Theoretical study of triphenylamine-based organic dyes with mono-, di-, and tri-anchoring groups for dye-sensitized solar cells

In this report, DFT and TDDFT calculations used to explain the noticeable difference in the power conversion efficiencies (PCEs) of three reported triphenylamine-based organic dyes with mono-, di-, and tri-cyanoacrylic acid units as anchoring groups, and thiophene unit as pi-bridges used as potential dyes in dye-sensitized solar cell applications. These dyes were showed the superiority of the multi-anchoring systems over the mono- anchoring system. The light was shed on the effect of various functionals, e.g., B3LYP, CAM-B3LYP,, M06, PBE0 and BHandHLYP on absorption spectra. The CAM-B3LYP functional was depicted the best among numerous adopted functionals which reproduced the experimental absorption data. The geometries, electronic structures and UV-Vis absorption spectra of the isolated dyes and their adsorbed dye@TiO2 complexes were calculated. The associated physical parameters that control the effectiveness of the cell, such as the light harvesting efficiency (LHE), hole and electron reorganization energies (lambda(hole), and lambda(ele)), and the driving force for electron injection (Delta G(inj)) and dye regeneration (Delta G(reg)) were computed and analyzed in order to determine their contributions to the open-circuit photovoltage (V-oc) or the short-circuit photocurrent density (J(sc)), and correlate the structure with property. Also, charge transfer descriptors, such as the charge transfer distance (D-CT), the amount of charge transferred (q(CT)), and dipole moment (mu(CT)) were investigated. The results reveal that the increasing of anchoring groups made significant influence on the PCE of the cells. The effect of mono-, di-, and tri-substituted anchored cyanoacrylic acid group(s) on the triphenylamine-TiO2 complexes has been explored on the frontier molecular orbitals, binding energies and absorption wavelengths. Among all the studied derivatives, electron injection energy barrier for TPA3T3A has been found the smallest which revealed that electron injection in this compound might be superior to other counterparts. Low-lying lowest unoccupied molecular orbital of TPA3T3A would favor it to stabilize thermodynamically. Moreover, TPA3T3A@TiO2 cluster was observed the most stable adsorption complex especially anchoring of TPA3T3A sensitizer at TiO2 cluster from B-side. Comprehensible intramolecular charge transfer has been observed from occupied to unoccupied molecular orbitals. The red shift in the absorption spectra has been perceived by increasing the number of anchoring groups, i.e., cyanoacrylic acid. The type-II band gap alignment between triphenylamine-based organic sensitizers and TiO2 is revealing that photovoltaic efficiency in these studied derivatives would be enhanced. Our quantum chemical analysis explored that these TPA derivatives especially tri-anchored dye would be proficient to be used in photovoltaic devices which would enhance the solar cell efficiency.