Density functional theory (DFT) and time-dependent DFT calculations have been employed to model Zn meso-tetraphenylporphyrin (ZnTPP) complexes having different b-substituents, in order to design an efficient sensitizer for dye-sensitized solar cells. To calculate the excited states of the porphyrin analogues, at least the TD-B3LYP/6-31G* level of theory is needed to replicate the experimental absorption spectra. Solvation results were found to be invariant with respect to the type of model used (PCM vs. C-PCM). Most of the electronic transitions based on Gouterman's four-orbital model of ZnTPP-A and ZnTPP-B are p -p* transitions, so that cell efficiency can be enhanced by increasing the p-conjugation and electron-withdrawing capability of the bsubstituent. This proposition was tested by inserting thiophene into the b-substituent of ZnTPP-A to form a new analogue, ZnTPP-C. Compared with ZnTPP-A and ZnTPP-B, ZnTPP-C has a smaller band gap, which brings LUMO closer to the conduction band of TiO 2 , and a red-shifted absorption spectrum with higher extinction coefficients, especially in the Q-band position.
One of the most significant aspects in the development of dye-sensitized solar cells is the exploration and design of high-efficiency and low-cost dyes. This paper reports the theoretical design of various triphenylamine analogues, wherein the central nitrogen moiety establishes an sp(2)-hybridization, which endows a significant participation in the charge-transfer properties. Density functional theory (DFT) and time-dependent DFT methodologies were utilized to investigate the geometry, electronic structure, photochemical properties, and electrochemical properties of these dyes. Different exchange-correlation functionals were initially evaluated to establish a proper methodology for calculating the excited-state energy of the reference dye, known as DIA3. Consequently, TD-LC-ωPBE with a damping parameter of 0.175 Bohr(-1) best correlates with the experimental value. Four new dyes, namely, Dhk1, Dhk2, Dhk3, and Dhk4, were designed by modifying the rigidity of the donor moiety. According to the results, altering the type and position of binding in the donor group leads to distinct planarity of the dyes, which significantly affects their properties. The designed Dhk4 dye showed more red-shifted and broadened absorption spectra owing to the enhanced coplanarity between its donor and π-bridge moiety, which brings an advantage for its potential use as sensitizer for photovoltaic applications.
A series of distorted push-pull meso-substituted porphyrin analogues with different acceptor groups and additional electron-donating substituents are investigated as organic sensitizers for application in dye-sensitized solar cells (DSSCs) using density functional theory (DFT) and time-dependent DFT approach. The donor was modified by interchanging methyl group with methoxy and extending the p-conjugation. The acceptor group was assessed based on cyanoacrylic (A analogues) or methylenemalonic (B analogues) acid groups. Benchmark calculations using YD1 as reference indicated that the best method to depict the excitation energies was with TDxB97X-D exchange-correlation (xc) functional while the computational protocol for computing redox potentials was found to be with the M06-2X xc functional based on vertical DSCF method. The absorption spectra of all the porphyrin analogues were red-shifted and produced higher oscillator strengths, especially at the Q-bands as compared to the reference molecule. Among the analogues, A2-OMe and B2-OMe are good candidates for sensitizers in DSSCs due to its larger hyperpolarizabilities, better light-harvesting efficiencies, proper matching of the ground-state oxidation potentials with respect to the I À =I À 3 redox couple, and higher dipole moment of the adsorbed analogues. This study further enhances the role of theoretical calculations in the molecular design of sensitizers for DSSCs in an effort to produce a highly efficient dye.
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