RESEARCH & LAB

A series of p-extended salophen-type Schiff-base zinc(II) complexes, e.g., zinc-salophen complexes (ZSC), were investigated toward potential applications for dye-sensitized solar cells. The ZSC dyes adopt linear-, X-, or p-shaped geometries either with the functionalization of 1 donor/1 acceptor or 2 donors/2 acceptors to achieve a push–pull type molecular structure. The frontier molecular orbitals, light-harvesting properties as well as charge transfer characters against regio-specific substitution of donor/acceptor groups were studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results reveal that all ZSC dyes of D-ZnS-p-A geometry (where D, S, and A denote to donor, salophen ligand, and acceptor, respectively) exhibit relatively lower HOMO energy compared to the structurally resembled porphyrin dye YD2-o-C8. Natural transition orbital (NTO) and electron–hole separation (EHS) approaches clearly differentiate the linear type YD-series dyes from CL-, AJ1-, and AJ2-series dyes because of poor charge transfer (CT) properties. In contrast, the p-shaped AJ2-series and X-shaped AJ1-series dyes outperform the others in a manner of stronger CT characteristics, broadened UV-vis absorption as well as tunable bandgap simply via substitution of p-ethynylbenzoic acids (EBAs) and arylamine donors at salophen 7,8- and 2,3,12,13-positions, respectively. Both EHS and calculated exciton binding energies suggest the strength of CT character for ZSC dyes with an amino donor in the trend TPA > AN > DPA. This work has provided clear illustration toward molecular design of efficient dyes featuring a zinc-salophen backbone.

In the study of dye-sensitized solar cells (DSCs), developing computational predictions of key properties of the dyes is highly desirable for identifying promising candidates. In this work, we report first-principle-based, theoretically estimated charge-recombination (CR) rates from TiO2 to the cationic dye for 2 pairs of organic dye molecules. A recently developed multistate fragment charge difference (msFCD) scheme, together with long-range-corrected time-dependent density functional theory, was used to calculate the electronic coupling. The msFCD scheme removes the local excitation components in the charge-transfer states, and generates acceptable diabatic states. The range-separated ωPBE and BNL functionals were useful for the charge-transfer problem, and results were largely independent of TiO2–dye binding modes. The rates obtained for CR to oxidized dye (CRD) followed a trend similar to experimental results. In general, a difference in the reorganization energy (λ) and the free energy (ΔG0) had a large effect on electron transfer rates. However, electronic coupling strength could also have a dominant role over (λ+ΔG0) in the CRD rate. We report a generally applicable ab initio approach to predict CRD rate and explored the potential roles of coupling factors in the performance of DSCs.

A series of zinc porphyrin dyes YD22–YD28 were synthesized and used for dye-sensitized solar cells. Dyes YD26–YD28 consist of zinc porphyrin (ZnP) as core unit, arylamine (Am) as electron-donating group, and p-ethynylbenzoic acid (EBA) as an electron-withdrawing/-anchoring group. The dyes YD22–YD25 contain additional phenylethynylene group (PE) bridged between Am and ZnP units. The influence of the PE unit on molecular properties as well as photovoltaic performances were investigated via photophysical and electrochemical studies and density functional calculations. With the insertion of PE unit, the dyes YD22–YD25 possess better light-harvesting properties in terms of significantly red-shifted Q-band absorption. The conversion efficiencies for dyes YD22–YD25 are better than those of dyes YD26–YD28 owing to larger JSC output. Natural transition orbitals and Mulliken charge analysis were used to analyze the electron injection efficiency for porphyrin dyes upon time-dependent DFT calculations. The results indicated that insertion of additional PE unit is beneficial to higher JSC by means of improved light-harvesting property due to broadened and red-shifted absorption.