Connection style and spectroscopic properties: Theoretical understanding of the interface between N749 and TiO2 in DSSCs

• Four connecting models in N749 and TiO2 system are studied.
• The electronic structures of complex systems are explored by TDDFT.
• Different connecting styles between dye and TiO2 affect their spectra remarkably.

N749 has been widely investigated in the dye-sensitized solar cells (DSSCs) for its high incident photon-to-current conversion efficiency (IPCE) in the long wavelength range. Absorption spectra of the N749–TiO2 photoanode have been theoretically examined in this article. Depending on rational models between dye and TiO2 film, the experimental absorption peaks have been well reproduced through density functional theory (DFT) and time-dependent DFT (TDDFT) approaches. The calculation results show that types of N749 connecting to TiO2 electrode affect the utilization of solar energy in DSSCs. The way of electron injection can be distinguished based on the character of the interfacial charge transfer. In addition, the deprotonation of dye molecule also has a significant effect on the absorption spectrum. It is demonstrated that the dyes with closely-lying unoccupied orbitals, allowing for enhanced orbital-coupling, would improve the total photo-to-current conversion efficiency in DSSCs.

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