Systematic Linkage Engineering of Double-Branched Metal-Free Indolo[2,3-b]Quinoxaline Photosensitizers for High-Efficiency Dye-Sensitized Solar Cells

A systematic linkage engineering within double-branched metal-free organic sensitizer remains underexplored, particularly for understanding how donor-donor linkage mode affects photosensitizer performance in dye-sensitized solar cell (DSSC). While double-branched D-D-π-A sensitizers are known to outperform single-branch analogues, the relative impact of branching through different donor sites (D1-D1 vs. D2-D2) within the same indolo[2,3-b]quinoxaline scaffold has not been systematically established. This work directly addresses that gap by designing and synthesizing three novel indolo[2,3-b]quinoxaline-based photosensitizers (NQ-1–NQ-3) containing indole as auxiliary donor (D1), fused indolo[2,3-b]quinoxaline as a second donor (D2) and 4-(2-cyanoacetamido) benzoic acid as spacer-acceptor (π–A) unit to construct D1–D2–π–A system. This work presents a systematic comparison of the single D1–D2–π–A system (NQ-1) with two double-branched system featuring D1–D1 (NQ-2) and D2–D2 (NQ-3) hexyl linkage modes. Comprehensive characterization methods including UV-Vis, cyclic voltammetry (CV), incident-photon-to-current efficiency (IPCE), current density voltage (J-V) and electrochemical impedance spectroscopy (EIS) were utilized. These dyes exhibited strong visible-light absorption at 572–608 nm, particularly NQ-2 with the longest-wavelength absorption maximum at 608 nm due to enhanced ICT, extended conjugation, favorable D1–D1 electronic coupling, and nearly planar optimized geometry. The frontier molecular orbital distributions of HOMO-LUMO and molecular electrostatic potential (MEP) were calculated using density functional theory (DFT) with CAM-B3LYP/6–31G(d,p) level of theory. Among fabricated DSSCs, NQ-2-based device in presence of chenodeoxycholic acid (CDCA) displayed the highest IPCE response around 90% (500–598 nm), with best power efficiency, reaching 12.65%, outperforming NQ-1, NQ-3 and the benchmark N719 dyes. Moreover, EIS revealed that NQ-2-based device possessed the strongest charge recombination resistance (60.81 Ω cm−2) and longest electron lifetime (τeff = 41.35 ms), suggesting effective suppression of interfacial charge recombination. These results indicate that the higher performance of NQ-2 is ascribed to the D1–D1 topology in double-branched system highlighting that donor–donor linkage engineering as a powerful method to develop highly efficient fused indoloquinoxaline sensitizers for DSSCs.