Novel D–A−π–A-Type Organic Dyes Containing a Ladderlike Dithienocyclopentacarbazole Donor for Effective Dye-Sensitized Solar Cells

A novel ladderlike fused-ring donor, dithienocyclopentacarbazole (DTCC) derivative, is used to design and synthesize three novel donor–acceptor−π–acceptor-type organic dyes (C1–C3) via facile direct arylation reactions, in which the DTCC derivative substituted by four p-octyloxyphenyl groups is served as the electron donor and the carboxylic acid group is used as the electron acceptor or anchoring group. To fine-tune the optical, electrochemical, and photovoltaic properties of the three dyes, various auxiliary acceptors, including benzo­[2,1,3]­thiadiazole (BT), 5,6-difluorobenzo­[2,1,3]­thiadiazole (DFBT), and pyridal­[2,1,3]­thiadiazole (PT), are incorporated into the dye backbones. The results indicate that all of the three dyes exhibit strong light-capturing ability in the visible region and obtain relatively high molar extinction coefficients (>31 000 M^–1 cm^–1) due to their strong charge transfer (CT) from donor to acceptor. Moreover, theoretical model calculations demonstrate fully separated highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels for the three dyes, which is helpful for efficient charge separation and electron injection. Using the three dyes as sensitizers, conventional dye-sensitized solar cells (DSSCs) based on liquid iodide/triiodide redox electrolytes are fabricated. Our results indicate that the BT-containing dye C1 affords the highest power conversion efficiency of up to 6.75%, much higher than that of the DFBT-containing dye C2 (5.40%) and the PT-containing dye C3 (1.85%). To our knowledge, this is the first example reported in the literature where the DTCC unit has been used to develop novel organic dyes for DSSC applications. Our work unambiguously demonstrates that the ladderlike DTCC derivatives are the superb electron-donating blocks for the development of high-performance organic dyes