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

Abstract

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 (TiO2)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–(TiO2)38 systems that were transferred to the TiO2 moieties upon excitation, allowing us to investigate their electron injection mechanisms. We found that the dye–(TiO2)38 systems followed different direct and indirect mechanisms of electron injection to TiO2 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 TiO2 moiety upon excitation and have driving forces for excited-state electron injection, followed direct electron injection mechanisms to TiO2. These results are compatible with their higher experimentally observed short-circuit currents (Jsc) 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 TiO2; in addition, SQ2 and SQ4 do not provide driving force for electron injection. SQ12, SQ2, and SQ4 dyes had lower experimentally observed Jsc values than those of the other SQ dyes. The calculated probabilities of electron density being delocalized into TiO2 and driving force for excited-state electron injection from these studied SQ dyes are compatible with their experimentally observed Jsc values. This study provides insight into the electron injection mechanisms of SQ-derived dyes adsorbed on TiO2 upon photoexcitation. Furthermore, our calculations and findings give clues for designing new SQ-derived sensitizers for DSC applications.