited-state properties of coumarin dyes for solar cells, highlighting the importance of exchange corrections for accurate predictions compared to traditional functionals.

Contribution

It demonstrates the effectiveness of LC-TDDFT with optimized parameters in accurately modeling charge-transfer excitations in dye molecules, outperforming B3LYP.

Findings

LC-TDDFT provides consistent charge-transfer excitation descriptions.

B3LYP overestimates dipole moments and underestimates excitation energies.

Properly optimized mu values are crucial for accurate results.

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 paramenter, mu. To investigate the acceptable range of mu and 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 (CC2) calculations. When using a properly-optimized value of mu, 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 excitations 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.