A Computational Investigation of Organic Dyes for Dye-Sensitized Solar Cells: Benchmark, Strategies, and Open Issues

A comprehensive theoretical study on the electronic absorption spectra of a representative group of organic dyes (L0, D4, D5, C217, and JK2) employed in dye-sensitized solar cell devices is reported. A benchmark evaluation on different time-dependent density functional theory (TDDFT) approaches with respect to high-level correlated coupled cluster (CC) and multireference perturbation theory (MRPT) benchmark calculations is performed in the gas phase. The benchmark results indicate that TDDFT calculations using the hybrid MPW1K and the long-range correct CAM-B3LYP functionals represent a valuable tool of comparable accuracy to that of the much more computationally demanding ab initio methods. Thus, the problem of the comparison between the calculated excitation energies and the measured absorption maximum wavelengths has been addressed employing the MPW1K functional and including the solvation effects by a polarizable continuum model. The present results show that taking into account the chemical and physical phenomena occurring in solution (i.e., protonation/deprotonation of the carboxylic function and the explicit solute solvent interactions) is of crucial importance for a meaningful comparison between the calculated and the experimental absorption spectra. Our investigation paves the way to the reliable computational design and predictive screening of organic dye sensitizers, even before their synthesis, in analogy to what has been achieved for transition-metal complexes.