Meta versus para substituent effect of organic dyes for sensitized solar cells

A series of new metal-free organic donor-bridge-acceptor dyes comprising a triphenylamine moiety as the electron donor, and a cyanoacrylic acid (dye series A) or a carboxylic acid moiety (dye series B) as the electron acceptor were synthesized and utilized for dye-sensitized solar cells (DSSCs). The triphenylamine moiety was linked to the main chromophore through either a para or a meta position across a phenyl group in order to examine the difference of structural effect. Both types of compounds exhibited apparent solvatochromic shift in their fluorescence spectra, showing the ready formation of charge separated states in both the para and meta-isomers. Quantum mechanical calculations were performed by using the density functional theory (DFT) at the B3LYP/6-31G(d,p) level to gain insight into the electron distributions surrounding the para and meta-substituted triphenylamine groups. The movement of an electron from the donor to the acceptor under the irradiation of light can be depicted by the time-dependent DFT (TDDFT) calculations, and the results were compared with experimental observations. These dyes were used effectively as sensitizers in DSSCs after being absorbed on surface of nanocrystalline TiO2. The incident photo-to-current conversion efficiency (IPCE) spectra of the para isomers are broader than that of the corresponding meta isomers, and the short-circuit current density (Jsc) values of the former is also higher than the latter. Dyes of series A exhibited overall conversion efficiencies of 1.27–4.12%, depending on the relative values of Jsc, under AM 1.5 G irradiation (100 mW cm−2). For dyes of series B, the para isomers showed overall better performance than the meta isomers, i.e., higher values for all three parameters Jsc, Voc (open-circuit voltage), and ff (fill factor), thus led to a higher conversion efficiency.

Graphical abstract.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Dyes with either a para or a meta connectivity across a phenyl group are compared for DSSC performance. ► The excited state lifetimes of meta-isomers are longer than the corresponding para ones, indicating a more stabilized charge separation upon photo-excitation. ► The para isomers showed overall better performance than the meta isomers. ► At certain wavelength region where the meta-isomers exhibit high absorption, their IPCE values are higher than those of para-isomers.