Computational Study of Absorption Energies of Organic Sensitizers Used in Photovoltaic Applications

An assessment of several range-separated (RSH) DFT functionals, such as LC-omega PBE, LC-BLYP, omega B97X-D, and CAM-B3LYP, in predicting the excited-state energies of organic sensitizers containing 2-cyano-3-(thiophen-2-yl)acrylic acid was performed. The choice of RSH DFT functional with an optimized separation parameter, omega, is crucial for calculating the excitation energies of organic dyes with extended pi-conjugation. The optimized omega for TD-LC-omega PBE, TD-LC-BLYP, and TD omega B97X-D RSH fimctionals are the same for both HQ-1 and HQ-2, where HQ-2 has an added thiophene moiety at the pi-bridge. On the other hand, for the TD-CAM-B3LYP RSH functional, HQ-2 yielded a larger omega than HQ-1. For ground-state geometry optimization, those DFT functionals with 50% Hartree-Fock exchange showed a good correlation with the MP2-optimized geometries. Among the methods used, TD-LC-omega PBE/6-31+G(d)//mPWPW91 alpha 50/6-31G(d) in a solvent effect with omega = 0.20 Bohr(-1) gave the best accuracy of < 0.03 eV for the analogues with spatial overlap of 0.42 5 <= Lambda(HL) 5 <= 0.52. This study highlights the importance of a proper assessment of omega based on its charge-transfer properties when calculating the excited-state energies of the analogues. This paves the way for the proper screening of candidate analogues used in dye-sensitized solar cells in an effort to produce highly efficient solar cells.