Theoretical characterization of ruthenium complexes containing functionalized bithiophene ligands for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) have gained widespread interest for their potential as low-cost solar energy conversion devices. One of the key issues is the design of higher efficient light-absorbing dyes. In this paper, we present a theoretical characterization of ruthenium complexes containing functionalized bithiophene (btp) ligands (CYC–B1 and CYC–B11) based on density functional theory (DFT) calculations. Molecular geometries, electronic structures, and optical absorption spectra are investigated both in the gas phase and in dimethylformamide (DMF) solution. Frontier orbital analysis shows the three highest HOMOs are composed of nonbonding combinations of the Ru t2g orbitals with the p orbital and lone pairs of the SCN ligands, while the six lowest LUMOs are the π* combinations of the 4,4’-dicarboxy-2,2’-bipyridine (dcbpy) and/or btp-functionalized bipyridine (bpy) ligands calculated in the gas phase. Inclusion of solvent results in great changes in energies and compositions of the molecular orbitals of these complexes. The spectra are assigned to the intraligand π → π* transitions of the dcbpy ligand in the ultraviolet region, whereas in the visible region the spectra show multitransition character of metal-to-ligand charge transfer (MLCT), interligand π → π*, and intraligand π → π*. Our results clarify the role of the functionalized btp ligands on the absorption properties of the dyes.

Graphical abstractThe molecular geometries, electronic structures, and optical absorption spectra of the ruthenium dye sensitizers containing functionalized bithiophene ligands have been investigated using the DFT/TD-DFT approach both in the gas phase and in dimethylformamide solution. The role of the functionalized bithiophene ligands has been clarified with respect to the absorption properties of the dyes.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Design of higher efficient dyes is crucial to boost dye-sensitized solar cells. ► DFT/TD-DFT calculations characterize the role of bithiophene (btp) in ruthenium dyes. ► Adding btp tunes molecular energy levels, enhances spectral response, and increases oscillator strength.