Molecular Engineering of Quinodimethane-based Dyes with Different Anchor Groups for Dye Sensitized Solar Cells (DSSCs)

This paper mainly works on changing different anchoring groups of quinodimethane-based dyes, which represent a twisting phenyl ring during the excitation process, for dye-sensitized solar cells (DSSCs). Five different groups, (1) cyanoacrylic acid (-CHC(CN)COOH), (2) carboxylic acid (-COOH), (3) pyridine (-C5H5N), (4) sulfonic acid (-SO3H), (5) phosphoric acid (-PO3H2), were adopted as anchors on the quinodimethane moieties forming dyes 1-5, to further explore their effects on optoelectronic properties via molecular engineering methods. Density functional theory (DFT) and time-dependent DFT (TD-DFT) were used to calculate their electronic structures and electro-optical properties. Computational results show that all dyes show suitable molecular orbital energy levels between the conductive band of TiO2 and the potentials I-/I3- redox couple so that all of 1-5 can be potential candidates for DSSCs application. Also, TDDFT results exhibit that dye 1 has the widest absorption band in UV-visible region, with the first excitation energies of 2.782 eV, while other dyes show similar first excitation energies about 0.2 eV higher than that of dye 1. In particular, dye 3 with pyridine as anchor group shows a very excellent charge transfer properties from the highest occupied molecular orbitals (HOMO) to the lowest unoccupied molecular orbitals (LUMO), with the highest HOMO level of -5.16 eV, which offers a strong driving force for dye regeneration, and a LUMO level of -2.36 eV, which enables excited electrons successfully to be injected in to titania film. Also, this pyridine anchor group offers a larger π conjugation, which may improve the ability of electron injecting and further strengthen the photovoltaic performance in DSSCs.