Theoretical investigation on structural and electronic properties of organic dye C258 on TiO₂(101) surface in dye-sensitized solar cells

The structural and electronic properties of an organic dye C258 before and after being adsorbed onto a TiO₂(101) surface by two adsorption modes, monodentate (Mha) and bidentate bridging (BBH), have been investigated in detail. The combination of density functional tight-binding (DFTB), density functional theory (DFT), and time-dependent DFT (TDDFT) approaches have been employed. DFT calculations show that C258 has remarkable charge-transfer characteristics, which favors fast electron injection from the excited dye to the conduction band of TiO₂. A detailed analysis of the adsorbate contributions of the dye molecule to band states of TiO₂ shows a strong coupling of the adsorbate orbitals with the substrate orbitals. Significant electronic transfer characteristics across the interface reveal a direct electron injection mechanism arising from the electronic excitation of the anchoring group of C258 to the conduction bands of TiO₂. The adsorption energy and the electron density distribution demonstrate that the BBH structure is more stable and has a stronger coupling with TiO₂ than the Mha pattern, which is able to better promote the electron injection to increase the efficiency of dye-sensitized solar cells (DSSCs).