Efficient triphenylamine-based dyes featuring dual-role carbazole, fluorene and spirobifluorene moieties

Three triphenylamine-based organic dyes SD6, SD7, and SD8 containing the bulky dual-role moieties (fluorene, carbazole, and spirobifluorene) in the molecular frameworks were synthesized, characterized and applied in dye-sensitized solar cells (DSSCs). The effects of dual-role moieties of organic dyes on their photophysical, electrochemical, and photovoltaic properties have been investigated in detail. These dyes exhibit strong charge transfer absorption bands in the visible region. Their redox potential levels were estimated by cyclic voltammetry and found to suit well to the charge flow in DSSCs. Inserting the dual-role moieties as the secondary donor between the triphenylamine and thiophene units increased the electron density of the donor groups, therefore reduced the HOMO-LUMO band gaps. The combination of ultraviolet-visible (UV-vis) region broad absorption bands with fairly high extinction coefficients and appropriate redox properties observed in these triphenylamine-based dyes make them promising dyes for DSSCs. For a typical solar cell device based on dye SD6, the maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach to ∼73%, with a short-circuit photocurrent density (J_sc) of 14.25 mA/cm², an open-circuit photovoltage (V_oc) of 0.70 V, and a fill factor (FF) of 0.705, which corresponds to a power conversion efficiency (PCE) of 7.03%.