Computational and Spectroscopic Analysis of β‑Indandione Modified Zinc Porphyrins

Porphyrins have characteristic optical properties which give them the potential to be used in a range of applications. In this study, a series of β-indandione modified zinc porphyrins, systematically changed in terms of linker length and substituent, resulted in absorption spectra that are dramatically different than that observed for the parent zinc porphyrin (ZnTXP, 5,10,15,20-tetrakis­(3,5-dimethylphenyl)­porphyrinato zinc­(II)). These changes include strong absorptions at 420, 541, and 681 nm (110.2, 57.5, and 29.2 mM^–1 cm^–1, respectively) for the most perturbed compound. Computational studies were conducted and showed the different optical effects are due to a reorganization of molecular orbitals (MOs) away from Gouterman’s four-orbital model. The substituent effects alter both unoccupied and occupied MOs. An increased length of linker group raised the energy of the HOMO–2 such that it plays a significant role in the observed transitions. The degenerate LUMO (e_g) set are split by substitution, and this splitting may be increased by use of a propylidenodinitrile group, which shows the lowest-energy transitions and the greatest spectral perturbation from the parent zinc porphyrin complex. These data are supported by resonance Raman spectroscopy studies which show distinct enhancement of phenyl modes for high-energy transitions and indandione modes for lower-energy transitions

A Nonconjugated Bridge in Dimer-Sensitized Solar Cells Retards Charge Recombination without Decreasing Charge Injection Efficiency

Dye sensitized solar cells (DSSCs) employing a dimer porphyrin, which was synthesised with two porphyrin units connected without conjugation, have shown that both porphyrin components can contribute to photocurrent generation, that is, more than 50 % internal quantum efficiency. In addition, the open-circuit voltage (<i>V</i>_oc) of the DSSCs was higher than that of DSSCs using monomer porphyrins. In this paper, we first optimized cell structure and fabrication conditions. We obtained more than 80% incident photon to current conversion efficiency from the dimer porphyrin sensitized DSSCs and higher <i>V</i>_oc and energy conversion efficiency than monomer porphyrin sensitized solar cells. To examine the origin of the higher <i>V</i>_oc, we measured electron lifetime in the DSSCs with various conditions, and found that the dimer system increased the electron lifetime by improving the steric blocking effect of the dye layer, whilst the lack of a conjugated linker prevents an increase in the attractive force between conjugated sensitizers and the acceptor species in the electrolyte. The results support a hypothesis; dispersion force is one of the factors influencing the electron lifetime in DSSCs

Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light-Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells

The origin of simultaneous improvements in the short-circuit current density (<i>J</i>_sc) and open-circuit voltage (<i>V</i>_oc) of porphyrin dye-sensitized TiO₂ solar cells following white light illumination was studied by systematic variation of several different device parameters. Reduction of the dye surface loading resulted in greater relative performance enhancements, suggesting open space at the TiO₂ surface expedites the process. Variation of the electrolyte composition and subsequent analysis of the conduction band potential shifts suggested that a light-induced replacement of surface-adsorbed lithium (Li⁺) ions with dimethylpropylimidazolium (DMPIm⁺) ions was responsible for an increased electron lifetime by decreasing the recombination with the redox mediator. Variation of the solvent viscosity was found to affect the illumination time required to generate increased performance, while similar performance enhancements were not replicated by application of negative bias under dark conditions, indicating the light exposure effect was initiated by formation of dye cation molecules following photoexcitation. The substituents and linker group on the porphyrin chromophore were both varied, with light exposure producing increased electron lifetime and <i>V</i>_oc for all dyes; however, increased <i>J</i>_sc values were only measured for dyes containing binding moieties with multiple carboxylic acids. It was proposed that the initial injection limitation and/or fast recombination process in these dyes arises from the presence of lithium at the surface, and the improved injection and/or retardation of fast recombination after light exposure is caused by the Li⁺ removal by cation exchange under illumination