Synthesis of Carbazole-Benzimidazole Dyes with Size and Acid–Base Complementarity for High-Performance Ternary co-Sensitized Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have gained substantial interest as cost-effective and solution processible alternatives to silicon photovoltaics, yet the spectral and surface-coverage limitations of single-dye sensitization continue to constrain device efficiency. Herein, a dual complementarity strategy combining molecular size and acid–base character is introduced as a new co-sensitization design principle for high-performance DSSCs. Thus, carbazole/benzimidazole (IM-1) photosensitizer has been synthesized as acidic dye by incorporating 4-vinylphthalic acid into our previous synthesized dye (IMZ-CZB). Moreover, two basic dyes IM-2 and IM-3 as co-sensitizers were synthesized through the reaction of benzene-1,2-diamine with two different aldehydes; 4-pyridinecarboxaldehyde and 4-(pyridin-4-yl)benzaldehyde respectively. IM-1 represents a bulky D2-D-π-A carbazole bis-benzimidazole sensitizing dye molecule, which is linked via an acidic phthalic acid group. On the other hand, IM-2 is a smaller dye molecule and IM-3 is an intermediate-size molecule, enabling them to geometrically occupy the interstitial voids between adsorbed IM-1 molecules. Photophysical and electrochemical measurements were systematically conducted. The results showed that the ternary IM-1+IM-2+IM-3 system exhibited the broadest ICT absorption (505 nm) and the highest molar extinction coefficient (49,300 M−1 cm−1), translating into the highest incident photon-to-current efficiency (IPCE) of 88% extending to 512 nm among all configurations. Density functional theory (DFT) calculations confirmed HOMO/LUMO energy levels of −5.17/−2.95 eV for IM-1 supporting spontaneous electron injection. The DSSC devices were fabricated using individual IM-1, binary (IM-1 + IM-2 and IM-1 + IM-3) and ternary (IM-1 + IM-2 + IM-3) co-sensitizer system. Among them, DSSC based on the ternary system achieved the highest power conversion efficiency (PCE) of 6.00% corresponding to a 66% improvement relative to the individually sensitized IM-1 device fabricated in the present study. Based on electrochemical impedance spectroscopy studies, the ternary dye cell exhibited the maximum value for recombination resistance and electron lifetime (63.7 Ω and 17.19 ms, respectively), indicating the inhibition of interfacial charge recombination. Furthermore, the ternary dye cell preserved 93% efficiency of its original value even after continuous illumination for 1000 h. These results highlight the importance of integrating both size and acid–base complementarity in the rational design of co-sensitizer systems for high-performance DSSCs.