Synergistic design of carbazole-bearing (D–π)₂-D–π–A architectures for dye-sensitized solar cells: Experimental and theoretical evaluations

The performance of metal-free organic dyes in dye-sensitized solar cells (DSSCs) can be significantly enhanced through targeted molecular engineering. In this work, we report the design, synthesis, and comprehensive evaluation of four novel organic dyes (IMZ-1 to IMZ-4) based on a (D–π)₂-D–π–A architecture. These dyes incorporate 3,6-disubstituted carbazole moieties as primary electron-donating units and benzimidazole-based auxiliary donors with and without N-hexyl substitution linked via selectively engineered π-bridges consisting of thiophene or furan units. The chemical structures of IMZ 1-4 were comprehensively confirmed using 1H, 13C.NMR, MS spectrometry, and elemental analysis. The impact of these changes on photophysical behavior, electronic structure, interfacial charge motion, and device stability was investigated systematically through combined experimental (UV-Vis, CV) and theoretical (DFT, TD-DFT) studies. Density functional theory (DFT) and time-dependent DFT (TD-DFT) revealed that thiophene-bridged dyes (IMZ-1, IMZ-3) have stronger π-electron delocalization and lower energy gaps (2.41 and 2.44 eV) than their furan analogs (IMZ-2, IMZ-4, 2.46 and 2.57 eV). Moreover, Frontier molecular orbital analysis (FMO) concluded a perfect charge isolation between LUMO and HOMO indicating efficient intramolecular charge transfer (ICT). In comparison with benchmark, simulated UV–Vis spectra and the experimental pattern absorption of IMZ 1-4 corroborated a red-shifted of absorption ranged 370-483 nm. For all IMZ dyes, electrochemical and photovoltaic studies shown markedly better charge injection and higher recombination resistance particularly IMZ-3 (Rrec=21.27 Ω) due to the optimal structural balance between electronic coupling and inhibition of aggregation. Devices based IMZ 1-4 achieved considerable power conversion efficiency (PCE) between 4.15 % and 5.53 %, particularly IMZ-3 (PCE: 5.53 %) with remarkable long-term stability of operation (99.7 % PCE retention over 1000 hours). These findings indicate that the synergy between π-bridge conjugation, molecular planarity, and alkyl-induced steric modulation is key to defining dye performance. Our findings present a strategic roadmap for the design of high-efficiency, long-lasting, and tunable organic sensitizers for dye-sensitized solar cells (DSSCs).

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