Enhanced Efficiency of Organic Dye-Sensitized Solar Cells: Triphenylamine Derivatives

Abstract

A general TDDFT procedure has been set up that accurately evaluates the UV/vis absorption spectra of a series of new conjugated metal-free organic dyes based on the triphenylamine (TPA) moiety, which have recently been developed for dye-sensitized solar cells (DSSCs). It turns out that the BHandH functional, combined with the 6-311+G(2d,2p) basis set, gives reliable auxochromic shifts when the bulk solvation effects are included in the model. Indeed, the theoretical procedure provides λmax with a mean absolute deviation limited to ∼0.1 eV only. In addition, we give insights into the geometrical and electronic structures of the dyes, and we unravel the structural modifications allowing to optimize the properties of TPA-based DSSCs. This investigation aims at improving the electron-injection process, as well as the light-harvesting efficiency (LHE) of the dyes. To this purpose, we considered a set of about 20 new dyes, and starting from the TPC-1 structure, the following modifications help to get better electron injection and light-harvesting properties: (i) the extension of the bridging group by addition of an ethylene subunit between the two phenyl groups (TPC-14); (ii) the 16-COOH, 15-OMe, 1a,6-diCN functionalization (TPC-18); (iii) moving the terminal cyano acceptor from the 16 to the 15 position, while introducing two −OMe functions in 11 and 13 positions and/or grafting two −CN groups in 1a and 6 positions on the TPA moiety (TPC-20). These specific modifications induce a maximal increase of the LHE and a more exoenergic free enthalpy of injection (−2.20 eV compared to −1.84 eV for TPC-1). Finally, TPC-23 (which results from the TPC-14/TPC-20 combination) shows an improvement of both the spectroscopic and energetic parameters. Moreover, the molecular topology analysis demonstrates that the coplanarity between the anchoring and the bridging unit is broken, that is, the positive charge is not directly in contact with the TiO2 surface, and the recombination reaction is therefore inhibited.

Acene‐Modified Triphenylamine Dyes for Dye‐Sensitized Solar Cells: A Computational Study

“…The improvements in CPU resources now allow the study, at correlated levels of approximation, of the absorption spectra of large molecular species such as the dyes used in DSSCs. The state-of-the-art computational methodologies based on DFT and TDDFT provide accurate results and reproduce well the optical properties of various Ru II complexes [26][27][28] and metal-free organic dyes, [29][30][31][32][33][34] along with their ground-and excited-state oxidation potentials. The "cyanine-like" molecules, such as TPA dyes, show electronic excitations with large charge-transfer character, which make them difficult to study by DFT calculations.…”

“…Consistent with previous calculations, the B3LYP functional will underestimate the lowest excitation energy for the TPA derivatives. [32] For TPA-AC3, B3LYP predicts l max with a large error of 1 eV relative to the experimental data. Tretiak and Magyar have examined the performance of various TDDFT functionals for the description of electronic excitations in large molecules and clusters.…”

“…However, it has been reported that the usage of "long-range-corrected" functionals and hybrid functionals can provide reasonable optical properties. [32,33] All calculations were performed with the Gaussian 03 and Gaussian 09 packages. [35,36] The ground-state and excited-state geometries of the acene-modified TPA dyes were optimized in the gas phase using DFT or TDDFT calculations with the hybrid B3LYP functional and the standard 6-31G(d,p) basis set.…”

“…The functional and basis set used here are satisfying choices for the geometry optimization of the TPA dyes. [32,37] For the TDDFT calculations, performed on the B3LYP optimized ground-state geometries, four different ex- change-correlation functionals were used: the three hybrid B3LYP, [38] PBE0, [39] and BHandH [40] functionals, including 20, 25, and 50 % of Hartree-Fock (HF) exchange, respectively, and CAM-B3LYP, [41] which is Handy and co-workers' long-range-corrected version of B3LYP using the Coulomb-attenuating method. The visible spectra are very sensitive to the basis set extension, and the simultaneous addition of extra polarization, diffuse, and valence functions would lead to % 15 nm change of spectra.…”

“…However, extra d and f functions would result in negligible refinements. [32] As a compromise between accuracy and basis set size, we chose 6-311G + (2d,2p) for the TDDFT calculations. The DSSCs work in the solvent phase, and the iodine/iodide couple is used as regenerator of the DSSCs.…”