Improvement of the efficiency of thiophene-bridged compounds for dye-sensitized solar cells

A quantum-chemical study is conducted in order to provide UV/Vis absorption spectra (with a ±0.20 eV accuracy) and oxidation potentials (±0.50 eV accuracy) of a series of conjugated metal-free organic dyes containing triphenylamine (TPA) and thiophene (TH) moieties. These compounds have recently been developed for dye sensitized solar cells (DSSCs), and are here compared to the tetrahydroquinoline (THQ) class of dyes. Our theoretical results reveal that TPAs provide a larger ΔG^inject. variability than THQ dyes, and we have therefore chosen to optimize the former structures. Our procedure made it possible to get insights into the geometrical and electronic structures of the dyes, and to unravel the structural modifications needed to optimize the properties of TPA-based DSSCs. In particular, we propose ways to improve the electron injection process, as well as the light harvesting efficiency (LHE) of the dyes. On this purpose, we considered a large set of original compounds, and starting from the TPA structure, were shown to increase the efficiency of the dye: (i) the 18-OH,-COOH, 13,15-diOMe, 1a,1b-diCN functionalization of TPA-2; (ii) the 1a,1b-diCN, 14,15-diOMe,17-CN,18-H,-COOH functionalization of TPA-1, these specific groups inducing a strongly exergonic free enthalpy of injection; (iii) the 18-diCOOH substitution of TPA-2 improves the LHE without suffering a deterioration of the exergonic character of the free enthalpy of injection. Moreover, the molecular topology analysis demonstrates that, due to the lost of coplanarity between the anchoring and the bridging unit, the positive charge is not directly brought in contact with the TiO ₂ surface, consequently limiting the recombination reaction.