The molecular geometries, electronic structures, and absorption spectra of two organic dyes, 3-(5-(4-(IDB)phenyl)thiophene-2-yl)-2-cyanoacrylic acid (IDB-1), and 3-(5-(4-(IDB)styryl) thiophene-2-yl)-2-cyanoacrylic acid (IDB-2), before and after binding to TiO2 cluster were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods to understand the effect of enhanced π-conjugation of organic dye on the energy-to-electricity conversion efficiency (η) of dye-sensitized solar cell (DSSC), where, IDB=10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl. The introduction of vinyl unit into the π-spacer enhances the coplanarity and subsequently red-shifts, intensifies, and broadens the absorption spectrum of IDB-2, resulting in the stronger electronic coupling between dye and TiO2 conduction band, thus the more efficient electron transfer. From the theoretical evaluation of electron injection driving force (D), light-harvesting efficiency (LHE), and shift of TiO2 conduction band (ΔEcb), we successfully interpreted the experimentally observed efficiency difference between IDB-1 and IDB-2. Under this theoretical procedure, several novel D-π-A dyes namely IDB-3, IDB-4, and IDB-5, were designed. Our calculated results reveal that IDB-5 has the improved Jsc and Voc compared with IDB-2 because it performs nicely on the three key parameters (D, LHE, and ΔEcb). This work highlight the importance of using dimethyl-substituted cyclopentadithiophene group as π-spacer in achieving more efficient dyes for DSSC. We hope these discussions can provide fundamental guidelines for the optimization of existing cell efficiency as well as the design of novel high-efficiency organic dyes.
