New D–π–A Dyes Based on Indolo[3,2,1-jk]carbazole and Thienopyrrolo[3,2,1-jk]carbazole for Application in Dye-Sensitized Solar Cells

Abstract

Six new D-π-A type molecules were designed, synthesized, and investigated. The donor moieties were rigid, tripod-shaped polycyclic heteroaromatic indolo[3,2,1-jk]carbazole (ICz) and thieno[2′,3′:4,5]pyrrolo[3,2,1-jk]carbazole (TPCz) skeletons, while the acceptor groups were cyanoacrylic acid and rhodanine-3-acetic, and the linkers were phenyl, thiophene, and an acetylene bridge. TPCz is a stronger donor than ICz, which destabilises the HOMO orbital in the molecules and lowers the energy band gap. The optical and electrochemical properties were thoroughly analysed by comparing experimental data with DFT and TD-DFT calculations. The potential of the synthesized compounds for photovoltaic applications was investigated in dye-sensitized solar cells. It was found that the device sensitized with the TPCz molecule containing a cyanoacrylic acid unit exhibited the best performance compared to the others; however, the power conversion efficiency (PCE) was low, 1.31%. This molecule appears to act as a complementary sensitizer to the commercial N719 dye. Therefore, additional device engineering strategies were implemented, including co-sensitization with N719, incorporation of a co-adsorbent (chenodeoxycholic acid), introduction of a blocking layer, and operation under reduced illumination conditions. These modifications resulted in a significant enhancement of PCE, reaching 12.30% under reduced illumination (10 mW cm⁻²).