Exploring the sensitization properties of thienyl-functionalized tripyrrole Ru(II) complexes on TiO2 (101) surface: a theoretical study

Ruthenium(II) complexes, as the dye sensitizer in the solar cell system, has attracted great interests. In the present study, based on the ruthenium(II) complex N749, new sensitizers have been designed theoretically to increase the stability and the efficiency of dye-sensitized solar cell (DSSC). By investigating the ground state geometries, electronic structures, and spectroscopic properties by density functional theory (DFT) and time-dependent DFT, the orbital components and absorption transition have been obtained. The effect of tripyrrin ligand in the designed new sensitizers can be demonstrated from our results. The results show that the absorption spectra are systematically broadened and red-shifted with the increase sizes of the pyrrole ligands. The important unoccupied orbitals referred to charge transfer are mainly from di/tripyrrin derivative groups. Consequently, the charge transfer to the di/tripyrrin derivative groups has been strengthened. According to our study, the di/tripyrrin derivative ligand is more efficient than the NCS− ligand in absorbing visible light. The calculation results also indicate that the electronic structures of the N749 derived sensitizers are significantly influenced by the different substituted positions of the thienyl groups on di/tripyrrin ligands. Thus, the efficiency of DSSCs would be different. Our research predicted that the Ru(II) complexes containing 5,10-(2-thienyl)-4,6,9,11-tripyrrin ligand may enhance the visible light absorption of DSSC. This is in accordance with the corresponding experiment. These results are expected to assist the molecular design for new dyes in future DSSCs.