Design of (2Z)-2-cyano-2-[2-[(E)-2-[5-[(E)-2-(4-dimethylaminophenyl)vinyl]-2-thienyl]vinyl]pyran-4-ylidene]acetic acid derivatives as D-π-A dye sensitizers in molecular photovoltaics: a density functional theory approach

The use of computational methods such as density functional theory (DFT) in material design has attracted considerable attention aimed at achieving efficient dye-sensitized solar cells (DSSCs). A series of novel (2Z)-2-cyano-2-[2-[(E)-2-[5-[(E)-2-(4-dimethylaminophenyl)vinyl]-2-thienyl]vinyl]pyran-4-ylidene]acetic acid derivatives were simulated using DFT and time-dependent DFT for calculations of molecular properties, electronic properties, optical properties, population analysis, global reactivity indices and light harvesting efficiency (LHE). The results showed that incorporation of an F/CH3 substituent on the acceptor unit increased/decreased the charge density on the acceptor unit, thereby increasing/lowering its tendency to accept electrons from the donor unit through a pi-conjugated linker due to the electron-withdrawing/electron-donating effect of F/CH3; this ultimately increased/decreased the highest occupied molecular orbital and lowest occupied molecular orbital (HOMO-LUMO) energy band gap (E-g). The para-amino substituents on the donor unit drastically increased the natural bond orbital (NBO) charges of both the donor and acceptor units of the dyes, i.e. para-CH3 < para-NH2 < para-N(CH3)(2); this agreed well with the ordering of the band gap. Generally, dyes with para-N(CH3)(2) on the donor subunit have longer light absorption wavelengths, a low E-g and a high LHE, which could lead to enhancement of the photocurrent and charge transfer in DSSCs.