Organic Sensitizers Based on Hexylthiophene-Functionalized Indolo[3,2-b]carbazole for Efficient Dye-Sensitized Solar Cells

Two new organic sensitizers, MKZ-21 and MKZ-22, comprising 5,11-dioctylindolo[3,2-b]carbazole moiety as the electron donor, n-hexyl-substituted oligothiophene units as the π-conjugated bridge, and cyanoacrylic acid group as the electron acceptor were designed and synthesized for application in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). Theoretical calculation indicates that HOMO−LUMO excitation moves the electron distribution from the indolo[3,2,-b]carbazole moiety to the cyanoacrylic acid (cyanoacrylate) group. Compared with a sensitizer MK-1 based on a carbazole unit as the electron donor, both experimental results and theoretical calculations prove that, when connected into the similar donor-π-acceptor structure, indolo[3,2-b]carbazole unit has stronger electron donating ability than the carbazole unit. For the functionalized organic sensitizers MKZ-21 and MKZ-22, upon anchoring onto TiO2 film, MKZ-21 exhibited a better photovoltaic performance: a monochromatic incident photon-to-current conversion efficiency (IPCE) of 83%, a short-circuit photocurrent density (Jsc) of 15.4 mA cm−2, an open-circuit voltage (Voc) of 0.71 V, and a fill factor (FF) of 0.67, corresponding to an overall conversion efficiency (η) of 7.3% under standard AM 1.5G irradiation (100 mW cm−2), which suggests that the sensitizers based on indolo[3,2,-b]carbazole unit are promising candidates for DSSCs.