Synthesis, characterization, and DFT studies of metal-free hydrazide co-sensitizers with N719 for Pt-free DSSCs achieving 12.53% outdoor and 26.66% indoor efficiency

Two metal-free organic sensitizers, MTP-1 and SPZ-1 , incorporating methoxytriphenylamine and phenylphenothiazine donor units, respectively, with a carboxyhydrazide-based acceptor/anchoring segment, were synthesized, characterized, and evaluated as co-sensitizers with N719 for dye-sensitized solar cells. The structures of the dyes were confirmed using various spectroscopic analysis. UV–Vis absorption studies revealed that MTP-1 exhibited a more red-shifted intramolecular charge-transfer band, higher molar absorptivity, and broader visible-light absorption than SPZ-1 , indicating stronger donor–acceptor electronic communication. Co-sensitization with N719 significantly enhanced the optical response of TiO₂ photoanodes, with the absorbance following the order MTP-1 + N719 > SPZ-1 + N719 > N719 , confirming complementary light harvesting between the organic ICT transitions and the MLCT absorption of N719. DFT calculations further supported the experimental findings, showing effective spatial separation between HOMO and LUMO levels, directional charge transfer from the donor moieties toward the carboxyhydrazide anchoring unit, and stable bidentate adsorption on TiO₂. To improve redox regeneration and charge transport, a Pt-free PEDOT/SnSe-rGO composite counter electrode was employed. Under AM 1.5G illumination, the MTP-1 + N719:PEDOT/SnSe-rGO device achieved the highest efficiency of 12.53%, outperforming the corresponding Pt-based device and N719 -only systems. Remarkably, under indoor illumination at 1000 lx, the same device delivered an outstanding efficiency of 26.66%, with enhanced V OC , J SC , and fill factor. IPCE and impedance analyses confirmed improved photon-to-current conversion, accelerated interfacial charge transfer, and suppressed recombination in the PEDOT/SnSe-rGO- based cells. These results demonstrate that combining metal-free hydrazide co-sensitizers, DFT-guided donor engineering, and Pt-free counter-electrode design is an effective strategy for developing high-performance DSSCs for both outdoor and indoor energy-harvesting applications.