Electron Transfer of Squaraine-Derived Dyes Adsorbed on TiO₂ Clusters in Dye-Sensitized Solar Cells: A Density Functional Theory Investigation

Because the intense absorptions of squaraine (SQ) dyes in the red/near-infrared spectral regions closely match the spectral response of sunlight, SQ dyes have great potential for use in dye-sensitized solar cells (DSCs). In this study, we employed density functional theory (DFT) and time-dependent DFT to investigate the structural, optical, and electron transfer properties of seven recently reported SQ-derived dyes adsorbed on a (TiO 2 ) 38 cluster having an anatase (101) surface, as a model for corresponding DSCs. In particular, we calculated the proportions of the electron densities in the dye–(TiO 2 ) 38 systems that were transferred to the TiO 2 moieties upon excitation, allowing us to investigate their electron injection mechanisms. We found that the dye–(TiO 2 ) 38 systems followed different direct and indirect mechanisms of electron injection to TiO 2 depending on the localization of the excited state electron density and the driving force for excited-state electron injection. JD10 and YR6 owning two intense absorption bands, which have significant proportions of electron density delocalized into the TiO 2 moiety upon excitation and have driving forces for excited-state electron injection, followed direct electron injection mechanisms to TiO 2 . These results are compatible with their higher experimentally observed short-circuit currents ( J sc ) than those of the SQ dyes. In contrast, SQ12, SQ2, and SQ4 followed the indirect electron injection mechanism due to their negligible proportion of electron density injected into TiO 2; in addition, SQ2 and SQ4 do not provide driving force for electron injection. SQ12, SQ2, and SQ4 dyes had lower experimentally observed J sc values than those of the other SQ dyes. The calculated probabilities of electron density being delocalized into TiO 2 and driving force for excited-state electron injection from these studied SQ dyes are compatible with their experimentally observed J sc values. This study provides insight into the electron injection mechanisms of SQ-derived dyes adsorbed on TiO 2 upon photoexcitation. Furthermore, our calculations and findings give clues for designing new SQ-derived sensitizers for DSC applications.