Push–pull carbazole twin dyads as efficient sensitizers/co-sensitizers for DSSC application: effect of various anchoring groups on photovoltaic performance†

To investigate the effect of various anchoring groups of organic sensitizers on fundamental processes occurring inside DSSCs and their overall performance, we designed and synthesized nine new double donor–acceptor (D–A) type organic dyes DCH_1–9 comprising carbazole-based twin molecules as electron donors, with a non-conjugated linear alkyl chain as an extended linker featuring multiple acceptor units. Their photophysical, thermal, electrochemical, and theoretical properties were examined to gain a deeper understanding of the structure–property relationship. Photophysical results revealed that all dyes display λ_abs and λ_emi in the range of 400–470 nm and 500–560 nm, respectively, with a bandgap in the range of 2.46–2.74 eV. The push–pull structure with extended conjugation results in strong fluorescence characteristics. Photophysical and electrochemical studies confirm their thermodynamic feasibility for electron injection, recombination, and dye regeneration in cells. Quantum chemical simulations further provided insights into their structural, electronic, and optical parameters. New DSSCs were fabricated employing dyes DCH_1–9 as sensitizers/co-sensitizers. The cell sensitized with DCH₁ achieved the highest power conversion efficiency (PCE) of 2.45% under standard AM 1.5 solar conditions. Furthermore, co-sensitization of DCH_1–9 with the Ru-based HD-2 sensitizer resulted in an improved PCE of 8.82% for DCH₂, surpassing HD-2 alone (6.79%). EIS studies were conducted to further explore their energy conversion processes. Conclusively, these investigations highlight the significant potential of dyes carrying carbazole twin molecules with different anchoring units in enhancing the overall performance of DSSCs.