Synergistic charge-transfer dynamics of novel D-D-A-π-A framework containing indoline-benzo[d][1,2,3]thiadiazole based push-pull sensitizers: from structural engineering to performance metrics in photovoltaic solar cells

Context Dye-sensitized solar cells (DSSCs) present a convincing substitute for conventional silicon-based solar cells because of their possible lower manufacturing costs and versatile uses. Electron injection and dye regeneration processes are important in meeting the need for photosensitizers with improved efficiency and stability. Aimed at enhancing the performance and efficiency of DSSCs, this study focuses on the structural engineering to performance metrics of novel indoline-benzo[d][1,2,3]thiadiazole based push-pull sensitizers (LHZ1 to LHZ9) with D-D-A-π-A framework. The current study provides insights into the photovoltaic and optoelectronic properties of the investigated dyes, which are significantly influenced by the modification of auxiliary donors (D), internal acceptors with thiophene as a spacer, and cyanoacrylic acid (A) as the terminal acceptor. These modifications enhance rapid charge transfer among the dyes, highlighting the critical role of dye-semiconductor interactions. Methods The suitability of developed sensitizers for DSSCs applications is confirmed by executing quantum methods like NBO, TDM, FMO, DOS, E_b, ΔG^reg, ΔG^inject, VRP, and ICT parameters q^CT (e^-), D^CT (\(\overset{\circ }{A}\)), H index (\(\overset{\circ }{A}\)), ∆(\(\overset{\circ }{A}\)), t index (\(\overset\circ A\)), and μ^CT (D). All of the investigated dyes have HOMO levels lower than the electrode I-/I₃-‘s redox potential (−4.8 eV) and LUMO values that are appropriately higher than the conduction band of TiO₂ (−4.0 eV). The novel dyes showed a closing of the energy gap (2.38–1.84 eV). The LHZ7 and LHZ8 molecules with the lowest E_g (1.97 eV and 1.84 eV) demonstrated the highest absorption (up to 746 nm > 402 nm for LHZ), which was caused by the insertion effect of varied donors and internal acceptors. Almost all photosensitizers appeared with remarkable properties, i.e., red-shifted absorption maxima (746 nm), lowest E_x (1.66 eV), E_b (0.02 eV), and highest values of LHE (0.958). The TDM analysis revealed high charge density on HOMO of donor and LUMO of acceptors in designed dyes. DOS analysis revealed that the donor parts of the molecules delocalized the highest occupied molecular orbitals of dye particles. The electronic properties predicted by the NBO analysis showed that donor groups donate high and faster transfer of charge, and internal acceptor groups rapidly accept them. The electron injection (ΔG^inject) and dye regeneration (ΔG^reg) analysis of photosensitizers attached with TiO₂ proved efficient charge transfer properties from the donor of newly designed dyes onto the conduction band of TiO₂. This study, also supported by the thermodynamic stability of dyes with negative values of Gibbs free energy, revealed that the performance of the designed dyes is augmented by modifying the donor and internal acceptors of the reference photosensitizer for effective application in the experimental community. All of the dyes are suitable for DSSCs based on the calculated parameters. Still, the LHZ9 dye proved proficient in applying dye-sensitized solar cells due to its remarkable properties, i.e., lowest gap and red-shifted absorption maxima. Graphical abstract