Ternary co-sensitization with SQ-2 and indole-based dyes boosts DSSC efficiency by 49% Over N719 benchmark

In this study, four novel metal-free organic sensitizers (FM-1 to FM-4) based on an ethylindole donor core and various electron-accepting units, 4-carboxycyanoacetamide, rodanine-3-acetic acid, thiazolidine, and thiobarbituric acid, were synthesized and characterized for application in dye-sensitized solar cells (DSSCs). The dyes exhibited strong intramolecular charge transfer (ICT) with absorption maxima ranging from to 425–455 nm and optical band gaps between 2.47 and 2.68 eV. Electrochemical analysis revealed HOMO and LUMO energy levels from − 5.79 to − 6.19 eV and − 3.32 to − 3.51 eV, ensuring favorable alignment for efficient electron injection into TiO 2 and dye regeneration by the I − /I 3 − redox couple. Among all the dyes, FM-1 demonstrated the lowest band gap (2.47 eV) and strongest ICT character. Co-sensitization of FM dyes with N719 broadened the absorption spectrum and improved device performance, with binary FM-1/N719 achieving a power conversion efficiency ( PCE ) of 10.77%, Voc = 0.773 V, and J SC = 24.00 mA.cm −2 . Incorporation of the unsymmetrical squaraine dye SQ-2 in a ternary system (FM-1 + N719 + SQ-2) further enhanced light harvesting across 300–700 nm and reduced charge recombination, achieving a record PCE of 11.76%, a 49% improvement over the N719-only cell (7.90%). EIS analysis confirmed increased charge transfer resistance, and IPCE spectra showed an extended photoresponse up to 700 nm. These findings reveal the synergistic effect of SQ-2 and indole-based dyes in boosting the photovoltaic efficiency and highlight ternary co-sensitization as a promising strategy for high-performance metal-free DSSCs.