Organic Dyes Containing Carbazole as Donor and π‑Linker: Optical, Electrochemical, and Photovoltaic Properties

A series of new metal free organic dyes containing carbazole as donor and π-linker have been synthesized and characterized as effective sensitizers for dye sensitized solar cells (DSSCs). The carbazole functionalized at C-2 and C-7 served as electron-rich bridge. The donor property of the carbazole is substantially enhanced on introduction of <i>tert</i>-butyl groups at C-3 and C-6 positions and the oxidation propensity of the dyes increased on insertion of thiophene unit in the conjugation pathway. These structural modifications fine-tuned the optical and electrochemical properties of the dyes. Additionally, the presence of <i>tert</i>-butyl groups on the carbazole nucleus minimized the intermolecular interactions which benefited the performance of DSSCs. The dyes served as efficient sensitizers in DSSCs owing to their promising optical and electrochemical properties. The efficiency of DSSCs utilizing these dyes as sensitizers ranged from 4.22 to 6.04%. The <i>tert</i>-butyl groups were found to suppress the recombination of injected electrons which contributed to the increment in the photocurrent generation (<i>J</i>_SC) and open circuit voltage (<i>V</i>_OC). A dye with carbazole donor functionalized with <i>tert</i>-butyl groups and the conjugation bridge composed of 2,7-disubstituted carbazole and thiophene fragments exhibited higher <i>V</i>_OC value. However, the best device efficiency was observed for a dye with unsubstituted carbazole donor and the π-linker featuring carbazole and bithiophene units due to the high photocurrent generation arising from the facile injection of photogenerated electrons into the conduction band of titanium dioxide (TiO₂) facilitated by the low-lying LUMO