Synthesis and Characterization of D-D--A-Type Organic Dyes Bearing Carbazole-Carbazole as a Donor Moiety (D-D) for Efficient Dye-Sensitized Solar Cells

A series of new D-D--A-type organic dyes - CCTnA (n = 1-3), CCT3N and CCT2PA, bearing the 3-(3,6-di-tert-butylcarbazol-N-yl)-N-dodecylcarbazol-6-yl system as an electron donor moiety (D-D) - were synthesized by convenient methods and successfully utilized as dye sensitizers for dye-sensitized solar cells (DSSCs). The central -conjugated bridges were made of oligothiophene and oligothiophene-phenylene units, whereas the acceptor groups were either cyanoacrylic acid or cyanoacrylamide. Detailed investigation into the relationship between the structures, spectral and electrochemical properties, and performances of the DSSCs is described. The DSSC devices performed remarkably well, with typical overall conversion efficiencies of 3.60-5.69%, and optimal incident photon-to-current conversion efficiencies (IPCEs) exceeding 80%. The devices containing oligothiophene bridging groups performed better than those with oligothiophene-phenylene bridging groups. Of solar cells based on these dyes, the CCT3A-based one gave a maximum IPCE value of 84%, a short-circuit photocurrent density (J(sc)) of 11.31 mAcm(-2), an open-circuit voltage (V-oc) of 0.71 V and a fill factor (FF) of 0.71, corresponding to an overall conversion efficiency () of 5.69% (>96% of that of the reference N719-based cell, = 5.92%). This work suggests that the organic dyes based on donor moieties or donor molecular architectures of this type are promising candidates for improvement of the performances of DSSCs.