Carbazole-Dendrimer-Based Donor−π–Acceptor Type Organic Dyes for Dye-Sensitized Solar Cells: Effect of the Size of the Carbazole Dendritic Donor

A series of novel D−π–A type organic dyes, namely, <b>G<i>n</i>TA</b> (<i>n</i> = 1–4), containing carbazole dendrons up to fourth generation as a donor, bithiophene as π-linkage, and cyanoacrylic acid as acceptor were synthesized and characterized for applications in dye-sensitized solar cells (DSSCs). The photophysical, thermal, electrochemical, and photovoltaic properties of the new dyes as dye sensitizers were investigated, and the effects of the carbazole dendritic donors on these properties were evaluated. Results demonstrated that increasing the size or generation of the carbazole dendritic donor of the dye molecules enhances their total light absorption abilities and unluckily reduces the amount of dye uptake per unit TiO₂ area because of their high molecular volumes. The latter was found to have a strong effect on the power conversion efficiency of DSSCs. Importantly, electrochemical impedance spectroscopy (EIS) revealed that the size or generation of the donor had a significant influence on a charge-transfer resistance for electron recombination at the TiO₂/electrolyte interface, causing a difference in open circuit voltage (<i>V</i>_oc) of the solar cells. Among them, dye <b>G1TA</b> containing first generation dendron as a donor (having lowest molecular volume) exhibited the highest power conversion efficiency of 5.16% (<i>J</i>_sc = 9.89 mA cm^–2, <i>V</i>_oc = 0.72 V, ff = 0.73) under simulated AM 1.5 irradiation (100 mW cm^–2)