Novel Organic Dyes Featuring Spiro[dibenzo[3,4:6,7]cyclohepta[1,2‑b]quinoxaline-10,9′-fluorene] (SDBQX) as a Rigid Moiety for Dye-Sensitized Solar Cells

Three novel organic dyes coded as FHD4, FHD5, and FHD6 featuring spiro­[di­benzo­[3,4:6,7]­cyclo­hepta­[1,2-b]­quinoxaline-10,9′-fluorene] (SDBQX) moieties were developed for dye-sensitized solar cells (DSSCs). The fluorenyl moiety of SDBQX is perpendicular to the quinoxaline moiety, which is beneficial in inhibiting the H-aggregation in DSSCs. The band gap energies according to DFT calculations showed a good correlation with the transition energy calculated from the absorption spectra, which indicates that the DFT calculations would be an effective method to predict the absorption spectra range for FHD-type dyes. Broad spectral coverage and a high molar extinction were observed in the absorption spectrum of FHD4, which leads to the best power conversion efficiency that was obtained for the FHD4-based DSSC. Coadsorption of CDCA improved the power conversion efficiency slightly for FHD4-based DSSCs (from 4.61 to 4.69%) and FHD6-based DSSCs (from 3.59 to 3.69%). The coadsorption of CDCA decreased the dye loading amount of FHD5 significantly, while the power conversion efficiency increased significantly from 3.18 to 3.73%. Finally, we have developed SDBQX as a new architecture for developing efficient organic dyes for DSSC applications.