Theoretical investigation of novel phenothiazine-based D–π–A conjugated organic dyes as dye-sensitizer in dye-sensitized solar cells

The structural and electronic properties of four phenothiazine-based sensitizers (TMH–CNcis, TMH–CNtrans, TMH–COOH, and TMH–2COOH) have been examined by means of density functional theory (DFT) and time-dependent DFT calculations. And the periodic DFT method is used to investigate the adsorption of dyes on the TiO2 anatase (1 0 1) surface within the DMol3 code. The results promise that anchor dyes with strong withdrawing CN group can effectively injected electrons to the conduction band of semiconductor TiO2 surface implying that TMH–CNcis and TMH–CNtrans will show better performance among four dyes. Particularly, the adsorption energies and charge density difference maps are calculated to investigate the possible adsorption modes of the dye on the TiO2 surface. After establishing the preferred anchoring configurations, we performed a detailed analysis on the electronic structures of the dye-TiO2 complexes to explore the absorption spectra, charge distribution and the composition of the density of states (DOS), providing a quantitative description of the variation in electronic coupling induced by the different anchoring group. The results show that after binding to the semiconductor, the absorption spectra of four dyes-TiO2 complexes are all red-shifted significantly as compared to that of the isolated dyes, and the TMH–CNtrans–TiO2 will transfer more electrons during the photoexcitation, showing an obvious charge transfer characteristic.