New Carbazole-Based Sensitizers for p-Type DSSCs: Impact of the Position of Acceptor Units on Device Performance

We report the design, synthesis, and characterization of two new carbazole-based organic dyes <bold>PC1-2</bold> as potential sensitizers for NiO-based p-type dye-sensitized solar cells (p-DSSCs). The D-A-pi-A' configured <bold>PC1</bold> dye comprises a thienyl unit as a pi-spacer and a malononitrile as an end-capping acceptor unit, whereas in <bold>PC2</bold>(,) the cyanovinylene group serves as an acceptor unit and a thienyl group acts as a donor unit in a D-A-D configuration. These molecules achieved excellent solubility due to their long-branched alkyl chains. The current work encompasses their structural, photophysical, thermal, electrochemical, theoretical, and photoelectrochemical studies, establishing structure-property relationships. <bold>PC1-2</bold> exhibit lambda(abs) and lambda(emi) in the range of 389-404 and 448-515 nm, respectively, with a band gap in the range of 2.88-2.92 eV. Electrochemical studies confirm the feasibility of electron injection, regeneration, and recombination. The introduction of an additional electron-withdrawing group (cyanovinylene group) on the dye <bold>PC1</bold> skeleton endows it with a higher dye loading capacity, high hole injection, and a strengthened intramolecular charge transfer (ICT) effect, resulting in a redshifted ICT absorption with a higher molar extinction coefficient. Among the two new dyes, the device based on <bold>PC1</bold> achieved the highest power conversion efficiency (PCE) of 0.027% with a short-circuit current density (J(SC)) of 1.29 mAmiddotcm(-2), open-circuit voltage (V-OC) of 67 mV, and fill factor (FF) of 31%, whereas the device with dye <bold>PC2</bold> performed less efficiently (PCE: 0.018%, J(SC): 0.92 mAmiddotcm(-2), V-OC: 68 mV, and FF: 30%). Conclusively, the study provides insights into the intricacies involved in the structural modification of carbazole-based p-type dyads for the development of highly efficient DSSCs.