Tailoring of energy levels in (2Z)-2-cyano-2-[2-[(E)-2-[2-[(E)-2-(p-tolyl)vinyl]thieno[3,2-b]thiophen-5-yl]vinyl]pyran-4-ylidene]acetic acid derivatives via conjugate bridge and fluorination of acceptor units for effective D-π-A dye-sensitized solar cells: DFT-TDDFT approach

Density functional theory (DFT) was employed to investigate the role of fused thiophene and bridged thiophene pi-linkers as well as acceptor unit fluorination in modifying the properties of dye sensitizers for dye-sensitized solar cells (DSSCs). A series of novel (2Z)-2-cyano-2-[2-[(E)-2-[2-[(E)-2-(p-tolyl)vinyl]thieno[3,2-b]thiophen-5-yl]vinyl]pyran-4-ylidene]acetic acid derivatives were simulated using DFT and time-dependent density functional theory to calculate their electronic and optical properties, population analysis, global reactivity index and light harvesting efficiency. The results showed that dyes with bridged thiophene pi-linker have narrower energy bandgap (E (g)) and longer absorption wavelength (lambda (max)) than those with fused thiophene pi-linker. Also, fluorination of the acceptor unit of the dyes enhanced the electron accepting ability of 2-cyano-2-pyran-4-ylidene-acetic acid by lowering the lowest unoccupied molecular orbital (LUMO) energy, which leads to lower E (g), lower chemical hardness (eta), and longer wavelength. Therefore, incorporation of fluorine atoms at the acceptor unit makes the conduction-band potential more favorable, leading to effective charge separation and charge transfer between donor and acceptor.