Silole-Analog MK-3 Dyes with a Large Bathochromic Shift for Organic Solar Cells

Substituting the middle thiophene ring in the MK-3 dye with a silole ring, the B3LYP calculations have shown that the absorption wavelengths of the new dyes, Me2Si-MK-3 and F2Si-MK-3, are bathochromically shifted by about 100 nm from 436 nm (MK-3). This large red shift is due mainly to a higher HOMO energy level in the Me2Si-MK-3 dye (-5.19 eV) with respect to the MK-3 dye (-5.30 eV) but to a lower LUMO energy level in the F2Si-MK-3 dye (-3.41 eV) compared to the MK-3 dye (-3.09 eV). These results suggest a wide range of alternatives in altering absorption wavelengths in the design of organic dyes for solar cells. Additionally, the calculations have shown the absorption intensities of the proposed new dyes are higher than that of the MK-3 dye. A pragmatic technique is developed to accurately describe the photon excitation of dye molecules by considering higher energy molecular orbital involvement. All three set of data, the HOMO-LUMO gap, the TD-B3LYP, and the calibrated TD-B3LYP, show the consistent red shifts for the new dye molecules with respect to the MK-3 dye.