Tuning the photophysical properties of triphenylamine pyrazine-based dyes: role of π-spacers in DSSCs with iodine and copper-based redox shuttle

Abstract

Context: A series of donor-π-acceptor dyes were designed based on the structure of the experimentally reported TPP dye, which incorporates Triphenylamine (TPA) as the donor, Pyrazine as the π-bridge, and a carboxylic acid group as the acceptor. To enhance the photovoltaic performance of dye-sensitized solar cells, five new dyes (TPP1-TPP5) were modelled by introducing different alterations to the π-conjugated bridge. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were carried out to examine how the alteration in the π-spacer influences the optical, electronic, and photovoltaic performance. All dyes displayed negative Gibbs free energy values for electron injection into TiO₂, confirming the thermodynamic favourability of the charge transfer process. Short-circuit current density (J_SC) was found as the highest for TPP3 and TPP4, outperforming the other TPP dyes with 1.40 mA cm^-2 and 1.87 mA cm^-2. Furthermore, with the lowest dye regeneration of ΔG_reg = 0.46 eV and a comparable open circuit voltage (V_oc) of 1.17 eV, TPA4 demonstrated higher regeneration kinetics. Natural bond order analysis was conducted to assess the bond strength and examine the molecular orbitals associated with the donor, π-spacer and acceptor unit. All the modelled dyes found strong non-linear optical characteristics having the linear polarizability (α) amplitudes greater than the first-order total polarizability (β_total) relative to the experimental dye. Light harvesting efficiency of the modelled dye TPP4 was found the maximum (89%) among the studied dyes. These findings show that π-spacer alteration is an effective strategy for improving overall dye performance in DSSCs.

Methods: Optimization of all species by using Gaussian16 with functional B3LYP and basis set 6-311G (d,p). NBO analysis was performed to explore the interactions between the filled orbitals of one part and the vacant orbitals of another part. TDDFT studies were performed using ORCA4.2 with Zeroth-Order Regular Approximation for accounting relativistic effects to calculate excitation energies.

Keywords: DFT/TDDFT; Donor-π-Acceptor; LHE; NBO; NLO; Redox shuttle.