Unveiling the auxiliary acceptor role of azo spacers in triphenylamine-based sensitizers: A computational insight into spectral broadening and charge transfer mechanisms

A computational study was conducted on eight novel triphenylamine-based donor–π–acceptor sensitizers to elucidate the auxiliary acceptor mechanism of electron-deficient spacers in dye-sensitized solar cells. The investigated molecular systems incorporate alkenyl, azomethine, or azo spacers positioned either between the donor and the π bridge (AN series) or between the π bridge and the cyanoacrylic acceptor (BN series). Density functional theory (DFT) calculations at the M06/6-31G(d) level and time-dependent DFT calculations using M06-2X/6-31G(d) were employed to investigate the structural, electronic, optical, and photovoltaic properties of the dyes. The results reveal that the BN-series dyes, particularly BN4 containing an azo spacer, exhibit superior performance, including a reduced HOMO–LUMO gap of 2.19 eV, bathochromically shifted absorption extending to 625 nm, a stabilized LUMO energy of –2.97 eV, and enhanced ground-state polarization with Δq(D–A) = 0.55 e - . The BN architecture transforms the conventional D–π–A framework into a donor–π–auxiliary acceptor–acceptor (D–π–A′–A) configuration, in which the spacer acts as an electron-withdrawing modulator that broadens the effective acceptor domain. Additional NBO, molecular electrostatic potential, and conceptual DFT analyses further support this mechanism. BN4 also exhibits favorable photovoltaic descriptors, including efficient electron injection (ΔGinj = 0.81 eV), strong dye regeneration thermodynamics (ΔGreg = 0.37 eV), high light-harvesting efficiency (LHE = 0.96), and an extended excited-state lifetime (τ = 4.20 ns). These findings provide useful structure–property relationships for the rational design of high-performance metal-free organic sensitizers. • Evaluation of azo, azomethine, and vinylene spacers shows that topological position has a greater impact on bandgap tuning than chemical identity. • Placing electron-deficient spacers near the acceptor in the BN series forms a D-π-A'-A architecture, which significantly stabilizes the LUMO. • NBO analysis shows enhanced ground-state charge separation in the BN series, facilitating intramolecular charge transfer. • All designed dyes retain adequate driving forces for electron injection and dye regeneration, even with red-shifted absorption. • Inserting azo groups near the acceptor is an effective strategy for developing high-efficiency organic sensitizers.